Make no_mangle on foreign items explicit instead of implicit #144678

jdonszelmann · 2025-07-30T11:24:44Z

for a followup PR I'm working on I need some foreign items to mangle. I could add a new attribute: no_no_mangle or something silly like that but by explicitly putting no_mangle in the codegen fn attrs of foreign items we can default it to no_mangle and then easily remove it when we don't want it.

I guess you'd know about this r? @bjorn3. Shouldn't be too hard to review :)

Builds on #144655 which should merge first.

rustbot · 2025-07-30T11:24:50Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

loge

bjorn3 · 2025-07-30T11:34:44Z

Can you please squash the last two commits?

jdonszelmann · 2025-07-30T13:53:56Z

Can do

compiler/rustc_symbol_mangling/src/lib.rs

compiler/rustc_passes/src/reachable.rs

bjorn3 · 2025-07-31T12:41:35Z

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -192,6 +192,10 @@ impl CodegenFnAttrs {
    /// * `#[export_name(...)]` is present
    /// * `#[linkage]` is present
    ///
+    /// Note that this returns true for foreign items.
+    /// However, in some places that care about `contains_extern_indicator`, foreign items
+    /// (in an `extern` block) should explicitly be ignored.


Is it possible to make this not return true for foreign items? Foreign items don't need to be exported and the point of this method is to check if an item needs to be exported.

it's annoying since we don't have a did or TyCtxt here. We could, but that'd involve adding another flag in the codegen fn attributes. I decided against that but it's perfectly possible.

bjorn3 · 2025-08-01T08:52:04Z

@bors r+

bors · 2025-08-01T08:52:07Z

📌 Commit 9003a3b has been approved by bjorn3

It is now in the queue for this repository.

…jorn3 Make no_mangle on foreign items explicit instead of implicit for a followup PR I'm working on I need some foreign items to mangle. I could add a new attribute: `no_no_mangle` or something silly like that but by explicitly putting `no_mangle` in the codegen fn attrs of foreign items we can default it to `no_mangle` and then easily remove it when we don't want it. I guess you'd know about this r? `@bjorn3.` Shouldn't be too hard to review :) Builds on rust-lang#144655 which should merge first.

Rollup of 13 pull requests Successful merges: - #143857 (Port #[macro_export] to the new attribute parsing infrastructure) - #144070 (Implement `hash_map` macro ) - #144322 (Add lint against dangling pointers from local variables) - #144667 (`AlignmentEnum` should just be `repr(usize)` now) - #144678 (Make no_mangle on foreign items explicit instead of implicit) - #144790 (Multiple bounds checking elision failures) - #144794 (Port `#[coroutine]` to the new attribute system) - #144805 (compiletest: Preliminary cleanup of `ProcRes` printing/unwinding) - #144808 (`Interner` arg to `EarlyBinder` does not affect auto traits) - #144816 (Update E0562 to account for the new impl trait positions) - #144822 (Return a struct with named fields from `hash_owner_nodes`) - #144824 (Updated test links in compiler) - #144829 (Use full flag name in strip command for Darwin) r? `@ghost` `@rustbot` modify labels: rollup

Zalathar · 2025-08-03T09:46:57Z

Trying to diagnose rollup failure #144846 (comment)

@bors try jobs=test-various

rust-bors · 2025-08-03T09:47:02Z

⌛ Trying commit 9003a3b with merge 50d1271…

To cancel the try build, run the command @bors try cancel.

Make no_mangle on foreign items explicit instead of implicit try-job: test-various

rust-bors · 2025-08-03T10:38:50Z

💔 Test failed (CI). Failed jobs:

bors build finished (web logs, extended logs)
try - test-various (web logs, extended logs)

Zalathar · 2025-08-03T10:38:56Z

@bors r-

jdonszelmann · 2025-08-11T13:44:08Z

@bors2 try jobs=test-various

Make no_mangle on foreign items explicit instead of implicit try-job: test-various

rust-bors · 2025-08-11T13:44:13Z

⌛ Trying commit 5aa8fb1 with merge b47c284…

To cancel the try build, run the command @bors try cancel.

rust-bors · 2025-08-11T15:21:50Z

☀️ Try build successful (CI)
Build commit: b47c284 (b47c28460e3041feeec2dd6f6fcfb2ddc7bf294e, parent: 577166503aee7290e09374da21f4045c455acfd5)

jdonszelmann · 2025-08-11T17:07:38Z

@bjorn3 that seems promising?

compiler/rustc_symbol_mangling/src/lib.rs

jdonszelmann · 2025-08-12T10:10:42Z

@bors2 try jobs=test-various

Make no_mangle on foreign items explicit instead of implicit try-job: test-various

rust-bors · 2025-08-12T11:48:16Z

☀️ Try build successful (CI)
Build commit: a1039d3 (a1039d3cbd63a0b67b7310bc436492f1055f7702, parent: a1531335fe2807715fff569904d99602022643a7)

jdonszelmann · 2025-08-12T12:40:59Z

@bjorn seems ok?

jdonszelmann · 2025-08-12T12:41:26Z

@rustbot ready

bjorn3 · 2025-08-12T13:35:11Z

@bors r+

bors · 2025-08-12T13:35:14Z

📌 Commit 7aa8707 has been approved by bjorn3

It is now in the queue for this repository.

bors · 2025-08-12T16:20:27Z

⌛ Testing commit 7aa8707 with merge 8e62bfd...

bors · 2025-08-12T20:03:49Z

☀️ Test successful - checks-actions
Approved by: bjorn3
Pushing 8e62bfd to master...

github-actions · 2025-08-12T20:07:18Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing d9dba3a (parent) -> 8e62bfd (this PR)

Test differences

Show 4 test diffs

4 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 8e62bfd311791bfd9dca886abdfbab07ec54d8b4 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-apple-1: 7151.0s -> 10604.1s (48.3%)
x86_64-apple-2: 4279.4s -> 5341.4s (24.8%)
pr-check-2: 2178.7s -> 2596.1s (19.2%)
aarch64-gnu-llvm-19-2: 2237.2s -> 2639.2s (18.0%)
dist-apple-various: 3911.0s -> 4436.4s (13.4%)
x86_64-rust-for-linux: 2620.5s -> 2939.6s (12.2%)
x86_64-gnu-llvm-19: 2441.7s -> 2738.6s (12.2%)
x86_64-gnu-tools: 3288.6s -> 3666.9s (11.5%)
aarch64-gnu-debug: 4455.3s -> 4961.8s (11.4%)
i686-gnu-nopt-1: 7275.8s -> 8053.4s (10.7%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-08-12T21:14:29Z

Finished benchmarking commit (8e62bfd): comparison URL.

Overall result: ❌ regressions - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.6%]	8
Regressions ❌ (secondary)	0.2%	[0.1%, 0.5%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.2%, 0.6%]	8

Max RSS (memory usage)

Results (primary -1.3%, secondary 2.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	1

Cycles

Results (secondary 3.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 464.325s -> 465.671s (0.29%)
Artifact size: 377.45 MiB -> 377.43 MiB (-0.01%)

jdonszelmann · 2025-08-12T22:21:21Z

sigh, ye that's significant I'm afraid :/

bjorn3 · 2025-08-13T14:36:04Z

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -443,6 +443,27 @@ fn apply_overrides(tcx: TyCtxt<'_>, did: LocalDefId, codegen_fn_attrs: &mut Code
    if tcx.should_inherit_track_caller(did) {
        codegen_fn_attrs.flags |= CodegenFnAttrFlags::TRACK_CALLER;
    }
+
+    // Foreign items by default use no mangling for their symbol name.
+    if tcx.is_foreign_item(did) {


I suspect the extra query call here is the cause for the perf regression.

rustbot assigned bjorn3 Jul 30, 2025

rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) 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 Jul 30, 2025

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Rollup of 13 pull requests #144846

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Auto merge of #144678 - jdonszelmann:no-mangle-extern, r=<try>

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Make no_mangle on foreign items explicit instead of implicit try-job: test-various

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Auto merge of #144678 - jdonszelmann:no-mangle-extern, r=<try>

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Make no_mangle on foreign items explicit instead of implicit try-job: test-various

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Auto merge of #144678 - jdonszelmann:no-mangle-extern, r=<try>

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Make no_mangle on foreign items explicit instead of implicit try-job: test-various

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bors merged commit 8e62bfd into rust-lang:master Aug 12, 2025
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Make no_mangle on foreign items explicit instead of implicit #144678

Make no_mangle on foreign items explicit instead of implicit #144678

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Test differences

Job duration changes

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rust-timer commented Aug 12, 2025

Overall result: ❌ regressions - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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