error[E0277]: the trait bound `Mut: IsDowngradeFrom<No>` is not satisfied
  --> $DIR/autobad-dg-no-mut.rs:15:37
   |
15 |   let e: &mut partial!(X mut a) = d.as_mut();
   |                                     ^^^^^^ the trait `IsDowngradeFrom<No>` is not implemented for `Mut`
   |
   = help: the trait `IsDowngradeFrom<Mut>` is implemented for `Mut`
note: required because of the requirements on the impl of `AsMut<X__Partial<Mut>>` for `X__Partial<No>`
  --> $DIR/autobad-dg-no-mut.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)