Rewrite: seperated Decode and BorrowDecode

[VictorKoenders]

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.

[codecov]

Codecov Report

Merging #526 (f7f5700) into trunk (5b19220) will decrease coverage by 0.81%.
The diff coverage is 32.23%.

@@            Coverage Diff             @@
##            trunk     #526      +/-   ##
==========================================
- Coverage   55.33%   54.52%   -0.82%     
==========================================
  Files          48       49       +1     
  Lines        4223     4422     +199     
==========================================
+ Hits         2337     2411      +74     
- Misses       1886     2011     +125     

Impacted Files	Coverage Δ
derive/src/attribute.rs	0.00% <0.00%> (ø)
derive/src/derive_enum.rs	0.00% <0.00%> (ø)
derive/src/derive_struct.rs	0.00% <0.00%> (ø)
src/atomic.rs	100.00% <ø> (+100.00%)	⬆️
src/features/impl_std.rs	65.85% <0.00%> (-9.15%)	⬇️
src/features/serde/mod.rs	47.82% <0.00%> (-4.56%)	⬇️
src/varint/decode_unsigned.rs	69.41% <ø> (ø)
tests/atomic.rs	100.00% <ø> (ø)
tests/basic_types.rs	99.33% <ø> (ø)
tests/issues/issue_431.rs	100.00% <ø> (ø)
... and 19 more

---

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[ZoeyR]

Will this require anyone who implements Decode to also implement BorrowDecode? i.e. is it considered an error to implement just Decode?

[VictorKoenders]

They should implement both, but it's not enforced in the type system. I'm not sure if we should considering it an error

[VictorKoenders]

Some cases we should test:

• Arc<str>: Arc<str> can't be bincoded #527
• Cow<'a, str>: https://github.com/ajeetdsouza/zoxide/blob/main/src/db/dir.rs#L83-L88