mark THIR use as candidate for constness check

Author: dingxiangfei2009

compiler/rustc_mir_build/src/builder/expr/as_rvalue.rs

@@ -661,8 +661,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// operations that can be const-folded today.
     fn check_constness(&self, mut kind: &'a ExprKind<'tcx>) -> bool {
         loop {
+            debug!(?kind, "check_constness");
             match kind {
-                &ExprKind::PointerCoercion {
+                &ExprKind::ValueTypeAscription { source: eid, user_ty: _, user_ty_span: _ }
+                | &ExprKind::Use { source: eid }
+                | &ExprKind::PointerCoercion {
                     cast: PointerCoercion::Unsize,
                     source: eid,
                     is_from_as_cast: _,

tests/ui/coercion/no_local_for_coerced_const-issue-143671.rs

@@ -29,10 +29,18 @@ const Y: X<'static, i32> = X { f: &0 };
 fn main() {
     let _: [X<'static, dyn Display>; 0] = [Y; 0];
     coercion_on_weak_in_const();
+    coercion_on_weak_as_cast();
 }
 
 fn coercion_on_weak_in_const() {
     const X: Weak<i32> = Weak::new();
     const Y: [Weak<dyn Send>; 0] = [X; 0];
     let _ = Y;
 }
+
+fn coercion_on_weak_as_cast() {
+    const Y: X<'static, i32> = X { f: &0 };
+    // What happens in the following code is that
+    // a constant is explicitly coerced into
+    let _a: [X<'static, dyn Display>; 0] = [Y as X<'static, dyn Display>; 0];
+}