Tracking Issue for core::cell::{Ref,RefMut}::try_map

Feature gate: #![feature(refcell_try_map)]

This is a tracking issue for core::cell::{Ref,RefMut}::try_map

Public API

impl<'b, T> Ref<'b, T> {
    pub fn try_map<U: ?Sized, E>(
        orig: Ref<'b, T>,
        f: impl FnOnce(&T) -> Result<&U, E>,
    ) -> Result<Ref<'b, U>, (Self, E)> ;
}

impl<'b, T> RefMut<'b, T> {
    pub fn try_map<U: ?Sized, E>(
        mut orig: RefMut<'b, T>,
        f: impl FnOnce(&mut T) -> Result<&mut U, E>,
    ) -> Result<RefMut<'b, U>, (Self, E)>;
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

Implementation: Add Ref/RefMut try_map method #118087
First ACP: ACP: Add {Ref,RefMut}::try_map method libs-team#341 (ignored by t-libs-api for 17 months)
Second ACP: ACP: std::cell::{Ref,RefMut}::try_map libs-team#586 (accepted within 10 days, lol)
Separate error type: refcell_try_map: add a separate error type #153093
Expanding to cover more types: TBD
Final comment period (FCP)¹
Stabilization PR: Stabilize refcell_try_map #152092

Unresolved Questions

None yet.

https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

Rollup merge of rust-lang#118087 - GrigorenkoPV:refcell_try_map, r=Ma…

Unrolled build for #118087

Rollup merge of #118087 - GrigorenkoPV:refcell_try_map, r=Mark-Simula…

Rollup merge of rust-lang#118087 - GrigorenkoPV:refcell_try_map, r=Ma…

@rustbot

Implementation PR is merged, so @rustbot label -S-tracking-unimplemented

Rollup merge of #118087 - GrigorenkoPV:refcell_try_map, r=Mark-Simula…

Nominating for libs-api to consider whether an FCP is warranted or we have questions before starting it.

This was extensively discussed in last week's @rust-lang/libs-api meeting. There were concerns around the fact that the return value doesn't work with ? due to it being a tuple. Instead, the following signature was proposed:

pub fn try_map<U, E>(
    orig: Ref<'b, T>,
    f: impl FnOnce(&T) -> Result<&U, E>,
) -> Result<Ref<'b, U>, TryMapError<'b, T, E>>

impl From<TryMapError<E>> for E { ... }

This would allow ? to work seamlessly since ? calls .into() on the error type.

Also, another thing that was discussed in the meeting is that we would like this API to also be provided on MutexGuard, RwLockReadGuard, RwLockWriteGuard and ReentrantLockGuard. Generally these should mirror the API of RefCell guards, since they basically work the same way except for working across threads.

Instead, the following signature was proposed:

From #153093 (comment) It seems like adding impl From<TryMapError<E>> for E { ... } causes type inference failures in some cases due to the single-impl rule no longer applying, e.g.

use std::fmt;
struct Foo;
impl fmt::Debug for Foo {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut w = |name: &str| { // can fix by -> fmt::Result on closure
            f.write_str(name)?;
            Ok(()) // compiler can no longer infer that E = fmt::Error
        };
        w("hello")?;
        Ok(())
    }
}

fn main() {
    dbg!(Foo);
}

error[E0282]: type annotations needed
 --> b.rs:7:13
  |
7 |             Ok(()) // compiler can no longer infer that this is fmt::Result
  |             ^^ cannot infer type of the type parameter `E` declared on the enum `Result`
  |
help: consider specifying the generic arguments
  |
7 |             Ok::<(), E>(()) // compiler can no longer infer that this is fmt::Result
  |               +++++++++

error: aborting due to 1 previous error

Should a crater run be done?

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Tracking Issue for `core::cell::{Ref,RefMut}::try_map` #143801

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Tracking Issue for core::cell::{Ref,RefMut}::try_map #143801

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karolzwolak commented on Aug 24, 2025

Mark-Simulacrum commented on Feb 14, 2026

Amanieu commented on Feb 24, 2026

Amanieu commented on Feb 24, 2026

zachs18 commented on Jun 12, 2026

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Tracking Issue for `core::cell::{Ref,RefMut}::try_map` #143801