Add a new concat metavar expr

[c410-f3r]

Revival of #111930

Giving it another try now that #117050 was merged.

With the new rules, meta-variable expressions must be referenced with a dollar sign ($) and this can cause misunderstands with $concat.

macro_rules! foo {
    ( $bar:ident ) => {
        const ${concat(VAR, bar)}: i32 = 1;
    };
}

// Will produce `VARbar` instead of `VAR_123`
foo!(_123);

In other words, forgetting the dollar symbol can produce undesired outputs.

cc #29599
cc #124225

[rustbot]

r? @WaffleLapkin

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[tgross35]

@rustbot label +A-macros

[tgross35]

Looks pretty good to me. I started some discussion in wg-macros https://rust-lang.zulipchat.com/#narrow/stream/404510-wg-macros/topic/concat_idents.20as.20metavar.20.24.7B.2E.2E.2E.7D.

I don't think you need to mention the RFC in the title/commit message since this specific function wasn't called out in the RFC.

[WaffleLapkin]

(please use @rustbot review once my review is needed, this is currently S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. )

[tgross35]

@c410-f3r is this ready for review? I didn't intend for my comments to be blocking

[c410-f3r]

Yeap, it is ready for review

@rustbot labels -S-waiting-on-author +S-waiting-on-review

[petrochenkov]

r=me after addressing #118958 (comment).
@rustbot author

[tgross35]

There still isn't a test ensuring that this syntax is rejected without the feature gate, feature-gate-macro-metavar-expr-concat.rs is run-pass #118958 (comment)

Edit: thanks, this looks good now

[tgross35]

Spurious failure #126430

[c410-f3r]

@rustbot review

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

.......... (120/124)
...F       (124/124)


/checkout/tests/rustdoc-gui/help-page.goml help-page... FAILED
[ERROR] `tests/rustdoc-gui/utils.goml` line 16
    from `tests/rustdoc-gui/help-page.goml` line 23
    from `tests/rustdoc-gui/help-page.goml` line 32: Error: Evaluation failed: The following errors happened: [localStorage item "rustdoc-theme" (of value "ayu") != "null"]: for command `assert-local-storage: {"rustdoc-theme": |theme|}`
Error: ()
Build completed unsuccessfully in 0:03:32
  local time: Fri Jun 14 02:02:41 UTC 2024
  network time: Fri, 14 Jun 2024 02:02:41 GMT

[petrochenkov]

@bors r+

[bors]

📌 Commit 4b82afb has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4b82afb with merge f8e5660...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing f8e5660 to master...

[tgross35]

Woohoo! Thanks for all the work here, I think everyone has been needing this for a long while :)

[rust-timer]

Finished benchmarking commit (f8e5660): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

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Bootstrap: 671.574s -> 671.997s (0.06%)
Artifact size: 319.78 MiB -> 320.43 MiB (0.20%)