Squash closure cast error into fn ptr cast error

Author: oli-obk

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -164,14 +164,11 @@ fn check_rvalue(
             }
         }
         Rvalue::Cast(CastKind::UnsafeFnPointer, _, _) |
+        Rvalue::Cast(CastKind::ClosureFnPointer, _, _) |
         Rvalue::Cast(CastKind::ReifyFnPointer, _, _) => Err((
             span,
             "function pointer casts are not allowed in const fn".into(),
         )),
-        Rvalue::Cast(CastKind::ClosureFnPointer, _, _) => Err((
-            span,
-            "closures are not allowed in const fn".into(),
-        )),
         Rvalue::Cast(CastKind::Unsize, _, _) => Err((
             span,
             "unsizing casts are not allowed in const fn".into(),