Remove the deprecated unstable `concat_idents!` macro #142704

tgross35 · 2025-06-19T06:49:53Z

In rust-lang/rust#137653, the lang and libs-API teams did a joint FCP to deprecate
and eventually remove the long-unstable concat_idents! macro. The
deprecation is landing in 1.88, so do the removal here (target version
1.90).

This macro has been superseded by the more recent ${concat(...)}
metavariable expression language feature, which avoids some of the
limitations of concat_idents!. The metavar expression is unstably
available under the macro_metavar_expr_concat feature.

History is mildly interesting here: concat_idents! goes back to 2011
when it was introduced with 513276e ("Add #concat_idents[] and
#ident_to_str[]"). The syntax looks a bit different but it still works
about the same:

let asdf_fdsa = "<.<";
assert(#concat_idents[asd,f_f,dsa] == "<.<");

assert(#ident_to_str[use_mention_distinction]
       == "use_mention_distinction");

(That test existed from introduction until its removal here.)

Closes: #29599

rustbot · 2025-06-19T06:49:58Z

r? @WaffleLapkin

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tgross35 · 2025-06-19T06:53:30Z

This shouldn't need any specific approval because of the completed FCP at #137653 (comment), but just as a heads up: @rust-lang/lang @rust-lang/libs

fee1-dead · 2025-06-19T07:02:27Z

should we do a crater run?

tgross35 · 2025-06-19T07:06:12Z

I don't think we necessarily need to act on any crater results, since anyone using this should already be getting the deprecation warnings. But it also doesn't hurt to see what those results might be.

@bors2 try

rust-bors · 2025-06-19T07:06:16Z

⌛ Trying commit 938b6f5 with merge 1f5783c…

To cancel the try build, run the command @bors2 try cancel.

Remove the deprecated `concat_idents!` macro In [#137653], the lang and libs-API teams did a joint FCP to deprecate and eventually remove the long-unstable `concat_idents!` macro. The deprecation is landing in 1.88, so do the removal here (target version 1.90). This macro has been superseded by the more recent `${concat(...)}` metavariable expression language feature, which avoids some of the limitations of `concat_idents!`. The metavar expression is unstably available under the [`macro_metavar_expr_concat`] feature. History is mildly interesting here: `concat_idents!` goes back to 2011 when it was introduced with 513276e ("Add #concat_idents[] and #ident_to_str[]"). The syntax looks a bit different but it still works about the same: let asdf_fdsa = "<.<"; assert(#concat_idents[asd,f_f,dsa] == "<.<"); assert(#ident_to_str[use_mention_distinction] == "use_mention_distinction"); (That test existed from introduction until its removal here.) Closes: #29599 [#137653]: #137653 [`macro_metavar_expr_concat`]: #124225

rust-bors · 2025-06-19T09:36:18Z

☀️ Try build successful (CI)
Build commit: 1f5783c (1f5783cbeb5da8b38566d718631d016891e9c912, parent: 8a65ee08296b36342bf7c3cdc15312ccbc357227)

tgross35 · 2025-06-19T09:38:57Z

@craterbot check

craterbot · 2025-06-19T09:39:21Z

👌 Experiment pr-142704 created and queued.
🤖 Automatically detected try build 1f5783c
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-06-19T11:27:21Z

🚧 Experiment pr-142704 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

fee1-dead

After this PR the feature appears unknown to end users. That's not the best migration UX; so we should probably add this to the list in compiler/rustc_feature/src/removed.rs (AFAIK, no other way to do this for removed lib features).

library/core/src/prelude/v1.rs

tests/ui/derives/nonsense-input-to-debug.rs

fee1-dead · 2025-06-22T11:06:55Z

tests/ui/unpretty/exhaustive.rs

    /// TyKind::MacCall
-    #[expect(deprecated)] // concat_idents is deprecated
    fn ty_mac_call() {
-        let _: concat_idents!(T);
-        let _: concat_idents![T];
-        let _: concat_idents! { T };
+        let _ = concat!("T");
+        let _ = concat!["T"];
+        let _ = concat! { "T" };


This is incorrect; the test requires a macro at type position (TyKind::MacCall). this probably needs a helper macro.

I am not exactly sure what this is testing, but I added a macro that forwards a $ty metavar.

rustbot · 2025-06-24T08:01:13Z

There are changes to the tidy tool.

cc @jieyouxu

tgross35 · 2025-06-24T08:05:16Z

Comments should be addressed, added concat_idents to a new section in the list of removed compiler features. This will need a squash before merge.

@rustbot ready

fee1-dead

r=me after squash

`concat_idents!` is in the process of being removed, but a few things it is used to test will still be relevant. Migrate these tests to something other than `concat_idents`.

These tests were updated in the previous commit; while they are being cleaned up, move them to a non-issue directory.

In [137653], the lang and libs-API teams did a joint FCP to deprecate and eventually remove the long-unstable `concat_idents!` macro. The deprecation is landing in 1.88, so do the removal here (target version 1.90). This macro has been superseded by the more recent `${concat(...)}` metavariable expression language feature, which avoids some of the limitations of `concat_idents!`. The metavar expression is unstably available under the [`macro_metavar_expr_concat`] feature. History is mildly interesting here: `concat_idents!` goes back to 2011 when it was introduced with 513276e ("Add #concat_idents[] and about the same: let asdf_fdsa = "<.<"; assert(#concat_idents[asd,f_f,dsa] == "<.<"); assert(#ident_to_str[use_mention_distinction] == "use_mention_distinction"); (That test existed from introduction until its removal here.) Closes: https://www.github.com/rust-lang/rust/issues/29599 [137653]: https://www.github.com/rust-lang/rust/pull/137653 [`macro_metavar_expr_concat`]: https://www.github.com/rust-lang/rust/issues/124225

tgross35 · 2025-06-24T11:12:08Z

Thanks!

@bors r=fee1-dead

bors · 2025-06-24T11:12:11Z

📌 Commit 0e4de4c has been approved by fee1-dead

It is now in the queue for this repository.

Rollup of 7 pull requests Successful merges: - #137268 (Allow comparisons between `CStr`, `CString`, and `Cow<CStr>`.) - #142704 (Remove the deprecated unstable `concat_idents!` macro) - #142742 ([win][aarch64] Fix linking statics on Arm64EC, take 2) - #142843 (Enable reproducible-build-2 for Windows MSVC) - #142916 (rustdoc-json: Add test for `#[optimize(..)]`) - #142919 (rustdoc-json: Add test for `#[cold]`) - #142944 (Stats output tweaks) Failed merges: - #142825 (Port `#[track_caller]` to the new attribute system) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #142704 - tgross35:remove-concat_idents, r=fee1-dead Remove the deprecated unstable `concat_idents!` macro In [#137653], the lang and libs-API teams did a joint FCP to deprecate and eventually remove the long-unstable `concat_idents!` macro. The deprecation is landing in 1.88, so do the removal here (target version 1.90). This macro has been superseded by the more recent `${concat(...)}` metavariable expression language feature, which avoids some of the limitations of `concat_idents!`. The metavar expression is unstably available under the [`macro_metavar_expr_concat`] feature. History is mildly interesting here: `concat_idents!` goes back to 2011 when it was introduced with 513276e ("Add #concat_idents[] and #ident_to_str[]"). The syntax looks a bit different but it still works about the same: let asdf_fdsa = "<.<"; assert(#concat_idents[asd,f_f,dsa] == "<.<"); assert(#ident_to_str[use_mention_distinction] == "use_mention_distinction"); (That test existed from introduction until its removal here.) Closes: #29599 [#137653]: #137653 [`macro_metavar_expr_concat`]: #124225

…ee1-dead Remove the deprecated unstable `concat_idents!` macro In [rust-lang#137653], the lang and libs-API teams did a joint FCP to deprecate and eventually remove the long-unstable `concat_idents!` macro. The deprecation is landing in 1.88, so do the removal here (target version 1.90). This macro has been superseded by the more recent `${concat(...)}` metavariable expression language feature, which avoids some of the limitations of `concat_idents!`. The metavar expression is unstably available under the [`macro_metavar_expr_concat`] feature. History is mildly interesting here: `concat_idents!` goes back to 2011 when it was introduced with 513276e ("Add #concat_idents[] and #ident_to_str[]"). The syntax looks a bit different but it still works about the same: let asdf_fdsa = "<.<"; assert(#concat_idents[asd,f_f,dsa] == "<.<"); assert(#ident_to_str[use_mention_distinction] == "use_mention_distinction"); (That test existed from introduction until its removal here.) Closes: rust-lang#29599 [rust-lang#137653]: rust-lang#137653 [`macro_metavar_expr_concat`]: rust-lang#124225

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tgross35 mentioned this pull request Jun 19, 2025

Tracking issue for concat_idents #29599

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tgross35 mentioned this pull request Jun 19, 2025

Deprecate the unstable concat_idents! #137653

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ojeda mentioned this pull request Jun 19, 2025

Rust unstable features needed for the kernel Rust-for-Linux/linux#2

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 19, 2025

cyrgani mentioned this pull request Jun 19, 2025

concat_idents cannot be used in pattern position #50408

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traviscross added the T-lang Relevant to the language team label Jun 19, 2025

aDotInTheVoid changed the title ~~Remove the deprecated concat_idents! macro~~ Remove the deprecated unstable concat_idents! macro Jun 19, 2025

tgross35 added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. and removed T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jun 19, 2025

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fee1-dead mentioned this pull request Jun 23, 2025

track renamed and removed library feature gates #141617

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tgross35 force-pushed the remove-concat_idents branch from c4e45d1 to 9b1f7ad Compare June 24, 2025 10:23

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tgross35 added 3 commits June 24, 2025 10:54

Migrate some tests away from concat-idents

f8ab934

`concat_idents!` is in the process of being removed, but a few things it is used to test will still be relevant. Migrate these tests to something other than `concat_idents`.

Move some issues-* tests to better homes

c978c89

These tests were updated in the previous commit; while they are being cleaned up, move them to a non-issue directory.

tgross35 force-pushed the remove-concat_idents branch from 9b1f7ad to 0e4de4c Compare June 24, 2025 11:07

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 24, 2025

GuillaumeGomez mentioned this pull request Jun 24, 2025

Rollup of 7 pull requests #142962

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bors merged commit 0377330 into rust-lang:master Jun 24, 2025
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tgross35 deleted the remove-concat_idents branch June 24, 2025 18:49

dereckson mentioned this pull request Sep 24, 2025

[v0.4] concat_idents feature has been removed rwf2/Rocket#2964

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erwinwolff mentioned this pull request Oct 30, 2025

Cannot build due to error in devise_core 2.1 rwf2/Rocket#2974

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Remove the deprecated unstable concat_idents! macro #142704

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Remove the deprecated unstable `concat_idents!` macro #142704

Remove the deprecated unstable `concat_idents!` macro #142704

tgross35 commented Jun 19, 2025 •

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