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#[unsafe(no_mangle)] erroneously accepted inside thread_local! #146557

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#[unsafe(no_mangle)] erroneously accepted inside thread_local!#146557
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A-attributesArea: Attributes (`#[…]`, `#![…]`)A-macrosArea: All kinds of macros (custom derive, macro_rules!, proc macros, ..)C-bugCategory: This is a bug.P-mediumMedium priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.

Description

@Jules-Bertholet
Jules-Bertholet
opened on Sep 14, 2025

I tried this code:

thread_local! {
    #[unsafe(no_mangle)]
    static FOO: () = const { () };
}

I expected to see this happen:

error: const items should never be `#[no_mangle]`

(The macro expands to a const item that has the #[unsafe(no_mangle)] applied to it.)

Instead, this happened:

It compiled successfully. The attribute is ignored

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (02c7b1a7a 2025-09-13)
binary: rustc
commit-hash: 02c7b1a7ac1d739663878030510508372e46f254
commit-date: 2025-09-13
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.1

@rustbot label A-attributes A-macros T-compiler

Edit: see #146557 (comment) for root cause

Activity

  1. Jules-Bertholet
    added
    C-bugCategory: This is a bug.
    on Sep 14, 2025
  2. rustbot
    added
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    A-attributesArea: Attributes (`#[…]`, `#![…]`)
    A-macrosArea: All kinds of macros (custom derive, macro_rules!, proc macros, ..)
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    on Sep 14, 2025
  3. jieyouxu
    removed
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    on Sep 16, 2025

  4. jieyouxu commented on Sep 16, 2025

    @jieyouxu
    jieyouxu
    on Sep 16, 2025
    Member

    Accepted since 1.82.0.

    thread_local! {
    #[no_mangle]
    static FOO: () = const { () };
}

    accepted since 1.59.0 when thread_local_const_init was stabilized.

  5. Jules-Bertholet commented on Sep 16, 2025

    @Jules-Bertholet
    Jules-Bertholet
    on Sep 16, 2025
    ContributorAuthor

    Without the const, it’s since 1.29.0. (I don’t think this is an issue with the thread_local macro specifically, but I haven’t been able to make another reproducer yet.)

    @rustbot label regression-from-stable-to-stable

  6. rustbot
    added
    regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on Sep 16, 2025
  7. jieyouxu
    added
    P-mediumMedium priority
    and removed
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on Sep 16, 2025

  8. Jules-Bertholet commented on Sep 16, 2025

    @Jules-Bertholet
    Jules-Bertholet
    on Sep 16, 2025
    ContributorAuthor

    cargo bisect-rustc:

    searched toolchains nightly-2018-06-01 through nightly-2018-08-01

    Regression in nightly-2018-07-22

  9. Jules-Bertholet commented on Sep 16, 2025

    @Jules-Bertholet
    Jules-Bertholet
    on Sep 16, 2025
    ContributorAuthor

    Ahhh… this is probably just #52467 + #146567. unused_attributes and no_mangle_const_items are FCW, not hard errors.

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    A-attributesArea: Attributes (`#[…]`, `#![…]`)A-macrosArea: All kinds of macros (custom derive, macro_rules!, proc macros, ..)C-bugCategory: This is a bug.P-mediumMedium priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.

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