Support $interpolation?

[Goncalerta]

Currently, this crate follows the syntax of dtolnay/quote, where the symbol # is used for interpolations. I wonder if this symbol was chosen because $ would clash with macro_rules!.

The (unstable) libproc_macro::quote uses $ rather than #. Would allowing this symbol as an alias to # be beneficial? Using $ would allow a smoother transition to libproc once quote is stable.

[dtolnay]

The choice of # is handy when writing a proc macro that emits macro_rules macros. If you go with $, you will want to come up with some syntax for escaping them, which is not needed for #.

[Goncalerta]

Interesting point. It didn't occur to me that someone could want to write a proc macro that emits macro_rules.

[eddyb]

I mentioned the implementation of proc_macro::quote! in the original thread (dtolnay/request-for-implementation#8 (comment)).

What it does is allow using $$ to escape $, which is probably fine, although I'd prefer \$ myself.

@dtolnay How do you deal with #[...] attribute syntax? Does it work out by simply not overlapping with #(...) or any other interpolation?

[dtolnay]

The quote crate does not have any syntax for escaping anything, and I don't recall anyone asking for escaping in the past 2 years. It allows #[...] without needing an escape.

For \$ presumably you would also want an escape sequence for \ so that a macro can emit literally \$ as two tokens. Er, is the problem that \ is not allowed in macro input?

error: unknown start of token: \
 --> src/main.rs:2:25
  |
2 |     println!(stringify!(\));
  |                         ^

[eddyb]

Yeah, \ would have to be officially introduced, and you probably can't even allow it at all without giving it some semantics in macro_rules (because you can't add those later, and change the behavior of code using \).

This is a bit sad, as we could've introduced \ a while back, and properly supported higher-order macros even in cases where repetitions are used, but we didn't.

It overall makes me have no preference between using # and having to deal with $$.