Feature: MaybeItemFn for better rust-analyzer experience

[Diggsey]

Currently when an attribute macro parses a function body it will cause rust-analyzer's completion to stop working within the function.

This is because the code within the function will fail to parse, and then the attribute macro will just expand to a compile error.

To avoid this, attribute macros can use a custom type to only parse the function signature, and leave the function body as an opaque token-stream: Diggsey/sqlxmq@ea408f7#diff-5796541ab2132b1f9f5e924468f24b542a59297c91a7543be0a5702926b7d3ecR95-R131

A similar approach has also been used in other crates like tokio.

It would be convenient if syn could provide a type like MaybeItemFn.

[dtolnay]

I would prefer not to build this into syn. I consider the behavior that this is working around to be a rust-analyzer bug that needs to be fixed on their end. dtolnay/async-trait#178 has some related context.

[Diggsey]

@dtolnay
Ok, let's imagine a world where RA correctly handles this. I think it's still worth having this functionality (ability to parse only a function signature) in syn, because many attribute macros don't care about the function body at all.

I could imagine a scenario where you have two attribute macros:

#[uses_fn_signature]
#[bf2rust]
fn foo() {
    [->+<]
}

In this case, if uses_fn_signature tries to parse the function body it will fail because it's not valid rust, but this is ignoring the fact that the #[bf2rust] attribute will transform it into something that is valid. The user would need to swap the order of attributes.

A perhaps more realistic example would be one where #[uses_fn_signature] is implemented against an older edition of rust, and is missing some new syntax from its parser. If this attribute only looks at the signature, then it won't stop you from using the newer features within the function body.

In summary, parsing more than is strictly necessary is redundant work and makes the attribute more fragile. I think there's value in making good practices easier.