Tracking Issue for `cell_get_cloned`

View all comments

Feature gate: #![feature(cell_get_cloned)]

This is a tracking issue for Cell::get_cloned for T: CloneFromCopy (e.g. Rc<T>, Arc<T>, etc.).

ACP: https://github.com/rust-lang/libs-team/issues/572

Public API

/// Safety: can call `Self::clone(&*ManuallyDrop::new(ptr::read(ptr_to_self)))`
#[marker]
pub unsafe trait CloneFromCell: Clone {}

// Can't do this, unsound
// unsafe impl<T: Copy> CloneFromCell for T {}

unsafe impl<T: ?Sized> CloneFromCell for Rc<T> {}
unsafe impl<T: ?Sized> CloneFromCell for Arc<T> {}
unsafe impl<T: ?Sized> CloneFromCell for rc::Weak<T> {}
unsafe impl<T: ?Sized> CloneFromCell for sync::Weak<T> {}

// basically all std types that don't have indirection:
unsafe impl<T: CloneFromCell, const N: usize> CloneFromCell for [T; N] {}
unsafe impl<T: CloneFromCell> CloneFromCell for Option<T> {}
unsafe impl<T: CloneFromCell, E: CloneFromCell> CloneFromCell for Result<T, E> {}
// ...

impl<T: CloneFromCell> Cell<T> {
    pub fn get_cloned(&self) -> Self;
}

Steps / History

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

• Implementation: https://github.com/rust-lang/rust/pull/145685
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Should this remain get_cloned or move to the Clone impl for Cell? https://github.com/rust-lang/libs-team/issues/572#issuecomment-3180625110
• Concern: the soundness conditions here are extremely tricky to get right. We need to have a warning about this.
• The soundness requirement on CloneFromCopy as stated above is likely wrong.

cc @rust-lang/libs-api @rust-lang/lang

Start with the tracking issue, the Cell::get_cloned API sketch, and implementation PR 145685. Read the linked libs-team discussion and unresolved questions before making changes, especially the proposed method location and soundness requirements. Done means the implementation, final comment period, and stabilization PR have been completed.