[crater only] Syntactically reject equality predicates#153513
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[crater only] Syntactically reject equality predicates
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Equality predicates have been syntactically valid since PR #39158 / nightly-2017-02-02 / 1.16 (released 2017-03-16, 9 years ago), both forms that is:
$ty = $tyand$ty == $ty. They're not registered as an unstable feature despite having a tracking issue (#20041). Naturally, they don't have a feature gate. Of course, we reject them post-expansion, so they are still semantically invalid.Parser scaffolding for
$ident = $tywas added in RUST-19391 (2014) which was then generalized to$ty = $tyin RUST-20002 (2014) and extended to additionally cover$ty == $tyin RUST-39158 (2017). As mentioned, the last PR also made them grammatical.RUST-87471 (2021) attempted to start impl'ing typeck'ing but it was closed due to concerns: #87471 (comment) (already back in 2017 there were concerns: https://github.com/rust-lang/rust/pull/39158/changes#r97811244).
In 2022, T-lang discussed this feature during a meeting and raised concerns: #20041 (comment). However, they were inclined to accept a restricted form, namely
T::AssocTy = $ty(Tis a (self) ty param or a self ty alias) and<$ty as Trait>::AssocTy = $tysince that's trivial to support in HIR ty lowering (this is still accurate).I'm very interested in making equality predicates syntactically invalid again for now (whether through a hard error or a pre-expansion feature gate error) (even if we end up permitting & impl'ing the proposed restricted forms, it's still valuable to gate this feature because we still need to decide between
=and==, we ought not have both). So let's crater this.