Tracking Issue for `cell_get_cloned`

[frank-king]

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 {}

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

// 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<T> {}
unsafe impl<T: CloneFromCopy, E: CloneFromCopy> CloneFromCopy for Result<T, E> {}
// ...

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

Steps / History

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

• Implementation: #...
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

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

Footnotes

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

[theemathas]

unsafe impl<T: Copy> CloneFromCopy for T {}

This impl seems problematic. Cell::get_cloned has to call T::clone, which may run arbitrary user code, even for Copy types.

[frank-king]

unsafe impl<T: Copy> CloneFromCopy for T {}

This impl seems problematic. Cell::get_cloned has to call T::clone, which may run arbitrary user code, even for Copy types.

T is copied (via ptr::read) before calling T::clone, and thus there's no reference to the copied T, so T::clone cannot do harm to it.