Rollup merge of #128424 - fee1-dead-contrib:fx-clean, r=oli-obk

minor `effects` cleanups

* remove the fixme comment about not needing defaults because it turns out we do need defaults (if I made it None instead it would ice a bunch of tests)
* remove the part that special cased trait args when lowering them. This is now historical because effects doesn't add host args to traits anymore (we use associated types now)

compiler/rustc_ast_lowering/src/item.rs

@@ -1668,7 +1668,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             }),
                         )),
                     )),
-                    // FIXME(effects) we might not need a default.
                     default: Some(default_ct),
                     is_host_effect: true,
                     synthetic: true,

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -253,32 +253,6 @@ pub fn lower_generic_args<'tcx: 'a, 'a>(
             match (args_iter.peek(), params.peek()) {
                 (Some(&arg), Some(&param)) => {
                     match (arg, &param.kind, arg_count.explicit_late_bound) {
-                        (
-                            GenericArg::Const(hir::ConstArg {
-                                is_desugared_from_effects: true,
-                                ..
-                            }),
-                            GenericParamDefKind::Const { is_host_effect: false, .. }
-                            | GenericParamDefKind::Type { .. }
-                            | GenericParamDefKind::Lifetime,
-                            _,
-                        ) => {
-                            // FIXME(effects): this should be removed
-                            // SPECIAL CASE FOR DESUGARED EFFECT PARAMS
-                            // This comes from the following example:
-                            //
-                            // ```
-                            // #[const_trait]
-                            // pub trait PartialEq<Rhs: ?Sized = Self> {}
-                            // impl const PartialEq for () {}
-                            // ```
-                            //
-                            // Since this is a const impl, we need to insert a host arg at the end of
-                            // `PartialEq`'s generics, but this errors since `Rhs` isn't specified.
-                            // To work around this, we infer all arguments until we reach the host param.
-                            args.push(ctx.inferred_kind(&args, param, infer_args));
-                            params.next();
-                        }
                         (GenericArg::Lifetime(_), GenericParamDefKind::Lifetime, _)
                         | (
                             GenericArg::Type(_) | GenericArg::Infer(_),