Rollup merge of #120139 - compiler-errors:fnonce-shim, r=BoxyUwU

Do not normalize closure signature when building `FnOnce` shim

It is not necessary to normalize the closure signature when building an `FnOnce` shim for an `Fn`/`FnMut` closure. That closure shim is just calling `FnMut::call_mut(&mut self)` anyways.

It's also somewhat sketchy that we were ever doing this to begin with, since we're normalizing with a `ParamEnv::reveal_all()` param-env, which is definitely not right with possibly polymorphic substs.

This cuts out a tiny bit of unnecessary work in `Instance::resolve` and simplifies the signature because now we can unconditionally return an `Instance`.

compiler/rustc_ty_utils/src/instance.rs

@@ -265,7 +265,12 @@ fn resolve_associated_item<'tcx>(
                 match *rcvr_args.type_at(0).kind() {
                     ty::Closure(closure_def_id, args) => {
                         let trait_closure_kind = tcx.fn_trait_kind_from_def_id(trait_id).unwrap();
-                        Instance::resolve_closure(tcx, closure_def_id, args, trait_closure_kind)
+                        Some(Instance::resolve_closure(
+                            tcx,
+                            closure_def_id,
+                            args,
+                            trait_closure_kind,
+                        ))
                     }
                     ty::FnDef(..) | ty::FnPtr(..) => Some(Instance {
                         def: ty::InstanceDef::FnPtrShim(trait_item_id, rcvr_args.type_at(0)),