RPIT hidden types can be ill-formed

[aliemjay]

The following UB shouldn't compile:

type Static<'a> = &'static &'a ();
trait Extend<'a> {
    fn extend(self, _: &'a str) -> &'static str;
}
impl<'a> Extend<'a> for Static<'a> {
    fn extend(self, s: &'a str) -> &'static str {
        s
    }
}
fn boom<'a>(arg: Static<'_>) -> impl Extend<'a> {
    arg
}
fn main() {
    let y = boom(&&()).extend(&String::from("temporary"));
    println!("{}", y);
}

This regressed in v1.61. I guess it is #94081.

Discovered when investigating #114572.

[lqd]

It does indeed bisect to nightly-2022-03-31 which contains #94081:

• Rollup of 4 pull requests #95448
• update miri #95455
• Lazy type-alias-impl-trait take two #94081
• Rollup of 5 pull requests #95466
• Strict Provenance MVP #95241
• allow arbitrary inherent impls for builtin types in core #94963
• Sync rustfmt subtree #95458
• Yet more parse_tt improvements #95425

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-critical

[apiraino]

Downgrading priority as per compiler meeting (notes)