Make rebase_path() aware of Windows drive letter capitalization

tools/gn/filesystem_utils.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/filesystem_utils.h"
 
 #include <algorithm>
 
 #include "base/files/file_util.h"
 #include "base/logging.h"
@@ skipping 511 matching lines @@
   if (IsPathAbsolute(input) && !IsSlash(input[0]) && IsSlash(dest[0])) {
     std::string corrected_input(1, dest[0]);
     corrected_input.append(input);
     return MakeRelativePath(corrected_input, dest);
   }
   if (IsPathAbsolute(dest) && !IsSlash(dest[0]) && IsSlash(input[0])) {
     std::string corrected_dest(1, input[0]);
     corrected_dest.append(dest);
     return MakeRelativePath(input, corrected_dest);
   }
+
+  // Make sure that both absolute paths use the same drive letter case.
+  if (IsPathAbsolute(input) && IsPathAbsolute(dest) && input.size() > 1 &&
+      dest.size() > 1) {
+    int letter_pos = base::IsAsciiAlpha(input[0]) ? 0 : 1;
+    if (input[letter_pos] != dest[letter_pos] &&
+        base::ToUpperASCII(input[letter_pos]) ==
+            base::ToUpperASCII(dest[letter_pos])) {
+      std::string corrected_input = input;
+      corrected_input[letter_pos] = dest[letter_pos];
+      return MakeRelativePath(corrected_input, dest);
+    }
+  }
 #endif
 
   std::string ret;
 
   // Skip the common prefixes of the source and dest as long as they end in
   // a [back]slash.
   size_t common_prefix_len = 0;
   size_t max_common_length = std::min(input.size(), dest.size());
   for (size_t i = common_prefix_len; i < max_common_length; i++) {
     if (IsSlash(input[i]) && IsSlash(dest[i]))
@@ skipping 373 matching lines @@
 
 SourceDir GetCurrentOutputDir(const Scope* scope) {
   return GetOutputDirForSourceDirAsOutputFile(
       scope->settings(), scope->GetSourceDir()).AsSourceDir(
           scope->settings()->build_settings());
 }
 
 SourceDir GetCurrentGenDir(const Scope* scope) {
   return GetGenDirForSourceDir(scope->settings(), scope->GetSourceDir());
 }