Rollup merge of #139669 - nnethercote:overhaul-AssocItem, r=oli-obk

Overhaul `AssocItem`

`AssocItem` has multiple fields that only make sense some of the time. E.g. the `name` can be empty if it's an RPITIT associated type. It's clearer and less error prone if these fields are moved to the relevant `kind` variants.

r? ``@fee1-dead``

compiler/rustc_mir_transform/src/check_undefined_transmutes.rs

@@ -42,7 +42,7 @@ impl<'a, 'tcx> UndefinedTransmutesChecker<'a, 'tcx> {
         if self.tcx.is_const_fn(def_id)
             || matches!(
                 self.tcx.opt_associated_item(def_id),
-                Some(AssocItem { kind: AssocKind::Const, .. })
+                Some(AssocItem { kind: AssocKind::Const { .. }, .. })
             )
         {
             let fn_sig = function.ty(self.body, self.tcx).fn_sig(self.tcx).skip_binder();

compiler/rustc_mir_transform/src/shim.rs

@@ -66,7 +66,7 @@ fn make_shim<'tcx>(tcx: TyCtxt<'tcx>, instance: ty::InstanceKind<'tcx>) -> Body<
             let call_mut = tcx
                 .associated_items(fn_mut)
                 .in_definition_order()
-                .find(|it| it.kind == ty::AssocKind::Fn)
+                .find(|it| it.is_fn())
                 .unwrap()
                 .def_id;