opaques in dead code and choosing 'static

[lcnr]

after rust-lang/rust#138785:

#![feature(rustc_attrs)]
#![rustc_hidden_type_of_opaques]

use std::ops::Deref;
fn foo<'a, 'b>() -> impl Deref<Target = u32> + use<'a, 'b> {
    unimplemented!() as &'a u32
}

results in

error: &'static u32
 --> src/main.rs:5:21
  |
5 | fn foo<'a, 'b>() -> impl Deref<Target = u32> + use<'a, 'b> {
  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We never do anything withthe user type annotation mentioning 'a as its in dead code, so we end up with opaque<'a, 'b> = &'unconstrained u32 in the storage. The member constraint 'unconstrained member ['a, 'b, 'static] just picks 'static at this point as it's the smallest region related to all other choice regions.

This should be moved as an issue to the rust-lang/rust repo once this behavior is happening on stable