Derive macros break with where clauses

[SabrinaJewson]

Test case:

#[derive(bincode::Encode)]
struct Test<T>
where
    T: Clone,
{
    a: T,
}

Output:

error: expected one of `{`, lifetime, or type, found `,`
 --> src/main.rs:3:10
  |
3 | #[derive(bincode::Encode)]
  |          ^^^^^^^^^^^^^^^ expected one of `{`, lifetime, or type
  |
  = note: this error originates in the derive macro `bincode::Encode` (in Nightly builds, run with -Z macro-backtrace for more info)

error: proc-macro derive produced unparseable tokens
 --> src/main.rs:3:10
  |
3 | #[derive(bincode::Encode)]
  |          ^^^^^^^^^^^^^^^

[VictorKoenders]

There is an attribute you can use called bounds, I only just noticed we've not documented this.

There is an example in tests/issues/issue_431.rs that uses borrow_decode_bounds. There are these 4 attributes:

• bounds, used by all 3 derives
• decode_bounds, used by #[derive(Decode)]
• borrow_decode_bounds, used by #[derive(BorrowDecode)]
• encode_bounds, used by #[derive(Encode)]

You can check target/test_Encode.rs to see what bounds would be needed.

This is assuming either virtue or bincode-derive doesn't do something weird

[SabrinaJewson]

Thanks — the following does compile:

#[derive(bincode::Encode)]
#[bincode(encode_bounds = "T: bincode::Encode + Clone")]
struct Test<T>
where
    T: Clone,
{
    a: T,
}

I still think it would be nice for bincode to just automatically support where bounds, but if it must be unsupported could the error message be improved?

[purpleposeidon]

It appears it does attempt to support automatic bounds.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dtolnay]

Bincode is still generating syntactically invalid code in this situation. That's a bug independent of whether encode_bounds exists.