NLL and closures: higher-ranked lifetime bounds are not enforced in type tests

[aliemjay]

This code compiles after #95565 although it shouldn't: (playground)

fn assert_static<T>() where for<'a> T: 'a, {}

fn test<T>() {
    || {
        assert_static::<T>();
    };
}

This is similar to #98095 in that we're not taking into account placeholder regions when "promoting" type-tests from the closure to its caller:

rust/compiler/rustc_borrowck/src/region_infer/mod.rs

Line 940 in ddcbba0

for ur in self.scc_values.universal_regions_outlived_by(r_scc) {

@rustbot label T-types regression-from-stable-to-beta C-bug A-NLL I-unsound

[aliemjay]

I guess it would be an easy fix
@rustbot claim

[steffahn]

The unsoundness exploitation in #98095 can easily be adapted for this issue, so it is in fact a soundness issue: playground

[apiraino]

issue touched in today's T-compiler meeting, notes from Zulip chat. Will also be discussed tomorrow during the T-types meeting.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-critical