make target feature ABI check a hard error on ARM by RalfJung · Pull Request #161280 · rust-lang/rust

RalfJung · 2026-08-18T09:27:34Z

With LLVM 24, we now (finally) get hard errors from the backend for "you want ABI X but the target features required for that ABI are missing", at least on the ARM backend:

error: <unknown>:0:0: in function _RNvXsc_Cs4Af2OiBEA1T_8minicoreiNtB5_3Add3add i32 (i32, i32): calling convention is hard-float, but floating-point registers are unavailable

That's not a pretty error, so let's make rust emit a hard error before we even get there -- just on ARM for now, matching LLVM. We have emitted a future-compat error for this since Rust 1.86 (#136147, #134794). I'm not making it a hard error everywhere since for other targets we are still figuring out the exact things we have to check, and it's better to do that without risking new hard errors.

For ARM, the exact check we are doing is:

every target must set llvm_floatabi to either "soft" or "hard"
on softfloat targets, no check is needed
on hardfloat targets, the "fpregs" target feature is required and the the "soft-float" target feature is forbidden

Since this is a new hard error, this probably requires FCP. I am not sure for which team -- @rust-lang/lang is often involved for target feature things, but this is mostly about rejecting invalid -C flags which are handled by t-compiler (@davidtwco @BoxyUwU).

Fixes #161276
Tracking issue #116344
Cc @TimNN

rustbot · 2026-08-18T09:27:39Z

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RalfJung · 2026-08-18T10:09:40Z

Pinging maintainers of affected targets
@rustbot ping arm-maintainers

rustbot · 2026-08-18T10:09:42Z

Error: This team (arm-maintainers) cannot be pinged via this command; it may need to be added to triagebot.toml on the default branch.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.

RalfJung · 2026-08-18T14:44:08Z

Cc @adamgemmell @davidtwco @Jamesbarford @lqd

Skgland · 2026-08-18T16:42:46Z

on hardfloat targets, the "fpregs" target feature is required

On the llvm pr adding the error there is a comment llvm/llvm-project#111334 (comment) arguing against this general requirement.
Arguing this should only error if hard-float and soft-float ABI disagree on a function signature.

RalfJung · 2026-08-18T16:49:58Z

We've been emitting the warning without regard for which function signatures are actually used, and so far nobody complained about those warnings AFAIK. Doing this for function signatures would require completely overhauling the check we do (and that would be hard to do without impacting compile times). Also, since the underlying ABI of many types is not guaranteed, checking the function signature would expose internal rustc implementation details in a way where changing such details (which we are explicitly permitted to do any time) could lead to compilation failures, so a proper check here would require developing an entire new set of guarantees around which types are definitely not using float registers for their ABI.

davidtwco

Looks good to me, we can always remove an error if we can find a better solution later so this just seems like an improvement over giving people an LLVM error

View changes since this review

tmandry · 2026-08-19T15:39:06Z

@rfcbot fcp merge lang

rust-rfcbot · 2026-08-19T15:39:08Z

@tmandry 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.

traviscross · 2026-08-19T15:40:16Z

Thanks @RalfJung.

@rfcbot reviewed

rust-rfcbot · 2026-08-29T15:40:51Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

RalfJung · 2026-08-29T21:15:51Z

Can I take this as an "r=me once FCP passes"?

In case not --
r? @workingjubilee

rustbot · 2026-08-29T21:15:56Z

workingjubilee is currently at their maximum review capacity.
They may take a while to respond.

rustbot · 2026-08-29T21:16:25Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

davidtwco · 2026-09-03T12:02:11Z

r? @davidtwco

@bors r+

rust-bors · 2026-09-03T12:02:15Z

📌 Commit dd4b00d has been approved by davidtwco

It is now in the queue for this repository.

@davidtwco

…re-arm, r=davidtwco make target feature ABI check a hard error on ARM With LLVM 24, we now (finally) get hard errors from the backend for "you want ABI X but the target features required for that ABI are missing", at least on the ARM backend: ``` error: <unknown>:0:0: in function _RNvXsc_Cs4Af2OiBEA1T_8minicoreiNtB5_3Add3add i32 (i32, i32): calling convention is hard-float, but floating-point registers are unavailable ``` That's not a pretty error, so let's make rust emit a hard error before we even get there -- just on ARM for now, matching LLVM. We have emitted a future-compat error for this since Rust 1.86 (rust-lang#136147, rust-lang#134794). I'm not making it a hard error everywhere since for other targets we are still figuring out the exact things we have to check, and it's better to do that without risking new hard errors. For ARM, the exact check we are doing is: - every target must set `llvm_floatabi` to either "soft" or "hard" - on softfloat targets, no check is needed - on hardfloat targets, the "fpregs" target feature is *required* and the the "soft-float" target feature is *forbidden* Since this is a new hard error, this probably requires FCP. I am not sure for which team -- @rust-lang/lang is often involved for target feature things, but this is mostly about rejecting invalid `-C` flags which are handled by t-compiler (@davidtwco @BoxyUwU). Fixes rust-lang#161276 Tracking issue rust-lang#116344 Cc @TimNN

@davidtwco

…re-arm, r=davidtwco make target feature ABI check a hard error on ARM With LLVM 24, we now (finally) get hard errors from the backend for "you want ABI X but the target features required for that ABI are missing", at least on the ARM backend: ``` error: <unknown>:0:0: in function _RNvXsc_Cs4Af2OiBEA1T_8minicoreiNtB5_3Add3add i32 (i32, i32): calling convention is hard-float, but floating-point registers are unavailable ``` That's not a pretty error, so let's make rust emit a hard error before we even get there -- just on ARM for now, matching LLVM. We have emitted a future-compat error for this since Rust 1.86 (rust-lang#136147, rust-lang#134794). I'm not making it a hard error everywhere since for other targets we are still figuring out the exact things we have to check, and it's better to do that without risking new hard errors. For ARM, the exact check we are doing is: - every target must set `llvm_floatabi` to either "soft" or "hard" - on softfloat targets, no check is needed - on hardfloat targets, the "fpregs" target feature is *required* and the the "soft-float" target feature is *forbidden* Since this is a new hard error, this probably requires FCP. I am not sure for which team -- @rust-lang/lang is often involved for target feature things, but this is mostly about rejecting invalid `-C` flags which are handled by t-compiler (@davidtwco @BoxyUwU). Fixes rust-lang#161276 Tracking issue rust-lang#116344 Cc @TimNN

…uwer Rollup of 12 pull requests Successful merges: - #161227 (implement `Add` and `Sub` for `Complex`) - #161280 (make target feature ABI check a hard error on ARM) - #161893 (Add custom allocator support to `(try_)map` on `UniqueArc` and `UniqueRc`) - #162154 (fix[154166]: closure debug capture print) - #161951 (Windows: add fallback if `canonicalize` fails) - #162173 (fix supposedly unreachable `bug!` being reachable) - #162180 (remove outdated next-solver handling) - #162191 (core: mark float `ClampBounds` methods as `#[inline]`) - #162195 (docs(time): clarify exact seconds for week and day) - #162199 (docs(time): clarify exact seconds for hour and minute) - #162222 (coverage: Small cleanups in `extract_hir_info`) - #162230 (Pass -Z merge-functions=disabled in tests/codegen-llvm/intrinsics/unchecked_math.rs)

@davidtwco

Rollup merge of #161280 - RalfJung:abi-required-target-feature-arm, r=davidtwco make target feature ABI check a hard error on ARM With LLVM 24, we now (finally) get hard errors from the backend for "you want ABI X but the target features required for that ABI are missing", at least on the ARM backend: ``` error: <unknown>:0:0: in function _RNvXsc_Cs4Af2OiBEA1T_8minicoreiNtB5_3Add3add i32 (i32, i32): calling convention is hard-float, but floating-point registers are unavailable ``` That's not a pretty error, so let's make rust emit a hard error before we even get there -- just on ARM for now, matching LLVM. We have emitted a future-compat error for this since Rust 1.86 (#136147, #134794). I'm not making it a hard error everywhere since for other targets we are still figuring out the exact things we have to check, and it's better to do that without risking new hard errors. For ARM, the exact check we are doing is: - every target must set `llvm_floatabi` to either "soft" or "hard" - on softfloat targets, no check is needed - on hardfloat targets, the "fpregs" target feature is *required* and the the "soft-float" target feature is *forbidden* Since this is a new hard error, this probably requires FCP. I am not sure for which team -- @rust-lang/lang is often involved for target feature things, but this is mostly about rejecting invalid `-C` flags which are handled by t-compiler (@davidtwco @BoxyUwU). Fixes #161276 Tracking issue #116344 Cc @TimNN

rustbot assigned TaKO8Ki Aug 18, 2026

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 Aug 18, 2026

RalfJung added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Aug 18, 2026

RalfJung force-pushed the abi-required-target-feature-arm branch 2 times, most recently from ac3223c to ae04514 Compare August 18, 2026 09:31

RalfJung mentioned this pull request Aug 18, 2026

compiletest compile-flags forwarding between current test crate and auxiliaries/minicore is iffy #147881

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RalfJung mentioned this pull request Aug 18, 2026

doc/rustc: clarify how to contact arm-maintainers #153720

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RalfJung force-pushed the abi-required-target-feature-arm branch from ae04514 to b684a89 Compare August 18, 2026 16:30

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davidtwco approved these changes Aug 19, 2026

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RalfJung added I-lang-nominated Nominated for discussion during a lang team meeting. I-compiler-nominated Nominated for discussion during a compiler team meeting. labels Aug 19, 2026

traviscross added P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang T-lang Relevant to the language team I-lang-radar Items that are on lang's radar and will need eventual work or consideration. labels Aug 19, 2026

traviscross removed the I-lang-nominated Nominated for discussion during a lang team meeting. label Aug 19, 2026

rust-rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Aug 29, 2026

rust-rfcbot added the to-announce Announce this issue on triage meeting label Aug 29, 2026

rustbot mentioned this pull request Aug 29, 2026

Tracking issue for release notes of #161280: make target feature ABI check a hard error on ARM #161983

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rustbot assigned workingjubilee and unassigned TaKO8Ki Aug 29, 2026

make target feature ABI check a hard error on ARM

dd4b00d

RalfJung force-pushed the abi-required-target-feature-arm branch from 5d56e98 to dd4b00d Compare August 29, 2026 21:16

rustbot assigned davidtwco Sep 3, 2026

rust-bors Bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Sep 3, 2026

rustbot unassigned workingjubilee Sep 3, 2026

rust-bors Bot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 3, 2026

apiraino removed the I-compiler-nominated Nominated for discussion during a compiler team meeting. label Sep 3, 2026

JonathanBrouwer mentioned this pull request Sep 3, 2026

Rollup of 12 pull requests #162249

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This was referenced Sep 3, 2026

Rollup of 13 pull requests #162251

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Rollup of 12 pull requests #162254

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rust-bors Bot merged commit 36b19a5 into rust-lang:main Sep 4, 2026
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rustbot added this to the 1.100.0 milestone Sep 4, 2026

RalfJung deleted the abi-required-target-feature-arm branch September 4, 2026 11:56

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