Do not resolve directory junctions for `-fdiagnostics-absolute-paths` on Windows.

If the source file path contains directory junctions, and we resolve them when
printing diagnostic messages, these paths look independent for an IDE.
For example, both Visual Studio and Visual Studio Code open separate editors
for such paths, which is not only inconvenient but might even result in losing
changes made in one of them.

Differential Revision: https://reviews.llvm.org/D59415

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361598 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Frontend/TextDiagnostic.cpp

@@ -765,7 +765,28 @@ void TextDiagnostic::emitFilename(StringRef Filename, const SourceManager &SM) {
     const DirectoryEntry *Dir = SM.getFileManager().getDirectory(
         llvm::sys::path::parent_path(Filename));
     if (Dir) {
+      // 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
+      SmallString<4096> DirName = Dir->getName();
+      llvm::sys::fs::make_absolute(DirName);
+      llvm::sys::path::native(DirName);
+      llvm::sys::path::remove_dots(DirName, /* remove_dot_dot */ true);
+#else
       StringRef DirName = SM.getFileManager().getCanonicalName(Dir);
+#endif
       llvm::sys::path::append(AbsoluteFilename, DirName,
                               llvm::sys::path::filename(Filename));
       Filename = StringRef(AbsoluteFilename.data(), AbsoluteFilename.size());

test/Frontend/absolute-paths-windows.test

@@ -0,0 +1,9 @@
+// REQUIRES: system-windows
+// RUN: rm -rf %t.dir
+// RUN: mkdir -p %t.dir\real
+// RUN: cmd /c mklink /j %t.dir\junc %t.dir\real
+// RUN: echo "wrong code" > %t.dir\real\foo.cpp
+// RUN: not %clang_cc1 -fsyntax-only -fdiagnostics-absolute-paths %t.dir\junc\foo.cpp 2>&1 | FileCheck %s
+
+// CHECK-NOT: .dir\real\foo.cpp
+// CHECK: .dir\junc\foo.cpp

test/Frontend/lit.local.cfg

@@ -1 +1 @@
-config.suffixes = ['.c', '.cpp', '.m', '.mm', '.ll', '.cl']
+config.suffixes = ['.c', '.cpp', '.m', '.mm', '.ll', '.cl', '.test']