[clang-tidy] Avoid processing declarations in system headers

Currently, clang-tidy processes the entire TranslationUnit, including
declarations in system headers. However, the work done in system
headers is discarded at the very end when presenting results, unless
the SystemHeaders option is active.

This is a lot of wasted work, and makes clang-tidy very slow.
In comparison, clangd only processes declarations in the main file,
and it's claimed to be 10x faster than clang-tidy:

https://github.com/lljbash/clangd-tidy

To solve this problem, we can apply a similar solution done in clangd
into clang-tidy. We do this by changing the traversal scope from the
default TranslationUnitDecl, to only contain the top-level declarations
that are _not_ part of system headers. We do this by prepending a new
ASTConsumer to the list of consumers: this new consumer sets the
traversal scope in the ASTContext, which is later used by the
MatchASTConsumer.

Note: this behavior is not active if the user requests warnings from
system headers via the SystemHeaders option.

Note2: out of all the unit tests, only one of them fails:

readability/identifier-naming-anon-record-fields.cpp

This is because the limited traversal scope no longer includes the
"IndirectFieldDecl" that appears in the AST when having a global
scope anonymous union.

I have not found a way to make this one work. However, it does seem
like a very niche use case, and the benefits of a 10x faster clang-tidy
largely outweigh the false negative now introduced by this patch. This
use case is therefore removed from the unit test to make it pass.

Note3: I have purposely decided to make this new feature enabled by
default, instead of adding a new "opt-in/opt-out" flag. Having a new
flag would mean duplicating all our tests to ensure they work in both
modes, which would be infeasible. Having it enabled by default allow
people to get the benefits immediately. Given that all unit tests pass,
the risk for regressions is low. Even if that's the case, the only
issue would be false negatives (fewer things are detected), which
are much more tolerable than false positives.

Credits: original implementation by @njames93, here:
https://reviews.llvm.org/D150126

This implementation is simpler in the sense that it does not consider
HeaderFilterRegex to filter even further. A follow-up patch could
include the functionality if wanted.

Fixes llvm#52959

Author: Carlos Gálvez

clang-tools-extra/clang-tidy/ClangTidy.cpp

@@ -339,6 +339,56 @@ class ClangTidyASTConsumer : public MultiplexConsumer {
   void anchor() override {};
 };
 
+/// ASTConsumer that filters top-level declarations that are in system headers,
+/// and sets the AST traversal scope to only cover the declarations in user
+/// headers. This makes all clang-tidy checks avoid spending time processing
+/// declarations in system headers. The results are discarded anyway when
+/// presenting the results.
+class IgnoreSystemHeadersConsumer : public ASTConsumer {
+public:
+  void Initialize(ASTContext &Context) override {
+    // Make sure the main file ID always gets included.
+    Cache.insert(
+        std::make_pair(Context.getSourceManager().getMainFileID(), true));
+  }
+
+  bool HandleTopLevelDecl(DeclGroupRef DG) override {
+    for (Decl *D : DG) {
+      if (shouldKeepDecl(D))
+        Decls.push_back(D);
+    }
+    return true;
+  }
+
+  void HandleTranslationUnit(ASTContext &Ctx) override {
+    Ctx.setTraversalScope(Decls);
+  }
+
+private:
+  llvm::DenseMap<clang::FileID, bool> Cache;
+  std::vector<Decl *> Decls;
+
+  bool shouldKeepDecl(Decl *D) {
+    auto &SM = D->getASTContext().getSourceManager();
+    FileID FID = SM.getDecomposedExpansionLoc(D->getLocation()).first;
+
+    // Invalid file, keep the declaration to be on the safe side
+    if (FID.isInvalid() || FID == FileID::getSentinel())
+      return true;
+
+    // Check the cache
+    auto [Item, Inserted] = Cache.try_emplace(FID, true);
+    if (!Inserted)
+      return Item->second;
+
+    // If not in the cache, check and update
+    SrcMgr::SLocEntry Entry =
+        SM.getLocalSLocEntry(static_cast<unsigned>(FID.getHashValue()));
+    Item->second = !SrcMgr::isSystem(Entry.getFile().getFileCharacteristic());
+    return Item->second;
+  }
+};
+
 } // namespace
 
 ClangTidyASTConsumerFactory::ClangTidyASTConsumerFactory(
@@ -449,6 +499,10 @@ ClangTidyASTConsumerFactory::createASTConsumer(
   }
 
   std::vector<std::unique_ptr<ASTConsumer>> Consumers;
+
+  if (!Context.getOptions().SystemHeaders.value_or(false))
+    Consumers.push_back(std::make_unique<IgnoreSystemHeadersConsumer>());
+
   if (!Checks.empty())
     Consumers.push_back(Finder->newASTConsumer());
 

clang-tools-extra/docs/ReleaseNotes.rst

@@ -88,6 +88,10 @@ Improvements to clang-query
 Improvements to clang-tidy
 --------------------------
 
+- It no longer processes declarations from system headers by default, greatly
+  improving performance (up to 10x speed-up). This behavior is disabled if the
+  `SystemHeaders` option is enabled.
+
 New checks
 ^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-anon-record-fields.cpp

@@ -33,24 +33,6 @@
 // RUN:     readability-identifier-naming.LocalConstantPointerPrefix: 'lc_', \
 // RUN:   }}'
 
-static union {
-  int global;
-// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for global variable 'global'
-// CHECK-FIXES: {{^}}  int g_global;{{$}}
-
-  const int global_const;
-// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: invalid case style for global constant 'global_const'
-// CHECK-FIXES: {{^}}  const int GLOBAL_CONST;{{$}}
-
-  int *global_ptr;
-// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: invalid case style for global pointer 'global_ptr'
-// CHECK-FIXES: {{^}}  int *GlobalPtr_Ptr;{{$}}
-
-  int *const global_const_ptr;
-// CHECK-MESSAGES: :[[@LINE-1]]:14: warning: invalid case style for global constant pointer 'global_const_ptr'
-// CHECK-FIXES: {{^}}  int *const GLOBAL_CONST_PTR_Ptr;{{$}}
-};
-
 namespace ns {
 
 static union {

clang-tools-extra/test/clang-tidy/infrastructure/file-filter.cpp

@@ -66,19 +66,12 @@ class A { A(int); };
 // CHECK4-NOT: warning:
 // CHECK4-QUIET-NOT: warning:
 
-// CHECK: Suppressed 3 warnings (3 in non-user code)
 // CHECK: Use -header-filter=.* to display errors from all non-system headers.
 // CHECK-QUIET-NOT: Suppressed
-// CHECK2: Suppressed 1 warnings (1 in non-user code)
-// CHECK2: Use -header-filter=.* {{.*}}
 // CHECK2-QUIET-NOT: Suppressed
-// CHECK3: Suppressed 2 warnings (2 in non-user code)
-// CHECK3: Use -header-filter=.* {{.*}}
 // CHECK3-QUIET-NOT: Suppressed
 // CHECK4-NOT: Suppressed {{.*}} warnings
-// CHECK4-NOT: Use -header-filter=.* {{.*}}
 // CHECK4-QUIET-NOT: Suppressed
-// CHECK6: Suppressed 2 warnings (2 in non-user code)
 // CHECK6: Use -header-filter=.* {{.*}}
 
 int x = 123;

clang-tools-extra/test/clang-tidy/infrastructure/system-headers.cpp

@@ -11,9 +11,9 @@
 // RUN: clang-tidy -help | FileCheck -check-prefix=CHECK-OPT-PRESENT %s
 
 // RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='.*' -system-headers=true %s -- -isystem %S/Inputs/system-headers 2>&1 | FileCheck -check-prefix=CHECK-SYSTEM-HEADERS %s
-// RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='.*' -system-headers=false %s -- -isystem %S/Inputs/system-headers 2>&1 | FileCheck -check-prefix=CHECK-NO-SYSTEM-HEADERS %s
+// RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='.*' -system-headers=false %s -- -isystem %S/Inputs/system-headers 2>&1 | FileCheck -check-prefix=CHECK-NO-SYSTEM-HEADERS --allow-empty %s
 // RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='.*' -config='SystemHeaders: true' %s -- -isystem %S/Inputs/system-headers 2>&1 | FileCheck -check-prefix=CHECK-SYSTEM-HEADERS %s
-// RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='.*' -config='SystemHeaders: false' %s -- -isystem %S/Inputs/system-headers 2>&1 | FileCheck -check-prefix=CHECK-NO-SYSTEM-HEADERS %s
+// RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='.*' -config='SystemHeaders: false' %s -- -isystem %S/Inputs/system-headers 2>&1 | FileCheck -check-prefix=CHECK-NO-SYSTEM-HEADERS --allow-empty %s
 
 #include <system_header.h>
 // CHECK-SYSTEM-HEADERS: system_header.h:1:13: warning: single-argument constructors must be marked explicit