Tracking Issue for `macro_metavar_expr_concat`

[c410-f3r]

This is a tracking issue for the experimental feature macro metavariable expression concat (RFC pending).
The feature gate for the issue is #![feature(macro_metavar_expr_concat)].

Provides a modern alternative to the concat_idents! macro.

#![feature(macro_metavar_expr_concat)]

macro_rules! many_idents {
    ($a:ident, $c:ident) => {
        const ${concat($a, B, $c, D)}: i32 = 1;
    };
}

fn main() {
    many_idents!(A, C);
    assert_eq!(ABCD, 1);
}

About experimental features

An experimental feature is one that has not yet had an RFC. The idea is to allow implementation work to proceed to better inform an upcoming RFC. Experimental features cannot be stabilized without first having an RFC. The existence of an experimental feature does not indicate lang team consensus that the feature is desirable, only that there is a problem that is worthy of being solved and that the idea has enough merit to consider exploring. See the lang team process page for more details.

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 / History

• Implementation (Add a new concat metavar expr #118958)
• Write an RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Should the syntax use commas? Add a new concat metavar expr #118958 (comment)
• Should numeric literals be allowed? (yes) Add a new concat metavar expr #118958 (comment)
• Which span should the resulting identifier have? Should it be related to the macro, the concatenated values, or customizable? Add a new concat metavar expr #118958 (comment)
• What should be done with r#foo raw identifiers? Add a new concat metavar expr #118958 (comment)

References

• Tracking Issue for RFC 3086: macro metavariable expressions #83527

[fmease]

This still needs a T-lang shepherd, right?

[c410-f3r]

This still needs a T-lang shepherd, right?

Unfortunately I am not familiar or aware of this procedure.

[fmease]

All language features have to go through the RFC process eventually. However, the implementation can be done before an RFC has been written by following T-lang's experimental feature process which requires a T-lang member to support — shepherd — the venture. Only “small” language (not compiler) “fixes” don't need to follow the RFC process, they just need a T-lang FCP.

[fmease]

T-lang, this feature is seeking a shepherd, therefore I'm nominating this for you.

An implementation is already under way (#118958). This feature adds a new kind of macro metavariable expression (the latter (overarching) feature has been approved by the merged RFC 3086, see also its tracking issue and stabilization PR (proposed FCP, disposition: merge)). The feature represents a more potent alternative / potential successor to the unstable built-in macro concat_idents (the latter has stalled, blocked on design concerns, see its tracking issue).

Please consult the tracking issue description for more context.

[joshtriplett]

Happy to second this with my T-lang hat on.

@c410-f3r Good luck, and please reach out to #t-lang on Zulip if you have any questions. Feel free to @ me there if needed.

[c410-f3r]

Thank you @joshtriplett