treat no_mangle_generic_items as hard error instead of lint warning by HerrCai0907 · Pull Request #154585 · rust-lang/rust

HerrCai0907 · 2026-03-30T13:02:53Z

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In rust-lang/miri#4929 (comment), rustc should reject the no_mangled generic function.
This PR treat is as a hard error

HerrCai0907 · 2026-03-30T13:03:12Z

r? @oli-obk

rustbot · 2026-03-30T13:03:18Z

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

oli-obk · 2026-03-30T13:29:40Z

Please make it a hard error and not a lint at all if crater doesn't show any significant regressions.

For crater purposes this works.

oli-obk · 2026-03-30T13:30:08Z

@bors try

set no_mangle_generic_items deny by default

rust-bors · 2026-03-30T15:42:41Z

☀️ Try build successful (CI)
Build commit: ac4c361 (ac4c3613be2acd5395b65d85e5657b81e3c982f0, parent: 4cf5f9580233c36f6bc8db76e282ba8a1c1ea491)

oli-obk · 2026-03-30T17:02:38Z

@craterbot check

craterbot · 2026-03-30T17:03:01Z

👌 Experiment pr-154585 created and queued.
🤖 Automatically detected try build ac4c361
🔍 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 · 2026-04-04T18:55:30Z

🚧 Experiment pr-154585 is now running

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

craterbot · 2026-04-06T17:23:19Z

🎉 Experiment pr-154585 is completed!
📊 5 regressed and 2 fixed (871888 total)
📊 5110 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

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-154585/retry-regressed-list.txt ↩

oli-obk · 2026-04-08T10:13:50Z

I reached out to the jni-mangle author, who used a generic function with no_mangle in a test file, but never actually called it (it wouldn't mangle anyway).

The rxdp failure is because (while there are no generics involved in the function signature) the body uses the generic parameter T from the impl block. (well the failure happens regardless of use in the body, but that's why the functions can't be free functions). I opened a PR to fix it, but the crate also has no dependents, no release in 5 years and the last change to the repo was 3 years ago.

So, let's turn this lint into a hard error. Then we need to write an explainer for T-lang and then we can nominate it.

rustbot · 2026-04-10T05:07:23Z

The Miri subtree was changed

cc @rust-lang/miri

rust-log-analyzer · 2026-04-10T05:40:55Z

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

##[endgroup]
Executing "/scripts/stage_2_test_set1.sh"
+ /scripts/stage_2_test_set1.sh
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip compiler --skip src
PR_CI_JOB set; skipping tidy
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
---
failures:

---- [ui] tests/ui/backtrace/line-tables-only.rs stdout ----

error: test did not exit with success! code=Some(101) so test would pass with `run-fail`
status: exit status: 101
command: cd "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/backtrace/line-tables-only" && RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" RUST_TEST_THREADS="4" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/backtrace/line-tables-only/a"
stdout: none
--- stderr -------------------------------
Backtrace [
    { fn: "<std::backtrace::Backtrace>::create" },
    { fn: "line_tables_only_helper::capture_backtrace::{closure#0}", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 17 },
    { fn: "line_tables_only_helper::baz::<line_tables_only_helper::capture_backtrace::{closure#0}>", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 4 },
    { fn: "line_tables_only_helper::bar::<line_tables_only_helper::capture_backtrace::{closure#0}>", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 8 },
    { fn: "line_tables_only_helper::foo::<line_tables_only_helper::capture_backtrace::{closure#0}>", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 12 },
    { fn: "capture_backtrace", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 17 },
    { fn: "main", file: "/checkout/tests/ui/backtrace/line-tables-only.rs", line: 43 },
    { fn: "call_once<fn(), ()>", file: "/rustc/FAKE_PREFIX/library/core/src/ops/function.rs", line: 250 },
    { fn: "__rust_begin_short_backtrace<fn(), ()>", file: "/rustc/FAKE_PREFIX/library/std/src/sys/backtrace.rs", line: 166 },
    { fn: "std::rt::lang_start::<()>::{closure#0}", file: "/rustc/FAKE_PREFIX/library/std/src/rt.rs", line: 206 },
    { fn: "std::rt::lang_start_internal" },
    { fn: "main" },
    { fn: "__libc_start_main" },
    { fn: "_start" },
]
Backtrace [
    { fn: "<std::backtrace::Backtrace>::create" },
    { fn: "line_tables_only_helper::capture_backtrace::{closure#0}", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 17 },
    { fn: "line_tables_only_helper::baz::<line_tables_only_helper::capture_backtrace::{closure#0}>", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 4 },
    { fn: "line_tables_only_helper::bar::<line_tables_only_helper::capture_backtrace::{closure#0}>", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 8 },
    { fn: "line_tables_only_helper::foo::<line_tables_only_helper::capture_backtrace::{closure#0}>", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 12 },
    { fn: "capture_backtrace", file: "/checkout/tests/ui/backtrace/auxiliary/line-tables-only-helper.rs", line: 17 },
    { fn: "main", file: "/checkout/tests/ui/backtrace/line-tables-only.rs", line: 43 },
    { fn: "call_once<fn(), ()>", file: "/rustc/FAKE_PREFIX/library/core/src/ops/function.rs", line: 250 },
    { fn: "__rust_begin_short_backtrace<fn(), ()>", file: "/rustc/FAKE_PREFIX/library/std/src/sys/backtrace.rs", line: 166 },
    { fn: "std::rt::lang_start::<()>::{closure#0}", file: "/rustc/FAKE_PREFIX/library/std/src/rt.rs", line: 206 },
    { fn: "std::rt::lang_start_internal" },
    { fn: "main" },
    { fn: "__libc_start_main" },
    { fn: "_start" },
]

thread 'main' (36099) panicked at /checkout/tests/ui/backtrace/line-tables-only.rs:38:5:
backtrace does not contain expected line 11
stack backtrace:

rust-bors · 2026-05-21T13:55:23Z

☔ The latest upstream changes (presumably #156794) made this pull request unmergeable. Please resolve the merge conflicts.

RalfJung · 2026-06-24T15:56:57Z

Nominating for @rust-lang/lang:

The no_mangle_generic_items lint was added 10 years ago (#30269) to lint against no_mangle attributes on generic items, which fundamentally make no sense. I am not sure why it was not made an FCW; I suspect the FCW machinery did not even exist yet back then. Anyway it is high time that we move towards erroring on such nonsensical code:

#[no_mangle]
fn no_mangle_generic<T>() {}

A crater run showed only 3 true regressions, 2 of them on crates.io:

rxdp has been fixed. No new release has been made but the crate is also basically unused.
jni-mangle has been fixed, and then the repo has been archived. Also basically an unused crate.

So we suggest that we skip the FCW stage and immediately move to a hard error here.

RalfJung · 2026-06-24T16:00:21Z

+        assert_contains(&backtrace, "foo", "line-tables-only-helper.rs", 6);
    }
-    assert_contains(&backtrace, "bar", "line-tables-only-helper.rs", 10);
-    assert_contains(&backtrace, "baz", "line-tables-only-helper.rs", 5);
+    assert_contains(&backtrace, "bar", "line-tables-only-helper.rs", 11);
+    assert_contains(&backtrace, "baz", "line-tables-only-helper.rs", 6);


Suggested change

assert_contains(&backtrace, "foo", "line-tables-only-helper.rs", 6);

}

assert_contains(&backtrace, "bar", "line-tables-only-helper.rs", 10);

assert_contains(&backtrace, "baz", "line-tables-only-helper.rs", 5);

assert_contains(&backtrace, "bar", "line-tables-only-helper.rs", 11);

assert_contains(&backtrace, "baz", "line-tables-only-helper.rs", 6);

assert_contains(&backtrace, "foo", "line-tables-only-helper.rs", 12);

}

assert_contains(&backtrace, "bar", "line-tables-only-helper.rs", 8);

assert_contains(&backtrace, "baz", "line-tables-only-helper.rs", 4);

That's based on the CI messages, I'd suggest running the tests locally though.

View changes since the review

traviscross · 2026-06-25T06:05:48Z

Makes sense to me. Thanks @HerrCai0907, @oli-obk, and @RalfJung. I propose we make this a hard error and accept this (minimal) observed breakage.

@rfcbot fcp merge lang

rust-rfcbot · 2026-06-25T06:05:51Z

@traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

ehuss · 2026-06-25T13:57:31Z

There is a waiting docs PR at rust-lang/reference#1904. Am I correct that the new rule should state the following?

r[abi.no_mangle.generic]
It is an error to use `no_mangle` on an item with generic type or constant parameters.

RalfJung · 2026-06-25T14:10:00Z

Yes that sounds right. (Though IMO the reference should list where the attribute is allowed, not where it is forbidden.)

set no_mangle_generic_items deny by default

3b4b402

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fmease added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Mar 30, 2026

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rust-bors Bot pushed a commit that referenced this pull request Mar 30, 2026

Auto merge of #154585 - HerrCai0907:fix/no_mangle_generic_items, r=<try>

ac4c361

set no_mangle_generic_items deny by default

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 30, 2026

fix review

40a9cfe

craterbot removed the S-waiting-on-crater Status: Waiting on a crater run to be completed. label Apr 6, 2026

craterbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 6, 2026

oli-obk mentioned this pull request Apr 8, 2026

Remove ineffective no_mangle doyshinda/rxdp#1

Merged

RalfJung reviewed Apr 10, 2026

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Comment thread src/tools/miri/test-cargo-miri/issue-rust-86261/src/lib.rs

HerrCai0907 changed the title ~~set no_mangle_generic_items deny by default~~ treat no_mangle_generic_items as hard error instead of lint warning Apr 25, 2026

This was referenced Jun 24, 2026

cargo miri test --all in test-cargo-miri fails rust-lang/miri#5128

Closed

Tracking Issue for future-incompatibility lint no_mangle_generic_items #158365

Closed

RalfJung added I-lang-nominated Nominated for discussion during a lang team meeting. S-waiting-on-t-lang Status: Awaiting decision from T-lang labels Jun 24, 2026

RalfJung reviewed Jun 24, 2026

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traviscross added the needs-reference-pr This language change needs an approved Reference PR to proceed. label Jun 25, 2026

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