Tracking Issue for cell_get_cloned (original) (raw)

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: rust-lang/libs-team#572

Public API

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

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

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

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

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

Steps / History

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

• Implementation: add CloneFromCell and Cell::get_cloned #145685
• Final comment period (FCP)1
• Stabilization PR

Unresolved Questions

• Should this remain get_cloned or move to the Clone impl for Cell? Add Cell::get for Rc-like shallow-copying types libs-team#572 (comment)
• 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

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