Structs

The opportunistic region resolver opportunistically resolves regions variables to the variable with the least variable id. It is used when normalizing projections to avoid hitting the recursion limit by creating many versions of a predicate for types that in the end have to unify.
The opportunistic resolver can be used at any time. It simply replaces type/const variables that have been unified with the things they have been unified with (similar to shallow_resolve, but deep). This is useful for printing messages etc but also required at various points for correctness.
The unresolved type finder walks a type searching for type variables that don’t yet have a value. The first unresolved type is stored. It does not construct the fully resolved type (which might involve some hashing and so forth).

Functions

Full type resolution replaces all type and region variables with their concrete results. If any variable cannot be replaced (never unified, etc) then an Err result is returned.