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Auto merge of #105409 - compiler-errors:closure-infer-cycle, r=jackh726

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Don't deduce a signature that makes a closure cyclic

Sometimes when elaborating supertrait bounds for closure signature inference, we end up deducing a closure signature that is cyclical because either a parameter or the return type references a projection mentioning `Self` that also has escaping bound vars, which means that it's not eagerly replaced with an inference variable.

Interestingly, this is not *just* related to my PR that elaborates supertrait bounds for closure signature deduction. The committed test `supertrait-hint-cycle-3.rs` shows **stable** code that is fixed by this PR:

```rust
trait Foo<'a> {
    type Input;
}

impl<F: Fn(u32)> Foo<'_> for F {
    type Input = u32;
}

fn needs_super<F: for<'a> Fn(<F as Foo<'a>>::Input) + for<'a> Foo<'a>>(_: F) {}

fn main() {
    needs_super(|_: u32| {});
}
```

Fixes #105401
Fixes #105396

r? types
This commit is contained in:
bors 2023-01-05 03:59:31 +00:00
commit 03b9e1d154
7 changed files with 162 additions and 10 deletions
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21
compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
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@ -33,7 +33,7 @@ use rustc_infer::infer::error_reporting::TypeErrCtxt;
use rustc_infer::infer::{InferOk, TypeTrace};
use rustc_middle::traits::select::OverflowError;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
use rustc_middle::ty::error::ExpectedFound;
use rustc_middle::ty::error::{ExpectedFound, TypeError};
use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
use rustc_middle::ty::print::{with_forced_trimmed_paths, FmtPrinter, Print};
use rustc_middle::ty::{
@ -1215,6 +1215,25 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
}
}
OutputTypeParameterMismatch(
found_trait_ref,
expected_trait_ref,
terr @ TypeError::CyclicTy(_),
) => {
let self_ty = found_trait_ref.self_ty().skip_binder();
let (cause, terr) = if let ty::Closure(def_id, _) = self_ty.kind() {
(
ObligationCause::dummy_with_span(tcx.def_span(def_id)),
TypeError::CyclicTy(self_ty),
)
} else {
(obligation.cause.clone(), terr)
};
self.report_and_explain_type_error(
TypeTrace::poly_trait_refs(&cause, true, expected_trait_ref, found_trait_ref),
terr,
)
}
OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);