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Error on wrong targets for no_mangle and export_name - #164789

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Error on wrong targets for no_mangle and export_name#164789
JonathanBrouwer wants to merge 2 commits into
rust-lang:mainfrom
JonathanBrouwer:export-name-target

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@JonathanBrouwer

@JonathanBrouwer JonathanBrouwer commented Oct 23, 2026 •
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This PR makes applying the #[no_mangle] and #[export_name] attributes an error, rather than a warning.

It also makes applying the attributes to a provided trait function consistently an error.
This is necessary because the current behavior is very buggy, and it is not clear to me what the correct behavior should be.

History:

  • Historically, export_name used to error on all targets except fields, arms, macro defs, and macro calls.
  • Historically, no_mangle has always warned on invalid targets. It has always had buggy behaviour on provided trait functions.
  • A year ago, Rework target checking for built-in attributes #145085 introduced a bug which allows export_name on provided trait functions. The behaviour is buggy.

The specific changes this PR makes are:

  • export_name goes from warning to error on fields, arms, macro defs, and macro calls
  • export_name goes from allowed to error on provided trait functions
  • no_mangle goes from warning to error on all targets except the allowed targets: free functions, statics, inherent methods and trait impl methods.

Fixes #164571

cc @mejrs

@rustbot rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 23, 2026
@JonathanBrouwer JonathanBrouwer mentioned this pull request Oct 23, 2026
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@JonathanBrouwer JonathanBrouwer added S-waiting-on-crater Status: Waiting on a crater run to be completed. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. needs-crater This change needs a crater run to check for possible breakage in the ecosystem. labels Oct 23, 2026
@JonathanBrouwer

JonathanBrouwer commented Oct 23, 2026

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This needs a crater run
@bors try

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rust-bors Bot pushed a commit that referenced this pull request Oct 23, 2026
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Auto merge of #164789 - JonathanBrouwer:export-name-target, r=<try>
4d7c0cd 
Error on wrong targets for `no_mangle` and `export_name`,
@JonathanBrouwer JonathanBrouwer changed the title Error on wrong targets for no_mangle and export_name, Error on wrong targets for no_mangle and export_name Oct 23, 2026
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☀️ Try build successful (CI)
Build commit: 4d7c0cd (4d7c0cd298ace72490220301fa543d63a154f8ce)
Base parent: 0cefa27 (0cefa27fe3deee26fc98ee44249406358d9f0d26)

@JonathanBrouwer

JonathanBrouwer commented Oct 23, 2026

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@craterbot check

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craterbot commented Oct 23, 2026

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👌 Experiment pr-164789 created and queued.
🤖 Automatically detected try build 4d7c0cd
⚠️ Try build based on commit 5c4119b, but latest commit is 6717514. Did you forget to make a new try build?
🔍 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 craterbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Oct 23, 2026
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@mejrs

mejrs commented Oct 23, 2026

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rustc_codegen_ssa has a bunch of code defending against invalid codegen attrs, being able to clean that up would be nice. (for example, #159374 adds some)

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rust-bors Bot commented Oct 25, 2026

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☔ The latest upstream changes (presumably #164894) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

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craterbot commented Oct 29, 2026

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🚧 Experiment pr-164789 is now running

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

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craterbot commented Oct 31, 2026

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🎉 Experiment pr-164789 is completed!
📊 212 regressed and 2 fixed (1201864 total)
📊 6001 spurious results on the retry-regressed-list.txt, consider a retry1 if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

  1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-164789/retry-regressed-list.txt ↩

@craterbot craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Oct 31, 2026
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mejrs commented Oct 31, 2026

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It looks like many errors are actually from the cpp crate:

https://github.com/mystor/rust-cpp/blob/ac7272466d0927f630c2a926a63a98af9c2623c3/cpp_macros/src/lib.rs#L327

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mejrs commented Oct 31, 2026

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and from rust-analyzer:
https://crates.io/crates/ra_ap_test_utils/0.0.39/code/src/mark.rs#L37

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JonathanBrouwer commented Nov 1, 2026

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Crater analysis:

small crates (crates that directly broke, or crates with <5 dependents)

20 foreign static
13 foreign fn
9 foreign mod
6 struct
2 type alias
2 required trait method
2 struct,enum
1 use
1 enum
1 trait
1 use,typealias,struct,

brotli

12 crates
on struct, enum, use statement

cosnmic-space

5 crates
on foreign module

hdi

56 crates
on foreign module

i-slint-backend-qt

27 crates
on foreign static

qmetaobject

25 crates
on foreign static

rs-x11-hash

5 crates
on foreign module

sapp-linux

24 crates
on foreign function

@fmease fmease 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 Nov 3, 2026
@mejrs mejrs mentioned this pull request Nov 3, 2026
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mejrs commented Nov 3, 2026

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The breakage in i-slint-backend-qt and qmetaobject is actually in cpp, I've filed a pr there: mystor/rust-cpp#117

@mejrs mejrs mentioned this pull request Nov 3, 2026
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ogoffart pushed a commit to mystor/rust-cpp that referenced this pull request Nov 4, 2026
@mejrs
Remove invalid no_mangle attribute (#117)
d373785 
This is invalid as the `no_mangle` attribute has no meaning on foreign statics.

This crate surfaced in a recent [crater run](rust-lang/rust#164789), where 56 crates started failing with errors like:
```
error: the `no_mangle` attribute cannot be used on foreign statics
   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/i-slint-backend-qt-0.3.5/qt_accessible.rs:116:9
    |
116 | /         cpp!(unsafe [obj as "QObject*"] {
117 | |             QTimer::singleShot(0, [obj = QPointer(obj)]() {
118 | |                 if (!obj)
119 | |                     return;
...   |
129 | |             });
130 | |         });
    | |__________^
    |
    = help: the `no_mangle` attribute can be applied to functions and statics
    = note: this error originates in the macro `__cpp_closure_impl` which comes from the expansion of the macro `cpp` (in Nightly builds, run with -Z macro-backtrace for more info)
```

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A-attributes Area: Attributes (`#[…]`, `#![…]`) needs-crater This change needs a crater run to check for possible breakage in the ecosystem. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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export_name on provided trait associated functions has funny behavior

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