For anyone altering code or reviewing changes in libsyntax, please consider the affect your changes will have on tools and syntax extensions. Although we don’t offer stability guarantees, libsyntax is public API and is used by many syntax extensions and tools. If you change API you are making a breaking change. This has costs and we should be sure that the benefits outweigh those costs.
Just because libsyntax is unstable, does not mean we can make breaking changes without consequences. We should treat libsyntax essentially like we treated the standard libraries before 1.0. Breaking changes are fine, but there must be justification.
There have been several PRs landed in the past few weeks which are very low benefit ‘tidying up’ patches, but which have broken large numbers of syntax extensions. More worrying is that I have observed several ‘rubber stamp’ reviews where it doesn’t look like the reviewer has considered stability of the API at all. Please, lets do better than this!