OptWithInfcx naming nits, trait bound simplifications

[compiler-errors]

• Use an associated type Interner on InferCtxtLike to remove a redundant interner parameter (I: Interner, Infcx: InferCtxtLike<I> -> Infcx: InferCtxtLike).
• Remove double-Option between infcx: Option<Infcx> and fn universe_of_ty(&self, ty: ty::InferTy) -> Option<ty::UniverseIndex>. We don't need the infcx to be optional if we can provide a "noop" (NoInfcx) implementation that just always returns None for universe index.
  • Also removes the core::convert::Infallible implementation which I found a bit weird...
• Some naming nits with params.
  • I found InferCtxt + InfCtx and Infcx to be a lot of different ways to spell "inference context", so I got rid of the InfCtx type parameter name in favor of Infcx which is a more standard name.
  • I found OptWithInfcx to be a bit redundant -> WithInfcx.

I'm making these changes because I intend to reuse the InferCtxtLike trait for uplifting the canonicalizer into a new trait -- conveniently, the information I need for uplifting the canonicalizer also is just the universe information of a type var, so it's super convenient 😸

r? @BoxyUwU or @lcnr

[BoxyUwU]

I didnt actually want the universe_of_x to return an Option. it was just not yet physically possible to get the universe always. In an ideal world we'd be able to tell what the universe of a var is even after its been unified with something in a lower universe and those three methods would all just be -> ty::UniverseIndex

[compiler-errors]

@BoxyUwU: We could make infcx optional again if it's possible in the future to have universe_of_* -> UniverseIndex directly. For now, though, I feel like it's redundant.

[BoxyUwU]

r? @lcnr
because im not going to r+ this

[lcnr]

@bors r+ rollup

[bors]

📌 Commit 269d5a3 has been approved by lcnr

It is now in the queue for this repository.