Support paste in arbitrary name-value attributes

[scalexm]

Fixes #56.

[scalexm]

Ping @dtolnay, I would like to know whether you like the idea, or if you think this is inappropriate for paste.

[dtolnay]

I am on board with this. The PR is marked experimental. Are you planning changes or is this ready for review?

[scalexm]

I’ve tested it on my side and it covers my use case, so ready for review. Cc @frol.

[dtolnay]

It looks like this turns all values in key/value attributes into strings, which is likely to break some macros.

#[proc_macro_attribute]
pub fn debug(_args: TokenStream, input: TokenStream) -> TokenStream {
    println!("{}", input);
    TokenStream::new()
}

paste! {
    #[debug]
    #[xyz = 123]
    struct S;
}

#[xyz = "123"] struct S ;

[scalexm]

I restored the previous behavior which was to have at least two tokens after the = sign.

[dtolnay]

Thanks!

[frol]

@scalexm @dtolnay Thank you!