add llvm writable attribute conditionally#155207

quiode · 2026-04-12T18:54:57Z

This PR tries to address rust-lang/unsafe-code-guidelines#584 (comment). It is part of a bachelor thesis supervised by @JoJoDeveloping and @RalfJung, for more information, see: Project_Description.pdf.
If the new -Zllvm-writable flag is set, the llvm writable attribute is inserted for all mutable borrows. This can be conditionally turned off on a per-function basis using the #[rustc_no_writable] attribute. The new Undefined Behaviour introduced by this can detected by Miri, which is implemented here: rust-lang/miri#4947.

Two library functions already received the #[rustc_no_writable] attribute, as they are known to cause problems under the Tree Borrows aliasing model with implicit writes enabled.

rustbot · 2026-04-12T18:55:01Z

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-04-12T18:55:03Z

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quiode · 2026-04-12T18:55:08Z

r? @RalfJung

rustbot · 2026-04-12T18:55:12Z

RalfJung is not on the review rotation at the moment.
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RalfJung

This generally looks good, thanks! I just hope this larger ArgAttribute won't cause us problems. Cc @nikic for the LLVM parts, in case you want to take a 2nd look.

I am not sure if there's anything special to look out for in the new attribute infrastructure; @jdonszelmann would be great if you could take a brief look at that part.

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RalfJung · 2026-04-14T13:52:14Z

    // The subset of llvm::Attribute needed for arguments, packed into a bitfield.
    #[derive(Clone, Copy, Default, Hash, PartialEq, Eq, HashStable_Generic)]
-    pub struct ArgAttribute(u8);
+    pub struct ArgAttribute(u16);


Ah, this is unfortunate. We should benchmark this to ensure the larger bitfield isn't a problem.

rustbot · 2026-04-14T14:03:04Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

JonathanBrouwer · 2026-04-14T19:39:32Z

+
+impl<S: Stage> NoArgsAttributeParser<S> for RustcNoWritableParser {
+    const PATH: &[Symbol] = &[sym::rustc_no_writable];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Warn;


I think it's nicest for all new attributes to be Error on duplicate
I'll go make a PR to give this const a default value

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(Made #155308)

JonathanBrouwer

r=me on the attribute changes, didn't check the other part of this PR

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JonathanBrouwer · 2026-04-14T19:47:48Z

@bors try @rust-timer queue

rust-timer · 2026-04-14T19:47:52Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

add llvm writable attribute conditionally

rust-bors · 2026-04-14T20:02:47Z

💔 Test for 37fa495 failed: CI. Failed job:

try - dist-x86_64-linux-quick (web logs, enhanced plaintext logs)

JonathanBrouwer · 2026-04-14T20:05:34Z

^ The PR needs to be rebased

Co-authored-by: Ralf Jung <post@ralfj.de>

rustbot · 2026-04-15T15:17:07Z

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.

quiode · 2026-04-15T15:39:33Z

@rustbot ready

rustbot assigned mati865 Apr 12, 2026

rustbot assigned RalfJung and unassigned mati865 Apr 12, 2026

quiode force-pushed the writable branch from 245ff9c to 0c53594 Compare April 12, 2026 19:29

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RalfJung reviewed Apr 14, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 14, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 14, 2026

rust-bors bot pushed a commit that referenced this pull request Apr 14, 2026

Auto merge of #155207 - quiode:writable, r=<try>

37fa495

add llvm writable attribute conditionally

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quiode and others added 11 commits April 15, 2026 16:58

add llvm writable attribute conditionally

fff1def

reword rustc_no_writable description in builtin_attrs.rs

a25d7b4

Co-authored-by: Ralf Jung <post@ralfj.de>

reword llvm_writable flag description

eaf2c07

Co-authored-by: Ralf Jung <post@ralfj.de>

assume rustc_no_writable when def_id does not exist

4405e0f

rework logic for setting writable

1be119b

rename def_id in arg_attrs_for_rust_scalar to determined_fn_def_id

cd4ad73

Update src/doc/unstable-book/src/compiler-flags/llvm-writable.md

b93142e

Co-authored-by: Ralf Jung <post@ralfj.de>

test box and unsafe_pinned not getting writable

ee6131d

tidy

13b8e42

error on duplicate, remove completely once 155308 is merged

abc467a

apply updates from rebase

091196c

quiode force-pushed the writable branch from 514b3eb to 091196c Compare April 15, 2026 15:17

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 15, 2026

quiode mentioned this pull request Apr 15, 2026

tree borrows: implicit writes rust-lang/miri#4947

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