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Adjust rc::Weak::from_raw to support unsized T

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This commit is contained in:
CAD97 2020-07-08 16:23:38 -04:00
parent 85109afee9
commit 728938346b
1 changed files with 12 additions and 11 deletions
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23
library/alloc/src/rc.rs
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@ -1841,17 +1841,18 @@ impl<T> Weak<T> {
/// [`new`]: Weak::new
#[stable(feature = "weak_into_raw", since = "1.45.0")]
pub unsafe fn from_raw(ptr: *const T) -> Self {
if ptr.is_null() {
Self::new()
} else {
// See Rc::from_raw for details
unsafe {
let offset = data_offset(ptr);
let fake_ptr = ptr as *mut RcBox<T>;
let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
}
}
// SAFETY: data_offset is safe to call, because this pointer originates from a Weak.
// See Weak::as_ptr for context on how the input pointer is derived.
let offset = unsafe { data_offset(ptr) };
// Reverse the offset to find the original RcBox.
// SAFETY: we use wrapping_offset here because the pointer may be dangling (iff T: Sized).
let ptr = unsafe {
set_data_ptr(ptr as *mut RcBox<T>, (ptr as *mut u8).wrapping_offset(-offset))
};
// SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
}
}