[Rewrite][NFC] Add FIXMEs and tests for RemoveLineIfEmpty bug

I'd like to add these comments to warn others of problems I
encountered when trying to use `RemoveLineIfEmpty`.  I originally
tried to fix the problem, but I realized I could implement the
functionality more easily and efficiently in my calling code where I
can make the simplifying assumption that there are no prior edits to
the line from which text is being removed.  While I've lost the
motivation to write a fix, which doesn't look easy, I figure a warning
to others is better than silence.

I've added a unit test to demonstrate the problem.  I don't know how
to mark it as an expected failure, so I just marked it disabled.

Reviewed By: jkorous

Differential Revision: https://reviews.llvm.org/D61466

llvm-svn: 369049

1 files changed, 11 insertions, 0 deletions

diff --git a/clang/lib/Rewrite/Rewriter.cpp b/clang/lib/Rewrite/Rewriter.cpp
index 881399e98e3..33718b7721c 100644
--- a/clang/lib/Rewrite/Rewriter.cpp
+++ b/clang/lib/Rewrite/Rewriter.cpp
@@ -96,6 +96,17 @@ void RewriteBuffer::RemoveText(unsigned OrigOffset, unsigned Size,
     }
     if (posI != end() && *posI == '\n') {
       Buffer.erase(curLineStartOffs, lineSize + 1/* + '\n'*/);
+      // FIXME: Here, the offset of the start of the line is supposed to be
+      // expressed in terms of the original input not the "real" rewrite
+      // buffer.  How do we compute that reliably?  It might be tempting to use
+      // curLineStartOffs + OrigOffset - RealOffset, but that assumes the
+      // difference between the original and real offset is the same at the
+      // removed text and at the start of the line, but that's not true if
+      // edits were previously made earlier on the line.  This bug is also
+      // documented by a FIXME on the definition of
+      // clang::Rewriter::RewriteOptions::RemoveLineIfEmpty.  A reproducer for
+      // the implementation below is the test RemoveLineIfEmpty in
+      // clang/unittests/Rewrite/RewriteBufferTest.cpp.
       AddReplaceDelta(curLineStartOffs, -(lineSize + 1/* + '\n'*/));
     }
   }