Investigate proc_macro::quote_span integration

[Aaron1011]

With PR rust-lang/rust#84278, Rust now has the ability to quote a Span to a TokenStream. This allows us to obtain spans pointing into a proc-macro crate:

error[E0412]: cannot find type `MissingType` in this scope
  --> $DIR/auxiliary/span-from-proc-macro.rs:37:20
   |
LL | pub fn error_from_attribute(_args: TokenStream, _input: TokenStream) -> TokenStream {
   | ----------------------------------------------------------------------------------- in this expansion of procedural macro `#[error_from_attribute]`
...
LL |             field: MissingType
   |                    ^^^^^^^^^^^ not found in this scope
   |
  ::: $DIR/span-from-proc-macro.rs:8:1
   |
LL | #[error_from_attribute]
   | ----------------------- in this macro invocation

Currently, this is integrated with the unstable proc_macro::quote! macro. However, as virtually everyone uses the quote crate instead, it would be nice to add span-quoting support here (behind a nightly feature).

The span-quoting API is used via proc_macro::quote_span, and produces a TokenStream. I've provided an example of using it here.

I'm not sure how to best way to integrate this API into the quote crate, since it looks like a Span parameter isn't passed around in many places (presumably for performance reasons). However, I'd be happy to work on a PR if you support the overall idea, and know the general shape of the modifications that you'd like.

[Aaron1011]

cc @dtolnay - does the overall idea of supporting this in quote sound reasonable?

[dtolnay]

If I understand correctly, you are asking to reimplement quote as a procedural macro? If so, I am not interested in that. I think the benefits of this change would be minor or nonexistent for typical users of this crate and the compile time cost would greatly outweigh it in comparison.

[Aaron1011]

It should be possible to get most of the benefit of quote_span without re-implementing the quote crate. Instead, a helper proc-macro crate would be used to implement push_ident:

quote/src/lib.rs

Lines 1047 to 1049 in cbe5ded

	($tokens:ident $ident:ident) => {
	$crate::__private::push_ident(&mut $tokens, stringify!($ident));
	};

We would pass the captured $ident metavariable to the proc-macro:

quote_helper::quote_with_span!($ident)

The macro implementation would call proc_macro::quote_span with the Span taken from its input TokenStream. As a result, quote_with_span!($ident) would expand to something like Ident::new(<literal_ident_contents>, ::Span::recover_proc_macro_span(...))

This would allow the rest of quote to remain unchanged. We wouldn't be able to quote spans for the various operators special-cased by quote_token, but it should be enough to get many error messages to point to the proper place within a proc-macro crate.

[dtolnay]

Nice, that sounds promising -- but I would like this to be done as a different crate for now.

[dtolnay]

I'll close since I am not planning on pursuing this in quote for now. I am interested to hear how it goes in a different crate though.