Tracking issue for libsyntax breaking changes

[Manishearth]

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!

[BurntSushi]

I would also like to re-submit my plea that commits with breaking changes are tagged, and if possible, at least some message that hints at or explains how to fix breakages.

Other crates with macros in use (apparently) are docopt_macros and quickcheck_macros.

[Manishearth]

Yeah, they get tagged with [breaking-change]. With this bug in place they should also be posted here first and we can notify everyone well in advance before a rollup.

[Manishearth]

#32767 is a breaking change that will land soon, along with #32688.

I'd normally wait a few days before merging these so that everyone can fix their code early, but in this case #32688 already landed so there's not much I can do.

This will definitely break syntex and friends, cc @erickt. I've never looked at diesel's code so it may or may not break it, @sgrif you may want to have a look before this lands in the nightly.