Support for source_target_relative expansion in GN

tools/gn/substitution_writer.h

Index: tools/gn/substitution_writer.h
diff --git a/tools/gn/substitution_writer.h b/tools/gn/substitution_writer.h
index a450bbbc03c02f0ce13b8eb0cf0999705aac382d..a43af4419e19b5b69d9e2726902af2e177eae0c8 100644
--- a/tools/gn/substitution_writer.h
+++ b/tools/gn/substitution_writer.h
@@ -82,14 +82,17 @@ class SubstitutionWriter {
   // result isn't in the correct directory. The caller should validate this
   // first (see for example IsFileInOuputDir).

[brettw, 2016/10/05 19:47:50]

GN almost never supports random pointers being null, so I think it's worthwhile
to call out that target can be null really obviously. In each of these blocks
with comments above the function, can you append "The target can be null (see
class comment above)."

[Petr Hosek, 2016/10/11 00:59:49]

Done.

   static SourceFile ApplyPatternToSource(
+      const Target* target,
       const Settings* settings,
       const SubstitutionPattern& pattern,
       const SourceFile& source);
   static std::string ApplyPatternToSourceAsString(
+      const Target* target,
       const Settings* settings,
       const SubstitutionPattern& pattern,
       const SourceFile& source);
   static OutputFile ApplyPatternToSourceAsOutputFile(
+      const Target* target,
       const Settings* settings,
       const SubstitutionPattern& pattern,
       const SourceFile& source);
@@ -99,16 +102,19 @@ class SubstitutionWriter {
   // apply to multiple files and create a list. The result can either be
   // SourceFiles or OutputFiles.
   static void ApplyListToSource(
+      const Target* target,
       const Settings* settings,
       const SubstitutionList& list,
       const SourceFile& source,
       std::vector<SourceFile>* output);
   static void ApplyListToSourceAsString(
+      const Target* target,
       const Settings* settings,
       const SubstitutionList& list,
       const SourceFile& source,
       std::vector<std::string>* output);
   static void ApplyListToSourceAsOutputFile(
+      const Target* target,
       const Settings* settings,
       const SubstitutionList& list,
       const SourceFile& source,
@@ -117,16 +123,19 @@ class SubstitutionWriter {
   // Like ApplyListToSource but applies the list to all sources and replaces
   // rather than appends the output (this produces the complete output).
   static void ApplyListToSources(
+      const Target* target,
       const Settings* settings,
       const SubstitutionList& list,
       const std::vector<SourceFile>& sources,
       std::vector<SourceFile>* output);
   static void ApplyListToSourcesAsString(
+      const Target* target,
       const Settings* settings,
       const SubstitutionList& list,
       const std::vector<SourceFile>& sources,
       std::vector<std::string>* output);
   static void ApplyListToSourcesAsOutputFile(
+      const Target* target,
       const Settings* settings,
       const SubstitutionList& list,
       const std::vector<SourceFile>& sources,
@@ -139,6 +148,7 @@ class SubstitutionWriter {
   // for {{source}} will be written since that maps to Ninja's implicit $in
   // variable.
   static void WriteNinjaVariablesForSource(
+      const Target* target,
       const Settings* settings,
       const SourceFile& source,
       const std::vector<SubstitutionType>& types,
@@ -150,6 +160,7 @@ class SubstitutionWriter {
   // indicates the directory that the relative directories should be relative
   // to, otherwise it is ignored.
   static std::string GetSourceSubstitution(
+      const Target* target,
       const Settings* settings,
       const SourceFile& source,
       SubstitutionType type,