Overlap rules

[nikomatsakis]

We need to model the overlap rules (excluding, for now, specialization). These rules work roughly like this:

• For each impl I1 in the current crate...
  • For each impl I2 of the same trait in any crate...
    • Check whether I1 and I2 can be applied to the same types
    • And check whether predicates I1 and I2 hold, modulo #[fundamental] and locality concerns as described in https://rust-lang.github.io/rfcs/1023-rebalancing-coherence.html and https://rust-lang.github.io/rfcs/2451-re-rebalancing-coherence.html

Here is a list of work items:

• Implement the overlap check and detect when impls have non-unifiable types #90
• Report an error if there are two instances of the exact same impl #88
• Permit impls if the where-clauses are not provable, using the rules from re-balancing coherence
• Support fundamental traits
• Permit impls if the where-clauses have negative impls

[nikomatsakis]

The branch 2022-08-coherence includes some preliminary work here:

It introduces an overlap check and checks that the various impls that are in scope do not have types that successfully unify.

However, it has some problems:

• it fails to detect two instances of the same impl and report an error
• it doesn't include the check for where-clauses that are not provable as true