set no_mangle_generic_items deny by default

[HerrCai0907]

In rust-lang/miri#4929 (comment), rustc should reject the no_mangled generic function.
This PR change the default behavior from warn to deny.

[HerrCai0907]

r? @oli-obk

[rustbot]

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

[oli-obk]

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]

@bors try

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

Executing "/scripts/stage_2_test_set2.sh"
+ /scripts/stage_2_test_set2.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
downloading https://static.rust-lang.org/dist/2026-03-05/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz

[rust-bors]

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

[oli-obk]

@craterbot check

[craterbot]

👌 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]

🚧 Experiment pr-154585 is now running

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

[craterbot]

🎉 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

Footnotes

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

[oli-obk]

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.