Regression in #[allow(deprecated)] for `impl Trait` return type

[cramertj]

The following code compiles on stable but fails to compile on beta and nightly with the warning "use of deprecated item Deprecated":

#![deny(warnings)]

#[deprecated]
trait Deprecated {}

#[deprecated]
struct DeprecatedTy;

#[allow(deprecated)]
impl Deprecated for DeprecatedTy {}

#[allow(deprecated)]
fn foo() -> impl Deprecated { DeprecatedTy }

fn main() {
    foo();
}

Error:

error: use of deprecated item 'Deprecated'
  --> src/main.rs:13:18
   |
13 | fn foo() -> impl Deprecated { DeprecatedTy }
   |                  ^^^^^^^^^^
   |
note: lint level defined here
  --> src/main.rs:1:9
   |
1  | #![deny(warnings)]
   |         ^^^^^^^^
   = note: #[deny(deprecated)] implied by #[deny(warnings)]

error: aborting due to previous error

Possibly related to @oli-obk 's refactoring of impl Trait?

[oli-obk]

Possible. Is the deprecation checker running on HIR? because we now have a HIR existential type item, and I don't think we're copying lint attributes from the function over to it

[pietroalbini]

cc @Mark-Simulacrum @rust-lang/compiler

Beta promotion to stable should happen today...

[eddyb]

because we now have a HIR existential type item, and I don't think we're copying lint attributes from the function over to it

Ideally, the HIR would be nested in such a way that the lints implicitly apply to the existential.

[pietroalbini]

Bisected this, the cause of the regression is #51806. cc @oli-obk (author) @cramertj (reviewer)

[oli-obk]

Ideally, the HIR would be nested in such a way that the lints implicitly apply to the existential.

for that we'd either need some form of anonymous modules that fill their items into the outer module, or we'd move the existential type item into the function body (which you were explicitly against), or we'd add a hir::TyKind::Item which contains an actual hir::Item

[eddyb]

or we'd add a hir::TyKind::Item which contains an actual hir::Item

This is the closest to a real fix IMO, the path trick is cool but problematic.