mgca: Lower all const paths as ConstArgKind::Path

When `#![feature(min_generic_const_args)]` is enabled, we now lower all
const paths in generic arg position to `hir::ConstArgKind::Path`. We
then lower assoc const paths to `ty::ConstKind::Unevaluated` since we
can no longer use the anon const expression lowering machinery. In the
process of implementing this, I factored out `hir_ty_lowering` code that
is now shared between lowering assoc types and assoc consts.

This PR also introduces a `#[type_const]` attribute for trait assoc
consts that are allowed as const args. However, we still need to
implement code to check that assoc const definitions satisfy
`#[type_const]` if present (basically is it a const path or a
monomorphic anon const).

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -3250,7 +3250,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
         span: Span,
     ) -> Result<Diag<'a>, ErrorGuaranteed> {
-        if !self.tcx.features().generic_const_exprs() {
+        if !self.tcx.features().generic_const_exprs()
+            && !self.tcx.features().min_generic_const_args()
+        {
             let guar = self
                 .dcx()
                 .struct_span_err(span, "constant expression depends on a generic parameter")

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -85,6 +85,12 @@ pub fn is_const_evaluatable<'tcx>(
             }
             _ => bug!("unexpected constkind in `is_const_evalautable: {unexpanded_ct:?}`"),
         }
+    } else if tcx.features().min_generic_const_args() {
+        // This is a sanity check to make sure that non-generics consts are checked to
+        // be evaluatable in case they aren't cchecked elsewhere. This will NOT error
+        // if the const uses generics, as desired.
+        crate::traits::evaluate_const(infcx, unexpanded_ct, param_env);
+        Ok(())
     } else {
         let uv = match unexpanded_ct.kind() {
             ty::ConstKind::Unevaluated(uv) => uv,

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -410,7 +410,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let predicate = obligation.predicate.skip_binder();
 
         let mut assume = predicate.trait_ref.args.const_at(2);
-        // FIXME(min_generic_const_exprs): We should shallowly normalize this.
+        // FIXME(mgca): We should shallowly normalize this.
         if self.tcx().features().generic_const_exprs() {
             assume = crate::traits::evaluate_const(self.infcx, assume, obligation.param_env)
         }