RPIT associated type lifetime bound is ignored

[LHolten]

I tried this code:

trait Foo<'x> {
    type Out;
    fn foo(self) -> Self::Out;
}

struct Bar;

impl<'x> Foo<'x> for Bar {
    type Out = ();
    fn foo(self) -> Self::Out {
        todo!()
    }
}

fn make_static_foo<'x>(_: &'x ()) -> impl Foo<'x, Out: 'static> {
    Bar
}

fn assert_static<T: 'static>(_: T) {}

fn test() {
    let local = ();
    assert_static(make_static_foo(&local).foo());
}

I expect it to compile, but instead it gives the following error:

error[E0597]: `local` does not live long enough
  --> src/lib.rs:23:35
   |
22 |     let local = ();
   |         ----- binding `local` declared here
23 |     assert_static(make_static_foo(&local).foo());
   |                   ----------------^^^^^^-
   |                   |               |
   |                   |               borrowed value does not live long enough
   |                   argument requires that `local` is borrowed for `'static`
24 | }
   | - `local` dropped here while still borrowed

It looks like the Out: 'static bound on make_static_foo is just ignored.

The issue sounds similar to #106684, but it is actually the opposite:
In this issue the bound is in the RPIT instead of in the trait definition.

Meta

rustc --version --verbose:

rustc 1.85.0-nightly (7f75bfa1a 2024-12-30)
binary: rustc
commit-hash: 7f75bfa1ad4e9a9d33a179a90603001515e91991
commit-date: 2024-12-30
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.6

Same happens on stable

[traviscross]

Interestingly, this doesn't work:

trait Tr<'x> {
    type Ty;
    fn f(self) -> Self::Ty;
}

impl Tr<'_> for () {
    type Ty = ();
    fn f(self) -> Self::Ty {}
}

fn g(_: &()) -> impl Tr<'_, Ty: Sized + 'static> {} // <--

fn test(x: &()) -> impl Sized + 'static + use<> {
    g(x).f()
//~^ ERROR lifetime may not live long enough
}

But this does:

trait Tr<'x> {
    type Ty;
    fn f(self) -> Self::Ty;
}

impl Tr<'_> for () {
    type Ty = ();
    fn f(self) -> Self::Ty {}
}

fn g(_: &()) -> impl Tr<'_, Ty = impl Sized + 'static> {} // <--

fn test(x: &()) -> impl Sized + 'static + use<> {
    g(x).f()
}

Equivalently, this works:

Playground link

And this does not:

Playground link

cc @compiler-errors @oli-obk

[compiler-errors]

@traviscross: That's just the part of #116040 which was unsound to fix. A correct fix is a lot more work.

edit: nvm