error[E0277]: the trait bound `Const: IsDowngradeFrom<No>` is not satisfied
  --> $DIR/autobad-dg-no-const.rs:15:39
   |
15 |   let e: &mut partial!(X const a) = d.as_mut();
   |                                       ^^^^^^ the trait `IsDowngradeFrom<No>` is not implemented for `Const`
   |
   = help: the following other types implement trait `IsDowngradeFrom<P>`:
             <Const as IsDowngradeFrom<Const>>
             <Const as IsDowngradeFrom<Mut>>
note: required because of the requirements on the impl of `AsMut<X__Partial<Const>>` for `X__Partial<No>`
  --> $DIR/autobad-dg-no-const.rs:9:10
   |
9  | #[derive(PartialBorrow,Default)]
   |          ^^^^^^^^^^^^^
   = note: this error originates in the derive macro `PartialBorrow` (in Nightly builds, run with -Z macro-backtrace for more info)