Rollup merge of rust-lang#87770 - BoxyUwU:cec-drop-impl, r=lcnr

permit drop impls with generic constants in where clauses

Fixes rust-lang#79248

`==` is not sufficient to check for equality between unevaluated consts which causes the above issue because the const in `[(); N - 1]:` on the impl and the const in `[(); N - 1]:` on the struct def are not seen as equal. Any predicate that can contain an unevaluated const cant use `==` here as it will cause us to incorrectly emit an error.

I dont know much about chalk but it seems like we ought to be relating the `TypeWellFormedFromEnv` instead of `==` as it contains a `Ty` so I added that too...

r? `````@lcnr`````

compiler/rustc_typeck/src/check/dropck.rs

@@ -218,9 +218,9 @@ fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
 
         // This closure is a more robust way to check `Predicate` equality
         // than simple `==` checks (which were the previous implementation).
-        // It relies on `ty::relate` for `TraitPredicate` and `ProjectionPredicate`
-        // (which implement the Relate trait), while delegating on simple equality
-        // for the other `Predicate`.
+        // It relies on `ty::relate` for `TraitPredicate`, `ProjectionPredicate`,
+        // `ConstEvaluatable` and `TypeOutlives` (which implement the Relate trait),
+        // while delegating on simple equality for the other `Predicate`.
         // This implementation solves (Issue #59497) and (Issue #58311).
         // It is unclear to me at the moment whether the approach based on `relate`
         // could be extended easily also to the other `Predicate`.
@@ -235,6 +235,13 @@ fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
                 (ty::PredicateKind::Projection(a), ty::PredicateKind::Projection(b)) => {
                     relator.relate(predicate.rebind(a), p.rebind(b)).is_ok()
                 }
+                (
+                    ty::PredicateKind::ConstEvaluatable(def_a, substs_a),
+                    ty::PredicateKind::ConstEvaluatable(def_b, substs_b),
+                ) => tcx.try_unify_abstract_consts(((def_a, substs_a), (def_b, substs_b))),
+                (ty::PredicateKind::TypeOutlives(a), ty::PredicateKind::TypeOutlives(b)) => {
+                    relator.relate(predicate.rebind(a.0), p.rebind(b.0)).is_ok()
+                }
                 _ => predicate == p,
             }
         };

src/test/ui/const-generics/const_evaluatable_checked/drop_impl.rs

@@ -0,0 +1,16 @@
+//check-pass
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+struct Foo<const N: usize>
+where
+    [(); N + 1]: ;
+
+impl<const N: usize> Drop for Foo<N>
+where
+    [(); N + 1]: ,
+{
+    fn drop(&mut self) {}
+}
+
+fn main() {}