LibTooling getting FixItWriter to work

Hi!

I have been experimenting with libTooling for a while now. Looking at the clang-tools sources I managed to get ASTFrontendActions, ASTConsumer, ast_matchers, FixItHints, DiagnosticsEngines, etc. to work.

But I am stuck now with the clang::FixItRewriter class. Basically I want to do a simple source-to-source translation. The InPlace option in the clang::FixItOptions looks like the right thing to me. Then let the Rewriter do the work via its method WriteFixedFiles();

I have been basically following Peter Goldsborough's blog post and his presentations from
https://github.com/peter-can-talk/cppnow-2017

When I compile his minus-tool example, that rewrites any binary addition expression (from 4 + 3 to 4 - 3), it works on my linux box either with g++ 8.2.1 or with clang 7.0.0. However when I try my code, the DiganosticsEngine shows the Warnings and associated FixItHints, but no inplace rewriting is performed.

Any ideas on what to look out for?

Thanks a lot!
Uli