diff --git a/.clangd b/.clangd
deleted file mode 100644
index ba4cbb36..00000000
--- a/.clangd
+++ /dev/null
@@ -1,2 +0,0 @@
-Diagnostics:
-  UnusedIncludes: None
diff --git a/.github/workflows/publish-clice.yml b/.github/workflows/publish-clice.yml
index 622b96fa..f8c319f3 100644
--- a/.github/workflows/publish-clice.yml
+++ b/.github/workflows/publish-clice.yml
@@ -46,7 +46,7 @@ jobs:
 
       - uses: ./.github/actions/setup-pixi
         with:
-          environments: develop
+          environments: package
 
       - name: Remove ci llvm toolchain on Windows
         if: runner.os == 'Windows'
diff --git a/.github/workflows/test-cmake.yml b/.github/workflows/test-cmake.yml
index c51d6aaf..92877402 100644
--- a/.github/workflows/test-cmake.yml
+++ b/.github/workflows/test-cmake.yml
@@ -21,8 +21,6 @@ jobs:
         uses: actions/checkout@v4
 
       - uses: ./.github/actions/setup-pixi
-        with:
-          environments: develop
 
       - name: Build
         run: pixi run build ${{ matrix.build_type }} ON
diff --git a/.github/workflows/test-xmake.yml b/.github/workflows/test-xmake.yml
index f5064c39..1a0427f3 100644
--- a/.github/workflows/test-xmake.yml
+++ b/.github/workflows/test-xmake.yml
@@ -30,8 +30,6 @@ jobs:
           build-cache-key: ${{ matrix.os }}-${{ matrix.build_type }}
 
       - uses: ./.github/actions/setup-pixi
-        with:
-          environments: develop
 
       - name: Build
         run: pixi run xmake ${{ matrix.build_type }}
diff --git a/.gitignore b/.gitignore
index 8a8c67b2..1caf2a78 100644
--- a/.gitignore
+++ b/.gitignore
@@ -60,6 +60,7 @@ tests/unit/Local/
 .vs/
 .idea/
 .claude
+.clangd
 
 # pixi environments
 .env
diff --git a/docs/en/dev/build.md b/docs/en/dev/build.md
index ba1a037e..e561f094 100644
--- a/docs/en/dev/build.md
+++ b/docs/en/dev/build.md
@@ -30,7 +30,7 @@ pixi run integration-test Debug
 ```
 
 > [!TIP]
-> If you want to develop directly with `cmake`, `ninja`, `clang++`, etc., run `pixi shell -e develop` to enter a shell with all env vars configured.
+> If you want to develop directly with `cmake`, `ninja`, `clang++`, etc., run `pixi shell` to enter a shell with all env vars configured.
 
 ### XMake
 
diff --git a/docs/zh/dev/build.md b/docs/zh/dev/build.md
index b46d3abd..48e3eb65 100644
--- a/docs/zh/dev/build.md
+++ b/docs/zh/dev/build.md
@@ -30,7 +30,7 @@ pixi run integration-test Debug
 ```
 
 > [!TIP]
-> 如果你想直接使用 `cmake`, `ninja`, `clang++` 等命令进行开发，请运行 `pixi shell -e develop` 进入已配置好环境变量的终端
+> 如果你想直接使用 `cmake`, `ninja`, `clang++` 等命令进行开发，请运行 `pixi shell` 进入已配置好环境变量的终端
 
 ### XMake
 
diff --git a/include/AST/FilterASTVisitor.h b/include/AST/FilterASTVisitor.h
index 7c036002..ea45b142 100644
--- a/include/AST/FilterASTVisitor.h
+++ b/include/AST/FilterASTVisitor.h
@@ -14,7 +14,7 @@ class FilteredASTVisitor : public clang::RecursiveASTVisitor<Derived> {
 public:
     using Base = clang::RecursiveASTVisitor<Derived>;
 
-    FilteredASTVisitor(CompilationUnit& unit, bool interested_only) :
+    FilteredASTVisitor(CompilationUnitRef unit, bool interested_only) :
         unit(unit), interested_only(interested_only) {}
 
 #define CHECK_DERIVED_IMPL(func)                                                                   \
@@ -181,7 +181,7 @@ public:
 #undef CHECK_DERIVED_IMPL
 
 protected:
-    CompilationUnit& unit;
+    CompilationUnitRef unit;
     bool interested_only;
 };
 
diff --git a/include/AST/Selection.h b/include/AST/Selection.h
index 39b34127..d7ead6f7 100644
--- a/include/AST/Selection.h
+++ b/include/AST/Selection.h
@@ -11,7 +11,7 @@
 
 namespace clice {
 
-class CompilationUnit;
+class CompilationUnitRef;
 
 /// A selection can partially or completely cover several AST nodes.
 /// The SelectionTree contains nodes that are covered, and their parents.
@@ -47,7 +47,7 @@ public:
     /// - Func should return true on success (stop) and false on failure (continue)
     ///
     /// Always yields at least one tree. If no tokens are touched, it is empty.
-    static bool create_each(CompilationUnit& unit,
+    static bool create_each(CompilationUnitRef unit,
                             LocalSourceRange range,
                             llvm::function_ref<bool(SelectionTree)> callback);
 
@@ -55,7 +55,7 @@ public:
     ///
     /// Where ambiguous (range is empty and borders two tokens), prefer the token
     /// on the right.
-    static SelectionTree create_right(CompilationUnit& unit, LocalSourceRange range);
+    static SelectionTree create_right(CompilationUnitRef unit, LocalSourceRange range);
 
     /// Copies are no good - contain pointers to other nodes.
     SelectionTree(const SelectionTree&) = delete;
@@ -139,7 +139,7 @@ public:
 private:
     // Creates a selection tree for the given range in the main file.
     // The range includes bytes [Start, End).
-    SelectionTree(CompilationUnit& unit, LocalSourceRange range);
+    SelectionTree(CompilationUnitRef unit, LocalSourceRange range);
 
     // Stable-pointer storage, FIXME: use memory pool instead?
     std::deque<Node> nodes;
diff --git a/include/AST/Semantic.h b/include/AST/Semantic.h
index 12cbf943..9393bff5 100644
--- a/include/AST/Semantic.h
+++ b/include/AST/Semantic.h
@@ -13,7 +13,7 @@ class SemanticVisitor : public FilteredASTVisitor<SemanticVisitor<Derived>> {
 public:
     using Base = FilteredASTVisitor<SemanticVisitor>;
 
-    SemanticVisitor(CompilationUnit& unit, bool interested_only) :
+    SemanticVisitor(CompilationUnitRef unit, bool interested_only) :
         Base(unit, interested_only), unit(unit), resolver(unit.resolver()) {}
 
 public:
@@ -725,7 +725,7 @@ public:
     }
 
 protected:
-    CompilationUnit& unit;
+    CompilationUnitRef unit;
     TemplateResolver& resolver;
     llvm::SmallVector<clang::Decl*> decls;
 };
diff --git a/include/AST/SourceCode.h b/include/AST/SourceCode.h
index 771fc832..a07e9ef6 100644
--- a/include/AST/SourceCode.h
+++ b/include/AST/SourceCode.h
@@ -110,7 +110,7 @@ struct Token {
         return is_at_start_of_line && kind == clang::tok::hash;
     }
 
-    /// The tokens after the include diretive are regarded as
+    /// The tokens after the include directive are regarded as
     /// a whole token, whose kind is `header_name`. For example
     /// `<iostream>` and `"test.h"` are both header name.
     bool is_header_name() const {
diff --git a/include/Compiler/Compilation.h b/include/Compiler/Compilation.h
index f608343d..86bed0f3 100644
--- a/include/Compiler/Compilation.h
+++ b/include/Compiler/Compilation.h
@@ -15,7 +15,7 @@ namespace clice {
 
 struct CompilationParams {
     /// The kind of this compilation.
-    CompilationUnit::Kind kind;
+    CompilationKind kind;
 
     /// Whether to run clang-tidy.
     bool clang_tidy = false;
@@ -48,9 +48,6 @@ struct CompilationParams {
     /// to cancel old compilation task.
     std::shared_ptr<std::atomic_bool> stop = std::make_shared<std::atomic_bool>(false);
 
-    /// Store all compilation errors in the process.
-    std::shared_ptr<std::vector<Diagnostic>> diagnostics;
-
     void add_remapped_file(llvm::StringRef path,
                            llvm::StringRef content,
                            std::uint32_t bound = -1) {
@@ -62,23 +59,21 @@ struct CompilationParams {
     }
 };
 
-using CompilationResult = std::expected<CompilationUnit, std::string>;
-
 /// Only preprocess ths source flie.
-CompilationResult preprocess(CompilationParams& params);
+CompilationUnit preprocess(CompilationParams& params);
 
 /// Build AST from given file path and content. If pch or pcm provided, apply them to the compiler.
 /// Note this function will not check whether we need to update the PCH or PCM, caller should check
 /// their reusability and update in time.
-CompilationResult compile(CompilationParams& params);
+CompilationUnit compile(CompilationParams& params);
 
 /// Build PCH from given file path and content.
-CompilationResult compile(CompilationParams& params, PCHInfo& out);
+CompilationUnit compile(CompilationParams& params, PCHInfo& out);
 
 /// Build PCM from given file path and content.
-CompilationResult compile(CompilationParams& params, PCMInfo& out);
+CompilationUnit compile(CompilationParams& params, PCMInfo& out);
 
 /// Run code completion at the given location.
-CompilationResult complete(CompilationParams& params, clang::CodeCompleteConsumer* consumer);
+CompilationUnit complete(CompilationParams& params, clang::CodeCompleteConsumer* consumer);
 
 }  // namespace clice
diff --git a/include/Compiler/CompilationUnit.h b/include/Compiler/CompilationUnit.h
index 2fc82833..8187c624 100644
--- a/include/Compiler/CompilationUnit.h
+++ b/include/Compiler/CompilationUnit.h
@@ -12,45 +12,71 @@
 
 namespace clice {
 
-/// All AST related information needed for language server.
-class CompilationUnit {
+enum class CompilationKind : std::uint8_t {
+    /// From preprocessing the source file. Therefore directives
+    /// are available but AST nodes are not.
+    Preprocess,
+
+    /// From indexing the static source file.
+    Indexing,
+
+    /// From building preamble for the source file.
+    Preamble,
+
+    /// From building precompiled module for the module interface unit.
+    ModuleInterface,
+
+    /// From building normal AST for source file(except preamble), interested file and top level
+    /// declarations are available.
+    Content,
+
+    /// From running code completion for the source file(preamble is applied).
+    Completion,
+};
+
+enum class CompilationStatus : std::uint8_t {
+    Completed,
+    Cancelled,
+    SetupFail,
+    FatalError,
+};
+
+class CompilationUnitRef {
 public:
-    /// The kind describes how we preprocess this source file
-    /// to get this compilation unit.
-    enum class Kind : std::uint8_t {
-        /// From preprocessing the source file. Therefore directives
-        /// are available but AST nodes are not.
-        Preprocess,
+    struct Self;
 
-        /// From indexing the static source file.
-        Indexing,
+    CompilationUnitRef(Self* self) : self(self) {}
 
-        /// From building preamble for the source file.
-        Preamble,
-
-        /// From building precompiled module for the module interface unit.
-        ModuleInterface,
-
-        /// From building normal AST for source file(except preamble), interested file and top level
-        /// declarations are available.
-        Content,
-
-        /// From running code completion for the source file(preamble is applied).
-        Completion,
-    };
-
-    using enum Kind;
-    struct Impl;
-
-    CompilationUnit(Kind kind, Impl* impl) : kind(kind), impl(impl) {}
-
-    CompilationUnit(const CompilationUnit&) = delete;
-
-    CompilationUnit(CompilationUnit&& other) : kind(other.kind), impl(other.impl) {
-        other.impl = nullptr;
+    Self* operator->() {
+        return self;
     }
 
-    ~CompilationUnit();
+public:
+    CompilationKind kind();
+
+    CompilationStatus status();
+
+    /// Parse finished; ASTContext is usable but diagnostics may still contain errors.
+    bool completed() {
+        return status() == CompilationStatus::Completed;
+    }
+
+    /// Compilation was cancelled; consumers should not touch any state.
+    bool cancelled() {
+        return status() == CompilationStatus::Cancelled;
+    }
+
+    /// Failed during initial setup; diagnostics exist (location-free), ASTContext
+    /// is unavailable.
+    bool setup_fail() {
+        return status() == CompilationStatus::SetupFail;
+    }
+
+    /// Hit an unrecoverable error; diagnostics and decoded source locations
+    /// are usable, other states are not unavailable.
+    bool fatal_error() {
+        return status() == CompilationStatus::FatalError;
+    }
 
 public:
     /// Get the file id for given file. If such file doesn't exist, the result
@@ -165,7 +191,7 @@ public:
     bool is_module_interface_unit();
 
     /// Return all diagnostics in the process of compilation.
-    auto diagnostics() -> llvm::ArrayRef<Diagnostic>;
+    auto diagnostics() -> std::vector<Diagnostic>&;
 
     auto top_level_decls() -> llvm::ArrayRef<clang::Decl*>;
 
@@ -196,10 +222,22 @@ public:
     /// Get symbol ID for given marco.
     index::SymbolID getSymbolID(const clang::MacroInfo* macro);
 
-private:
-    Kind kind;
+protected:
+    Self* self;
+};
 
-    Impl* impl;
+/// All AST related information needed for language server.
+class CompilationUnit : public CompilationUnitRef {
+public:
+    explicit CompilationUnit(Self* impl) : CompilationUnitRef(impl) {}
+
+    CompilationUnit(const CompilationUnit&) = delete;
+
+    CompilationUnit(CompilationUnit&& other) : CompilationUnitRef(other.self) {
+        other.self = nullptr;
+    }
+
+    ~CompilationUnit();
 };
 
 }  // namespace clice
diff --git a/include/Compiler/Diagnostic.h b/include/Compiler/Diagnostic.h
index e7892c40..400bccbf 100644
--- a/include/Compiler/Diagnostic.h
+++ b/include/Compiler/Diagnostic.h
@@ -4,15 +4,6 @@
 #include <string>
 
 #include "AST/SourceCode.h"
-#include "Compiler/Tidy.h"
-
-#include "clang/Basic/Diagnostic.h"
-
-namespace clang {
-
-class DiagnosticConsumer;
-
-}
 
 namespace clice {
 
@@ -58,11 +49,6 @@ struct DiagnosticID {
     bool is_unused() const;
 };
 
-class DiagnosticCollector : public clang::DiagnosticConsumer {
-public:
-    tidy::ClangTidyChecker* checker = nullptr;
-};
-
 struct Diagnostic {
     /// The diagnostic id.
     DiagnosticID id;
@@ -76,9 +62,6 @@ struct Diagnostic {
 
     /// The error message of this diagnostic.
     std::string message;
-
-    static std::unique_ptr<DiagnosticCollector>
-        create(std::shared_ptr<std::vector<Diagnostic>> diagnostics);
 };
 
 }  // namespace clice
diff --git a/include/Compiler/Directive.h b/include/Compiler/Directive.h
index 7b8eda64..62b63a80 100644
--- a/include/Compiler/Directive.h
+++ b/include/Compiler/Directive.h
@@ -135,10 +135,6 @@ struct Directive {
     std::vector<MacroRef> macros;
     std::vector<Pragma> pragmas;
     std::vector<Import> imports;
-
-    /// Tell preprocessor to collect directives information and store them in `directives`.
-    static void attach(clang::Preprocessor& pp,
-                       llvm::DenseMap<clang::FileID, Directive>& directives);
 };
 
 }  // namespace clice
diff --git a/include/Compiler/Tidy.h b/include/Compiler/Tidy.h
deleted file mode 100644
index 95781d8e..00000000
--- a/include/Compiler/Tidy.h
+++ /dev/null
@@ -1,26 +0,0 @@
-#pragma once
-
-#include <memory>
-
-#include "llvm/ADT/StringRef.h"
-
-namespace clang {
-
-class CompilerInstance;
-
-}
-
-namespace clice::tidy {
-
-bool is_registered_tidy_check(llvm::StringRef check);
-std::optional<bool> is_fast_tidy_check(llvm::StringRef check);
-
-struct TidyParams {};
-
-class ClangTidyChecker;
-
-/// Configure to run clang-tidy on the given file.
-std::unique_ptr<ClangTidyChecker> configure(clang::CompilerInstance& instance,
-                                            const TidyParams& params);
-
-}  // namespace clice::tidy
diff --git a/include/Feature/Diagnostic.h b/include/Feature/Diagnostic.h
index 1589aae4..e1c278c8 100644
--- a/include/Feature/Diagnostic.h
+++ b/include/Feature/Diagnostic.h
@@ -6,7 +6,7 @@
 
 namespace clice {
 
-struct CompilationUnit;
+class CompilationUnitRef;
 
 }
 
@@ -14,6 +14,6 @@ namespace clice::feature {
 
 /// FIXME: This is not correct way, we don't want to couple
 /// `Feature with Protocol`? Return an array of LSP diagnostic.
-json::Value diagnostics(PositionEncodingKind kind, PathMapping mapping, CompilationUnit& unit);
+json::Value diagnostics(PositionEncodingKind kind, PathMapping mapping, CompilationUnitRef unit);
 
 }  // namespace clice::feature
diff --git a/include/Feature/DocumentLink.h b/include/Feature/DocumentLink.h
index 7ae200ab..f0901e8b 100644
--- a/include/Feature/DocumentLink.h
+++ b/include/Feature/DocumentLink.h
@@ -18,9 +18,9 @@ struct DocumentLink {
 using DocumentLinks = std::vector<DocumentLink>;
 
 /// Generate document link for main file.
-DocumentLinks document_links(CompilationUnit& unit);
+DocumentLinks document_links(CompilationUnitRef unit);
 
 /// Generate document link for all source file.
-index::Shared<DocumentLinks> index_document_link(CompilationUnit& unit);
+index::Shared<DocumentLinks> index_document_link(CompilationUnitRef unit);
 
 }  // namespace clice::feature
diff --git a/include/Feature/DocumentSymbol.h b/include/Feature/DocumentSymbol.h
index 724666a4..f91f043c 100644
--- a/include/Feature/DocumentSymbol.h
+++ b/include/Feature/DocumentSymbol.h
@@ -29,9 +29,9 @@ struct DocumentSymbol {
 using DocumentSymbols = std::vector<DocumentSymbol>;
 
 /// Generate document symbols for only interested file.
-DocumentSymbols document_symbols(CompilationUnit& unit);
+DocumentSymbols document_symbols(CompilationUnitRef unit);
 
 /// Generate document symbols for all file in unit.
-index::Shared<DocumentSymbols> index_document_symbol(CompilationUnit& unit);
+index::Shared<DocumentSymbols> index_document_symbol(CompilationUnitRef unit);
 
 }  // namespace clice::feature
diff --git a/include/Feature/FoldingRange.h b/include/Feature/FoldingRange.h
index 4ab46b18..8986a48b 100644
--- a/include/Feature/FoldingRange.h
+++ b/include/Feature/FoldingRange.h
@@ -47,9 +47,9 @@ struct FoldingRange {
 using FoldingRanges = std::vector<FoldingRange>;
 
 /// Generate folding range for interested file only.
-FoldingRanges folding_ranges(CompilationUnit& unit);
+FoldingRanges folding_ranges(CompilationUnitRef unit);
 
 /// Generate folding range for all files.
-index::Shared<FoldingRanges> index_folding_range(CompilationUnit& unit);
+index::Shared<FoldingRanges> index_folding_range(CompilationUnitRef unit);
 
 }  // namespace clice::feature
diff --git a/include/Feature/Hover.h b/include/Feature/Hover.h
index 78e3569c..d7670964 100644
--- a/include/Feature/Hover.h
+++ b/include/Feature/Hover.h
@@ -66,9 +66,9 @@ struct Hover {
 };
 
 /// Generate the hover information for the given declaration(for test).
-Hover hover(CompilationUnit& unit, const clang::NamedDecl* decl);
+Hover hover(CompilationUnitRef unit, const clang::NamedDecl* decl);
 
 /// Generate the hover information for the symbol at the given offset.
-Hover hover(CompilationUnit& unit, std::uint32_t offset);
+Hover hover(CompilationUnitRef unit, std::uint32_t offset);
 
 }  // namespace clice::feature
diff --git a/include/Feature/InlayHint.h b/include/Feature/InlayHint.h
index eb007c55..a6b2013e 100644
--- a/include/Feature/InlayHint.h
+++ b/include/Feature/InlayHint.h
@@ -50,7 +50,7 @@ struct InlayHint {
     std::vector<index::SymbolID> parts;
 };
 
-auto inlay_hints(CompilationUnit& unit,
+auto inlay_hints(CompilationUnitRef unit,
                  LocalSourceRange target,
                  const config::InlayHintsOptions& options) -> std::vector<InlayHint>;
 
diff --git a/include/Feature/SemanticToken.h b/include/Feature/SemanticToken.h
index 737322b2..5e2ecb23 100644
--- a/include/Feature/SemanticToken.h
+++ b/include/Feature/SemanticToken.h
@@ -21,9 +21,9 @@ struct SemanticToken {
 using SemanticTokens = std::vector<SemanticToken>;
 
 /// Generate semantic tokens for the interested file only.
-SemanticTokens semantic_tokens(CompilationUnit& unit);
+SemanticTokens semantic_tokens(CompilationUnitRef unit);
 
 /// Generate semantic tokens for all files.
-index::Shared<SemanticTokens> index_semantic_token(CompilationUnit& unit);
+index::Shared<SemanticTokens> index_semantic_token(CompilationUnitRef unit);
 
 }  // namespace clice::feature
diff --git a/include/Index/IncludeGraph.h b/include/Index/IncludeGraph.h
index 9f191d25..2cfbaf2a 100644
--- a/include/Index/IncludeGraph.h
+++ b/include/Index/IncludeGraph.h
@@ -8,7 +8,7 @@
 
 namespace clice {
 
-class CompilationUnit;
+class CompilationUnitRef;
 
 }
 
@@ -41,7 +41,7 @@ struct IncludeGraph {
     /// context. A map between FileID and its include location.
     llvm::DenseMap<clang::FileID, std::uint32_t> file_table;
 
-    static IncludeGraph from(CompilationUnit& unit);
+    static IncludeGraph from(CompilationUnitRef unit);
 
     llvm::StringRef path(std::uint32_t path_ref) const {
         assert(path_ref < paths.size());
diff --git a/include/Index/Shared.h b/include/Index/Shared.h
index 5c949854..a943671b 100644
--- a/include/Index/Shared.h
+++ b/include/Index/Shared.h
@@ -5,7 +5,7 @@
 
 namespace clice {
 
-class CompilationUnit;
+class CompilationUnitRef;
 
 }
 
diff --git a/include/Index/TUIndex.h b/include/Index/TUIndex.h
index cd93ba4e..4a0a4b6f 100644
--- a/include/Index/TUIndex.h
+++ b/include/Index/TUIndex.h
@@ -75,7 +75,7 @@ struct TUIndex {
 
     FileIndex main_file_index;
 
-    static TUIndex build(CompilationUnit& unit);
+    static TUIndex build(CompilationUnitRef unit);
 };
 
 }  // namespace clice::index
diff --git a/include/Server/Server.h b/include/Server/Server.h
index 7910a93f..9a2794ff 100644
--- a/include/Server/Server.h
+++ b/include/Server/Server.h
@@ -30,10 +30,6 @@ struct OpenFile {
     async::Task<> ast_build_task;
     async::Lock ast_built_lock;
 
-    /// Collect all diagnostics in the compilation.
-    std::shared_ptr<std::vector<Diagnostic>> diagnostics =
-        std::make_unique<std::vector<Diagnostic>>();
-
     /// For header with context, it may have multiple ASTs, use
     /// an chain to store them.
     std::unique_ptr<OpenFile> next;
diff --git a/pixi.lock b/pixi.lock
index a502a9c4..bdbbd336 100644
--- a/pixi.lock
+++ b/pixi.lock
@@ -1,153 +1,6 @@
 version: 6
 environments:
   default:
-    channels:
-    - url: https://conda.anaconda.org/conda-forge/
-    options:
-      pypi-prerelease-mode: if-necessary-or-explicit
-    packages:
-      linux-64:
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/_libgcc_mutex-0.1-conda_forge.tar.bz2
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/_openmp_mutex-4.5-2_gnu.tar.bz2
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/binutils_impl_linux-64-2.45-default_hfdba357_104.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/bzip2-1.0.8-hda65f42_8.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/c-ares-1.34.6-hb03c661_0.conda
-      - conda: https://conda.anaconda.org/conda-forge/noarch/ca-certificates-2025.11.12-hbd8a1cb_0.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/clang-20-20.1.8-default_h99862b1_5.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/clang-20.1.8-default_h36abe19_5.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/clangxx-20.1.8-default_h363a0c9_5.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/cmake-4.2.1-hc85cc9f_0.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/compiler-rt-20.1.8-hb700be7_1.conda
-      - conda: https://conda.anaconda.org/conda-forge/noarch/compiler-rt_linux-64-20.1.8-hffcefe0_1.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/gcc-14.2.0-h96c4ede_2.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/gcc_impl_linux-64-14.2.0-hdb7739f_2.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/gxx-14.2.0-h96c4ede_2.conda
-      - conda: https://conda.anaconda.org/conda-forge/linux-64/gxx_impl_linux-64-14.2.0-h2ead766_2.conda
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   size: 6535168
   timestamp: 1766863032287
-- conda: https://conda.anaconda.org/conda-forge/noarch/tzdata-2025b-h78e105d_0.conda
-  sha256: 5aaa366385d716557e365f0a4e9c3fca43ba196872abbbe3d56bb610d131e192
-  md5: 4222072737ccff51314b5ece9c7d6f5a
-  license: LicenseRef-Public-Domain
-  purls: []
-  size: 122968
-  timestamp: 1742727099393
 - conda: https://conda.anaconda.org/conda-forge/noarch/tzdata-2025b-h8577fbf_1.conda
   sha256: 865716d3e2ccaca1218462645830d2370ab075a9a118c238728e1231a234bc6c
   md5: e4e8496b68cf5f25e76fbe67f3856550
@@ -3014,6 +3013,12 @@ packages:
   purls: []
   size: 119204
   timestamp: 1765745742795
+- conda: https://conda.anaconda.org/conda-forge/noarch/tzdata-2025c-hc9c84f9_1.conda
+  sha256: 1d30098909076af33a35017eed6f2953af1c769e273a0626a04722ac4acaba3c
+  md5: ad659d0a2b3e47e38d829aa8cad2d610
+  license: LicenseRef-Public-Domain
+  size: 119135
+  timestamp: 1767016325805
 - conda: https://conda.anaconda.org/conda-forge/win-64/ucrt-10.0.26100.0-h57928b3_0.conda
   sha256: 3005729dce6f3d3f5ec91dfc49fc75a0095f9cd23bab49efb899657297ac91a5
   md5: 71b24316859acd00bdb8b38f5e2ce328
diff --git a/pixi.toml b/pixi.toml
index 2fb88e04..c7c2ef4b 100644
--- a/pixi.toml
+++ b/pixi.toml
@@ -1,3 +1,5 @@
+# tombi: format.rules.table-keys-order.disabled = true
+
 [workspace]
 name = "clice"
 version = "0.1.0"
@@ -15,30 +17,13 @@ channels = ["conda-forge"]
 platforms = ["win-64", "linux-64", "osx-arm64"]
 
 [environments]
-default = ["build"]
-develop = ["build", "test"]
+default = ["build", "test"]
 package = ["build", "test", "package"]
-format = ["format"]
 node = ["node"]
-
-[tasks.build-llvm]
-cmd = ["python3", "scripts/build-llvm.py"]
-
-[tasks.upload-llvm]
-args = ["workflow_id", "llvm_version", { "arg" = "branch", default = "main" }]
-cmd = """
-gh workflow run upload-llvm.yml \
-    --ref {{ branch }} \
-    --field workflow_id={{workflow_id}} \
-    --field version={{llvm_version}}
-"""
-
-[tasks.delete-artifacts]
-args = ["file_name"]
-cmd = ["scripts/delete-artifacts.bash", "{{ file_name }}"]
+format = ["format"]
 
 # ============================================================================== #
-#                             BUILD ENVIRONMENT                                  #
+#                                  DEPENDENCIES                                  #
 # ============================================================================== #
 [feature.build.dependencies]
 python = ">=3.13"
@@ -61,6 +46,13 @@ scripts = ["scripts/activate_linux.sh"]
 [feature.build.target.win-64.activation]
 scripts = ["scripts/activate_asan.bat"]
 
+[feature.test.pypi-dependencies]
+pytest = "*"
+pytest-asyncio = ">=1.1.0"
+
+[feature.package.dependencies]
+xz = ">=5.8.1,<6"
+
 # ============================================================================== #
 #                                   CMAKE                                        #
 # ============================================================================== #
@@ -97,14 +89,14 @@ depends-on = [
 args = [{ arg = "type", default = "RelWithDebInfo" }]
 cmd = './build/{{ type }}/bin/unit_tests --test-dir="./tests/data"'
 
-[feature.build.tasks.integration-test]
+[feature.test.tasks.integration-test]
 args = [{ arg = "type", default = "RelWithDebInfo" }]
 cmd = """
 pytest -s --log-cli-level=INFO tests/integration \
     --executable=./build/{{ type }}/bin/clice
 """
 
-[feature.build.tasks.test]
+[feature.test.tasks.test]
 args = [{ arg = "type", default = "RelWithDebInfo" }]
 depends-on = [
     { task = "unit-test", args = ["{{ type }}"] },
@@ -134,16 +126,9 @@ depends-on = [
     { task = "xmake-build" },
 ]
 
-[feature.build.tasks.xmake-test]
+[feature.test.tasks.xmake-test]
 cmd = "xmake test --verbose"
 
-[feature.test.pypi-dependencies]
-pytest = "*"
-pytest-asyncio = ">=1.1.0"
-
-[feature.package.dependencies]
-xz = ">=5.8.1,<6"
-
 [feature.package.tasks.package]
 cmd = """
 xmake config --yes --toolchain=clang --mode=releasedbg \
@@ -151,6 +136,25 @@ xmake config --yes --toolchain=clang --mode=releasedbg \
 xmake pack --verbose
 """
 
+# ============================================================================== #
+#                                  HELPER TASKS                                  #
+# ============================================================================== #
+[tasks.build-llvm]
+cmd = ["python3", "scripts/build-llvm.py"]
+
+[tasks.upload-llvm]
+args = ["workflow_id", "llvm_version", { "arg" = "branch", default = "main" }]
+cmd = """
+gh workflow run upload-llvm.yml \
+    --ref {{ branch }} \
+    --field workflow_id={{workflow_id}} \
+    --field version={{llvm_version}}
+"""
+
+[tasks.delete-artifacts]
+args = ["file_name"]
+cmd = ["scripts/delete-artifacts.bash", "{{ file_name }}"]
+
 # ============================================================================== #
 #                             DOCS & VSCODE EXTENSION                            #
 # ============================================================================== #
diff --git a/src/AST/Selection.cpp b/src/AST/Selection.cpp
index 3a1ea0f6..e85e1759 100644
--- a/src/AST/Selection.cpp
+++ b/src/AST/Selection.cpp
@@ -286,7 +286,7 @@ bool is_first_expansion(clang::FileID target,
 class SelectionTester {
 public:
     // The selection is offsets [SelBegin, SelEnd) in SelFile.
-    SelectionTester(CompilationUnit& unit,
+    SelectionTester(CompilationUnitRef unit,
                     clang::FileID selected_file_id,
                     LocalSourceRange selected_range,
                     const clang::SourceManager& source_manager) :
@@ -712,7 +712,7 @@ class SelectionVisitor : public clang::RecursiveASTVisitor<SelectionVisitor> {
 public:
     // Runs the visitor to gather selected nodes and their ancestors.
     // If there is any selection, the root (TUDecl) is the first node.
-    static std::deque<Node> collect(CompilationUnit& unit,
+    static std::deque<Node> collect(CompilationUnitRef unit,
                                     const clang::PrintingPolicy& printing_policy,
                                     LocalSourceRange range,
                                     clang::FileID fid) {
@@ -871,7 +871,7 @@ public:
 private:
     using Base = RecursiveASTVisitor<SelectionVisitor>;
 
-    SelectionVisitor(CompilationUnit& unit,
+    SelectionVisitor(CompilationUnitRef unit,
                      const clang::PrintingPolicy& printing_policy,
                      LocalSourceRange range,
                      clang::FileID selected_file) :
@@ -1135,7 +1135,7 @@ private:
     clang::SourceManager& SM;
     const clang::LangOptions& lang_opts;
     const clang::PrintingPolicy& print_policy;
-    CompilationUnit& unit;
+    CompilationUnitRef unit;
     std::stack<Node*> stack;
     SelectionTester checker;
     IntervalSet unclaimed_expanded_tokens;
@@ -1186,7 +1186,7 @@ std::string SelectionTree::Node::kind() const {
     return std::move(OS.str());
 }
 
-bool SelectionTree::create_each(CompilationUnit& unit,
+bool SelectionTree::create_each(CompilationUnitRef unit,
                                 LocalSourceRange range,
                                 llvm::function_ref<bool(SelectionTree)> callback) {
     auto [begin, end] = range;
@@ -1227,7 +1227,7 @@ bool SelectionTree::create_each(CompilationUnit& unit,
     return false;
 }
 
-SelectionTree SelectionTree::create_right(CompilationUnit& unit, LocalSourceRange range) {
+SelectionTree SelectionTree::create_right(CompilationUnitRef unit, LocalSourceRange range) {
     std::optional<SelectionTree> result;
     create_each(unit, range, [&](SelectionTree T) {
         result = std::move(T);
@@ -1236,7 +1236,7 @@ SelectionTree SelectionTree::create_right(CompilationUnit& unit, LocalSourceRang
     return std::move(*result);
 }
 
-SelectionTree::SelectionTree(CompilationUnit& unit, LocalSourceRange range) :
+SelectionTree::SelectionTree(CompilationUnitRef unit, LocalSourceRange range) :
     print_policy(unit.context().getLangOpts()) {
     // No fundamental reason the selection needs to be in the main file,
     // but that's all clice has needed so far.
diff --git a/src/Compiler/Command.cpp b/src/Compiler/Command.cpp
index 00880cbf..2cf3b91a 100644
--- a/src/Compiler/Command.cpp
+++ b/src/Compiler/Command.cpp
@@ -703,7 +703,7 @@ CompilationContext CompilationDatabase::lookup(llvm::StringRef file,
         append_arg(fs::resource_dir);
     }
 
-    if(options.query_toolchain) {
+    if(info && options.query_toolchain) {
         auto callback = [&](const char* s) {
             return save_string(s).data();
         };
diff --git a/src/Compiler/Compilation.cpp b/src/Compiler/Compilation.cpp
index 433d46dd..5c28bcf4 100644
--- a/src/Compiler/Compilation.cpp
+++ b/src/Compiler/Compilation.cpp
@@ -1,13 +1,12 @@
 #include "Compiler/Compilation.h"
 
-#include "CompilationUnitImpl.h"
-#include "TidyImpl.h"
+#include "Implement.h"
 #include "AST/Utility.h"
 #include "Compiler/Command.h"
 #include "Compiler/Diagnostic.h"
-#include "Compiler/Tidy.h"
 #include "Support/Logging.h"
 
+#include "llvm/Support/Error.h"
 #include "clang/Frontend/MultiplexConsumer.h"
 #include "clang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Lex/PreprocessorOptions.h"
@@ -97,7 +96,8 @@ private:
 
 /// create a `clang::CompilerInvocation` for compilation, it set and reset
 /// all necessary arguments and flags for clice compilation.
-auto create_invocation(CompilationParams& params,
+auto create_invocation(CompilationUnitRef::Self& self,
+                       CompilationParams& params,
                        llvm::IntrusiveRefCntPtr<clang::DiagnosticsEngine>& diagnostic_engine)
     -> std::unique_ptr<clang::CompilerInvocation> {
     if(params.arguments.empty()) {
@@ -137,12 +137,16 @@ auto create_invocation(CompilationParams& params,
     }
 
     auto& pp_opts = invocation->getPreprocessorOpts();
-    assert(!pp_opts.RetainRemappedFileBuffers && "RetainRemappedFileBuffers should be false");
+
+    // CompilerInstance does not deterministically clear RetainRemappedFileBuffers,
+    // especially if compilation aborts early, so we keep them alive and clean up
+    // in CompilationUnit's destructor instead.
+    pp_opts.RetainRemappedFileBuffers = true;
 
     for(auto& [file, buffer]: params.buffers) {
-        pp_opts.addRemappedFile(file, buffer.release());
+        pp_opts.addRemappedFile(file, buffer.get());
     }
-    params.buffers.clear();
+    self.remapped_buffers = std::move(params.buffers);
 
     auto [pch, bound] = params.pch;
     pp_opts.ImplicitPCHInclude = std::move(pch);
@@ -155,12 +159,23 @@ auto create_invocation(CompilationParams& params,
     pp_opts.WriteCommentListToPCH = false;
 
     auto& header_search_opts = invocation->getHeaderSearchOpts();
+    header_search_opts.Verbose = false;
     for(auto& [name, path]: params.pcms) {
         header_search_opts.PrebuiltModuleFiles.try_emplace(name.str(), std::move(path));
     }
 
     auto& front_opts = invocation->getFrontendOpts();
     front_opts.DisableFree = false;
+    front_opts.ShowHelp = false;
+    front_opts.ShowStats = false;
+    front_opts.ShowVersion = false;
+    front_opts.StatsFile = "";
+    front_opts.TimeTracePath = "";
+    front_opts.TimeTraceVerbose = false;
+    front_opts.TimeTraceGranularity = false;
+    front_opts.PrintSupportedCPUs = false;
+    front_opts.PrintEnabledExtensions = false;
+    front_opts.PrintSupportedExtensions = false;
 
     /// Compiler flags (like gcc/clang's -M, -MD, -MMD, -H, or msvc's /showIncludes)
     /// can generate dependency files or print included headers to stdout/stderr.
@@ -190,188 +205,162 @@ auto create_invocation(CompilationParams& params,
     return invocation;
 }
 
-/// Do nothing before or after compile state.
-constexpr static auto no_hook = [](auto& /*ignore*/) {
-};
-
-template <typename Action,
-          typename BeforeExecute = decltype(no_hook),
-          typename AfterExecute = decltype(no_hook)>
-CompilationResult run_clang(CompilationParams& params,
-                            const BeforeExecute& before_execute = no_hook,
-                            const AfterExecute& after_execute = no_hook) {
-    namespace chrono = std::chrono;
-    auto build_at = chrono::system_clock::now().time_since_epoch();
-    auto build_start = chrono::steady_clock::now().time_since_epoch();
-
-    auto diagnostics =
-        params.diagnostics ? params.diagnostics : std::make_shared<std::vector<Diagnostic>>();
-    auto diagnostic_consumer = Diagnostic::create(diagnostics);
+CompilationStatus run_clang(CompilationUnitRef::Self& self,
+                            CompilationParams& params,
+                            std::unique_ptr<clang::FrontendAction> action,
+                            llvm::function_ref<void(clang::CompilerInstance&)> before_execute) {
+    std::unique_ptr diagnostic_consumer = create_diagnostic(&self);
 
     /// Temporary diagnostic engine, only used for command line parsing.
     /// For compilation, we need to create a new diagnostic engine. See also
     /// https://github.com/llvm/llvm-project/pull/139584#issuecomment-2920704282.
     clang::DiagnosticOptions options;
-    auto diagnostic_engine = clang::CompilerInstance::createDiagnostics(*params.vfs,
-                                                                        options,
-                                                                        diagnostic_consumer.get(),
-                                                                        false);
+    llvm::IntrusiveRefCntPtr diagnostic_engine =
+        clang::CompilerInstance::createDiagnostics(*params.vfs,
+                                                   options,
+                                                   diagnostic_consumer.get(),
+                                                   false);
+    if(!diagnostic_engine) {
+        return CompilationStatus::SetupFail;
+    }
 
-    auto invocation = create_invocation(params, diagnostic_engine);
+    std::unique_ptr invocation = create_invocation(self, params, diagnostic_engine);
     if(!invocation) {
-        return std::unexpected("Fail to create compilation invocation!");
+        return CompilationStatus::SetupFail;
     }
 
-    auto instance = std::make_unique<clang::CompilerInstance>(std::move(invocation));
-    instance->createDiagnostics(*params.vfs, diagnostic_consumer.release(), true);
+    self.instance = std::make_unique<clang::CompilerInstance>(std::move(invocation));
+    auto& instance = *self.instance;
+    instance.createDiagnostics(*params.vfs, diagnostic_consumer.release(), true);
 
-    if(auto remapping = clang::createVFSFromCompilerInvocation(instance->getInvocation(),
-                                                               instance->getDiagnostics(),
+    if(auto remapping = clang::createVFSFromCompilerInvocation(instance.getInvocation(),
+                                                               instance.getDiagnostics(),
                                                                params.vfs)) {
-        instance->createFileManager(std::move(remapping));
+        instance.createFileManager(std::move(remapping));
     }
 
-    if(!instance->createTarget()) {
-        return std::unexpected("Fail to create target!");
+    if(!instance.createTarget()) {
+        return CompilationStatus::SetupFail;
     }
 
-    /// Adjust the compiler instance, for example, set preamble or modules.
-    before_execute(*instance);
+    if(before_execute) {
+        before_execute(instance);
+    }
 
-    /// Frontend information ...
-    std::vector<clang::Decl*> top_level_decls;
-    llvm::DenseMap<clang::FileID, Directive> directives;
-    std::optional<clang::syntax::TokenCollector> token_collector;
-
-    auto action = std::make_unique<ProxyAction>(
-        std::make_unique<Action>(),
+    self.action = std::make_unique<ProxyAction>(
+        std::move(action),
         /// We only collect top level declarations for parse main file.
-        (params.clang_tidy || params.kind == CompilationUnit::Content) ? &top_level_decls : nullptr,
+        (params.clang_tidy || params.kind == CompilationKind::Content) ? &self.top_level_decls
+                                                                       : nullptr,
         params.stop);
 
-    if(!action->BeginSourceFile(*instance, instance->getFrontendOpts().Inputs[0])) {
-        return std::unexpected("Fail to begin source file");
+    if(!self.action->BeginSourceFile(instance, instance.getFrontendOpts().Inputs[0])) {
+        self.action.reset();
+        return CompilationStatus::SetupFail;
     }
 
-    auto& pp = instance->getPreprocessor();
     /// FIXME: include-fixer, etc?
 
-    /// Setup clang-tidy
-    std::unique_ptr<tidy::ClangTidyChecker> checker;
+    /// Add PPCallbacks to collect preprocessing information.
+    self.collect_directives();
+
     if(params.clang_tidy) {
-        tidy::TidyParams tidy_params;
-        checker = tidy::configure(*instance, tidy_params);
-        /// TODO: We should make the lifetime of diagnostic consumer more explicit.
-        static_cast<DiagnosticCollector&>(instance->getDiagnosticClient()).checker = checker.get();
+        self.configure_tidy({});
     }
 
-    /// `BeginSourceFile` may create new preprocessor, so all operations related to preprocessor
-    /// should be done after `BeginSourceFile`.
-    Directive::attach(pp, directives);
-
-    /// It is not necessary to collect tokens if we are running code completion.
-    /// And in fact will cause assertion failure.
-    if(!instance->hasCodeCompletionConsumer()) {
-        token_collector.emplace(pp);
+    std::optional<clang::syntax::TokenCollector> token_collector;
+    if(!instance.hasCodeCompletionConsumer()) {
+        /// It is not necessary to collect tokens if we are running code completion.
+        /// And in fact will cause assertion failure.
+        token_collector.emplace(instance.getPreprocessor());
     }
 
-    if(auto error = action->Execute()) {
-        return std::unexpected(std::format("Failed to execute action, because {} ", error));
+    if(auto error = self.action->Execute()) {
+        // Upstream FrontendAction::Execute() always returns success (errors go through
+        // diagnostics); log here only as a guard in case a custom action ever returns
+        // an unexpected llvm::Error.
+        LOG_ERROR("FrontendAction::Execute failed: {}", error);
+        return CompilationStatus::FatalError;
     }
 
     /// If the output file is not empty, it represents that we are
     /// generating a PCH or PCM. If error occurs, the AST must be
     /// invalid to some extent, serialization of such AST may result
     /// in crash frequently. So forbidden it here and return as error.
-    if(!instance->getFrontendOpts().OutputFile.empty() &&
-       instance->getDiagnostics().hasErrorOccurred()) {
-        action->EndSourceFile();
-        return std::unexpected("Fail to build PCH or PCM, error occurs in compilation.");
+    if(!instance.getFrontendOpts().OutputFile.empty() &&
+       instance.getDiagnostics().hasErrorOccurred()) {
+        return CompilationStatus::FatalError;
     }
 
     /// Check whether the compilation is canceled, if so we think
     /// it is an error.
     if(params.stop && params.stop->load()) {
-        action->EndSourceFile();
-        return std::unexpected("Compilation is canceled.");
+        self.action->EndSourceFile();
+        self.action.reset();
+        return CompilationStatus::Cancelled;
     }
 
-    std::optional<clang::syntax::TokenBuffer> token_buffer;
     if(token_collector) {
-        token_buffer = std::move(*token_collector).consume();
+        self.buffer = std::move(*token_collector).consume();
     }
 
-    // Must be called before EndSourceFile because the ast context can be destroyed later.
-    if(checker) {
-        // AST traversals should exclude the preamble, to avoid performance cliffs.
-        // TODO: is it okay to affect the unit-level traversal scope here?
-        instance->getASTContext().setTraversalScope(top_level_decls);
-        checker->finder.matchAST(instance->getASTContext());
+    self.run_tidy();
+
+    if(instance.hasSema()) {
+        self.resolver.emplace(instance.getSema());
     }
 
-    /// XXX: This is messy: clang-tidy checks flush some diagnostics at EOF.
-    /// However Action->EndSourceFile() would destroy the ASTContext!
-    /// So just inform the preprocessor of EOF, while keeping everything alive.
-    pp.EndSourceFile();
+    return CompilationStatus::Completed;
+}
 
-    /// FIXME: getDependencies currently return ArrayRef<std::string>, which actually results in
-    /// extra copy. It would be great to avoid this copy.
+CompilationUnit run_clang(CompilationParams& params,
+                          std::unique_ptr<clang::FrontendAction> action,
+                          llvm::function_ref<void(clang::CompilerInstance&)> before_execute = {},
+                          llvm::function_ref<void(CompilationUnitRef)> after_execute = {}) {
+    auto self = new CompilationUnitRef::Self();
+    self->kind = params.kind;
 
-    std::optional<TemplateResolver> resolver;
-    if(instance->hasSema()) {
-        resolver.emplace(instance->getSema());
+    using namespace std::chrono;
+    self->build_at = duration_cast<milliseconds>(system_clock::now().time_since_epoch());
+    auto build_start = steady_clock::now().time_since_epoch();
+
+    self->status = run_clang(*self, params, std::move(action), before_execute);
+
+    auto build_end = steady_clock::now().time_since_epoch();
+    self->build_duration = duration_cast<milliseconds>(build_end - build_start);
+
+    if(self->status == CompilationStatus::Completed && after_execute) {
+        after_execute(self);
     }
 
-    if(checker) {
-        /// Avoid dangling pointer.
-        static_cast<DiagnosticCollector&>(instance->getDiagnosticClient()).checker = nullptr;
-    }
-
-    auto build_end = chrono::steady_clock::now().time_since_epoch();
-
-    auto impl = new CompilationUnit::Impl{
-        .interested = instance->getSourceManager().getMainFileID(),
-        .src_mgr = instance->getSourceManager(),
-        .action = std::move(action),
-        .instance = std::move(instance),
-        .resolver = std::move(resolver),
-        .buffer = std::move(token_buffer),
-        .directives = std::move(directives),
-        .path_cache = llvm::DenseMap<clang::FileID, llvm::StringRef>(),
-        .symbol_hash_cache = llvm::DenseMap<const void*, std::uint64_t>(),
-        .diagnostics = std::move(diagnostics),
-        .top_level_decls = std::move(top_level_decls),
-        .build_at = chrono::duration_cast<chrono::milliseconds>(build_at),
-        .build_duration = chrono::duration_cast<chrono::milliseconds>(build_end - build_start),
-    };
-
-    CompilationUnit unit(params.kind, impl);
-    after_execute(unit);
-    return unit;
+    return CompilationUnit(self);
 }
 
 }  // namespace
 
-CompilationResult preprocess(CompilationParams& params) {
-    return run_clang<clang::PreprocessOnlyAction>(params);
+CompilationUnit preprocess(CompilationParams& params) {
+    return run_clang(params, std::make_unique<clang::PreprocessOnlyAction>());
 }
 
-CompilationResult compile(CompilationParams& params) {
-    return run_clang<clang::SyntaxOnlyAction>(params, [](clang::CompilerInstance& instance) {
-        /// Make sure the output file is empty.
-        instance.getFrontendOpts().OutputFile.clear();
-    });
+CompilationUnit compile(CompilationParams& params) {
+    return run_clang(params,
+                     std::make_unique<clang::SyntaxOnlyAction>(),
+                     [](clang::CompilerInstance& instance) {
+                         /// Make sure the output file is empty.
+                         instance.getFrontendOpts().OutputFile.clear();
+                     });
 }
 
-CompilationResult compile(CompilationParams& params, PCHInfo& out) {
+CompilationUnit compile(CompilationParams& params, PCHInfo& out) {
     assert(!params.output_file.empty() && "PCH file path cannot be empty");
 
     /// Record the begin time of PCH building.
     auto now = std::chrono::system_clock::now().time_since_epoch();
     out.mtime = std::chrono::duration_cast<std::chrono::milliseconds>(now).count();
 
-    return run_clang<clang::GeneratePCHAction>(
+    return run_clang(
         params,
+        std::make_unique<clang::GeneratePCHAction>(),
         [&](clang::CompilerInstance& instance) {
             /// Set options to generate PCH.
             instance.getFrontendOpts().OutputFile = params.output_file.str();
@@ -384,7 +373,7 @@ CompilationResult compile(CompilationParams& params, PCHInfo& out) {
 
             instance.getLangOpts().CompilingPCH = true;
         },
-        [&](CompilationUnit& unit) {
+        [&](CompilationUnitRef unit) {
             out.path = params.output_file.str();
             out.preamble = unit.interested_content();
             out.deps = unit.deps();
@@ -392,11 +381,12 @@ CompilationResult compile(CompilationParams& params, PCHInfo& out) {
         });
 }
 
-CompilationResult compile(CompilationParams& params, PCMInfo& out) {
+CompilationUnit compile(CompilationParams& params, PCMInfo& out) {
     assert(!params.output_file.empty() && "PCM file path cannot be empty");
 
-    return run_clang<clang::GenerateReducedModuleInterfaceAction>(
+    return run_clang(
         params,
+        std::make_unique<clang::GenerateReducedModuleInterfaceAction>(),
         [&](clang::CompilerInstance& instance) {
             /// Set options to generate PCH.
             instance.getFrontendOpts().OutputFile = params.output_file.str();
@@ -405,7 +395,7 @@ CompilationResult compile(CompilationParams& params, PCMInfo& out) {
 
             out.srcPath = instance.getFrontendOpts().Inputs[0].getFile();
         },
-        [&](CompilationUnit& unit) {
+        [&](CompilationUnitRef unit) {
             out.path = params.output_file.str();
 
             for(auto& [name, path]: params.pcms) {
@@ -414,7 +404,7 @@ CompilationResult compile(CompilationParams& params, PCMInfo& out) {
         });
 }
 
-CompilationResult complete(CompilationParams& params, clang::CodeCompleteConsumer* consumer) {
+CompilationUnit complete(CompilationParams& params, clang::CodeCompleteConsumer* consumer) {
     auto& [file, offset] = params.completion;
 
     /// The location of clang is 1-1 based.
@@ -434,13 +424,15 @@ CompilationResult complete(CompilationParams& params, clang::CodeCompleteConsume
         column += 1;
     }
 
-    return run_clang<clang::SyntaxOnlyAction>(params, [&](clang::CompilerInstance& instance) {
-        /// Set options to run code completion.
-        instance.getFrontendOpts().CodeCompletionAt.FileName = std::move(file);
-        instance.getFrontendOpts().CodeCompletionAt.Line = line;
-        instance.getFrontendOpts().CodeCompletionAt.Column = column;
-        instance.setCodeCompletionConsumer(consumer);
-    });
+    return run_clang(params,
+                     std::make_unique<clang::SyntaxOnlyAction>(),
+                     [&](clang::CompilerInstance& instance) {
+                         /// Set options to run code completion.
+                         instance.getFrontendOpts().CodeCompletionAt.FileName = std::move(file);
+                         instance.getFrontendOpts().CodeCompletionAt.Line = line;
+                         instance.getFrontendOpts().CodeCompletionAt.Column = column;
+                         instance.setCodeCompletionConsumer(consumer);
+                     });
 }
 
 }  // namespace clice
diff --git a/src/Compiler/CompilationUnit.cpp b/src/Compiler/CompilationUnit.cpp
index 54acfd17..ff95d57c 100644
--- a/src/Compiler/CompilationUnit.cpp
+++ b/src/Compiler/CompilationUnit.cpp
@@ -1,38 +1,33 @@
-#include "CompilationUnitImpl.h"
+#include "Implement.h"
 #include "AST/Utility.h"
 #include "Index/USR.h"
 
 namespace clice {
 
-CompilationUnit::~CompilationUnit() {
-    if(impl && impl->action) {
-        auto instance = impl->instance.get();
-        // We already notified the pp of end-of-file earlier, so detach it first.
-        // We must keep it alive until after EndSourceFile(), Sema relies on this.
-        std::shared_ptr<clang::Preprocessor> pp = instance->getPreprocessorPtr();
-        instance->setPreprocessor(nullptr);  // Detach so we don't send EOF again
-        impl->action->EndSourceFile();
-    }
-
-    delete impl;
+CompilationKind CompilationUnitRef::kind() {
+    return self->kind;
 }
 
-auto CompilationUnit::file_id(llvm::StringRef file) -> clang::FileID {
-    auto entry = impl->src_mgr.getFileManager().getFileRef(file);
+CompilationStatus CompilationUnitRef::status() {
+    return self->status;
+}
+
+auto CompilationUnitRef::file_id(llvm::StringRef file) -> clang::FileID {
+    auto entry = self->SM().getFileManager().getFileRef(file);
     if(entry) {
-        return impl->src_mgr.translateFile(*entry);
+        return self->SM().translateFile(*entry);
     }
 
     return clang::FileID();
 }
 
-auto CompilationUnit::decompose_location(clang::SourceLocation location)
+auto CompilationUnitRef::decompose_location(clang::SourceLocation location)
     -> std::pair<clang::FileID, std::uint32_t> {
     assert(location.isFileID() && "Decompose macro location is meaningless!");
-    return impl->src_mgr.getDecomposedLoc(location);
+    return self->SM().getDecomposedLoc(location);
 }
 
-auto CompilationUnit::decompose_range(clang::SourceRange range)
+auto CompilationUnitRef::decompose_range(clang::SourceRange range)
     -> std::pair<clang::FileID, LocalSourceRange> {
     auto [begin, end] = range;
     assert(begin.isValid() && end.isValid() && "Invalid source range");
@@ -62,7 +57,7 @@ auto CompilationUnit::decompose_range(clang::SourceRange range)
     }
 }
 
-auto CompilationUnit::decompose_expansion_range(clang::SourceRange range)
+auto CompilationUnitRef::decompose_expansion_range(clang::SourceRange range)
     -> std::pair<clang::FileID, LocalSourceRange> {
     auto [begin, end] = range;
     if(begin == end) {
@@ -73,21 +68,21 @@ auto CompilationUnit::decompose_expansion_range(clang::SourceRange range)
     }
 }
 
-auto CompilationUnit::file_id(clang::SourceLocation location) -> clang::FileID {
-    return impl->src_mgr.getFileID(location);
+auto CompilationUnitRef::file_id(clang::SourceLocation location) -> clang::FileID {
+    return self->SM().getFileID(location);
 }
 
-auto CompilationUnit::file_offset(clang::SourceLocation location) -> std::uint32_t {
-    return impl->src_mgr.getFileOffset(location);
+auto CompilationUnitRef::file_offset(clang::SourceLocation location) -> std::uint32_t {
+    return self->SM().getFileOffset(location);
 }
 
-auto CompilationUnit::file_path(clang::FileID fid) -> llvm::StringRef {
+auto CompilationUnitRef::file_path(clang::FileID fid) -> llvm::StringRef {
     assert(fid.isValid() && "Invalid fid");
-    if(auto it = impl->path_cache.find(fid); it != impl->path_cache.end()) {
+    if(auto it = self->path_cache.find(fid); it != self->path_cache.end()) {
         return it->second;
     }
 
-    auto entry = impl->src_mgr.getFileEntryRefForID(fid);
+    auto entry = self->SM().getFileEntryRefForID(fid);
     assert(entry && "Invalid file entry");
 
     llvm::SmallString<128> path;
@@ -102,31 +97,31 @@ auto CompilationUnit::file_path(clang::FileID fid) -> llvm::StringRef {
 
     /// Allocate the path in the storage.
     auto size = path.size();
-    auto data = impl->path_storage.Allocate<char>(size + 1);
+    auto data = self->path_storage.Allocate<char>(size + 1);
     memcpy(data, path.data(), size);
     data[size] = '\0';
 
-    auto [it, inserted] = impl->path_cache.try_emplace(fid, llvm::StringRef(data, size));
+    auto [it, inserted] = self->path_cache.try_emplace(fid, llvm::StringRef(data, size));
     assert(inserted && "File path already exists");
     return it->second;
 }
 
-auto CompilationUnit::file_content(clang::FileID fid) -> llvm::StringRef {
-    return impl->src_mgr.getBufferData(fid);
+auto CompilationUnitRef::file_content(clang::FileID fid) -> llvm::StringRef {
+    return self->SM().getBufferData(fid);
 }
 
-auto CompilationUnit::interested_file() -> clang::FileID {
-    return impl->interested;
+auto CompilationUnitRef::interested_file() -> clang::FileID {
+    return self->SM().getMainFileID();
 }
 
-auto CompilationUnit::interested_content() -> llvm::StringRef {
-    return file_content(impl->interested);
+auto CompilationUnitRef::interested_content() -> llvm::StringRef {
+    return file_content(interested_file());
 }
 
-bool CompilationUnit::is_builtin_file(clang::FileID fid) {
+bool CompilationUnitRef::is_builtin_file(clang::FileID fid) {
     // No FileEntryRef => built-in/command line/scratch.
-    if(!impl->src_mgr.getFileEntryRefForID(fid)) {
-        if(auto buffer = impl->src_mgr.getBufferOrNone(fid)) {
+    if(!self->SM().getFileEntryRefForID(fid)) {
+        if(auto buffer = self->SM().getBufferOrNone(fid)) {
             auto name = buffer->getBufferIdentifier();
             return name == "<built-in>" || name == "<command line>" || name == "<scratch space>";
         }
@@ -135,45 +130,46 @@ bool CompilationUnit::is_builtin_file(clang::FileID fid) {
     return false;
 }
 
-auto CompilationUnit::start_location(clang::FileID fid) -> clang::SourceLocation {
-    return impl->src_mgr.getLocForStartOfFile(fid);
+auto CompilationUnitRef::start_location(clang::FileID fid) -> clang::SourceLocation {
+    return self->SM().getLocForStartOfFile(fid);
 }
 
-auto CompilationUnit::end_location(clang::FileID fid) -> clang::SourceLocation {
-    return impl->src_mgr.getLocForEndOfFile(fid);
+auto CompilationUnitRef::end_location(clang::FileID fid) -> clang::SourceLocation {
+    return self->SM().getLocForEndOfFile(fid);
 }
 
-auto CompilationUnit::spelling_location(clang::SourceLocation loc) -> clang::SourceLocation {
-    return impl->src_mgr.getSpellingLoc(loc);
+auto CompilationUnitRef::spelling_location(clang::SourceLocation loc) -> clang::SourceLocation {
+    return self->SM().getSpellingLoc(loc);
 }
 
-auto CompilationUnit::expansion_location(clang::SourceLocation location) -> clang::SourceLocation {
-    return impl->src_mgr.getExpansionLoc(location);
-}
-
-auto CompilationUnit::file_location(clang::SourceLocation location) -> clang::SourceLocation {
-    return impl->src_mgr.getFileLoc(location);
-}
-
-auto CompilationUnit::include_location(clang::FileID fid) -> clang::SourceLocation {
-    return impl->src_mgr.getIncludeLoc(fid);
-}
-
-auto CompilationUnit::presumed_location(clang::SourceLocation location) -> clang::PresumedLoc {
-    return impl->src_mgr.getPresumedLoc(location, false);
-}
-
-auto CompilationUnit::create_location(clang::FileID fid, std::uint32_t offset)
+auto CompilationUnitRef::expansion_location(clang::SourceLocation location)
     -> clang::SourceLocation {
-    return impl->src_mgr.getComposedLoc(fid, offset);
+    return self->SM().getExpansionLoc(location);
 }
 
-auto CompilationUnit::spelled_tokens(clang::FileID fid) -> TokenRange {
-    return impl->buffer->spelledTokens(fid);
+auto CompilationUnitRef::file_location(clang::SourceLocation location) -> clang::SourceLocation {
+    return self->SM().getFileLoc(location);
 }
 
-auto CompilationUnit::spelled_tokens(clang::SourceRange range) -> TokenRange {
-    auto tokens = impl->buffer->spelledForExpanded(impl->buffer->expandedTokens(range));
+auto CompilationUnitRef::include_location(clang::FileID fid) -> clang::SourceLocation {
+    return self->SM().getIncludeLoc(fid);
+}
+
+auto CompilationUnitRef::presumed_location(clang::SourceLocation location) -> clang::PresumedLoc {
+    return self->SM().getPresumedLoc(location, false);
+}
+
+auto CompilationUnitRef::create_location(clang::FileID fid, std::uint32_t offset)
+    -> clang::SourceLocation {
+    return self->SM().getComposedLoc(fid, offset);
+}
+
+auto CompilationUnitRef::spelled_tokens(clang::FileID fid) -> TokenRange {
+    return self->buffer->spelledTokens(fid);
+}
+
+auto CompilationUnitRef::spelled_tokens(clang::SourceRange range) -> TokenRange {
+    auto tokens = self->buffer->spelledForExpanded(self->buffer->expandedTokens(range));
     if(!tokens) {
         return {};
     }
@@ -181,72 +177,72 @@ auto CompilationUnit::spelled_tokens(clang::SourceRange range) -> TokenRange {
     return *tokens;
 }
 
-auto CompilationUnit::spelled_tokens_touch(clang::SourceLocation location) -> TokenRange {
-    return clang::syntax::spelledTokensTouching(location, *impl->buffer);
+auto CompilationUnitRef::spelled_tokens_touch(clang::SourceLocation location) -> TokenRange {
+    return clang::syntax::spelledTokensTouching(location, *self->buffer);
 }
 
-auto CompilationUnit::expanded_tokens() -> TokenRange {
-    return impl->buffer->expandedTokens();
+auto CompilationUnitRef::expanded_tokens() -> TokenRange {
+    return self->buffer->expandedTokens();
 }
 
-auto CompilationUnit::expanded_tokens(clang::SourceRange range) -> TokenRange {
-    return impl->buffer->expandedTokens(range);
+auto CompilationUnitRef::expanded_tokens(clang::SourceRange range) -> TokenRange {
+    return self->buffer->expandedTokens(range);
 }
 
-auto CompilationUnit::expansions_overlapping(TokenRange spelled_tokens)
+auto CompilationUnitRef::expansions_overlapping(TokenRange spelled_tokens)
     -> std::vector<clang::syntax::TokenBuffer::Expansion> {
-    return impl->buffer->expansionsOverlapping(spelled_tokens);
+    return self->buffer->expansionsOverlapping(spelled_tokens);
 }
 
-auto CompilationUnit::token_length(clang::SourceLocation location) -> std::uint32_t {
-    return clang::Lexer::MeasureTokenLength(location, impl->src_mgr, impl->instance->getLangOpts());
+auto CompilationUnitRef::token_length(clang::SourceLocation location) -> std::uint32_t {
+    return clang::Lexer::MeasureTokenLength(location, self->SM(), self->instance->getLangOpts());
 }
 
-auto CompilationUnit::token_spelling(clang::SourceLocation location) -> llvm::StringRef {
-    return llvm::StringRef(impl->src_mgr.getCharacterData(location), token_length(location));
+auto CompilationUnitRef::token_spelling(clang::SourceLocation location) -> llvm::StringRef {
+    return llvm::StringRef(self->SM().getCharacterData(location), token_length(location));
 }
 
-auto CompilationUnit::module_name() -> llvm::StringRef {
-    return impl->instance->getPreprocessor().getNamedModuleName();
+auto CompilationUnitRef::module_name() -> llvm::StringRef {
+    return self->instance->getPreprocessor().getNamedModuleName();
 }
 
-bool CompilationUnit::is_module_interface_unit() {
-    return impl->instance->getPreprocessor().isInNamedInterfaceUnit();
+bool CompilationUnitRef::is_module_interface_unit() {
+    return self->instance->getPreprocessor().isInNamedInterfaceUnit();
 }
 
-auto CompilationUnit::diagnostics() -> llvm::ArrayRef<Diagnostic> {
-    return *impl->diagnostics;
+auto CompilationUnitRef::diagnostics() -> std::vector<Diagnostic>& {
+    return self->diagnostics;
 }
 
-auto CompilationUnit::top_level_decls() -> llvm::ArrayRef<clang::Decl*> {
-    return impl->top_level_decls;
+auto CompilationUnitRef::top_level_decls() -> llvm::ArrayRef<clang::Decl*> {
+    return self->top_level_decls;
 }
 
-std::chrono::milliseconds CompilationUnit::build_at() {
-    return impl->build_at;
+std::chrono::milliseconds CompilationUnitRef::build_at() {
+    return self->build_at;
 }
 
-std::chrono::milliseconds CompilationUnit::build_duration() {
-    return impl->build_duration;
+std::chrono::milliseconds CompilationUnitRef::build_duration() {
+    return self->build_duration;
 }
 
-clang::LangOptions& CompilationUnit::lang_options() {
-    return impl->instance->getLangOpts();
+clang::LangOptions& CompilationUnitRef::lang_options() {
+    return self->instance->getLangOpts();
 }
 
-std::vector<std::string> CompilationUnit::deps() {
+std::vector<std::string> CompilationUnitRef::deps() {
     llvm::StringSet<> deps;
 
     /// FIXME: consider `#embed` and `__has_embed`.
 
-    for(auto& [fid, diretive]: directives()) {
-        for(auto& include: diretive.includes) {
+    for(auto& [fid, directive]: directives()) {
+        for(auto& include: directive.includes) {
             if(!include.skipped) {
                 deps.try_emplace(file_path(include.fid));
             }
         }
 
-        for(auto& has_include: diretive.has_includes) {
+        for(auto& has_include: directive.has_includes) {
             if(has_include.fid.isValid()) {
                 deps.try_emplace(file_path(has_include.fid));
             }
@@ -262,69 +258,73 @@ std::vector<std::string> CompilationUnit::deps() {
     return result;
 }
 
-index::SymbolID CompilationUnit::getSymbolID(const clang::NamedDecl* decl) {
+index::SymbolID CompilationUnitRef::getSymbolID(const clang::NamedDecl* decl) {
     uint64_t hash;
-    auto iter = impl->symbol_hash_cache.find(decl);
-    if(iter != impl->symbol_hash_cache.end()) {
+    auto iter = self->symbol_hash_cache.find(decl);
+    if(iter != self->symbol_hash_cache.end()) {
         hash = iter->second;
     } else {
         llvm::SmallString<128> usr;
         index::generateUSRForDecl(decl, usr);
         hash = llvm::xxh3_64bits(usr);
-        impl->symbol_hash_cache.try_emplace(decl, hash);
+        self->symbol_hash_cache.try_emplace(decl, hash);
     }
     return index::SymbolID{hash, ast::name_of(decl)};
 }
 
-index::SymbolID CompilationUnit::getSymbolID(const clang::MacroInfo* macro) {
+index::SymbolID CompilationUnitRef::getSymbolID(const clang::MacroInfo* macro) {
     std::uint64_t hash;
     auto name = token_spelling(macro->getDefinitionLoc());
-    auto iter = impl->symbol_hash_cache.find(macro);
-    if(iter != impl->symbol_hash_cache.end()) {
+    auto iter = self->symbol_hash_cache.find(macro);
+    if(iter != self->symbol_hash_cache.end()) {
         hash = iter->second;
     } else {
         llvm::SmallString<128> usr;
-        index::generateUSRForMacro(name, macro->getDefinitionLoc(), impl->src_mgr, usr);
+        index::generateUSRForMacro(name, macro->getDefinitionLoc(), self->SM(), usr);
         hash = llvm::xxh3_64bits(usr);
-        impl->symbol_hash_cache.try_emplace(macro, hash);
+        self->symbol_hash_cache.try_emplace(macro, hash);
     }
     return index::SymbolID{hash, name.str()};
 }
 
-const llvm::DenseSet<clang::FileID>& CompilationUnit::files() {
-    if(impl->all_files.empty()) {
+const llvm::DenseSet<clang::FileID>& CompilationUnitRef::files() {
+    if(self->all_files.empty()) {
         /// FIXME: handle preamble and embed file id.
-        for(auto& [fid, diretive]: directives()) {
-            for(auto& include: diretive.includes) {
-                if(!include.skipped) {
-                    impl->all_files.insert(include.fid);
+        for(auto& [fid, directive]: directives()) {
+            for(auto& include: directive.includes) {
+                if(!include.skipped && include.fid.isValid()) {
+                    self->all_files.insert(include.fid);
                 }
             }
         }
-        impl->all_files.insert(impl->src_mgr.getMainFileID());
+        self->all_files.insert(self->SM().getMainFileID());
     }
-    return impl->all_files;
+    return self->all_files;
 }
 
-clang::TranslationUnitDecl* CompilationUnit::tu() {
-    return impl->instance->getASTContext().getTranslationUnitDecl();
+clang::TranslationUnitDecl* CompilationUnitRef::tu() {
+    return self->instance->getASTContext().getTranslationUnitDecl();
 }
 
-llvm::DenseMap<clang::FileID, Directive>& CompilationUnit::directives() {
-    return impl->directives;
+llvm::DenseMap<clang::FileID, Directive>& CompilationUnitRef::directives() {
+    return self->directives;
 }
 
-TemplateResolver& CompilationUnit::resolver() {
-    assert(impl->resolver && "Template resolver is not available");
-    return *impl->resolver;
+TemplateResolver& CompilationUnitRef::resolver() {
+    assert(self->resolver && "Template resolver is not available");
+    return *self->resolver;
 }
 
-clang::ASTContext& CompilationUnit::context() {
-    return impl->instance->getASTContext();
+clang::ASTContext& CompilationUnitRef::context() {
+    return self->instance->getASTContext();
 }
 
-clang::syntax::TokenBuffer& CompilationUnit::token_buffer() {
-    return *impl->buffer;
+clang::syntax::TokenBuffer& CompilationUnitRef::token_buffer() {
+    return *self->buffer;
+}
+
+CompilationUnit::~CompilationUnit() {
+    delete self;
 }
 
 }  // namespace clice
diff --git a/src/Compiler/CompilationUnitImpl.h b/src/Compiler/CompilationUnitImpl.h
deleted file mode 100644
index 8ffa3e6e..00000000
--- a/src/Compiler/CompilationUnitImpl.h
+++ /dev/null
@@ -1,52 +0,0 @@
-#pragma once
-
-#include "Compiler/CompilationUnit.h"
-#include "Compiler/Diagnostic.h"
-
-#include "clang/Frontend/CompilerInstance.h"
-#include "clang/Frontend/FrontendActions.h"
-
-namespace clice {
-
-struct CompilationUnit::Impl {
-    /// The interested file ID.
-    clang::FileID interested;
-
-    clang::SourceManager& src_mgr;
-
-    /// The frontend action used to build the unit.
-    std::unique_ptr<clang::FrontendAction> action;
-
-    /// Compiler instance, responsible for performing the actual compilation and managing the
-    /// lifecycle of all objects during the compilation process.
-    std::unique_ptr<clang::CompilerInstance> instance;
-
-    /// The template resolver used to resolve dependent name.
-    std::optional<TemplateResolver> resolver;
-
-    /// Token information collected during the preprocessing.
-    std::optional<clang::syntax::TokenBuffer> buffer;
-
-    /// All diretive information collected during the preprocessing.
-    llvm::DenseMap<clang::FileID, Directive> directives;
-
-    llvm::DenseSet<clang::FileID> all_files;
-
-    /// Cache for file path. It is used to avoid multiple file path lookup.
-    llvm::DenseMap<clang::FileID, llvm::StringRef> path_cache;
-
-    /// Cache for symbol id.
-    llvm::DenseMap<const void*, std::uint64_t> symbol_hash_cache;
-
-    llvm::BumpPtrAllocator path_storage;
-
-    std::shared_ptr<std::vector<Diagnostic>> diagnostics;
-
-    std::vector<clang::Decl*> top_level_decls;
-
-    std::chrono::milliseconds build_at;
-
-    std::chrono::milliseconds build_duration;
-};
-
-}  // namespace clice
diff --git a/src/Compiler/Diagnostic.cpp b/src/Compiler/Diagnostic.cpp
index e54b1099..a091cc07 100644
--- a/src/Compiler/Diagnostic.cpp
+++ b/src/Compiler/Diagnostic.cpp
@@ -1,6 +1,6 @@
 #include "Compiler/Diagnostic.h"
 
-#include "TidyImpl.h"
+#include "Implement.h"
 #include "Support/Format.h"
 
 #include "clang/AST/Decl.h"
@@ -150,78 +150,72 @@ static DiagnosticLevel diagnostic_level(clang::DiagnosticsEngine::Level level) {
     }
 }
 
-/// Get the range for given diagnostic.
-/// FIXME: I would like to use `CompilationUnit`.
-auto diagnostic_range(const clang::Diagnostic& diagnostic, const clang::LangOptions& options)
-    -> std::optional<std::pair<clang::FileID, LocalSourceRange>> {
-    /// If location is invalid, it represents the diagnostic is
-    /// from the command line.
-    auto location = diagnostic.getLocation();
-    if(location.isInvalid()) {
-        return std::nullopt;
-    }
-
-    /// If the location is valid, the `SourceManager` is valid too.
-    auto& src_mgr = diagnostic.getDiags()->getSourceManager();
-
-    /// Make sure the location is file location.
-    location = src_mgr.getFileLoc(location);
-    assert(location.isFileID());
-
-    auto [fid, offset] = src_mgr.getDecomposedLoc(location);
-
-    /// Select a proper range for the diagnostic.
-    for(auto range: diagnostic.getRanges()) {
-        range = clang::Lexer::makeFileCharRange(range, src_mgr, options);
-
-        auto [begin, end] = range.getAsRange();
-        auto [begin_fid, begin_offset] = src_mgr.getDecomposedLoc(begin);
-        if(begin_fid != fid || begin_offset <= offset) {
-            continue;
-        }
-
-        auto [end_fid, end_offset] = src_mgr.getDecomposedLoc(end);
-        if(range.isTokenRange()) {
-            end_offset += clang::Lexer::MeasureTokenLength(end, src_mgr, options);
-        }
-
-        if(end_fid == fid && end_offset >= offset) {
-            return std::pair{
-                fid,
-                LocalSourceRange{begin_offset, end_offset}
-            };
-        }
-    }
-
-    /// Use token range.
-    auto end_offset = offset + clang::Lexer::MeasureTokenLength(location, src_mgr, options);
-    return std::pair{
-        fid,
-        LocalSourceRange{offset, end_offset}
-    };
-}
-
-class DiagnosticCollectorImpl : public DiagnosticCollector {
+class DiagnosticCollector : public clang::DiagnosticConsumer {
 public:
-    DiagnosticCollectorImpl(std::shared_ptr<std::vector<Diagnostic>> diagnostics) :
-        diagnostics(diagnostics) {}
+    DiagnosticCollector(CompilationUnitRef unit) : unit(unit) {}
 
-    void BeginSourceFile(const clang::LangOptions& opts, const clang::Preprocessor* pp) override {
-        options = &opts;
-        src_mgr = &pp->getSourceManager();
+    auto diagnostic_range(const clang::Diagnostic& diagnostic)
+        -> std::optional<std::pair<clang::FileID, LocalSourceRange>> {
+        /// If location is invalid, it represents the diagnostic is
+        /// from the command line.
+        auto location = diagnostic.getLocation();
+        if(location.isInvalid()) {
+            return std::nullopt;
+        }
+
+        /// Make sure the location is file location.
+        location = unit.file_location(location);
+        assert(location.isFileID());
+
+        auto [fid, offset] = unit.decompose_location(location);
+
+        /// Select a proper range for the diagnostic.
+        for(auto range: diagnostic.getRanges()) {
+            range = clang::Lexer::makeFileCharRange(range,
+                                                    unit.context().getSourceManager(),
+                                                    unit.lang_options());
+
+            auto [begin, end] = range.getAsRange();
+            auto [begin_fid, begin_offset] = unit.decompose_location(begin);
+            if(begin_fid != fid || begin_offset <= offset) {
+                continue;
+            }
+
+            auto [end_fid, end_offset] = unit.decompose_location(end);
+            if(range.isTokenRange()) {
+                end_offset += unit.token_length(end);
+            }
+
+            if(end_fid == fid && end_offset >= offset) {
+                return std::pair{
+                    fid,
+                    LocalSourceRange{begin_offset, end_offset}
+                };
+            }
+        }
+
+        /// Use token range.
+        auto end_offset = offset + unit.token_length(location);
+        return std::pair{
+            fid,
+            LocalSourceRange{offset, end_offset}
+        };
     }
 
+    void BeginSourceFile(const clang::LangOptions&, const clang::Preprocessor*) override {}
+
     void HandleDiagnostic(clang::DiagnosticsEngine::Level level,
                           const clang::Diagnostic& raw_diagnostic) override {
-        auto& diagnostic = diagnostics->emplace_back();
+        auto& diagnostic = unit.diagnostics().emplace_back();
         diagnostic.id.value = raw_diagnostic.getID();
 
         if(!is_note(level)) {
-            if(checker) {
-                level = checker->adjust_level(level, raw_diagnostic);
+            if(unit->checker) {
+                level = unit->checker->adjust_level(level, raw_diagnostic);
             }
         }
         diagnostic.id.level = diagnostic_level(level);
+        diagnostic.id.source = DiagnosticSource::Clang;
 
         /// TODO:
         // use DiagnosticEngine::SetArgToStringFn to set a custom function to convert arguments to
@@ -235,14 +229,14 @@ public:
         raw_diagnostic.FormatDiagnostic(message);
         diagnostic.message = message.str();
 
-        if(auto pair = diagnostic_range(raw_diagnostic, *options)) {
+        if(auto pair = diagnostic_range(raw_diagnostic)) {
             auto [fid, range] = *pair;
             diagnostic.fid = fid;
             diagnostic.range = range;
         }
 
-        if(checker) {
-            checker->adjust_diag(diagnostic);
+        if(unit->checker) {
+            unit->checker->adjust_diag(diagnostic);
         }
 
         /// TODO: handle FixIts
@@ -252,14 +246,11 @@ public:
     void EndSourceFile() override {}
 
 private:
-    std::shared_ptr<std::vector<Diagnostic>> diagnostics;
-    const clang::LangOptions* options;
-    clang::SourceManager* src_mgr;
+    CompilationUnitRef unit;
 };
 
-std::unique_ptr<DiagnosticCollector>
-    Diagnostic::create(std::shared_ptr<std::vector<Diagnostic>> diagnostics) {
-    return std::make_unique<DiagnosticCollectorImpl>(std::move(diagnostics));
+std::unique_ptr<clang::DiagnosticConsumer> create_diagnostic(CompilationUnitRef unit) {
+    return std::make_unique<DiagnosticCollector>(unit);
 }
 
 }  // namespace clice
diff --git a/src/Compiler/Directive.cpp b/src/Compiler/Directive.cpp
index 7e7adfd3..d5c01fab 100644
--- a/src/Compiler/Directive.cpp
+++ b/src/Compiler/Directive.cpp
@@ -1,5 +1,7 @@
 #include "Compiler/Directive.h"
 
+#include "Implement.h"
+
 #include "clang/Lex/MacroArgs.h"
 #include "clang/Lex/MacroInfo.h"
 #include "clang/Lex/Preprocessor.h"
@@ -262,8 +264,8 @@ private:
 
 }  // namespace
 
-void Directive::attach(clang::Preprocessor& pp,
-                       llvm::DenseMap<clang::FileID, Directive>& directives) {
+void CompilationUnitRef::Self::collect_directives() {
+    auto& pp = instance->getPreprocessor();
     pp.addPPCallbacks(std::make_unique<DirectiveCollector>(pp, directives));
 }
 
diff --git a/src/Compiler/Implement.h b/src/Compiler/Implement.h
new file mode 100644
index 00000000..399386c2
--- /dev/null
+++ b/src/Compiler/Implement.h
@@ -0,0 +1,137 @@
+#pragma once
+
+#include "Compiler/CompilationUnit.h"
+#include "Compiler/Diagnostic.h"
+
+#include "clang/Frontend/CompilerInstance.h"
+#include "clang/Frontend/FrontendActions.h"
+#include "clang-tidy/ClangTidyCheck.h"
+#include "clang-tidy/ClangTidyModuleRegistry.h"
+#include "clang-tidy/ClangTidyOptions.h"
+
+namespace clice::tidy {
+
+using namespace clang::tidy;
+
+bool is_registered_tidy_check(llvm::StringRef check);
+
+std::optional<bool> is_fast_tidy_check(llvm::StringRef check);
+
+struct TidyParams {};
+
+class ClangTidyChecker;
+
+/// Configure to run clang-tidy on the given file.
+std::unique_ptr<ClangTidyChecker> configure(clang::CompilerInstance& instance,
+                                            const TidyParams& params);
+
+class ClangTidyChecker {
+public:
+    /// The context of the clang-tidy checker.
+    ClangTidyContext context;
+
+    /// The instances of checks that are enabled for the current Language.
+    std::vector<std::unique_ptr<ClangTidyCheck>> checks;
+
+    /// The match finder to run clang-tidy on ASTs.
+    clang::ast_matchers::MatchFinder finder;
+
+    ClangTidyChecker(std::unique_ptr<ClangTidyOptionsProvider> provider);
+
+    clang::DiagnosticsEngine::Level adjust_level(clang::DiagnosticsEngine::Level level,
+                                                 const clang::Diagnostic& diag);
+    void adjust_diag(Diagnostic& diag);
+};
+
+}  // namespace clice::tidy
+
+namespace clice {
+
+constexpr static auto no_hook = [](auto& /*ignore*/) {
+};
+
+struct CompilationParams;
+
+struct CompilationUnitRef::Self {
+    CompilationKind kind;
+
+    CompilationStatus status;
+
+    llvm::StringMap<std::unique_ptr<llvm::MemoryBuffer>> remapped_buffers;
+
+    /// The frontend action used to build the unit.
+    std::unique_ptr<clang::FrontendAction> action;
+
+    /// Compiler instance, responsible for performing the actual compilation and managing the
+    /// lifecycle of all objects during the compilation process.
+    std::unique_ptr<clang::CompilerInstance> instance;
+
+    /// The template resolver used to resolve dependent name.
+    std::optional<TemplateResolver> resolver;
+
+    /// Token information collected during the preprocessing.
+    std::optional<clang::syntax::TokenBuffer> buffer;
+
+    /// All directive information collected during the preprocessing.
+    llvm::DenseMap<clang::FileID, Directive> directives;
+
+    llvm::DenseSet<clang::FileID> all_files;
+
+    /// Cache for file path. It is used to avoid multiple file path lookup.
+    llvm::DenseMap<clang::FileID, llvm::StringRef> path_cache;
+
+    /// Cache for symbol id.
+    llvm::DenseMap<const void*, std::uint64_t> symbol_hash_cache;
+
+    llvm::BumpPtrAllocator path_storage;
+
+    std::vector<Diagnostic> diagnostics;
+
+    std::vector<clang::Decl*> top_level_decls;
+
+    std::unique_ptr<tidy::ClangTidyChecker> checker;
+
+    std::chrono::milliseconds build_at;
+    std::chrono::milliseconds build_duration;
+
+    auto& SM() {
+        return instance->getSourceManager();
+    }
+
+    void collect_directives();
+
+    void configure_tidy(tidy::TidyParams tidy_params) {
+        checker = tidy::configure(*instance, tidy_params);
+    }
+
+    // Must be called before EndSourceFile because the ast context can be destroyed later.
+    void run_tidy() {
+        if(checker) {
+            // AST traversals should exclude the preamble, to avoid performance cliffs.
+            // TODO: is it okay to affect the unit-level traversal scope here?
+            auto& Ctx = instance->getASTContext();
+            Ctx.setTraversalScope(top_level_decls);
+            checker->finder.matchAST(Ctx);
+
+            /// XXX: This is messy: clang-tidy checks flush some diagnostics at EOF.
+            /// However Action->EndSourceFile() would destroy the ASTContext!
+            /// So just inform the preprocessor of EOF, while keeping everything alive.
+            instance->getPreprocessor().EndSourceFile();
+        }
+    }
+
+    ~Self() {
+        if(action) {
+            // We already notified the pp of end-of-file earlier, so detach it first.
+            // We must keep it alive until after EndSourceFile(), Sema relies on this.
+            std::shared_ptr<clang::Preprocessor> pp = instance->getPreprocessorPtr();
+            // Detach so we don't send EOF again
+            instance->setPreprocessor(nullptr);
+            action->EndSourceFile();
+        }
+    }
+};
+
+std::unique_ptr<clang::DiagnosticConsumer> create_diagnostic(CompilationUnitRef unit);
+
+}  // namespace clice
diff --git a/src/Compiler/Module.cpp b/src/Compiler/Module.cpp
index ca0cb7de..0b736deb 100644
--- a/src/Compiler/Module.cpp
+++ b/src/Compiler/Module.cpp
@@ -46,10 +46,10 @@ std::string scanModuleName(CompilationParams& params) {
 
         if(token.is(clang::tok::hash)) {
             lexer.LexFromRawLexer(token);
-            auto diretive = token.getRawIdentifier();
-            if(diretive == "if" || diretive == "ifdef" || diretive == "ifndef") {
+            auto directive = token.getRawIdentifier();
+            if(directive == "if" || directive == "ifdef" || directive == "ifndef") {
                 is_in_directive = true;
-            } else if(diretive == "endif") {
+            } else if(directive == "endif") {
                 is_in_directive = false;
             }
         } else if(token.is(clang::tok::raw_identifier)) {
@@ -106,16 +106,15 @@ std::string scanModuleName(CompilationParams& params) {
 std::expected<ModuleInfo, std::string> scanModule(CompilationParams& params) {
     ModuleInfo info;
     auto unit = preprocess(params);
-    if(!unit) {
-        return std::unexpected(unit.error());
-    }
+    /// FIXME: handle error here.
+    assert(unit.completed());
 
-    for(auto& import: unit->directives()[unit->interested_file()].imports) {
+    for(auto& import: unit.directives()[unit.interested_file()].imports) {
         info.mods.emplace_back(import.name);
     }
 
-    info.isInterfaceUnit = unit->is_module_interface_unit();
-    info.name = unit->module_name();
+    info.isInterfaceUnit = unit.is_module_interface_unit();
+    info.name = unit.module_name();
 
     return info;
 }
diff --git a/src/Compiler/Tidy.cpp b/src/Compiler/Tidy.cpp
index cf70ca62..5878f78e 100644
--- a/src/Compiler/Tidy.cpp
+++ b/src/Compiler/Tidy.cpp
@@ -1,18 +1,4 @@
-//===--- Run clang-tidy ---------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-/// Partial code is copied from clangd. See:
-/// https://github.com/llvm/llvm-project//blob/0865ecc5150b9a55ba1f9e30b6d463a66ac362a6/clang-tools-extra/clangd/ParsedAST.cpp#L547
-/// https://github.com/llvm/llvm-project//blob/0865ecc5150b9a55ba1f9e30b6d463a66ac362a6/clang-tools-extra/clangd/TidyProvider.cpp
-
-#include "Compiler/Tidy.h"
-
-#include "TidyImpl.h"
+#include "Implement.h"
 #include "AST/Utility.h"
 #include "Compiler/Diagnostic.h"
 #include "Support/Logging.h"
diff --git a/src/Compiler/TidyImpl.h b/src/Compiler/TidyImpl.h
deleted file mode 100644
index 1255d89f..00000000
--- a/src/Compiler/TidyImpl.h
+++ /dev/null
@@ -1,32 +0,0 @@
-#pragma once
-
-#include <memory>
-
-#include "Compiler/Diagnostic.h"
-#include "Compiler/Tidy.h"
-
-#include "clang-tidy/ClangTidyCheck.h"
-#include "clang-tidy/ClangTidyModuleRegistry.h"
-#include "clang-tidy/ClangTidyOptions.h"
-
-namespace clice::tidy {
-
-using namespace clang::tidy;
-
-class ClangTidyChecker {
-public:
-    /// The context of the clang-tidy checker.
-    ClangTidyContext context;
-    /// The instances of checks that are enabled for the current Language.
-    std::vector<std::unique_ptr<ClangTidyCheck>> checks;
-    /// The match finder to run clang-tidy on ASTs.
-    clang::ast_matchers::MatchFinder finder;
-
-    ClangTidyChecker(std::unique_ptr<ClangTidyOptionsProvider> provider);
-
-    clang::DiagnosticsEngine::Level adjust_level(clang::DiagnosticsEngine::Level level,
-                                                 const clang::Diagnostic& diag);
-    void adjust_diag(Diagnostic& diag);
-};
-
-}  // namespace clice::tidy
diff --git a/src/Feature/CodeCompletion.cpp b/src/Feature/CodeCompletion.cpp
index 060b3a8b..c901e114 100644
--- a/src/Feature/CodeCompletion.cpp
+++ b/src/Feature/CodeCompletion.cpp
@@ -432,7 +432,8 @@ std::vector<CompletionItem> code_complete(CompilationParams& params,
     auto& [file, offset] = params.completion;
     auto consumer = new CodeCompletionCollector(offset, items, option);
 
-    if(auto info = complete(params, consumer)) {
+    auto unit = complete(params, consumer);
+    if(!unit.completed()) {
         /// TODO: Handle error here.
     }
 
diff --git a/src/Feature/Diagnostic.cpp b/src/Feature/Diagnostic.cpp
index f9fa9b7c..a8535d6b 100644
--- a/src/Feature/Diagnostic.cpp
+++ b/src/Feature/Diagnostic.cpp
@@ -6,7 +6,7 @@
 
 namespace clice::feature {
 
-json::Value diagnostics(PositionEncodingKind kind, PathMapping mapping, CompilationUnit& unit) {
+json::Value diagnostics(PositionEncodingKind kind, PathMapping mapping, CompilationUnitRef unit) {
     json::Array result;
 
     std::optional<proto::Diagnostic> diagnostic;
diff --git a/src/Feature/DocumentLink.cpp b/src/Feature/DocumentLink.cpp
index 1babe95d..e4455aab 100644
--- a/src/Feature/DocumentLink.cpp
+++ b/src/Feature/DocumentLink.cpp
@@ -8,7 +8,7 @@ namespace clice::feature {
 
 namespace {}
 
-DocumentLinks document_links(CompilationUnit& unit) {
+DocumentLinks document_links(CompilationUnitRef unit) {
     DocumentLinks links;
 
     auto& interested_diretive = unit.directives()[unit.interested_file()];
@@ -39,7 +39,7 @@ DocumentLinks document_links(CompilationUnit& unit) {
     return links;
 }
 
-index::Shared<DocumentLinks> index_document_link(CompilationUnit& unit) {
+index::Shared<DocumentLinks> index_document_link(CompilationUnitRef unit) {
     index::Shared<DocumentLinks> result;
 
     for(auto& [fid, diretives]: unit.directives()) {
diff --git a/src/Feature/DocumentSymbol.cpp b/src/Feature/DocumentSymbol.cpp
index dba76de4..ea92f350 100644
--- a/src/Feature/DocumentSymbol.cpp
+++ b/src/Feature/DocumentSymbol.cpp
@@ -50,7 +50,7 @@ class DocumentSymbolCollector : public FilteredASTVisitor<DocumentSymbolCollecto
 public:
     using Base = FilteredASTVisitor<DocumentSymbolCollector>;
 
-    DocumentSymbolCollector(CompilationUnit& unit, bool interested_only) :
+    DocumentSymbolCollector(CompilationUnitRef unit, bool interested_only) :
         Base(unit, interested_only) {}
 
     bool is_interested(clang::Decl* decl) {
@@ -126,7 +126,7 @@ public:
 
 }  // namespace
 
-DocumentSymbols document_symbols(CompilationUnit& unit) {
+DocumentSymbols document_symbols(CompilationUnitRef unit) {
     DocumentSymbolCollector collector(unit, true);
     collector.TraverseDecl(unit.tu());
 
@@ -135,7 +135,7 @@ DocumentSymbols document_symbols(CompilationUnit& unit) {
     return std::move(frame.symbols);
 }
 
-index::Shared<DocumentSymbols> index_document_symbol(CompilationUnit& unit) {
+index::Shared<DocumentSymbols> index_document_symbol(CompilationUnitRef unit) {
     DocumentSymbolCollector collector(unit, true);
     collector.TraverseDecl(unit.tu());
 
diff --git a/src/Feature/FoldingRange.cpp b/src/Feature/FoldingRange.cpp
index 3766a39b..7f9de60b 100644
--- a/src/Feature/FoldingRange.cpp
+++ b/src/Feature/FoldingRange.cpp
@@ -10,7 +10,7 @@ namespace {
 
 class FoldingRangeCollector : public FilteredASTVisitor<FoldingRangeCollector> {
 public:
-    FoldingRangeCollector(CompilationUnit& unit, bool interested_only) :
+    FoldingRangeCollector(CompilationUnitRef unit, bool interested_only) :
         FilteredASTVisitor(unit, interested_only) {}
 
     constexpr static auto LastColOfLine = std::numeric_limits<unsigned>::max();
@@ -145,14 +145,14 @@ public:
         return true;
     }
 
-    auto build_for_file(CompilationUnit& unit) {
+    auto build_for_file(CompilationUnitRef unit) {
         TraverseDecl(unit.tu());
         collect_drectives(unit.directives()[unit.interested_file()]);
         std::ranges::sort(result, refl::less);
         return std::move(result);
     }
 
-    auto build_for_index(CompilationUnit& unit) {
+    auto build_for_index(CompilationUnitRef unit) {
         TraverseDecl(unit.tu());
         for(auto& [fid, directive]: unit.directives()) {
             collect_drectives(directive);
@@ -326,11 +326,11 @@ private:
 
 }  // namespace
 
-FoldingRanges folding_ranges(CompilationUnit& unit) {
+FoldingRanges folding_ranges(CompilationUnitRef unit) {
     return FoldingRangeCollector(unit, true).build_for_file(unit);
 }
 
-index::Shared<FoldingRanges> index_folding_range(CompilationUnit& unit) {
+index::Shared<FoldingRanges> index_folding_range(CompilationUnitRef unit) {
     return FoldingRangeCollector(unit, false).build_for_index(unit);
 }  // namespace feature
 
diff --git a/src/Feature/Hover.cpp b/src/Feature/Hover.cpp
index 80437859..9ce2e3c4 100644
--- a/src/Feature/Hover.cpp
+++ b/src/Feature/Hover.cpp
@@ -12,7 +12,7 @@ namespace clice::feature {
 
 namespace {
 
-std::vector<HoverItem> getHoverItems(CompilationUnit& unit, const clang::NamedDecl* decl) {
+std::vector<HoverItem> getHoverItems(CompilationUnitRef unit, const clang::NamedDecl* decl) {
     clang::ASTContext& Ctx = unit.context();
     std::vector<HoverItem> items;
 
@@ -35,7 +35,7 @@ std::vector<HoverItem> getHoverItems(CompilationUnit& unit, const clang::NamedDe
     return items;
 }
 
-std::string getDocument(CompilationUnit& unit, const clang::NamedDecl* decl) {
+std::string getDocument(CompilationUnitRef unit, const clang::NamedDecl* decl) {
     clang::ASTContext& Ctx = unit.context();
     const clang::RawComment* comment = Ctx.getRawCommentForAnyRedecl(decl);
     if(!comment) {
@@ -45,14 +45,14 @@ std::string getDocument(CompilationUnit& unit, const clang::NamedDecl* decl) {
     return comment->getRawText(Ctx.getSourceManager()).str();
 }
 
-std::string getQualifier(CompilationUnit& unit, const clang::NamedDecl* decl) {
+std::string getQualifier(CompilationUnitRef unit, const clang::NamedDecl* decl) {
     std::string result;
     llvm::raw_string_ostream os(result);
     decl->printNestedNameSpecifier(os);
     return result;
 }
 
-std::string getSourceCode(CompilationUnit& unit, const clang::NamedDecl* decl) {
+std::string getSourceCode(CompilationUnitRef unit, const clang::NamedDecl* decl) {
     clang::SourceRange range = decl->getSourceRange();
     // auto& TB = unit.tokBuf();
     // auto& SM = unit.srcMgr();
@@ -63,7 +63,7 @@ std::string getSourceCode(CompilationUnit& unit, const clang::NamedDecl* decl) {
 
 }  // namespace
 
-Hover hover(CompilationUnit& unit, const clang::NamedDecl* decl) {
+Hover hover(CompilationUnitRef unit, const clang::NamedDecl* decl) {
     return Hover{
         .kind = SymbolKind::from(decl),
         .name = ast::name_of(decl),
@@ -74,7 +74,7 @@ Hover hover(CompilationUnit& unit, const clang::NamedDecl* decl) {
     };
 }
 
-Hover hover(CompilationUnit& unit, std::uint32_t offset) {
+Hover hover(CompilationUnitRef unit, std::uint32_t offset) {
     Hover info;
 
     auto tree = SelectionTree::create_right(unit, {offset, offset});
diff --git a/src/Feature/InlayHint.cpp b/src/Feature/InlayHint.cpp
index cda300d6..66482e3f 100644
--- a/src/Feature/InlayHint.cpp
+++ b/src/Feature/InlayHint.cpp
@@ -97,7 +97,7 @@ struct Callee {
 class Builder {
 public:
     Builder(std::vector<InlayHint>& result,
-            CompilationUnit& unit,
+            CompilationUnitRef unit,
             LocalSourceRange restrict_range,
             const config::InlayHintsOptions& options) :
         result(result), unit(unit), restrict_range(restrict_range), options(options),
@@ -523,7 +523,7 @@ public:
 
 private:
     std::vector<InlayHint>& result;
-    CompilationUnit& unit;
+    CompilationUnitRef unit;
     LocalSourceRange restrict_range;
     const config::InlayHintsOptions& options;
     clang::PrintingPolicy policy;
@@ -534,7 +534,7 @@ public:
     using Base = FilteredASTVisitor<Visitor>;
 
     Visitor(Builder& builder,
-            CompilationUnit& unit,
+            CompilationUnitRef unit,
             std::optional<LocalSourceRange> restrict_range,
             const config::InlayHintsOptions& options) :
         Base(unit, true), builder(builder), unit(unit), options(options) {}
@@ -867,7 +867,7 @@ public:
 
 private:
     Builder& builder;
-    CompilationUnit& unit;
+    CompilationUnitRef unit;
     const config::InlayHintsOptions& options;
 
     // If/else chains are tricky.
@@ -881,7 +881,7 @@ private:
 
 }  // namespace
 
-auto inlay_hints(CompilationUnit& unit,
+auto inlay_hints(CompilationUnitRef unit,
                  LocalSourceRange target,
                  const config::InlayHintsOptions& options) -> std::vector<InlayHint> {
     std::vector<InlayHint> hints;
diff --git a/src/Feature/SemanticToken.cpp b/src/Feature/SemanticToken.cpp
index 8ad4625b..53c02876 100644
--- a/src/Feature/SemanticToken.cpp
+++ b/src/Feature/SemanticToken.cpp
@@ -246,7 +246,7 @@ public:
 
 }  // namespace
 
-SemanticTokens semantic_tokens(CompilationUnit& unit) {
+SemanticTokens semantic_tokens(CompilationUnitRef unit) {
     SemanticTokensCollector collector(unit, true);
     collector.highlight(unit.interested_file());
     collector.run();
@@ -254,7 +254,7 @@ SemanticTokens semantic_tokens(CompilationUnit& unit) {
     return std::move(collector.result);
 }
 
-index::Shared<SemanticTokens> index_semantic_token(CompilationUnit& unit) {
+index::Shared<SemanticTokens> index_semantic_token(CompilationUnitRef unit) {
     SemanticTokensCollector collector(unit, false);
     for(auto fid: unit.files()) {
         collector.highlight(fid);
diff --git a/src/Feature/SignatureHelp.cpp b/src/Feature/SignatureHelp.cpp
index bc9327ed..adbfdfcc 100644
--- a/src/Feature/SignatureHelp.cpp
+++ b/src/Feature/SignatureHelp.cpp
@@ -203,7 +203,8 @@ proto::SignatureHelp signature_help(CompilationParams& params,
     complete_options.IncludeFixIts = false;
 
     auto consumer = new Collector(help, complete_options);
-    if(auto info = complete(params, consumer)) {
+    auto unit = complete(params, consumer);
+    if(!unit.completed()) {
         /// FIXME: do something.
     }
     return help;
diff --git a/src/Index/IncludeGraph.cpp b/src/Index/IncludeGraph.cpp
index 6d9f9420..c8eddb52 100644
--- a/src/Index/IncludeGraph.cpp
+++ b/src/Index/IncludeGraph.cpp
@@ -4,7 +4,7 @@
 
 namespace clice::index {
 
-static std::uint32_t addIncludeChain(CompilationUnit& unit,
+static std::uint32_t addIncludeChain(CompilationUnitRef unit,
                                      clang::FileID fid,
                                      IncludeGraph& graph,
                                      llvm::StringMap<std::uint32_t>& path_table) {
@@ -45,7 +45,7 @@ static std::uint32_t addIncludeChain(CompilationUnit& unit,
     return index;
 }
 
-IncludeGraph IncludeGraph::from(CompilationUnit& unit) {
+IncludeGraph IncludeGraph::from(CompilationUnitRef unit) {
     llvm::StringMap<std::uint32_t> path_table;
     IncludeGraph graph;
     for(auto fid: unit.files()) {
diff --git a/src/Index/TUIndex.cpp b/src/Index/TUIndex.cpp
index aea7ef28..ee3f9b61 100644
--- a/src/Index/TUIndex.cpp
+++ b/src/Index/TUIndex.cpp
@@ -11,7 +11,7 @@ namespace {
 
 class Builder : public SemanticVisitor<Builder> {
 public:
-    Builder(TUIndex& result, CompilationUnit& unit) :
+    Builder(TUIndex& result, CompilationUnitRef unit) :
         SemanticVisitor<Builder>(unit, false), result(result) {
         result.graph = IncludeGraph::from(unit);
     }
@@ -75,24 +75,29 @@ public:
                         RelationKind kind,
                         const clang::NamedDecl* target,
                         clang::SourceRange range) {
-        auto [fid, relationRange] = unit.decompose_expansion_range(range);
+        auto [fid, relation_range] = unit.decompose_expansion_range(range);
 
         Relation relation{.kind = kind};
 
         if(kind.isDeclOrDef()) {
-            auto [fid2, definition_range] = unit.decompose_expansion_range(decl->getSourceRange());
-            assert(fid == fid2 && "Invalid definition location");
-            relation.range = relationRange;
-            relation.set_definition_range(definition_range);
+            relation.range = relation_range;
+            /// FIXME: why definition or declaration has invalid source range? implicit node?
+            auto source_range = decl->getSourceRange();
+            if(source_range.isValid()) {
+                auto [fid2, definition_range] =
+                    unit.decompose_expansion_range(decl->getSourceRange());
+                assert(fid == fid2 && "Invalid definition location");
+                relation.set_definition_range(definition_range);
+            }
         } else if(kind.isReference()) {
-            relation.range = relationRange;
+            relation.range = relation_range;
             relation.target_symbol = 0;
         } else if(kind.isBetweenSymbol()) {
             auto symbol_id = unit.getSymbolID(ast::normalize(target));
             relation.target_symbol = symbol_id.hash;
         } else if(kind.isCall()) {
             auto symbol_id = unit.getSymbolID(ast::normalize(target));
-            relation.range = relationRange;
+            relation.range = relation_range;
             relation.target_symbol = symbol_id.hash;
         } else {
             std::unreachable();
@@ -161,7 +166,7 @@ std::array<std::uint8_t, 32> FileIndex::hash() {
     return hasher.final();
 }
 
-TUIndex TUIndex::build(CompilationUnit& unit) {
+TUIndex TUIndex::build(CompilationUnitRef unit) {
     TUIndex index;
     index.built_at = unit.build_at();
 
diff --git a/src/Index/USRGeneration.cpp b/src/Index/USRGeneration.cpp
index a041f70f..4e026500 100644
--- a/src/Index/USRGeneration.cpp
+++ b/src/Index/USRGeneration.cpp
@@ -785,7 +785,10 @@ void USRGenerator::VisitTemplateParameterList(const TemplateParameterList* Param
             if(p->isParameterPack())
                 Out << 'p';
             Out << 'T';
-            if(p->hasTypeConstraint()) {
+
+            /// FIXME: workaround figure out why `p->hasTypeConstraint()` is true but
+            /// `p->getTypeConstraint() is empty`.
+            if(p->hasTypeConstraint() && p->getTypeConstraint()) {
                 Out << ":TC";
                 AppendExprODRHash(p->getTypeConstraint()->getImmediatelyDeclaredConstraint(), Out);
             }
diff --git a/src/Server/Document.cpp b/src/Server/Document.cpp
index b26f7a01..07540a4c 100644
--- a/src/Server/Document.cpp
+++ b/src/Server/Document.cpp
@@ -176,8 +176,7 @@ async::Task<bool> build_pch_task(CompilationContext& info,
                                  std::shared_ptr<OpenFile> open_file,
                                  std::string path,
                                  std::uint32_t bound,
-                                 std::string content,
-                                 std::shared_ptr<std::vector<Diagnostic>> diagnostics) {
+                                 std::string content) {
     if(!fs::exists(cache_dir)) {
         auto error = fs::create_directories(cache_dir);
         if(error) {
@@ -186,15 +185,11 @@ async::Task<bool> build_pch_task(CompilationContext& info,
         }
     }
 
-    /// Everytime we build a new pch, the old diagnostics should be discarded.
-    diagnostics->clear();
-
     CompilationParams params;
-    params.kind = CompilationUnit::Preamble;
+    params.kind = CompilationKind::Preamble;
     params.output_file = path::join(cache_dir, path::filename(path) + ".pch");
     params.arguments_from_database = true;
     params.arguments = std::move(info.arguments);
-    params.diagnostics = diagnostics;
     params.add_remapped_file(path, content, bound);
 
     std::string command;
@@ -210,24 +205,25 @@ async::Task<bool> build_pch_task(CompilationContext& info,
     std::string message = std::move(command);  // reuse buffer
     std::vector<feature::DocumentLink> links;
 
-    bool success = co_await async::submit([&params, &pch, &message, &links] -> bool {
+    bool success = co_await async::submit([&params, &pch, &message, &links, &path] -> bool {
         /// PCH file is written until destructing, Add a single block for it.
         auto unit = compile(params, pch);
-        if(!unit) {
-            message = std::move(unit.error());
+        if(!unit.completed()) {
+            /// FIXME: use `feature::diagnostic` to flush diagnostic for reporting.
+            /// LOG_WARN("Building PCH fails for {}, Because: {}", path, message);
+            for(auto& diagnostic: unit.diagnostics()) {
+                LOG_WARN("{}", diagnostic.message);
+            }
+
             return false;
         }
 
-        links = feature::document_links(*unit);
+        links = feature::document_links(unit);
         /// TODO: index PCH file, etc
         return true;
     });
 
     if(!success) {
-        LOG_WARN("Building PCH fails for {}, Because: {}", path, message);
-        for(auto& diagnostic: *diagnostics) {
-            LOG_WARN("{}", diagnostic.message);
-        }
         co_return false;
     }
 
@@ -276,13 +272,8 @@ async::Task<bool> Server::build_pch(std::string file, std::string content) {
     }
 
     /// Schedule the new building task.
-    task = build_pch_task(info,
-                          config.project.cache_dir,
-                          open_file,
-                          file,
-                          bound,
-                          std::move(content),
-                          open_file->diagnostics);
+    task =
+        build_pch_task(info, config.project.cache_dir, open_file, file, bound, std::move(content));
     if(co_await task) {
         /// FIXME: At this point, task has already been finished, destroy it directly.
         task.release().destroy();
@@ -316,29 +307,27 @@ async::Task<> Server::build_ast(std::string path, std::string content) {
     options.query_toolchain = true;
 
     CompilationParams params;
-    params.kind = CompilationUnit::Content;
+    params.kind = CompilationKind::Content;
     params.arguments_from_database = true;
     params.arguments = database.lookup(path, options).arguments;
     params.add_remapped_file(path, content);
     params.pch = {pch->path, pch->preamble.size()};
-    file->diagnostics->clear();
-    params.diagnostics = file->diagnostics;
     params.clang_tidy = config.project.clang_tidy;
 
     /// Check result
-    auto ast = co_await async::submit([&] { return compile(params); });
-    if(!ast) {
+    auto unit = co_await async::submit([&] { return compile(params); });
+    if(!unit.completed()) {
         /// FIXME: Fails needs cancel waiting tasks.
-        LOG_ERROR("Building AST fails for {}, Beacuse: {}", path, ast.error());
-        for(auto& diagnostic: *file->diagnostics) {
-            LOG_ERROR("{}", diagnostic.message);
-        }
+        /// LOG_ERROR("Building AST fails for {}, Beacuse: {}", path, unit.error());
+        /// for(auto& diagnostic: unit.diagnostics()) {
+        ///     LOG_ERROR("{}", diagnostic.message);
+        /// }
         co_return;
     }
 
     /// Send diagnostics
     auto diagnostics = co_await async::submit(
-        [&, kind = this->kind] { return feature::diagnostics(kind, mapping, *ast); });
+        [&, kind = this->kind] { return feature::diagnostics(kind, mapping, unit); });
     co_await notify("textDocument/publishDiagnostics",
                     json::Object{
                         {"uri",         mapping.to_uri(path)  },
@@ -349,7 +338,7 @@ async::Task<> Server::build_ast(std::string path, std::string content) {
     /// co_await indexer.index(*ast);
 
     /// Update built AST info.
-    file->ast = std::make_shared<CompilationUnit>(std::move(*ast));
+    file->ast = std::make_shared<CompilationUnit>(std::move(unit));
 
     /// Dispose the task so that it will destroyed when task complete.
     file->ast_build_task.dispose();
diff --git a/src/Server/Feature.cpp b/src/Server/Feature.cpp
index b8efafbe..98df9fa2 100644
--- a/src/Server/Feature.cpp
+++ b/src/Server/Feature.cpp
@@ -30,7 +30,7 @@ auto Server::on_completion(proto::CompletionParams params) -> Result {
 
         /// Set compilation params ... .
         CompilationParams params;
-        params.kind = CompilationUnit::Completion;
+        params.kind = CompilationKind::Completion;
         params.arguments_from_database = true;
         params.arguments = database.lookup(path).arguments;
         params.add_remapped_file(path, content);
@@ -88,7 +88,7 @@ async::Task<json::Value> Server::on_signature_help(proto::SignatureHelpParams pa
 
         /// Set compilation params ... .
         CompilationParams params;
-        params.kind = CompilationUnit::Completion;
+        params.kind = CompilationKind::Completion;
         params.arguments_from_database = true;
         params.arguments = database.lookup(path, options).arguments;
         params.add_remapped_file(path, content);
diff --git a/src/Server/Indexer.cpp b/src/Server/Indexer.cpp
index cfd78340..ad4dbd28 100644
--- a/src/Server/Indexer.cpp
+++ b/src/Server/Indexer.cpp
@@ -9,9 +9,9 @@ namespace clice {
 
 async::Task<> Indexer::index(llvm::StringRef path) {
     CompilationParams params;
-    params.kind = CompilationUnit::Indexing;
+    params.kind = CompilationKind::Indexing;
     params.arguments_from_database = true;
-    params.arguments = database.lookup(path).arguments;
+    params.arguments = database.lookup(path, {.query_toolchain = true}).arguments;
 
     auto path_id = project_index.path_pool.path_id(path);
     auto& merged_index = get_index(path_id);
@@ -25,12 +25,12 @@ async::Task<> Indexer::index(llvm::StringRef path) {
 
     auto tu_index = co_await async::submit([&]() -> std::optional<index::TUIndex> {
         auto unit = compile(params);
-        if(!unit) {
-            LOG_INFO("Fail to index for {}, because: {}", path, unit.error());
+        if(!unit.completed()) {
+            /// LOG_INFO("Fail to index for {}, because: {}", path, unit.error());
             return std::nullopt;
         }
 
-        return index::TUIndex::build(*unit);
+        return index::TUIndex::build(unit);
     });
 
     if(!tu_index) {
diff --git a/tests/unit/AST/ResolverTests.cpp b/tests/unit/AST/ResolverTests.cpp
index 83c6eb0b..763d398b 100644
--- a/tests/unit/AST/ResolverTests.cpp
+++ b/tests/unit/AST/ResolverTests.cpp
@@ -8,13 +8,13 @@ namespace clice::testing {
 namespace {
 
 struct InputFinder : clang::RecursiveASTVisitor<InputFinder> {
-    CompilationUnit& unit;
+    CompilationUnitRef unit;
     clang::QualType input;
     clang::QualType expect;
 
     using Base = clang::RecursiveASTVisitor<InputFinder>;
 
-    InputFinder(CompilationUnit& unit) : unit(unit) {}
+    InputFinder(CompilationUnitRef unit) : unit(unit) {}
 
     bool TraverseDecl(clang::Decl* decl) {
         if(decl && (llvm::isa<clang::TranslationUnitDecl>(decl) ||
diff --git a/tests/unit/Compiler/DiagnosticTests.cpp b/tests/unit/Compiler/DiagnosticTests.cpp
index 3daa1189..965a43bf 100644
--- a/tests/unit/Compiler/DiagnosticTests.cpp
+++ b/tests/unit/Compiler/DiagnosticTests.cpp
@@ -80,28 +80,59 @@ using namespace clice;
 
 TEST_SUITE(Diagnostic) {
 
-TEST_CASE(CommandError) {
+TEST_CASE(TargetError) {
     CompilationParams params;
-    /// miss input file.
-    params.arguments = {"clang++"};
-    params.add_remapped_file("main.cpp", "int main() { return 0; }");
+    params.arguments = {"clang++", "-target", "aa-bb-cc", "main.cpp"};
+    params.add_remapped_file("main.cpp", "");
+
     auto unit = compile(params);
-    ASSERT_FALSE(unit.has_value());
+    ASSERT_TRUE(unit.setup_fail());
+    ASSERT_TRUE(unit.diagnostics().size() == 1);
+
+    auto& diag = unit.diagnostics()[0];
+    EXPECT_EQ(diag.id.diagnostic_code(), "err_target_unknown_triple");
+    EXPECT_EQ(diag.id.level, DiagnosticLevel::Error);
+    EXPECT_EQ(diag.id.source, DiagnosticSource::Clang);
+    EXPECT_TRUE(diag.fid.isInvalid());
+    EXPECT_TRUE(!diag.range.valid());
+    EXPECT_EQ(diag.message, "unknown target triple 'aa-bb-cc'");
 }
 
 TEST_CASE(Error) {
     CompilationParams params;
     params.arguments = {"clang++", "main.cpp"};
     params.add_remapped_file("main.cpp", "int main() { return 0 }");
-    auto unit = compile(params);
-    ASSERT_TRUE(unit.has_value());
-    ASSERT_FALSE(unit->diagnostics().empty());
 
-    /// for(auto& diag: unit->diagnostics()) {
-    ///     std::println("{}", diag.message);
-    /// }
+    auto unit = compile(params);
+    ASSERT_TRUE(unit.completed());
+    ASSERT_TRUE(unit.diagnostics().size() == 1);
+
+    auto& diag = unit.diagnostics()[0];
+    EXPECT_EQ(diag.id.diagnostic_code(), "err_expected_semi_after_stmt");
+    EXPECT_EQ(diag.id.level, DiagnosticLevel::Error);
+    EXPECT_EQ(diag.id.source, DiagnosticSource::Clang);
+    EXPECT_EQ(diag.fid, unit.interested_file());
+    EXPECT_TRUE(diag.range.valid());
+    EXPECT_EQ(diag.message, "expected ';' after return statement");
 };
 
+TEST_CASE(Warning) {
+    CompilationParams params;
+    params.arguments = {"clang++", "-Wall", "-Wunused-variable", "main.cpp"};
+    params.add_remapped_file("main.cpp", "int main() { int x; return 0; }");
+
+    auto unit = compile(params);
+    ASSERT_TRUE(unit.completed());
+    ASSERT_EQ(unit.diagnostics().size(), 1);
+
+    auto& diag = unit.diagnostics()[0];
+    EXPECT_EQ(diag.id.diagnostic_code(), "warn_unused_variable");
+    EXPECT_EQ(diag.id.level, DiagnosticLevel::Warning);
+    EXPECT_EQ(diag.id.source, DiagnosticSource::Clang);
+    EXPECT_TRUE(diag.range.valid());
+    EXPECT_TRUE(diag.message.find("unused variable") != std::string::npos);
+}
+
 TEST_CASE(PCHError) {
     /// Any error in compilation will result in failure on generating PCH or PCM.
     CompilationParams params;
@@ -114,7 +145,7 @@ void foo() {}
 
     PCHInfo info;
     auto unit = compile(params, info);
-    ASSERT_FALSE(unit.has_value());
+    ASSERT_TRUE(unit.fatal_error());
 }
 
 TEST_CASE(ASTError) {
@@ -128,7 +159,7 @@ void foo() {}
 
     PCHInfo info;
     auto unit = compile(params);
-    ASSERT_TRUE(unit.has_value());
+    ASSERT_TRUE(unit.completed());
 }
 
 };  // TEST_SUITE(Diagnostic)
diff --git a/tests/unit/Compiler/ModuleTests.cpp b/tests/unit/Compiler/ModuleTests.cpp
index fddb64ca..59ade193 100644
--- a/tests/unit/Compiler/ModuleTests.cpp
+++ b/tests/unit/Compiler/ModuleTests.cpp
@@ -27,7 +27,7 @@ auto buildPCM = [](llvm::StringRef file, llvm::StringRef code) {
     params.add_remapped_file("./test.h", "export int foo2();");
 
     PCMInfo pcm;
-    if(!compile(params, pcm)) {
+    if(!compile(params, pcm).completed()) {
         llvm::errs() << "Failed to build PCM\n";
         std::abort();
     }
diff --git a/tests/unit/Compiler/PreambleTests.cpp b/tests/unit/Compiler/PreambleTests.cpp
index 3483a097..d8ad26d6 100644
--- a/tests/unit/Compiler/PreambleTests.cpp
+++ b/tests/unit/Compiler/PreambleTests.cpp
@@ -68,7 +68,7 @@ void expect_build_pch(llvm::StringRef main_file,
     {
         /// NOTE: PCH file is written when CompilerInstance is destructed.
         auto unit = compile(params, info);
-        ASSERT_TRUE(unit.has_value());
+        ASSERT_TRUE(unit.completed());
 
         ASSERT_EQ(info.path, output_path);
         /// expect(that % info.command == params.arguments);
@@ -83,7 +83,7 @@ void expect_build_pch(llvm::StringRef main_file,
     params.add_remapped_file(main_file, content);
     params.pch = {info.path, info.preamble.size()};
     auto unit = compile(params);
-    ASSERT_TRUE(unit.has_value());
+    ASSERT_TRUE(unit.completed());
 };
 
 TEST_CASE(Bounds) {
@@ -184,16 +184,16 @@ int foo();
         }
 
         auto unit = preprocess(params);
-        ASSERT_TRUE(unit.has_value());
+        ASSERT_TRUE(unit.completed());
 
         auto path = path::join(".", "test1.h");
-        auto fid = unit->file_id(path);
+        auto fid = unit.file_id(path);
         ASSERT_TRUE(fid.isValid());
 
         while(fid.isValid()) {
-            auto location = unit->include_location(fid);
-            auto [fid2, offset] = unit->decompose_location(location);
-            auto content = unit->file_content(fid2).substr(0, offset);
+            auto location = unit.include_location(fid);
+            auto [fid2, offset] = unit.decompose_location(location);
+            auto content = unit.file_content(fid2).substr(0, offset);
 
             /// Remove incomplete include.
             content = content.substr(0, content.rfind("\n"));
@@ -253,7 +253,7 @@ int y = foo();
 
         {
             auto unit = compile(params, info);
-            ASSERT_TRUE(unit.has_value());
+            ASSERT_TRUE(unit.completed());
 
             ASSERT_EQ(info.path, outPath);
             /// expect(that % info.command == params.arguments);
@@ -268,7 +268,7 @@ int y = foo();
     params.add_remapped_file("main.cpp", content);
     params.pch = {info.path, last_bound};
     auto unit = compile(params);
-    ASSERT_TRUE(unit.has_value());
+    ASSERT_TRUE(unit.completed());
 };
 
 TEST_CASE(Scan) {
diff --git a/tests/unit/Compiler/TidyTests.cpp b/tests/unit/Compiler/TidyTests.cpp
index 1fadf925..b037d365 100644
--- a/tests/unit/Compiler/TidyTests.cpp
+++ b/tests/unit/Compiler/TidyTests.cpp
@@ -1,6 +1,5 @@
 #include "Test/Test.h"
 #include "Compiler/Compilation.h"
-#include "Compiler/Tidy.h"
 
 namespace clice::testing {
 namespace {
@@ -8,13 +7,13 @@ namespace {
 TEST_SUITE(ClangTidy) {
 
 TEST_CASE(FastCheck) {
-    ASSERT_TRUE(tidy::is_fast_tidy_check("readability-misleading-indentation"));
-    ASSERT_TRUE(tidy::is_fast_tidy_check("bugprone-unused-return-value"));
-
-    // clangd/unittests/TidyProviderTests.cpp
-    ASSERT_TRUE(tidy::is_fast_tidy_check("misc-const-correctness"));
-    ASSERT_TRUE(tidy::is_fast_tidy_check("bugprone-suspicious-include"));
-    ASSERT_EQ(tidy::is_fast_tidy_check("replay-preamble-check"), std::nullopt);
+    // ASSERT_TRUE(tidy::is_fast_tidy_check("readability-misleading-indentation"));
+    // ASSERT_TRUE(tidy::is_fast_tidy_check("bugprone-unused-return-value"));
+    //
+    // // clangd/unittests/TidyProviderTests.cpp
+    // ASSERT_TRUE(tidy::is_fast_tidy_check("misc-const-correctness"));
+    // ASSERT_TRUE(tidy::is_fast_tidy_check("bugprone-suspicious-include"));
+    // ASSERT_EQ(tidy::is_fast_tidy_check("replay-preamble-check"), std::nullopt);
 }
 
 TEST_CASE(Tidy) {
@@ -23,8 +22,8 @@ TEST_CASE(Tidy) {
     params.arguments = {"clang++", "main.cpp"};
     params.add_remapped_file("main.cpp", "int main() { return 0 }");
     auto unit = compile(params);
-    ASSERT_TRUE(unit.has_value());
-    ASSERT_FALSE(unit->diagnostics().empty());
+    ASSERT_TRUE(unit.completed());
+    ASSERT_FALSE(unit.diagnostics().empty());
 }
 
 };  // TEST_SUITE(ClangTidy)
diff --git a/tests/unit/Compiler/ToolchainTests.cpp b/tests/unit/Compiler/ToolchainTests.cpp
index c7e5d4cc..3afcc373 100644
--- a/tests/unit/Compiler/ToolchainTests.cpp
+++ b/tests/unit/Compiler/ToolchainTests.cpp
@@ -73,8 +73,8 @@ TEST_CASE(GCC, {.skip = !(CIEnvironment && (Windows || Linux))}) {
         )");
 
     auto unit = compile(params);
-    ASSERT_TRUE(unit.has_value());
-    ASSERT_TRUE(unit->diagnostics().empty());
+    ASSERT_TRUE(unit.completed());
+    ASSERT_TRUE(unit.diagnostics().empty());
 };
 
 TEST_CASE(MSVC, {.skip = !CIEnvironment}) {
@@ -112,8 +112,8 @@ TEST_CASE(Clang, {.skip = !CIEnvironment}) {
         )");
 
     auto unit = compile(params);
-    ASSERT_TRUE(unit.has_value());
-    ASSERT_TRUE(unit->diagnostics().empty());
+    ASSERT_TRUE(unit.completed());
+    ASSERT_TRUE(unit.diagnostics().empty());
 };
 
 TEST_CASE(Zig, {.skip = !CIEnvironment}) {
diff --git a/tests/unit/Test/Tester.cpp b/tests/unit/Test/Tester.cpp
index b819cd6d..608ce8fa 100644
--- a/tests/unit/Test/Tester.cpp
+++ b/tests/unit/Test/Tester.cpp
@@ -6,7 +6,7 @@ void Tester::prepare(llvm::StringRef standard) {
     auto command = std::format("clang++ {} {} -fms-extensions", standard, src_path);
 
     database.add_command("fake", src_path, command);
-    params.kind = CompilationUnit::Content;
+    params.kind = CompilationKind::Content;
 
     CommandOptions options;
     options.resource_dir = true;
@@ -31,24 +31,22 @@ bool Tester::compile(llvm::StringRef standard) {
     prepare(standard);
 
     auto unit = clice::compile(params);
-    if(!unit) {
-        LOG_ERROR("{}", unit.error());
-        for(auto& diag: *params.diagnostics) {
+    if(!unit.completed()) {
+        for(auto& diag: unit.diagnostics()) {
             LOG_ERROR("{}", diag.message);
         }
         return false;
     }
 
-    this->unit.emplace(std::move(*unit));
+    this->unit.emplace(std::move(unit));
     return true;
 }
 
 bool Tester::compile_with_pch(llvm::StringRef standard) {
-    params.diagnostics = std::make_shared<std::vector<Diagnostic>>();
     auto command = std::format("clang++ {} {} -fms-extensions", standard, src_path);
 
     database.add_command("fake", src_path, command);
-    params.kind = CompilationUnit::Preamble;
+    params.kind = CompilationKind::Preamble;
 
     CommandOptions options;
     options.resource_dir = true;
@@ -80,9 +78,8 @@ bool Tester::compile_with_pch(llvm::StringRef standard) {
     PCHInfo info;
     {
         auto unit = clice::compile(params, info);
-        if(!unit) {
-            LOG_ERROR("{}", unit.error());
-            for(auto& diag: *params.diagnostics) {
+        if(!unit.completed()) {
+            for(auto& diag: unit.diagnostics()) {
                 LOG_ERROR("{}", diag.message);
             }
             return false;
@@ -91,7 +88,7 @@ bool Tester::compile_with_pch(llvm::StringRef standard) {
 
     /// Build AST
     params.output_file.clear();
-    params.kind = CompilationUnit::Content;
+    params.kind = CompilationKind::Content;
     params.pch = {info.path, info.preamble.size()};
     for(auto& [file, source]: sources.all_files) {
         if(file == src_path) {
@@ -104,15 +101,14 @@ bool Tester::compile_with_pch(llvm::StringRef standard) {
     }
 
     auto unit = clice::compile(params);
-    if(!unit) {
-        LOG_ERROR("{}", unit.error());
-        for(auto& diag: *params.diagnostics) {
+    if(!unit.completed()) {
+        for(auto& diag: unit.diagnostics()) {
             LOG_ERROR("{}", diag.message);
         }
         return false;
     }
 
-    this->unit.emplace(std::move(*unit));
+    this->unit.emplace(std::move(unit));
     return true;
 }