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Clarify when rc::data_offset is safe

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This commit is contained in:
CAD97 2020-06-30 12:47:23 -04:00
parent 0aecf3c74b
commit aed88e1804
2 changed files with 20 additions and 1 deletions
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10
src/liballoc/rc.rs
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@ -2116,6 +2116,16 @@ impl<T: ?Sized> AsRef<T> for Rc<T> {
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Rc<T> {}
/// Get the offset within an `ArcInner` for
/// a payload of type described by a pointer.
///
/// # Safety
///
/// This has the same safety requirements as `align_of_val_raw`. In effect:
///
/// - This function is safe for any argument if `T` is sized, and
/// - if `T` is unsized, the pointer must have appropriate pointer metadata
/// aquired from the real instance that you are getting this offset for.
unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
// Align the unsized value to the end of the `RcBox`.
// Because it is ?Sized, it will always be the last field in memory.

11
src/liballoc/sync.rs
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@ -2273,7 +2273,16 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Arc<T> {}
/// Computes the offset of the data field within `ArcInner`.
/// Get the offset within an `ArcInner` for
/// a payload of type described by a pointer.
///
/// # Safety
///
/// This has the same safety requirements as `align_of_val_raw`. In effect:
///
/// - This function is safe for any argument if `T` is sized, and
/// - if `T` is unsized, the pointer must have appropriate pointer metadata
/// aquired from the real instance that you are getting this offset for.
unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
// Align the unsized value to the end of the `ArcInner`.
// Because it is `?Sized`, it will always be the last field in memory.