error[E0277]: the trait bound `Const: CanSplitInto<Mut, Mut>` is not satisfied
  --> $DIR/autobad-split-const-mut-mut.rs:16:43
   |
16 |               &mut partial!(X mut a)) = i.into();
   |                                           ^^^^ the trait `CanSplitInto<Mut, Mut>` is not implemented for `Const`
   |
   = help: the following other types implement trait `CanSplitInto<R, S>`:
             <Const as CanSplitInto<Const, Const>>
             <Const as CanSplitInto<Const, No>>
             <Const as CanSplitInto<No, Const>>
             <Const as CanSplitInto<No, No>>
note: required because of the requirements on the impl of `From<&mut X__Partial<Const>>` for `(&mut X__Partial<Mut>, &mut X__Partial<Mut>)`
  --> $DIR/autobad-split-const-mut-mut.rs:9:10
   |
9  | #[derive(PartialBorrow,Default)]
   |          ^^^^^^^^^^^^^
   = note: required because of the requirements on the impl of `Into<(&mut X__Partial<Mut>, &mut X__Partial<Mut>)>` for `&mut X__Partial<Const>`
   = note: this error originates in the derive macro `PartialBorrow` (in Nightly builds, run with -Z macro-backtrace for more info)