Less greedily parse [const] bounds by fmease · Pull Request #146422 · rust-lang/rust (original) (raw)

Important

If you're coming here from any beta backport nomination thread on Zulip, only the last commit is truly relevant (the first commit doesn't need to be backported, it only contains test modifications)!

Don't consider [ to start a bound, only consider [const] in its entirety to do so. This drastically reduces (but doesn't eliminate!) the chance of real breakages. Like const, ~const and async before, [const] unavoidably brings along theoretical breakages, see preexisting tests: macro-const-trait-bound-theoretical-regression.rs and macro-async-trait-bound-theoretical-regression.rs.

Side note: It's unfortunate that we have to do this but apart from the known fact that MBE hurts forward compatibility, the [const] syntax is simply a bit scuffed (also CC'ing #146122, section (3)).

Fixes [after beta backport] #146417.

• 1st commit: Restore the original test intentions of several preexisting related tests that were unfortunately lost over time
  • I've added a bunch of SCREAMING comments to make it less likely to be lost again
  • CC PR Introduce const Trait (always-const trait bounds) #119099 which added most of these tests
  • CC #144409 (comment) for further context (NB: It's not the only PR that negatively affected the test intention)
• 2nd commit: Actually address the regression

r? @oli-obk or anyone