There are a number of issues with the current design where Decode depends on BorrowDecode. The most obvious one is the implementation of Cow.

Ideally we'd want:

impl<T> BorrowDecode for Cow<T> where T: BorrowDecode {
    fn borrow_decode<D: BorrowDecoder<'de>>(decoder: &mut D) -> Result<Self, DecodeError> {
        Ok(Cow::Borrowed(T::borrow_decode(decoder)?))
    }
}

impl<T> Decode for Cow<T> where T: Decode {
    fn decode<D: Decoder<'de>>(decoder: &mut D) -> Result<Self, DecodeError> {
        Ok(Cow::Owned(T::decode(decoder)?))
    }
}

But this is currently not possible.

This issue happens with other containers types as well. One issue is Arc<str> (#527), because Arc<T> requires T: Decode.

This pull request splits the implementation of Decode and BorrowDecode to be completely separate.

Some notes:

• I added a impl_borrow_decode macro that is publicly available, which can be used to automatically derive BorrowDecode for any type that implements Decode
  • This only works with basic types. Types with generics for example cannot use this macro
• #[derive(Decode)] now implies #[derive(BorrowDecode)] automatically.

As far as I can tell this is only a breaking change for people who implement Decode manually. Otherwise it should hopefully be a painless change.