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, 19 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/ty/assoc.rs b/compiler/rustc_middle/src/ty/assoc.rs
index 6309dd2e490..ce4c08aa485 100644
--- a/compiler/rustc_middle/src/ty/assoc.rs
+++ b/compiler/rustc_middle/src/ty/assoc.rs
@@ -3,7 +3,7 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Namespace};
 use rustc_hir::def_id::DefId;
 use rustc_macros::{Decodable, Encodable, HashStable};
-use rustc_span::{Ident, Symbol};
+use rustc_span::{Ident, Symbol, sym};
 
 use super::{TyCtxt, Visibility};
 use crate::ty;
@@ -108,6 +108,24 @@ impl AssocItem {
     pub fn is_impl_trait_in_trait(&self) -> bool {
         self.opt_rpitit_info.is_some()
     }
+
+    /// Returns true if:
+    /// - This trait associated item has the `#[type_const]` attribute,
+    /// - If it is in a trait impl, the item from the original trait has this attribute, or
+    /// - It is an inherent assoc const.
+    pub fn is_type_const_capable(&self, tcx: TyCtxt<'_>) -> bool {
+        if self.kind != ty::AssocKind::Const {
+            return false;
+        }
+
+        let def_id = match (self.container, self.trait_item_def_id) {
+            (AssocItemContainer::Trait, _) => self.def_id,
+            (AssocItemContainer::Impl, Some(trait_item_did)) => trait_item_did,
+            // Inherent impl but this attr is only applied to trait assoc items.
+            (AssocItemContainer::Impl, None) => return true,
+        };
+        tcx.has_attr(def_id, sym::type_const)
+    }
 }
 
 #[derive(Copy, Clone, PartialEq, Debug, HashStable, Eq, Hash, Encodable, Decodable)]