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Rollup merge of #109336 - compiler-errors:constrain-to-ct-err, r=BoxyUwU

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Constrain const vars to error if const types are mismatched

When equating two consts of different types, if either are const variables, constrain them to the correct const error kind.

This helps us avoid "successfully" matching a const against an impl signature but leaving unconstrained const vars, which will lead to incremental ICEs when we call const-eval queries during const projection.

Fixes #109296

The second commit in the stack fixes a regression in the first commit where we end up mentioning `[const error]` in an impl overlap error message. I think the error message changes for the better, but I could implement alternative strategies to avoid this without delaying the overlap error message...

r? `@BoxyUwU`
This commit is contained in:
Matthias Krüger 2023-03-21 19:00:12 +01:00 • committed by GitHub
commit 081c607b0a
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6 changed files with 51 additions and 13 deletions
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17
compiler/rustc_infer/src/infer/combine.rs
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@ -189,10 +189,19 @@ impl<'tcx> InferCtxt<'tcx> {
// the expected const's type. Specifically, we don't want const infer vars
// to do any type shapeshifting before and after resolution.
if let Err(guar) = compatible_types {
return Ok(self.tcx.const_error_with_guaranteed(
if relation.a_is_expected() { a.ty() } else { b.ty() },
guar,
));
// HACK: equating both sides with `[const error]` eagerly prevents us
// from leaving unconstrained inference vars during things like impl
// matching in the solver.
let a_error = self.tcx.const_error_with_guaranteed(a.ty(), guar);
if let ty::ConstKind::Infer(InferConst::Var(vid)) = a.kind() {
return self.unify_const_variable(vid, a_error);
}
let b_error = self.tcx.const_error_with_guaranteed(b.ty(), guar);
if let ty::ConstKind::Infer(InferConst::Var(vid)) = b.kind() {
return self.unify_const_variable(vid, b_error);
}
return Ok(if relation.a_is_expected() { a_error } else { b_error });
}
match (a.kind(), b.kind()) {