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).

1 files changed, 21 insertions, 0 deletions

diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index d6c0edf5ae7..91916b8f5fc 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -14,6 +14,7 @@ use rustc_attr_parsing::{AttributeKind, ReprAttr, find_attr};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{Applicability, DiagCtxtHandle, IntoDiagArg, MultiSpan, StashKey};
 use rustc_feature::{AttributeDuplicates, AttributeType, BUILTIN_ATTRIBUTE_MAP, BuiltinAttribute};
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalModDefId;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{
@@ -257,6 +258,9 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                         [sym::coroutine, ..] => {
                             self.check_coroutine(attr, target);
                         }
+                        [sym::type_const, ..] => {
+                            self.check_type_const(hir_id,attr, target);
+                        }
                         [sym::linkage, ..] => self.check_linkage(attr, span, target),
                         [sym::rustc_pub_transparent, ..] => self.check_rustc_pub_transparent(attr.span(), span, attrs),
                         [
@@ -2519,6 +2523,23 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
+    fn check_type_const(&self, hir_id: HirId, attr: &Attribute, target: Target) {
+        let tcx = self.tcx;
+        if target == Target::AssocConst
+            && let parent = tcx.parent(hir_id.expect_owner().to_def_id())
+            && self.tcx.def_kind(parent) == DefKind::Trait
+        {
+            return;
+        } else {
+            self.dcx()
+                .struct_span_err(
+                    attr.span(),
+                    "`#[type_const]` must only be applied to trait associated constants",
+                )
+                .emit();
+        }
+    }
+
     fn check_linkage(&self, attr: &Attribute, span: Span, target: Target) {
         match target {
             Target::Fn