Deprecate the unstable concat_idents!

[tgross35]

concat_idents has been around unstably for a long time, but there is now a better (but still unstable) way to join identifiers using ${concat(...)} syntax with macro_metavar_expr_concat. This resolves a lot of the problems with concat_idents and is on a better track toward stabilization, so there is no need to keep both versions around. concat_idents! still has a lot of use in the ecosystem so deprecate it before removing, as discussed in 1.

Link: #124225

[rustbot]

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[tgross35]

r? libs-api

[tgross35]

I'd like to propose deprecating it now (1.87) and removing in the next version (1.88), nominating for libs-api. Cc @rust-lang/lang since this is tightly coupled.

[traviscross]

Let's just FCP it. That's maybe weird for an unstable thing, but no more weird than doing a deprecation cycle for it.

@rfcbot fcp merge

[rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
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[nikomatsakis]

@rfcbot reviewed

[joshtriplett]

👍 for deprecating concat_idents!, in favor of concat from macro metavar expressions.

I saw that @tgross35 proposed rust-lang/rfcs#3649 to supersede the count and len mechanism. I want to make sure that RFC 3649 is only being proposed as an alternative to count and len, and that concat is still on track to continue.

[tgross35]

I saw that @tgross35 proposed rust-lang/rfcs#3649 to supersede the count and len mechanism. I want to make sure that RFC 3649 is only being proposed as an alternative to count and len, and that concat is still on track to continue.

That's correct; I think concat can move forward at any point, RFC 3649 just handles the group- and index-related things 👍

[traviscross]

@rustbot labels -I-lang-nominated

We discussed this in the lang call today. Thanks @tgross35 for pushing this forward.

As above, the one question that came up was whether this was related to our open question:

• Decide what we want about macro_metavar_expr #137581

But it's not, as macro_metavar_expr_concat is under a separate feature flag and isn't part of that proposed stabilization.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔