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UnsafePinned: update get() docs and signature to allow shared mutation #142162

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Jun 9, 2025
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UnsafePinned: update get() docs and signature to allow shared mutation #142162

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bafe406
UnsafePinned: update get() docs and signature to allow shared mutation
RalfJung Jun 7, 2025
bd0a81e
centralize aliasing rules discussion in UnsafeCell docs
RalfJung Jun 7, 2025
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2 changes: 1 addition & 1 deletion library/core/src/cell.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2170,7 +2170,7 @@ impl<T: ?Sized> UnsafeCell<T> {
/// This can be cast to a pointer of any kind.
/// Ensure that the access is unique (no active references, mutable or not)
/// when casting to `&mut T`, and ensure that there are no mutations
/// or mutable aliases going on when casting to `&T`
/// or mutable aliases going on when casting to `&T`.
///
/// # Examples
///
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19 changes: 11 additions & 8 deletions library/core/src/pin/unsafe_pinned.rs
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Expand Up @@ -86,11 +86,14 @@ impl<T: ?Sized> UnsafePinned<T> {
ptr::from_mut(self) as *mut T
}

/// Get read-only access to the contents of a shared `UnsafePinned`.
/// Get mutable access to the contents of a shared `UnsafePinned`.
///
/// Note that `&UnsafePinned<T>` is read-only if `&T` is read-only. This means that if there is
/// mutation of the `T`, future reads from the `*const T` returned here are UB! Use
/// [`UnsafeCell`] if you also need interior mutability.
/// This can be cast to a pointer of any kind.
/// Ensure that the access is unique (no active references, mutable or not)
/// when casting to `&mut T`, and ensure that there are no mutations
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@traviscross traviscross Jun 7, 2025 •
edited
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Since the second half of this, about casting to &T, makes note about ensuring there are no ongoing foreign writes (i.e. even if no active references exist), perhaps the first half should as well, since I'd expect casting to &mut T to have this same precondition.

/// or mutable aliases going on when casting to `&T`.
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Nit. This would be better, I'd suggest, mechanically wrapped.

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I personally prefer semantic linebreaks, since the primary way this will be read is in the actual compiled documentation form and so the internal format of our docs is best-off organized around tracking history.

I don't usually quibble much over it though.

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We use auto-wrapping almost everywhere, and semantic wrapping doesn't go well with out 100 column limit, so I think I'll re-wrap this.

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I'd suggest wording here along the lines of "...ensure that there are no ongoing mutations or live mutable references when casting to &T."

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FWIW this paragraph is copied verbatim from UnsafeCell::get. I was hoping we could just reuse it without spending a lot of time on editing.^^

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I wonder if we can apply the deduplicating technique of #141609 or if it's even worth it here? Probably not, it's just a few lines.

///
/// All the usual caveats around mutation shared state apply, see [`UnsafeCell`].
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Suggested change
/// All the usual caveats around mutation shared state apply, see [`UnsafeCell`].
/// All the usual caveats around mutation of shared state apply, see [`UnsafeCell`].

///
/// [`UnsafeCell`]: crate::cell::UnsafeCell
///
Expand All @@ -100,16 +103,16 @@ impl<T: ?Sized> UnsafePinned<T> {
///
/// unsafe {
/// let mut x = UnsafePinned::new(0);
/// let ptr = x.get(); // read-only pointer, assumes immutability
/// let ptr = x.get();
/// x.get_mut_unchecked().write(1);
/// ptr.read(); // UB!
/// assert_eq!(ptr.read(), 1);
/// }
/// ```
#[inline(always)]
#[must_use]
#[unstable(feature = "unsafe_pinned", issue = "125735")]
pub const fn get(&self) -> *const T {
ptr::from_ref(self) as *const T
pub const fn get(&self) -> *mut T {
self.value.get()
}

/// Gets an immutable pointer to the wrapped value.
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