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_const_eval/src/interpret/cast.rs

@@ -117,8 +117,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                             def_id,
                             args,
                             ty::ClosureKind::FnOnce,
-                        )
-                        .ok_or_else(|| err_inval!(TooGeneric))?;
+                        );
                         let fn_ptr = self.fn_ptr(FnVal::Instance(instance));
                         self.write_pointer(fn_ptr, dest)?;
                     }