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Pick one possible lifetime in case there are multiple choices #89327

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Oct 1, 2021
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Pick one possible lifetime in case there are multiple choices #89327

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17 changes: 16 additions & 1 deletion compiler/rustc_borrowck/src/region_infer/opaque_types.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -124,7 +124,22 @@ impl<'tcx> RegionInferenceContext<'tcx> {
ty::ReVar(vid) => {
// Find something that we can name
let upper_bound = self.approx_universal_upper_bound(vid);
self.definitions[upper_bound].external_name.unwrap_or(region)
let upper_bound = &self.definitions[upper_bound];
match upper_bound.external_name {
Some(reg) => reg,
None => {
// Nothing exact found, so we pick the first one that we find.
let scc = self.constraint_sccs.scc(vid);
for vid in self.rev_scc_graph.as_ref().unwrap().upper_bounds(scc) {
match self.definitions[vid].external_name {
None => {}
Some(&ty::ReStatic) => {}
Some(region) => return region,
}
}
region
}
}
}
_ => region,
})
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6 changes: 5 additions & 1 deletion src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unrelated.nll.stderr
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Expand Up @@ -4,7 +4,11 @@ error[E0700]: hidden type for `impl Trait` captures lifetime that does not appea
LL | fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'d, 'e>
| ^^^^^^^^^^^^^^^^^^
|
= note: hidden type `Ordinary<'_>` captures lifetime '_#9r
note: hidden type `Ordinary<'b>` captures the lifetime `'b` as defined on the function body at 16:21
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@lqd lqd Sep 29, 2021 •
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(This is pre-existing)

This message mentions Ordinary<'b> here as a "hidden type", shouldn't this be "hidden type for impl Trait" instead, or am I misunderstanding this diagnostic ?

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You are completely right.

We should completely rewrite the diagnostics around TAIT in general... they are all not too helpful. I'll open an issue to track the various ways in which it is not great

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It's maybe even impl Trait diagnostics in general, as there's no TAIT here, for example.

--> $DIR/ordinary-bounds-unrelated.rs:16:21
|
LL | fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'d, 'e>
| ^^

error: aborting due to previous error

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6 changes: 5 additions & 1 deletion src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.nll.stderr
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Expand Up @@ -4,7 +4,11 @@ error[E0700]: hidden type for `impl Trait` captures lifetime that does not appea
LL | fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b>
| ^^^^^^^^^^^^^^^^^^
|
= note: hidden type `Ordinary<'_>` captures lifetime '_#6r
note: hidden type `Ordinary<'b>` captures the lifetime `'b` as defined on the function body at 18:21
--> $DIR/ordinary-bounds-unsuited.rs:18:21
|
LL | fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b>
| ^^

error: aborting due to previous error

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