std: Clean up and improve errors for `is_*_feature_detected` macros - #165011

tgross35 · 2026-10-27T01:06:23Z

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This includes a handful of small changes around the internals of our feature detection macros:

Add UI tests, since this macro is pretty UI-forward but not yet covered anywhere
Remove misleading feature gates that aren't even being checked, e.g. what showed up in Tracking Issue for stdarch_riscv_feature_detection #111192 (comment).
Clean up some errors and improve the comments around all of this

This is done in a handful of separate commits, the messages have some more details.

r? libs

rustbot · 2026-10-27T01:06:26Z

Some changes occurred in std_detect

cc @Amanieu, @folkertdev, @sayantn

tgross35 · 2026-10-27T01:06:59Z

@bors try jobs=test-i686-gnu-1,test-i686-gnu-2

std: Clean up and improve errors for `is_*_feature_detected` macros try-job: test-i686-gnu-1 try-job: test-i686-gnu-2

std: Clean up and improve errors for `is_*_feature_detected` macros try-job: test-i686-gnu-1

It can be a bit tricky to follow these macros, but we still want to provide good diagnostics. Add tests making sure these don't change.

Because of rewrapping, this prints as: error: This macro cannot be used on the current target. You can prevent it from being used in other architectures by guarding it behind a cfg(any(target_arch = "aarch64", target_arch = "arm64ec")). Bridge newlines so it instead prints as this: error: This macro cannot be used on the current target. You can prevent it from being used in other architectures by guarding it behind a `cfg(any(target_arch = "aarch64", target_arch = "arm64ec"))`.

We have a handful of `#[unstable]` gates that aren't even being used, because all that matters is the re-export in `std`. For example, the RISC-V macro appears unstable, but has actually been stable since 1.78 <https://doc.rust-lang.org/1.78.0/std/arch/macro.is_riscv_feature_detected.html>.

tgross35 · 2026-10-27T10:59:47Z

@bors try jobs=test-i686-gnu-1

std: Clean up and improve errors for `is_*_feature_detected` macros try-job: test-i686-gnu-1

rust-bors · 2026-10-27T13:15:38Z

☀️ Try build successful (CI)
Build commit: a0dd189 (a0dd189f4a59792f71fc63cf586e0700c11752ff)
Base parent: 16d1ffa (16d1ffac17ad8b739e32483ac2e65fe7e1804977)

tgross35 · 2026-10-27T13:17:46Z

@rustbot ready

Rather than transmuting discriminants, use the macros to generate a list of all features that we can iterate instead. This also allows us to remove the sentinel `_last` entry.

LawnGnome · 2026-11-03T23:35:07Z

I appreciate the added commentary. My brain still hurts following this, but less than I think it would have otherwise.

Playing around with this, I don't see any obvious user-visible changes here other than better error messages, so 👍.

@bors r+ rollup=iffy

rust-bors · 2026-11-03T23:35:10Z

📌 Commit 5f7505c has been approved by LawnGnome

It is now in the queue for this repository.

…nGnome std: Clean up and improve errors for `is_*_feature_detected` macros This includes a handful of small changes around the internals of our feature detection macros: * Add UI tests, since this macro is pretty UI-forward but not yet covered anywhere * Remove misleading feature gates that aren't even being checked, e.g. what showed up in rust-lang#111192 (comment). * Clean up some errors and improve the comments around all of this This is done in a handful of separate commits, the messages have some more details.

Rollup of 9 pull requests Successful merges: - #165011 (std: Clean up and improve errors for `is_*_feature_detected` macros) - #164482 (Remove the label starting with 0 in the test binary_asm_labels_allowed.rs) - #165268 (Fix constant-pattern equality suggestion spans in macros) - #165318 (Fix explicit destructor-call suggestion spans in macros) - #164440 ([Outreachy] alloc: Add test coverage for try_clone_from_ref_in (Box, Arc, Rc)) - #165001 (Document allocator swap in BinaryHeap::append) - #165330 (Impl read_le/read_be for f16 and f128) - #165414 (Remove unusued query key impl) - #165440 (docker: bump node to v24.22.2 in shared script)

…nGnome std: Clean up and improve errors for `is_*_feature_detected` macros This includes a handful of small changes around the internals of our feature detection macros: * Add UI tests, since this macro is pretty UI-forward but not yet covered anywhere * Remove misleading feature gates that aren't even being checked, e.g. what showed up in rust-lang#111192 (comment). * Clean up some errors and improve the comments around all of this This is done in a handful of separate commits, the messages have some more details.

…uwer Rollup of 17 pull requests Successful merges: - #164752 (Apply the Cortex-A53 843419 CC mitigation to aarch64 targets ) - #165011 (std: Clean up and improve errors for `is_*_feature_detected` macros) - #165376 (Add `Forget` trait feature gate) - #164482 (Remove the label starting with 0 in the test binary_asm_labels_allowed.rs) - #165201 (Move a bunch of methods out of `AttributeExt`) - #165268 (Fix constant-pattern equality suggestion spans in macros) - #165318 (Fix explicit destructor-call suggestion spans in macros) - #164440 ([Outreachy] alloc: Add test coverage for try_clone_from_ref_in (Box, Arc, Rc)) - #165001 (Document allocator swap in BinaryHeap::append) - #165257 (Revert FCW on lint attributes on macro calls) - #165330 (Impl read_le/read_be for f16 and f128) - #165364 (Update books) - #165414 (Remove unusued query key impl) - #165424 (Update amdgpu datalayout for LLVM 24) - #165440 (docker: bump node to v24.22.2 in shared script) - #165443 (Add `Discriminant::ord`) - #165445 (Add an unstable option to increase the chain depth of the next solver overflow FCW)

rustbot added 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 Oct 27, 2026

rustbot assigned LawnGnome Oct 27, 2026