[Clang] [Diagnostics] Simplify filenames that contain '..' (#143520)

This can significantly shorten file paths to standard library headers,
e.g. on my system, `<ranges>` is currently printed as
```console
/usr/lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/ranges
```
but with this change, we instead print
```console
/usr/include/c++/15/ranges
```

This is of course just a heuristic; there are paths that would get longer
as a result of this, so we use whichever path ends up being shorter.

@AaronBallman pointed out that this might be problematic for network
file systems since path resolution might take a while, so this is enabled 
only for paths that are part of a local filesystem—though not on Windows
since there we noticed that the check itself is slow.

The file names are cached in `SourceManager`.

Author: Sirraide

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

@@ -12,8 +12,9 @@
 // RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='header\.h' %s -- -I %t 2>&1 | FileCheck --check-prefix=CHECK_HEADER %s
 // RUN: clang-tidy -checks='-*,google-explicit-constructor' -header-filter='header\.h' -quiet %s -- -I %t 2>&1 | FileCheck --check-prefix=CHECK_HEADER %s
 
-// Check that `-header-filter` operates on the same file paths as paths in
-// diagnostics printed by ClangTidy.
+// `-header-filter` operates on the actual file path that the user provided in
+// the #include directive; however, Clang's path name simplification causes the
+// path to be printed in canonicalised form here.
 #include "dir1/dir2/../header_alias.h"
-// CHECK_HEADER_ALIAS: dir1/dir2/../header_alias.h:1:11: warning: single-argument constructors
+// CHECK_HEADER_ALIAS: dir1/header.h:1:11: warning: single-argument constructors
 // CHECK_HEADER-NOT: warning:

clang/include/clang/Basic/SourceManager.h

@@ -824,6 +824,12 @@ class SourceManager : public RefCountedBase<SourceManager> {
 
   mutable std::unique_ptr<SrcMgr::SLocEntry> FakeSLocEntryForRecovery;
 
+  /// Cache for filenames used in diagnostics. See 'getNameForDiagnostic()'.
+  mutable llvm::StringMap<StringRef> DiagNames;
+
+  /// Allocator for absolute/short names.
+  mutable llvm::BumpPtrAllocator DiagNameAlloc;
+
   /// Lazily computed map of macro argument chunks to their expanded
   /// source location.
   using MacroArgsMap = std::map<unsigned, SourceLocation>;
@@ -1848,6 +1854,16 @@ class SourceManager : public RefCountedBase<SourceManager> {
   /// \return Location of the top-level macro caller.
   SourceLocation getTopMacroCallerLoc(SourceLocation Loc) const;
 
+  /// Retrieve the name of a file suitable for diagnostics.
+  // FIXME: Passing in the DiagnosticOptions here is a workaround for the
+  // fact that installapi does some weird things with DiagnosticsEngines,
+  // which causes the 'Diag' member of SourceManager (or at least the
+  // DiagnosticsOptions member thereof) to be a dangling reference
+  // sometimes. We should probably fix that or decouple the two classes
+  // to avoid this issue entirely.
+  StringRef getNameForDiagnostic(StringRef Filename,
+                                 const DiagnosticOptions &Opts) const;
+
 private:
   friend class ASTReader;
   friend class ASTWriter;

clang/lib/Basic/SourceManager.cpp

@@ -2390,3 +2390,75 @@ SourceManagerForFile::SourceManagerForFile(StringRef FileName,
   assert(ID.isValid());
   SourceMgr->setMainFileID(ID);
 }
+
+StringRef
+SourceManager::getNameForDiagnostic(StringRef Filename,
+                                    const DiagnosticOptions &Opts) const {
+  OptionalFileEntryRef File = getFileManager().getOptionalFileRef(Filename);
+  if (!File)
+    return Filename;
+
+  bool SimplifyPath = [&] {
+    if (Opts.AbsolutePath)
+      return true;
+
+    // Try to simplify paths that contain '..' in any case since paths to
+    // standard library headers especially tend to get quite long otherwise.
+    // Only do that for local filesystems though to avoid slowing down
+    // compilation too much.
+    if (!File->getName().contains(".."))
+      return false;
+
+    // If we're not on Windows, check if we're on a network file system and
+    // avoid simplifying the path in that case since that can be slow. On
+    // Windows, the check for a local filesystem is already slow, so skip it.
+#ifndef _WIN32
+    if (!llvm::sys::fs::is_local(File->getName()))
+      return false;
+#endif
+
+    return true;
+  }();
+
+  if (!SimplifyPath)
+    return Filename;
+
+  // This may involve computing canonical names, so cache the result.
+  StringRef &CacheEntry = DiagNames[Filename];
+  if (!CacheEntry.empty())
+    return CacheEntry;
+
+  // We want to print a simplified absolute path, i. e. without "dots".
+  //
+  // The hardest part here are the paths like "<part1>/<link>/../<part2>".
+  // On Unix-like systems, we cannot just collapse "<link>/..", because
+  // paths are resolved sequentially, and, thereby, the path
+  // "<part1>/<part2>" may point to a different location. That is why
+  // we use FileManager::getCanonicalName(), which expands all indirections
+  // with llvm::sys::fs::real_path() and caches the result.
+  //
+  // On the other hand, it would be better to preserve as much of the
+  // original path as possible, because that helps a user to recognize it.
+  // real_path() expands all links, which sometimes too much. Luckily,
+  // on Windows we can just use llvm::sys::path::remove_dots(), because,
+  // on that system, both aforementioned paths point to the same place.
+  SmallString<256> TempBuf;
+#ifdef _WIN32
+  TempBuf = File->getName();
+  llvm::sys::fs::make_absolute(TempBuf);
+  llvm::sys::path::native(TempBuf);
+  llvm::sys::path::remove_dots(TempBuf, /* remove_dot_dot */ true);
+#else
+  TempBuf = getFileManager().getCanonicalName(*File);
+#endif
+
+  // In some cases, the resolved path may actually end up being longer (e.g.
+  // if it was originally a relative path), so just retain whichever one
+  // ends up being shorter.
+  if (!Opts.AbsolutePath && TempBuf.size() > Filename.size())
+    CacheEntry = Filename;
+  else
+    CacheEntry = TempBuf.str().copy(DiagNameAlloc);
+
+  return CacheEntry;
+}

clang/lib/Frontend/SARIFDiagnostic.cpp

@@ -163,36 +163,7 @@ SARIFDiagnostic::addDiagnosticLevelToRule(SarifRule Rule,
 
 llvm::StringRef SARIFDiagnostic::emitFilename(StringRef Filename,
                                               const SourceManager &SM) {
-  if (DiagOpts.AbsolutePath) {
-    auto File = SM.getFileManager().getOptionalFileRef(Filename);
-    if (File) {
-      // We want to print a simplified absolute path, i. e. without "dots".
-      //
-      // The hardest part here are the paths like "<part1>/<link>/../<part2>".
-      // On Unix-like systems, we cannot just collapse "<link>/..", because
-      // paths are resolved sequentially, and, thereby, the path
-      // "<part1>/<part2>" may point to a different location. That is why
-      // we use FileManager::getCanonicalName(), which expands all indirections
-      // with llvm::sys::fs::real_path() and caches the result.
-      //
-      // On the other hand, it would be better to preserve as much of the
-      // original path as possible, because that helps a user to recognize it.
-      // real_path() expands all links, which is sometimes too much. Luckily,
-      // on Windows we can just use llvm::sys::path::remove_dots(), because,
-      // on that system, both aforementioned paths point to the same place.
-#ifdef _WIN32
-      SmallString<256> TmpFilename = File->getName();
-      llvm::sys::fs::make_absolute(TmpFilename);
-      llvm::sys::path::native(TmpFilename);
-      llvm::sys::path::remove_dots(TmpFilename, /* remove_dot_dot */ true);
-      Filename = StringRef(TmpFilename.data(), TmpFilename.size());
-#else
-      Filename = SM.getFileManager().getCanonicalName(*File);
-#endif
-    }
-  }
-
-  return Filename;
+  return SM.getNameForDiagnostic(Filename, DiagOpts);
 }
 
 /// Print out the file/line/column information and include trace.

clang/lib/Frontend/TextDiagnostic.cpp

@@ -738,39 +738,7 @@ void TextDiagnostic::printDiagnosticMessage(raw_ostream &OS,
 }
 
 void TextDiagnostic::emitFilename(StringRef Filename, const SourceManager &SM) {
-#ifdef _WIN32
-  SmallString<4096> TmpFilename;
-#endif
-  if (DiagOpts.AbsolutePath) {
-    auto File = SM.getFileManager().getOptionalFileRef(Filename);
-    if (File) {
-      // We want to print a simplified absolute path, i. e. without "dots".
-      //
-      // The hardest part here are the paths like "<part1>/<link>/../<part2>".
-      // On Unix-like systems, we cannot just collapse "<link>/..", because
-      // paths are resolved sequentially, and, thereby, the path
-      // "<part1>/<part2>" may point to a different location. That is why
-      // we use FileManager::getCanonicalName(), which expands all indirections
-      // with llvm::sys::fs::real_path() and caches the result.
-      //
-      // On the other hand, it would be better to preserve as much of the
-      // original path as possible, because that helps a user to recognize it.
-      // real_path() expands all links, which sometimes too much. Luckily,
-      // on Windows we can just use llvm::sys::path::remove_dots(), because,
-      // on that system, both aforementioned paths point to the same place.
-#ifdef _WIN32
-      TmpFilename = File->getName();
-      llvm::sys::fs::make_absolute(TmpFilename);
-      llvm::sys::path::native(TmpFilename);
-      llvm::sys::path::remove_dots(TmpFilename, /* remove_dot_dot */ true);
-      Filename = StringRef(TmpFilename.data(), TmpFilename.size());
-#else
-      Filename = SM.getFileManager().getCanonicalName(*File);
-#endif
-    }
-  }
-
-  OS << Filename;
+  OS << SM.getNameForDiagnostic(Filename, DiagOpts);
 }
 
 /// Print out the file/line/column information and include trace.

clang/test/Frontend/absolute-paths.c

@@ -8,9 +8,9 @@
 
 #include "absolute-paths.h"
 
-// Check whether the diagnostic from the header above includes the dummy
-// directory in the path.
-// NORMAL: SystemHeaderPrefix
+// Check that the bogus prefix we added is stripped out even if absolute paths
+// are disabled.
+// NORMAL-NOT: SystemHeaderPrefix
 // ABSOLUTE-NOT: SystemHeaderPrefix
 // CHECK: warning: non-void function does not return a value
 

clang/test/Frontend/simplify-paths.c

@@ -0,0 +1,18 @@
+// REQUIRES: shell
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t/a/b/
+// RUN: echo 'void x;' > %t/test.h
+// RUN: echo 'const void x;' > %t/header_with_a_really_long_name.h
+// RUN: ln -s %t/header_with_a_really_long_name.h %t/a/shorter_name.h
+//
+// RUN: %clang_cc1 -fsyntax-only -I %t %s 2> %t/output.txt || true
+// RUN: cat %t/output.txt | FileCheck %s
+
+// Check that we strip '..' by canonicalising the path...
+#include "a/b/../../test.h"
+// CHECK: simplify-paths.c.tmp/test.h:1:6: error: variable has incomplete type 'void'
+
+// ... but only if the resulting path is actually shorter.
+#include "a/b/../shorter_name.h"
+// CHECK: simplify-paths.c.tmp/a/b/../shorter_name.h:1:12: error: variable has incomplete type 'const void'