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weak-into-raw: as_raw -> as_ptr + dangling garbage 

* Rename Weak::as_raw to Weak::as_ptr for consistency with some other
  types.
* The as_ptr for a dangling Weak pointer might return whatever garbage
  (and takes that advantage to avoid a conditional).
* Don't guarantee to be able to do `Weak::from_raw(weak.as_ptr())` (even
  though it'll still work fine).
This commit is contained in:
Michal 'vorner' Vaner 2020-04-13 17:17:34 +02:00
parent 4d1fbaccb8
commit 80ccddc0ed
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GPG Key ID: F700D0C019E4C66F
2 changed files with 30 additions and 52 deletions
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41
src/liballoc/rc.rs
View File

@ -1644,8 +1644,8 @@ impl<T> Weak<T> {
/// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
///
/// The pointer is valid only if there are some strong references. The pointer may be dangling
/// or even [`null`] otherwise.
/// The pointer is valid only if there are some strong references. The pointer may be dangling,
/// unaligned or even [`null`] otherwise.
///
/// # Examples
///
@ -1658,31 +1658,22 @@ impl<T> Weak<T> {
/// let strong = Rc::new("hello".to_owned());
/// let weak = Rc::downgrade(&strong);
/// // Both point to the same object
/// assert!(ptr::eq(&*strong, weak.as_raw()));
/// assert!(ptr::eq(&*strong, weak.as_ptr()));
/// // The strong here keeps it alive, so we can still access the object.
/// assert_eq!("hello", unsafe { &*weak.as_raw() });
/// assert_eq!("hello", unsafe { &*weak.as_ptr() });
///
/// drop(strong);
/// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
/// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
/// // undefined behaviour.
/// // assert_eq!("hello", unsafe { &*weak.as_raw() });
/// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
/// ```
///
/// [`null`]: ../../std/ptr/fn.null.html
#[unstable(feature = "weak_into_raw", issue = "60728")]
pub fn as_raw(&self) -> *const T {
match self.inner() {
None => ptr::null(),
Some(inner) => {
let offset = data_offset_sized::<T>();
let ptr = inner as *const RcBox<T>;
// Note: while the pointer we create may already point to dropped value, the
// allocation still lives (it must hold the weak point as long as we are alive).
// Therefore, the offset is OK to do, it won't get out of the allocation.
let ptr = unsafe { (ptr as *const u8).offset(offset) };
ptr as *const T
}
}
pub fn as_ptr(&self) -> *const T {
let offset = data_offset_sized::<T>();
let ptr = self.ptr.cast::<u8>().as_ptr().wrapping_offset(offset);
ptr as *const T
}
/// Consumes the `Weak<T>` and turns it into a raw pointer.
@ -1691,7 +1682,7 @@ impl<T> Weak<T> {
/// can be turned back into the `Weak<T>` with [`from_raw`].
///
/// The same restrictions of accessing the target of the pointer as with
/// [`as_raw`] apply.
/// [`as_ptr`] apply.
///
/// # Examples
///
@ -1712,10 +1703,10 @@ impl<T> Weak<T> {
/// ```
///
/// [`from_raw`]: struct.Weak.html#method.from_raw
/// [`as_raw`]: struct.Weak.html#method.as_raw
/// [`as_ptr`]: struct.Weak.html#method.as_ptr
#[unstable(feature = "weak_into_raw", issue = "60728")]
pub fn into_raw(self) -> *const T {
let result = self.as_raw();
let result = self.as_ptr();
mem::forget(self);
result
}
@ -1730,9 +1721,8 @@ impl<T> Weak<T> {
///
/// # Safety
///
/// The pointer must have originated from the [`into_raw`] (or [`as_raw`], provided there was
/// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
/// count.
/// The pointer must have originated from the [`into_raw`] and must still own its potential
/// weak reference count.
///
/// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
/// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
@ -1765,7 +1755,6 @@ impl<T> Weak<T> {
/// [`upgrade`]: struct.Weak.html#method.upgrade
/// [`Rc`]: struct.Rc.html
/// [`Weak`]: struct.Weak.html
/// [`as_raw`]: struct.Weak.html#method.as_raw
/// [`new`]: struct.Weak.html#method.new
/// [`forget`]: ../../std/mem/fn.forget.html
#[unstable(feature = "weak_into_raw", issue = "60728")]

41
src/liballoc/sync.rs
View File

@ -1340,8 +1340,8 @@ impl<T> Weak<T> {
/// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
///
/// The pointer is valid only if there are some strong references. The pointer may be dangling
/// or even [`null`] otherwise.
/// The pointer is valid only if there are some strong references. The pointer may be dangling,
/// unaligned or even [`null`] otherwise.
///
/// # Examples
///
@ -1354,31 +1354,22 @@ impl<T> Weak<T> {
/// let strong = Arc::new("hello".to_owned());
/// let weak = Arc::downgrade(&strong);
/// // Both point to the same object
/// assert!(ptr::eq(&*strong, weak.as_raw()));
/// assert!(ptr::eq(&*strong, weak.as_ptr()));
/// // The strong here keeps it alive, so we can still access the object.
/// assert_eq!("hello", unsafe { &*weak.as_raw() });
/// assert_eq!("hello", unsafe { &*weak.as_ptr() });
///
/// drop(strong);
/// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
/// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
/// // undefined behaviour.
/// // assert_eq!("hello", unsafe { &*weak.as_raw() });
/// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
/// ```
///
/// [`null`]: ../../std/ptr/fn.null.html
#[unstable(feature = "weak_into_raw", issue = "60728")]
pub fn as_raw(&self) -> *const T {
match self.inner() {
None => ptr::null(),
Some(inner) => {
let offset = data_offset_sized::<T>();
let ptr = inner as *const ArcInner<T>;
// Note: while the pointer we create may already point to dropped value, the
// allocation still lives (it must hold the weak point as long as we are alive).
// Therefore, the offset is OK to do, it won't get out of the allocation.
let ptr = unsafe { (ptr as *const u8).offset(offset) };
ptr as *const T
}
}
pub fn as_ptr(&self) -> *const T {
let offset = data_offset_sized::<T>();
let ptr = self.ptr.cast::<u8>().as_ptr().wrapping_offset(offset);
ptr as *const T
}
/// Consumes the `Weak<T>` and turns it into a raw pointer.
@ -1387,7 +1378,7 @@ impl<T> Weak<T> {
/// can be turned back into the `Weak<T>` with [`from_raw`].
///
/// The same restrictions of accessing the target of the pointer as with
/// [`as_raw`] apply.
/// [`as_ptr`] apply.
///
/// # Examples
///
@ -1408,10 +1399,10 @@ impl<T> Weak<T> {
/// ```
///
/// [`from_raw`]: struct.Weak.html#method.from_raw
/// [`as_raw`]: struct.Weak.html#method.as_raw
/// [`as_ptr`]: struct.Weak.html#method.as_ptr
#[unstable(feature = "weak_into_raw", issue = "60728")]
pub fn into_raw(self) -> *const T {
let result = self.as_raw();
let result = self.as_ptr();
mem::forget(self);
result
}
@ -1427,9 +1418,8 @@ impl<T> Weak<T> {
///
/// # Safety
///
/// The pointer must have originated from the [`into_raw`] (or [`as_raw'], provided there was
/// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
/// count.
/// The pointer must have originated from the [`into_raw`] and must still own its potential
/// weak reference count.
///
/// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
/// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
@ -1458,7 +1448,6 @@ impl<T> Weak<T> {
/// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
/// ```
///
/// [`as_raw`]: struct.Weak.html#method.as_raw
/// [`new`]: struct.Weak.html#method.new
/// [`into_raw`]: struct.Weak.html#method.into_raw
/// [`upgrade`]: struct.Weak.html#method.upgrade