Move wasm throw intrinsic back to unwind

[purplesyringa]

Fixes #148246, less invasive than the previously proposed #148269. Removes the publicly visible unstable intrinsic tracked in #122465 since it's not clear how to export it in a sound manner.

r? @bjorn3

---

rustc assumes that regular extern "Rust" functions unwind only if the unwind panic runtime is linked. throw was annotated as such, but unwound unconditionally. This could cause UB when a crate built with -C panic=abort called throw from core built with -C panic=unwind, since no terminator was added to handle the panic arising from calling an allegedly non-unwinding extern "Rust" function.

rustc was taught to recognize this condition since #144225 and prevented such linkage, but this caused regressions in
#148246, since this meant that Emscripten projects could not be built with -C panic=abort without recompiling std.

The most straightforward solution would be to move throw into the panic_unwind crate, so that it's only compiled if the panic runtime is guaranteed to be unwind, but this is messy due to our architecture. Instead, move it into unwind::wasm, which is only compiled for bare-metal targets that default to panic = "abort", rendering the issue moot.

[rustbot]

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[bjorn3]

@bors r+

[bors]

📌 Commit 420544a has been approved by bjorn3

It is now in the queue for this repository.

[bors]

⌛ Testing commit 420544a with merge 8205e6b...

[bors]

☀️ Test successful - checks-actions
Approved by: bjorn3
Pushing 8205e6b to master...

[github-actions]

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 6906167 (parent) -> 8205e6b (this PR)

Test differences

Show 4 test diffs

Stage 1

• [ui] tests/ui/wasm/wasm-link-to-panic-abort-issue-148246.rs: [missing] -> ignore (only executed when the architecture is wasm32) (J1)

Stage 2

• [ui] tests/ui/wasm/wasm-link-to-panic-abort-issue-148246.rs: [missing] -> ignore (only executed when the architecture is wasm32) (J0)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: aarch64-apple, aarch64-gnu, aarch64-gnu-llvm-20-1, aarch64-msvc-1, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-gnu, x86_64-gnu-debug, x86_64-gnu-gcc, x86_64-gnu-llvm-20, x86_64-gnu-llvm-20-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1
• J1: x86_64-gnu-llvm-20-3

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 8205e6b75ec656305ac235d4726d2c7a1ddcef14 --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

1. x86_64-gnu-llvm-20-2: 6016.0s -> 5106.5s (-15.1%)
2. aarch64-gnu-llvm-20-2: 2569.6s -> 2196.4s (-14.5%)
3. x86_64-gnu-tools: 3783.8s -> 3324.7s (-12.1%)
4. pr-check-1: 1677.3s -> 1479.0s (-11.8%)
5. dist-apple-various: 3362.9s -> 3755.6s (+11.7%)
6. i686-gnu-2: 6279.5s -> 5552.7s (-11.6%)
7. dist-x86_64-apple: 7544.2s -> 6678.3s (-11.5%)
8. dist-aarch64-msvc: 5221.7s -> 5754.4s (+10.2%)
9. x86_64-gnu-aux: 6749.2s -> 7423.0s (+10.0%)
10. aarch64-gnu: 7036.5s -> 6334.3s (-10.0%)

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]

Finished benchmarking commit (8205e6b): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	-3.0%	[-3.0%, -3.0%]	1

Max RSS (memory usage)

Results (primary 0.3%, secondary -3.3%)

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

	mean	range	count
Regressions ❌ (primary)	4.0%	[4.0%, 4.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.5%	[-3.5%, -3.5%]	1
Improvements ✅ (secondary)	-3.3%	[-4.6%, -2.1%]	5
All ❌✅ (primary)	0.3%	[-3.5%, 4.0%]	2

Cycles

Results (primary -3.6%, secondary 2.6%)

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)	2.6%	[2.3%, 2.9%]	2
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.6%	[-3.6%, -3.6%]	1

Binary size

Results (primary -1.1%, secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Bootstrap: 475.565s -> 475.886s (0.07%)
Artifact size: 390.38 MiB -> 390.36 MiB (-0.01%)