uniquifying region constraints

[lcnr]

Given a goal like u32: Trait<'a, 'a>, should this be canonicalized to exists<'0> u32: Trait<'0, '0> or exists<'0, '1> u32: Trait<'0, '1>? Using the first variant causes issues in MIR typeck (#27). Going with the second approach will force us to use a semantic lookup for opaque types (#17).

https://rust-lang.zulipchat.com/#narrow/stream/364551-t-types.2Ftrait-system-refactor/topic/mir.20typeck.20and.20relying.20on.20region.20equality

We ended up going back to uniquifying region constraints in rust-lang/rust#114117. This feels like the only way to avoid the ICEs in MIR typeck from #27