See also: https://internals.rust-lang.org/t/please-consider-stability-of-libsyntax/2947

We decided to batch up breaking changes to oft-used unstable compiler internals (mainly, libsyntax) until they're large enough; so that the impact of these breaking changes is minimized. (For example, every time libsyntax breaks, aster and transitively everyone using serde on nightly has to fix it)

Let's try to coordinate these in the future through this issue. We can list breaking PRs here and batch them up when we feel that they're large enough (or when there's another inevitable breaking change about to merge). Merge them when there's enough time for the folks below to know about it, and preferably make PRs to the larger projects below beforehand.

CCing stakeholders:

• libsyntax: @nrc @huonw @eddyb
• clippy: me, @llogiq
• diesel: @sgrif
• phf: @sfackler
• regex_macros: @BurntSushi, @alexcrichton
• rust-cpp, other plugins: @mystor

Add more if you know of them!