make RefCell unstably const by Daniel-Aaron-Bloom · Pull Request #137843 · rust-lang/rust (original) (raw)

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service andprivacy statement. We’ll occasionally send you account related emails.

Already on GitHub?Sign in to your account

Conversation45 Commits1 Checks10 Files changed

Conversation

This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.Learn more about bidirectional Unicode characters

[ Show hidden characters]({{ revealButtonHref }})

Now that we can do interior mutability in const, most of the RefCell API can be const fn. The main exceptions are APIs which use FnOnce (RefCell::replace_with and Ref[Mut]::[filter_]map[_split]) and RefCell::take which calls Default::default.

Tracking issue: #137844

r? @Amanieu

rustbot has assigned @Amanieu.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

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

Mar 1, 2025

Daniel-Aaron-Bloom changed the title~~Draft: make Cell more unstably const~~ make RefCell more unstably const

Mar 1, 2025

@rustbot label -T-libs +T-libs-api
r? libs-api

rustbot added T-libs-api

Relevant to the library API team, which will review and decide on the PR/issue.

and removed T-libs

Relevant to the library team, which will review and decide on the PR/issue.

labels

Mar 1, 2025

Daniel-Aaron-Bloom changed the title~~make RefCell more unstably const~~ make RefCell unstably const

Mar 1, 2025

Cc @rust-lang/wg-const-eval

The main problem is dropping. Please make sure to also add a test actually using a RefCell; that will probably not work since the Ref/RefMut Drop impls are not const.

@compiler-errors what is the state of Drop in const? Can one impl const Drop and it will "just work"? Are const Destruct bounds ready for use in std (for APIs that are stable but const-unstable)?

Since we probably all want to hear the answer to that question...

cc @rust-lang/lang

Are const Destruct bounds ready for use in std (for APIs that are stable but const-unstable)?

No. Until we know what const traits syntax to implement, adding more const trait impls is not something we want. The libstd churn is not worth it

Can one impl const Drop and it will "just work"?

Yes

@Daniel-Aaron-Bloom okay so please remove the fn const_drop hack and instead try impl const Drop for BorrowRef and BorrowRefMut. If that does not work, then I think it is better to wait until that works; nobody will be able to use RefCell in const otherwise anyway (not even on nightly).

Should I also remove try to remove my added private function RefMut::as_mut and use const_deref instead?

Oh that's what those functions are for?

Sure, seems fine to me as long no incomplete nightly features have to be enabled for this.

This comment has been minimized.

oli-obk removed the S-blocked

Status: Blocked on something else such as an RFC or other implementation work.

label

Jun 2, 2025

📌 Commit 1f1000f has been approved by oli-obk

It is now in the queue for this repository.

bors 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

Jun 26, 2025

bors added a commit that referenced this pull request

Jun 27, 2025

…rors

Rollup of 18 pull requests

Successful merges:

#137843 (make RefCell unstably const)
#140942 (const-eval: allow constants to refer to mutable/external memory, but reject such constants as patterns)
#142549 (small iter.intersperse.fold() optimization)
#142637 (Remove some glob imports from the type system)
#142647 ([perf] Compute hard errors without diagnostics in impl_intersection_has_impossible_obligation)
#142700 (Remove incorrect comments in Weak)
#142927 (Add note to find_const_ty_from_env)
#142967 (Fix RwLock::try_write documentation for WouldBlock condition)
#142986 (Port #[export_name] to the new attribute parsing infrastructure)
#143001 (Rename run always )
#143010 (Update browser-ui-test version to 0.20.7)
#143015 (Add sym::macro_pin diagnostic item for core::pin::pin!())
#143033 (Expand const-stabilized API links in relnotes)
#143041 (Remove cache for citool)
#143056 (Move an ACE test out of the GCI directory)
#143059 (Fix 1.88 relnotes)
#143067 (Tracking issue number for iter_macro)
#143073 (Fix some fixmes that were waiting for let chains)