[llvm][mustache] Align standalone partial indentation with spec (#159185)

The current implementaion did not correctly handle indentation for
standalone partial tags. It was only applied to lines following a
newline, instead of the first line of a partial's content. This was
fixed by updating the AddIndentation implementaion to prepend the
indentation to the first line of the partial.

Author: ilovepi

llvm/lib/Support/Mustache.cpp

@@ -282,18 +282,15 @@ void stripTokenAhead(SmallVectorImpl<Token> &Tokens, size_t Idx) {
 // For example:
 //  The template string
 //  " \t{{#section}}A{{/section}}"
-// would be considered as having no text ahead and would be render as
+// would be considered as having no text ahead and would be render as:
 //  "A"
-// The exception for this is partial tag which requires us to
-// keep track of the indentation once it's rendered.
 void stripTokenBefore(SmallVectorImpl<Token> &Tokens, size_t Idx,
                       Token &CurrentToken, Token::Type CurrentType) {
   Token &PrevToken = Tokens[Idx - 1];
   StringRef PrevTokenBody = PrevToken.TokenBody;
   StringRef Unindented = PrevTokenBody.rtrim(" \r\t\v");
   size_t Indentation = PrevTokenBody.size() - Unindented.size();
-  if (CurrentType != Token::Type::Partial)
-    PrevToken.TokenBody = Unindented.str();
+  PrevToken.TokenBody = Unindented.str();
   CurrentToken.setIndentation(Indentation);
 }
 
@@ -439,7 +436,8 @@ class AddIndentationStringStream : public raw_ostream {
 public:
   explicit AddIndentationStringStream(llvm::raw_ostream &WrappedStream,
                                       size_t Indentation)
-      : Indentation(Indentation), WrappedStream(WrappedStream) {
+      : Indentation(Indentation), WrappedStream(WrappedStream),
+        NeedsIndent(true) {
     SetUnbuffered();
   }
 
@@ -448,10 +446,15 @@ class AddIndentationStringStream : public raw_ostream {
     llvm::StringRef Data(Ptr, Size);
     SmallString<0> Indent;
     Indent.resize(Indentation, ' ');
+
     for (char C : Data) {
+      if (NeedsIndent && C != '\n') {
+        WrappedStream << Indent;
+        NeedsIndent = false;
+      }
       WrappedStream << C;
       if (C == '\n')
-        WrappedStream << Indent;
+        NeedsIndent = true;
     }
   }
 
@@ -460,6 +463,7 @@ class AddIndentationStringStream : public raw_ostream {
 private:
   size_t Indentation;
   llvm::raw_ostream &WrappedStream;
+  bool NeedsIndent;
 };
 
 class Parser {

llvm/unittests/Support/MustacheTest.cpp

@@ -998,7 +998,7 @@ TEST(MustachePartials, StandaloneIndentation) {
   std::string Out;
   raw_string_ostream OS(Out);
   T.render(D, OS);
-  EXPECT_NE("\\\n  |\n  <\n  ->\n  |\n/\n", Out);
+  EXPECT_EQ("\\\n  |\n  <\n  ->\n  |\n/\n", Out);
 }
 
 TEST(MustacheLambdas, BasicInterpolation) {