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

[Aaron Ballman via cfe-commits]

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@@ -738,12 +738,20 @@ 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) {
+  auto File = SM.getFileManager().getOptionalFileRef(Filename);
+
+  // 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.
+  auto AlwaysSimplify = [&] {
+    return File->getName().contains("..") &&
+           llvm::sys::fs::is_local(File->getName());
+  };
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AaronBallman wrote:

Are we sure that `is_local()` isn't going to cause a performance issue by itself? I notice the Windows implementation looks like it could be expensive: https://github.com/llvm/llvm-project/blob/bdcbe67400b8b3d31d89df6c74d06ab80d8c7862/llvm/lib/Support/Windows/Path.inc#L313

https://github.com/llvm/llvm-project/pull/143520