Remove `Copy` supertrait from `VaList` by Jules-Bertholet · Pull Request #160221 · rust-lang/rust

Jules-Bertholet · 2026-07-30T15:12:46Z

… And adjust next_arg's safety comment accordingly. This leaves us more flexibility for the future.

Zulip discussion: https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/.60.3A.20Copy.60.20bound.20on.20.60VaArgSafe.60

@rustbot label T-libs-api

rustbot · 2026-07-30T15:12:51Z

r? @clarfonthey

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nia-e · 2026-07-30T15:18:53Z

r? me

r=me if CI passes

nia-e · 2026-07-30T15:23:52Z

@bors delegate+

rust-bors · 2026-07-30T15:23:55Z

✌️ @Jules-Bertholet, you can now approve this pull request!

If @nia-e told you to "r=me" after making some further change, then please make that change and post @bors r=nia-e.

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tgross35 · 2026-07-30T16:14:00Z

+    /// - If `T` is not [`Copy`], then it must not have already been read using `next_arg`
+    /// on a [`clone`][VaList::clone]d copy of this `VaList`.
+    /// (Currently, all types implementing [`VaArgSafe`] also implement [`Copy`],
+    /// but this may change in the future.)


Miri has been enforcing the other conditions, does it already catch this?

Cc @rust-lang/miri

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I wouldn't expect Miri to directly enforce this, as it is library UB and not language UB. Similar to ptr::reading a valid instance of an arbitrary type

Good point, the UB would probably manifest upon drop and be caught.

This cannot be triggered because VaArgSafe is only implemented for Copy types currently.

I personally have little appetite for opening that up much further. There are some obscure numeric types that we will likely add in time (f16, f128, x87 f80, ppc f128, Complex<T>, bf16), but portability for some of those is already not great (e.g. clang and gcc diverge), and for anything beyond that I just don't think it is needed.

(I'm fine with this change though, if we believe this is a better way to handle it)

My personal preference would be the exact opposite, have impl<T> VaArgSafe for T {} and rely on lints to flag incorrect usage instead. But that is a discussion for another day

i think cutting the user-visible behaviour is good, so i concur fully with removing the copy bound esp if it's a change we can undo if needed in the future & makes the stabilised api surface smaller. i would certainly raise more eyebrows at a blanket impl of the likes suggested but that's offtopic ^^

nia-e · 2026-07-30T17:43:28Z

@bors r+ rollup

rust-bors · 2026-07-30T17:43:31Z

📌 Commit 6537605 has been approved by nia-e

It is now in the queue for this repository.

…uwer Rollup of 25 pull requests Successful merges: - #160204 (Sync from portable simd 2026 07 30) - #138230 (Add `raw_borrows_via_references` lint) - #158057 (Don't escape U+FF9E and U+FF9F in `escape_debug_ext`) - #160015 (refactor(mir-transform): Merge `can_be_overridden`, `is_required` and `is_enabled` into one) - #160031 (std: make positioned I/O unsupported on VxWorks) - #160125 (Fix typing mode handling in transmute checks and rustc_dump_layout) - #160152 (Create on-demand CI job for testing EC2 instances) - #160232 (rustdoc: fix ICE when a grapheme cluster joins a Prepend-class character to `_` or `:`) - #159214 (std: improve the documentation of the random feature) - #159818 (Resolve vars before calling `unnormalized_obligations`) - #159955 (Stop using higher-order macros to declare arenas) - #159958 (Fix avoid cycle for self referential return type notation) - #160040 (Split function parsing out of `item.rs` to a new module.) - #160044 (Add regression tests for fixed dead-code issues) - #160144 (renovate: group lockfiles PRs) - #160149 (Fix Windows on Arm PAC default) - #160164 (Derive `GenericTypeVisitable` for `RegionConstraint`) - #160175 (Try to recover less from incorrectly parsed const arg) - #160177 (A few more "predicate"-to-"clause" renamings) - #160181 (Mark `Tuple` and `FnPtr` traits `#[fundamental]`) - #160192 (Fix ICE for parsing issue with a closing brace) - #160209 (bootstrap: Remove method `Subcommand::kind`) - #160221 (Remove `Copy` supertrait from `VaList`) - #160223 (interpret: rename validate_operand → validate_place) - #160234 (Always use short ty path for call with missing arguments suggestion)

@rustbot

Rollup merge of #160221 - Jules-Bertholet:valist-noncopy, r=nia-e Remove `Copy` supertrait from `VaList` … And adjust `next_arg`'s safety comment accordingly. This leaves us more flexibility for the future. Zulip discussion: https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/.60.3A.20Copy.60.20bound.20on.20.60VaArgSafe.60 @rustbot label T-libs-api

…uwer Rollup of 25 pull requests Successful merges: - rust-lang/rust#160204 (Sync from portable simd 2026 07 30) - rust-lang/rust#138230 (Add `raw_borrows_via_references` lint) - rust-lang/rust#158057 (Don't escape U+FF9E and U+FF9F in `escape_debug_ext`) - rust-lang/rust#160015 (refactor(mir-transform): Merge `can_be_overridden`, `is_required` and `is_enabled` into one) - rust-lang/rust#160031 (std: make positioned I/O unsupported on VxWorks) - rust-lang/rust#160125 (Fix typing mode handling in transmute checks and rustc_dump_layout) - rust-lang/rust#160152 (Create on-demand CI job for testing EC2 instances) - rust-lang/rust#160232 (rustdoc: fix ICE when a grapheme cluster joins a Prepend-class character to `_` or `:`) - rust-lang/rust#159214 (std: improve the documentation of the random feature) - rust-lang/rust#159818 (Resolve vars before calling `unnormalized_obligations`) - rust-lang/rust#159955 (Stop using higher-order macros to declare arenas) - rust-lang/rust#159958 (Fix avoid cycle for self referential return type notation) - rust-lang/rust#160040 (Split function parsing out of `item.rs` to a new module.) - rust-lang/rust#160044 (Add regression tests for fixed dead-code issues) - rust-lang/rust#160144 (renovate: group lockfiles PRs) - rust-lang/rust#160149 (Fix Windows on Arm PAC default) - rust-lang/rust#160164 (Derive `GenericTypeVisitable` for `RegionConstraint`) - rust-lang/rust#160175 (Try to recover less from incorrectly parsed const arg) - rust-lang/rust#160177 (A few more "predicate"-to-"clause" renamings) - rust-lang/rust#160181 (Mark `Tuple` and `FnPtr` traits `#[fundamental]`) - rust-lang/rust#160192 (Fix ICE for parsing issue with a closing brace) - rust-lang/rust#160209 (bootstrap: Remove method `Subcommand::kind`) - rust-lang/rust#160221 (Remove `Copy` supertrait from `VaList`) - rust-lang/rust#160223 (interpret: rename validate_operand → validate_place) - rust-lang/rust#160234 (Always use short ty path for call with missing arguments suggestion)

Remove Copy supertrait from VaList

6537605

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 30, 2026

rustbot assigned clarfonthey Jul 30, 2026

rustbot added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Jul 30, 2026

rustbot assigned nia-e and unassigned clarfonthey Jul 30, 2026

tgross35 reviewed Jul 30, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 30, 2026

JonathanBrouwer mentioned this pull request Jul 30, 2026

Rollup of 25 pull requests #160238

Merged

rust-bors Bot merged commit 4487801 into rust-lang:main Jul 31, 2026
13 checks passed

rustbot added this to the 1.99.0 milestone Jul 31, 2026

folkertdev added the F-c_variadic `#![feature(c_variadic)]` label Jul 31, 2026

Jules-Bertholet deleted the valist-noncopy branch July 31, 2026 12:07

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