macro_metavar_expr_concat: support concatenating into string literals by mejrs · Pull Request #160570 · rust-lang/rust

mejrs · 2026-08-05T13:58:02Z

Usually if you want to concatenate string literals in macros you expand into some combination of concat! and stringify! invocations. However this does not work in nested MetaItems, since those are never expanded (unlike #[path = EXPR] attributes like #[doc = include_str!("README.md")].

This PR adds support for that using ${concat_str(...)}, similar to ${concat(...)}. It is unstable under the same macro_metavar_expr_concat feature. See the tests in this PR for examples.

My motivation for adding this is that I've seen (and had myself) some cases where this would have been really useful. Thus I'd like to add this experimentally.

rustbot · 2026-08-05T13:58:08Z

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mejrs · 2026-08-17T11:43:15Z

r? compiler

petrochenkov · 2026-08-18T16:13:04Z

since those are not eagerly expanded (unlike #[path = EXPR] attributes like #[doc = include_str!("README.md")].

Technically, include_str in the example is not eagerly expanded either.

petrochenkov · 2026-08-18T16:13:31Z

Besides the is_ident check, the logic here exactly follows the behavior of existing ${concat} metavar, which is pretty conservative, so I don't have any concerns about adding this.
This is unstable, but probably still needs some lang team sign off.
@rustbot author

rustbot · 2026-08-18T16:13:35Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

joshtriplett · 2026-09-02T15:28:08Z

We talked about this in today's T-lang meeting. We're generally fine with adding something like this.

I'm happy to champion this, alongside the existing macro metavar work. It sounds like you're planning to track it via the same tracking issue?

mejrs · 2026-09-13T10:15:59Z

Hm, this PR still has all the lang labels. r=me once there's a formal lang approval.

@joshtriplett is the lang team being OK with it and you championing it enough here? if so can you remove some lang labels? I'll edit the existing tracking issue after this is approved.

mejrs · 2026-09-22T11:39:16Z

per lang labels being gone,

@bors r=petrochenkov

rust-bors · 2026-09-22T11:39:19Z

📌 Commit 8e54ab5 has been approved by petrochenkov

It is now in the queue for this repository.

macro_metavar_expr_concat: support concatenating into string literals Usually if you want to concatenate string literals in macros you expand into some combination of concat! and stringify! invocations. However this does not work in nested MetaItems, since those are never expanded (unlike `#[path = EXPR]` attributes like `#[doc = include_str!("README.md")]`. This PR adds support for that using `${concat_str(...)}`, similar to `${concat(...)}`. It is unstable under the same `macro_metavar_expr_concat` feature. See the tests in this PR for examples. My motivation for adding this is that I've seen (and had myself) some cases where this would have been really useful. Thus I'd like to add this experimentally.

JonathanBrouwer · 2026-09-22T15:06:01Z

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #163164 (comment)

rust-bors · 2026-09-22T15:06:09Z

This pull request was unapproved.

This PR was contained in a rollup (#163164), which was unapproved.

View changes since this unapproval

rustbot · 2026-09-22T15:58:16Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

mejrs · 2026-09-22T18:16:44Z

@bors r=petrochenkov

rust-bors · 2026-09-22T18:16:47Z

📌 Commit 6c0f3aa has been approved by petrochenkov

It is now in the queue for this repository.

macro_metavar_expr_concat: support concatenating into string literals Usually if you want to concatenate string literals in macros you expand into some combination of concat! and stringify! invocations. However this does not work in nested MetaItems, since those are never expanded (unlike `#[path = EXPR]` attributes like `#[doc = include_str!("README.md")]`. This PR adds support for that using `${concat_str(...)}`, similar to `${concat(...)}`. It is unstable under the same `macro_metavar_expr_concat` feature. See the tests in this PR for examples. My motivation for adding this is that I've seen (and had myself) some cases where this would have been really useful. Thus I'd like to add this experimentally.

…uwer Rollup of 13 pull requests Successful merges: - #156949 (Detect missing else in let statement) - #160436 (stabilize `Box::take`) - #160570 (macro_metavar_expr_concat: support concatenating into string literals) - #162837 (Dedicated Display type for CStr::display) - #163099 (Use wrapping arithmetic in `from_str_radix`) - #163166 (Tiny cleanups to deferred liveness) - #161667 (Add `f16` inline ASM support for `nvptx64-nvidia-cuda`) - #163063 (Restore `Send` and `Sync` for `BorrowedCursor`) - #163097 (OpenBSD/sparc64 has switched from GCC to Clang) - #163126 (Skip redundant storage-conflict updates during coroutine layout) - #163135 (librustdoc: remove stale dep on base64) - #163146 (tests: Update `f16b` codegen test for LoongArch and RISC-V) - #163159 (treat inductive cycles as ambig)

Rollup merge of #160570 - mejrs:concat_str, r=petrochenkov macro_metavar_expr_concat: support concatenating into string literals Usually if you want to concatenate string literals in macros you expand into some combination of concat! and stringify! invocations. However this does not work in nested MetaItems, since those are never expanded (unlike `#[path = EXPR]` attributes like `#[doc = include_str!("README.md")]`. This PR adds support for that using `${concat_str(...)}`, similar to `${concat(...)}`. It is unstable under the same `macro_metavar_expr_concat` feature. See the tests in this PR for examples. My motivation for adding this is that I've seen (and had myself) some cases where this would have been really useful. Thus I'd like to add this experimentally.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 5, 2026

rustbot assigned wesleywiser Aug 5, 2026

mejrs added the F-macro_metavar_expr_concat `#![feature(macro_metavar_expr_concat)]` label Aug 5, 2026

mejrs force-pushed the concat_str branch from 5690f12 to b3724cc Compare August 11, 2026 12:00

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rustbot assigned petrochenkov and unassigned wesleywiser Aug 17, 2026

petrochenkov reviewed Aug 18, 2026

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Comment thread tests/ui/macros/metavar-expressions/concat-str-horror.rs Outdated

Comment thread compiler/rustc_expand/src/mbe/transcribe.rs Outdated

Comment thread compiler/rustc_expand/src/mbe/transcribe.rs

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 18, 2026

mejrs force-pushed the concat_str branch from b3724cc to a27b1e5 Compare August 19, 2026 11:28

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mejrs force-pushed the concat_str branch from a27b1e5 to 2f79297 Compare August 19, 2026 11:45

mejrs added I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination labels Aug 20, 2026

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petrochenkov approved these changes Aug 21, 2026

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petrochenkov added S-waiting-on-t-lang Status: Awaiting decision from T-lang and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 21, 2026

traviscross added the P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang label Aug 26, 2026

mejrs force-pushed the concat_str branch from 2f79297 to 09ba495 Compare September 2, 2026 10:39

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rust-bors Bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Sep 22, 2026

JonathanBrouwer mentioned this pull request Sep 22, 2026

Rollup of 8 pull requests #163164

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rust-bors Bot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 22, 2026

macro_metavar_expr_concat: support concatenating into string literals

6c0f3aa

mejrs force-pushed the concat_str branch from 8e54ab5 to 6c0f3aa Compare September 22, 2026 15:58

rust-bors Bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 22, 2026

This was referenced Sep 22, 2026

Rollup of 15 pull requests #163175

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Rollup of 13 pull requests #163178

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mejrs mentioned this pull request Sep 22, 2026

Tracking Issue for macro_metavar_expr_concat #124225

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rust-bors Bot merged commit 9c60af0 into rust-lang:main Sep 23, 2026
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rustbot added this to the 1.100.0 milestone Sep 23, 2026

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