Explicitly check type of array len const

Previously, this was implicitly checked when checking the anon const.
But now, since const args are no longer always anon consts, this needs
to be done manually.

Author: camelid

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -680,9 +680,19 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                 self.require_sized(subty, ObligationCauseCode::SliceOrArrayElem);
             }
 
-            ty::Array(subty, _) => {
+            ty::Array(subty, len) => {
                 self.require_sized(subty, ObligationCauseCode::SliceOrArrayElem);
-                // Note that we handle the len is implicitly checked while walking `arg`.
+                let cause = self.cause(ObligationCauseCode::Misc);
+                self.out.push(traits::Obligation::with_depth(
+                    tcx,
+                    cause,
+                    self.recursion_depth,
+                    self.param_env,
+                    ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(
+                        len,
+                        tcx.types.usize,
+                    ))),
+                ));
             }
 
             ty::Pat(subty, _) => {