align chalk and rustc dyn representation

[nikomatsakis]

chalk represents dyn Trait using binders, so that for dyn Debug, you have a predicate like <T> T: Debug (here, the bound variable T represents the self type).

rustc uses ExistentialPredicate.

The chalk way seems superior. Better to not have an extra form of predicate.

The goal would be to refactor rustc to use a binder + predicate.

[nikomatsakis]

We may want to refactor rustc to have a WhereClause; ExistentialPredicate is almost a Binder<WhereClause>

[csmoe]

@rustbot claim

[nikomatsakis]

@csmoe note that this probably requires an MCP.

[csmoe]

okay, I'll create that.