Tracking Issue for RFC 3086: macro metavariable expressions

[nikomatsakis]

This is a tracking issue for the RFC "3086" (rust-lang/rfcs#3086).
The feature gate for the issue is #![feature(macro_metavar_expr)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the RFC (cc @markbt -- has there been work done here already?)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved questions and bugs

• Figure out problems around hygiene

Implementation history

[markbt]

There is a working prototype on the markbt/rust/metavariable_expressions branch. This needs feature gating, and there are a couple of TODOs to resolve, but it's otherwise in reasonable shape. I'm planning to work on it over the coming weeks.

[markbt]

Update (2021-04-07)

I've not yet started work on productionizing the prototype on the markbt/rust/metavariable_expressions branch. I plan to start later this month, free time permitting.

[markbt]

Update (2021-05-09)

I still haven't started on this yet as some stuff came up last month that prevented from having the time to work on it. It's still in my plan to work on it, and hopefully I'll have some time soon.

[markbt]

Update (2021-06-29)

Still no progress, as I haven't had any spare time to work on this project. I'm still planning to work on it, and hopefully will get some time soon.

[joshtriplett]

@markbt If you don't expect to find the bandwidth in the near future, would you potentially be interested in seeking help in the form of another owner for this initiative? If you're still interested in driving this, that's fine, but if you'd like us to switch gears from pinging you to broadcasting that the project could use help, we'd be happy to do that.

[markbt]

I'd be happy with any help if there's someone available. I still plan to work on it, but personal stuff is getting in the way at the moment. Sorry about that.

To recap: I have a working prototype on my branch at https://github.com/markbt/rust/tree/metavariable_expressions . The next steps are to rebase that onto the latest master, and then polish it up so that it's ready for inclusion. Then there's also the doc work to make sure the new feature is documented well. Help with any of this would be appreciated.

[rslife]

Nesting ignores issue:

#![feature(macro_metavar_expr)]

struct Foo(i32);
struct Bar(i32);
struct Baz(i32);

macro_rules! mk_p {
    ($p_ty:ty $(, foo=$foo:ident)? $(, bar=$bar:ident)?) => {
        $(${ignore($foo)} mk_p!($p_ty, foo=$foo, foo_or_bar=foo_or_bar); )?
        $(${ignore($bar)} mk_p!($p_ty, bar=$bar, foo_or_bar=foo_or_bar); )?
    };
    ($p_ty:ty $(, foo=$foo:ident)? $(, bar=$bar:ident)? $(, baz=$baz:ident)? $(, foo_or_bar=$foo_or_bar:ident)?) => {
        impl $p_ty {
            fn p(&self) {
                $(
                    ${ignore($baz)}
                    eprintln!("baz={}", self.0);
                )?

                $(
                    ${ignore($foo_or_bar)}
                    let i = self.0 + 5;
                    $(
                        ${ignore($foo)}
                        eprintln!("foo={i}");
                    )?
                    $(
                        ${ignore($bar)}
                        eprintln!("bar={i}");
                    )?
                )?
            }
        }
    };
}

mk_p!(Foo, foo=foo);
mk_p!(Bar, bar=bar);
mk_p!(Baz, baz=baz);


fn main() {}

Error:

error: meta-variable `foo_or_bar` repeats 1 time, but `bar` repeats 0 times
  --> src/main.rs:20:18
   |
20 |                   $(
   |  __________________^
21 | |                     ${ignore($foo_or_bar)}
22 | |                     let i = self.0 + 5;
23 | |                     $(
...  |
30 | |                     )?
31 | |                 )?
   | |_________________^

error: meta-variable `foo_or_bar` repeats 1 time, but `foo` repeats 0 times
  --> src/main.rs:20:18
   |
20 |                   $(
   |  __________________^
21 | |                     ${ignore($foo_or_bar)}
22 | |                     let i = self.0 + 5;
23 | |                     $(
...  |
30 | |                     )?
31 | |                 )?
   | |_________________^

Is this a current limitation or intended behavior?

[i18nsite]

the resolved

use ignore!

old

can $index support $index for xxx ? for example ${task.index}

[c410-f3r]

#117050 was merged ~2 months ago and no related issues have been created since then.

Can we finally proceed with the stabilization of everything but $$? Does anyone still have any kind of blocking concern?

[c410-f3r]

A stabilization attempt is available at #122808

[safinaskar]

🚀 I found a way to evaluate concat_idents (and concat and few other built-in macros) before evaluating other macro, which takes concat_idents as an argument! I. e. I found a way to evaluate a!(concat_idents!(...)) such way, that concat_idents evaluates before a. Answer is crate https://crates.io/crates/with_builtin_macros !!! Thanks, @danielhenrymantilla ! In other words, with_builtin_macros is paste, but not only for concat_idents, but also for concat and some other macros.

Note: #[feature(macro_metavar_expr_concat)] is not complete solution (see below).

And in other words, with_builtin_macros allows one to achieve eager evaluation of macros in limited way.

Also, with_builtin_macros allows one to use concat_idents when defining new identifier.

Also, https://crates.io/crates/with_builtin_macros allows one to use concat_idents in stable Rust.

// (This code was not tested, may contain typos)

fn concat_idents!(a, b) () {} // Doesn't work

with_builtin_macros::with_eager_expansions! {
  fn #{ concat_idents!(a, b) } () {} // Works! Even on stable!
}

macro_rules! this_macro_accepts_ident {
  ($a:ident) => {}
}

// Doesn't work, because "this_macro_accepts_ident" evaluates before "concat_idents"
this_macro_accepts_ident!(concat_idents!(a, b));

with_builtin_macros::with_eager_expansions! {
  this_macro_accepts_ident!(#{ concat_idents!(a, b) }); // Works! Even on stable!
}

macro_rules! this_macro_accepts_literal {
  ($a:literal) => {}
}

// Doesn't work.
// Moreover, you cannot solve this problem using #[feature(macro_metavar_expr_concat)],
// because ${concat(...)} produces identifier, not string literal!!!
// Same applies to "paste"! "paste::paste!" deals with identifiers, not strings. So, with_builtin_macros is the only way!!!
this_macro_accepts_literal!(concat!("a", "b"));

with_builtin_macros::with_eager_expansions! {
  this_macro_accepts_literal!(#{ concat!("a", "b") }); // Works! Even on stable!
}