Implied bounds compat mode does not deduce sufficient outlives bounds

[compiler-errors]

I tried this code:

pub trait Iter {
    type Item;
}

impl<'x, I> Iter for I
where
    I: IntoIterator<Item = &'x ()>,
{
    type Item = &'x ();
}

pub struct Map<I>(I)
where
    I: Iter,
    I::Item: 'static;

pub fn test_borrowck<'x>(_: Map<Vec<&'x ()>>, s: &'x str) -> &'static str {
    s
}

fn main() {}

I expected to see it work.

Instead, it did not.

Meta

1.85 + beta + nightly for Feb 27, 2025

[compiler-errors]

The reason this fails is because of this if statement:

rust/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs

Line 195 in f45d4ac

if obligation.predicate.has_non_region_infer() {

Combined w/ the fact that we use this function during borrowck for implied bounds means that we don't end up being able to assume that 'x: 'static during wfcheck:

pub fn wf<'x>(_: Map<Vec<&'x ()>>, s: &'x str)
where
    &'static &'x ():,
{
}

(which wouldn't be WF if we didn't know that 'x: 'static, so #137633 is strictly a fix.)