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Rollup merge of #114267 - compiler-errors:rpitit-opaque-bounds, r=spastorino

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Map RPITIT's opaque type bounds back from projections to opaques

An RPITIT in a program's AST is eventually translated into both a projection GAT and an opaque. The opaque is used for default trait methods, like:

```
trait Foo {
  fn bar() -> impl Sized { 0i32 }
}
```

The item bounds for both the projection and opaque are identical, and both have a *projection* self ty. This is mostly okay, since we can normalize this projection within the default trait method body to the opaque, but it does two things:
1. it leads to bugs in places where we don't normalize item bounds, like `deduce_future_output_from_obligations`
2. it leads to extra match arms that are both suspicious looking and also easy to miss

This PR maps the opaque type bounds of the RPITIT's *opaque* back to the opaque's self type to avoid this quirk. Then we can fix the UI test for #108304 (1.) and also remove a bunch of match arms (2.).

Fixes #108304

r? `@spastorino`
This commit is contained in:
Matthias Krüger 2023-07-31 16:57:55 +02:00 • committed by GitHub
commit 692d764e53
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9 changed files with 54 additions and 72 deletions
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7
compiler/rustc_infer/src/infer/opaque_types.rs
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@ -619,13 +619,6 @@ impl<'tcx> InferCtxt<'tcx> {
{
hidden_ty
}
// FIXME(RPITIT): This can go away when we move to associated types
// FIXME(inherent_associated_types): Extend this to support `ty::Inherent`, too.
ty::Alias(ty::Projection, ty::AliasTy { def_id: def_id2, args: args2, .. })
if def_id == def_id2 && args == args2 =>
{
hidden_ty
}
_ => ty,
},
lt_op: |lt| lt,