Description
The canonicalization scheme created in #48411 works by first folding types into a canonical form, which is then hashed and interned. This is not necessary, and it shows up in the profiles: we could augment the TypeFolder trait to have a method for hashing as it goes (e.g., a hash_with method, and some hash_ty / hash_region callbacks). Then we can begin by hashing the value-to-be-canonicalized, producing a hash (this would be a kind of "stable hash", hence big enough to be presumed unique). We would then look for an interned value with that hash and return it if found, avoiding all the folding. If no interned value is found, then we would fold and put the resulting value into the interning table. Ideally, this hash can then be kept for later, making the "stable hash" code quite cheap.
(This might actually not make things faster, of course, we would want to measure.)