Rollup merge of rust-lang#80764 - CAD97:weak-unsized-as-ptr-again, r=…

…RalfJung

Re-stabilize Weak::as_ptr and friends for unsized T

As per [T-lang consensus](https://hackmd.io/7r3_is6uTz-163fsOV8Vfg), this uses a branch to handle the dangling case. The discussed optimization of only doing the branch in the T: ?Sized case is left for a followup patch, as doing so is not trivial (as it requires specialization) and not _obviously_ better (as it requires using `wrapping_offset` rather than `offset` more).

<details><summary>Basically said optimization</summary>

Specialize on `T: Sized`:

```rust
fn as_ptr(&self) -> *const T {
    if [ T is Sized ] || !is_dangling(ptr) {
        (ptr as *mut T).set_ptr_value( (ptr as *mut u8).wrapping_offset(data_offset) )
    } else {
        ptr::null()
    }
}

fn from_raw(*const T) -> Self {
    if [ T is Sized ] || !ptr.is_null() {
        let ptr = (ptr as *mut RcBox).set_ptr_value( (ptr as *mut u8).wrapping_offset(-data_offset) );
        Weak { ptr }
    } else {
        Weak::new()
    }
}
```

(but with more `set_ptr_value` to avoid `Sized` restrictions and maintain metadata.)

Written in this fashion, this is not a correctness-critical specialization (i.e. so long as `[ T is Sized ]` is false for unsized `T`, it can be `rand()` for sized `T` without breaking correctness), but it's still touchy, so I'd rather do it in another PR with separate review.

---
</details>

This effectively reverts rust-lang#80422 and re-establishes rust-lang#74160. T-libs [previously signed off](rust-lang#74160 (comment)) on this stable API change in rust-lang#74160.

library/alloc/src/lib.rs

@@ -120,6 +120,7 @@
 #![feature(receiver_trait)]
 #![cfg_attr(bootstrap, feature(min_const_generics))]
 #![feature(min_specialization)]
+#![feature(set_ptr_value)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]
 #![feature(staged_api)]

library/alloc/src/rc.rs

@@ -829,8 +829,8 @@ impl<T: ?Sized> Rc<T> {
  let offset = unsafe { data_offset(ptr) };
 
  // Reverse the offset to find the original RcBox.
- let fake_ptr = ptr as *mut RcBox<T>;
- let rc_ptr = unsafe { set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset)) };
+ let rc_ptr =
+  unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) };
 
  unsafe { Self::from_ptr(rc_ptr) }
  }
@@ -850,7 +850,7 @@ impl<T: ?Sized> Rc<T> {
  pub fn downgrade(this: &Self) -> Weak<T> {
  this.inner().inc_weak();
  // Make sure we do not create a dangling Weak
- debug_assert!(!is_dangling(this.ptr));
+ debug_assert!(!is_dangling(this.ptr.as_ptr()));
  Weak { ptr: this.ptr }
  }
 
@@ -1164,7 +1164,7 @@ impl<T: ?Sized> Rc<T> {
  Self::allocate_for_layout(
  Layout::for_value(&*ptr),
  |layout| Global.allocate(layout),
- |mem| set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>,
+ |mem| (ptr as *mut RcBox<T>).set_ptr_value(mem),
  )
  }
  }
@@ -1203,20 +1203,7 @@ impl<T> Rc<[T]> {
  )
  }
  }
-}
-
-/// Sets the data pointer of a `?Sized` raw pointer.
-///
-/// For a slice/trait object, this sets the `data` field and leaves the rest
-/// unchanged. For a sized raw pointer, this simply sets the pointer.
-unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
- unsafe {
- ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
- }
- ptr
-}
 
-impl<T> Rc<[T]> {
  /// Copy elements from slice into newly allocated Rc<\[T\]>
  ///
  /// Unsafe because the caller must either take ownership or bind `T: Copy`
@@ -1860,8 +1847,8 @@ impl<T> Weak<T> {
  }
 }
 
-pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
- let address = ptr.as_ptr() as *mut () as usize;
+pub(crate) fn is_dangling<T: ?Sized>(ptr: *mut T) -> bool {
+ let address = ptr as *mut () as usize;
  address == usize::MAX
 }
 
@@ -1872,7 +1859,7 @@ struct WeakInner<'a> {
  strong: &'a Cell<usize>,
 }
 
-impl<T> Weak<T> {
+impl<T: ?Sized> 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,
@@ -1902,15 +1889,15 @@ impl<T> Weak<T> {
  pub fn as_ptr(&self) -> *const T {
  let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
 
- // SAFETY: we must offset the pointer manually, and said pointer may be
- // a dangling weak (usize::MAX) if T is sized. data_offset is safe to call,
- // because we know that a pointer to unsized T was derived from a real
- // unsized T, as dangling weaks are only created for sized T. wrapping_offset
- // is used so that we can use the same code path for the non-dangling
- // unsized case and the potentially dangling sized case.
- unsafe {
- let offset = data_offset(ptr as *mut T);
- set_data_ptr(ptr as *mut T, (ptr as *mut u8).wrapping_offset(offset))
+ if is_dangling(ptr) {
+  // If the pointer is dangling, we return the sentinel directly. This cannot be
+  // a valid payload address, as the payload is at least as aligned as RcBox (usize).
+  ptr as *const T
+ } else {
+  // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
+  // The payload may be dropped at this point, and we have to maintain provenance,
+ // so use raw pointer manipulation.
+ unsafe { &raw const (*ptr).value }
  }
  }
 
@@ -1992,22 +1979,24 @@ 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 {
- // 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 (but only if T: Sized).
- let ptr = unsafe {
- set_data_ptr(ptr as *mut RcBox<T>, (ptr as *mut u8).wrapping_offset(-offset))
+ let ptr = if is_dangling(ptr as *mut T) {
+ // This is a dangling Weak.
+ ptr as *mut RcBox<T>
+ } else {
+ // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
+ // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
+ let offset = unsafe { data_offset(ptr) };
+ // Thus, we reverse the offset to get the whole RcBox.
+ // SAFETY: the pointer originated from a Weak, so this offset is safe.
+ unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
  };
 
  // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
  Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
  }
-}
 
-impl<T: ?Sized> Weak<T> {
  /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
  /// dropping of the inner value if successful.
  ///
@@ -2070,7 +2059,7 @@ impl<T: ?Sized> Weak<T> {
  /// (i.e., when this `Weak` was created by `Weak::new`).
  #[inline]
  fn inner(&self) -> Option<WeakInner<'_>> {
- if is_dangling(self.ptr) {
+ if is_dangling(self.ptr.as_ptr()) {
  None
  } else {
  // We are careful to *not* create a reference covering the "data" field, as
@@ -2325,21 +2314,19 @@ 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 `RcBox` for
-/// a payload of type described by a pointer.
+/// Get the offset within an `RcBox` for the payload behind 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
-/// acquired from the real instance that you are getting this offset for.
+/// The pointer must point to (and have valid metadata for) a previously
+/// valid instance of T, but the T is allowed to be dropped.
 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.
- // Note: This is a detail of the current implementation of the compiler,
- // and is not a guaranteed language detail. Do not rely on it outside of std.
+ // Align the unsized value to the end of the RcBox.
+ // Because RcBox is repr(C), it will always be the last field in memory.
+ // SAFETY: since the only unsized types possible are slices, trait objects,
+ // and extern types, the input safety requirement is currently enough to
+ // satisfy the requirements of align_of_val_raw; this is an implementation
+ // detail of the language that may not be relied upon outside of std.
  unsafe { data_offset_align(align_of_val_raw(ptr)) }
 }
 

library/alloc/src/rc/tests.rs

@@ -208,6 +208,30 @@ fn into_from_weak_raw() {
  }
 }
 
+#[test]
+fn test_into_from_weak_raw_unsized() {
+ use std::fmt::Display;
+ use std::string::ToString;
+
+ let arc: Rc<str> = Rc::from("foo");
+ let weak: Weak<str> = Rc::downgrade(&arc);
+
+ let ptr = Weak::into_raw(weak.clone());
+ let weak2 = unsafe { Weak::from_raw(ptr) };
+
+ assert_eq!(unsafe { &*ptr }, "foo");
+ assert!(weak.ptr_eq(&weak2));
+
+ let arc: Rc<dyn Display> = Rc::new(123);
+ let weak: Weak<dyn Display> = Rc::downgrade(&arc);
+
+ let ptr = Weak::into_raw(weak.clone());
+ let weak2 = unsafe { Weak::from_raw(ptr) };
+
+ assert_eq!(unsafe { &*ptr }.to_string(), "123");
+ assert!(weak.ptr_eq(&weak2));
+}
+
 #[test]
 fn get_mut() {
  let mut x = Rc::new(3);
@@ -294,6 +318,23 @@ fn test_unsized() {
  assert_eq!(foo, foo.clone());
 }
 
+#[test]
+fn test_maybe_thin_unsized() {
+ // If/when custom thin DSTs exist, this test should be updated to use one
+ use std::ffi::{CStr, CString};
+
+ let x: Rc<CStr> = Rc::from(CString::new("swordfish").unwrap().into_boxed_c_str());
+ assert_eq!(format!("{:?}", x), "\"swordfish\"");
+ let y: Weak<CStr> = Rc::downgrade(&x);
+ drop(x);
+
+ // At this point, the weak points to a dropped DST
+ assert!(y.upgrade().is_none());
+ // But we still need to be able to get the alloc layout to drop.
+ // CStr has no drop glue, but custom DSTs might, and need to work.
+ drop(y);
+}
+
 #[test]
 fn test_from_owned() {
  let foo = 123;

library/alloc/src/sync.rs

@@ -846,8 +846,7 @@ impl<T: ?Sized> Arc<T> {
  let offset = data_offset(ptr);
 
  // Reverse the offset to find the original ArcInner.
- let fake_ptr = ptr as *mut ArcInner<T>;
- let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
+ let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
 
  Self::from_ptr(arc_ptr)
  }
@@ -888,7 +887,7 @@ impl<T: ?Sized> Arc<T> {
  match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
  Ok(_) => {
  // Make sure we do not create a dangling Weak
- debug_assert!(!is_dangling(this.ptr));
+ debug_assert!(!is_dangling(this.ptr.as_ptr()));
  return Weak { ptr: this.ptr };
  }
  Err(old) => cur = old,
@@ -1131,7 +1130,7 @@ impl<T: ?Sized> Arc<T> {
  Self::allocate_for_layout(
  Layout::for_value(&*ptr),
  |layout| Global.allocate(layout),
- |mem| set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>,
+ |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
  )
  }
  }
@@ -1170,20 +1169,7 @@ impl<T> Arc<[T]> {
  )
  }
  }
-}
-
-/// Sets the data pointer of a `?Sized` raw pointer.
-///
-/// For a slice/trait object, this sets the `data` field and leaves the rest
-/// unchanged. For a sized raw pointer, this simply sets the pointer.
-unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
- unsafe {
- ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
- }
- ptr
-}
 
-impl<T> Arc<[T]> {
  /// Copy elements from slice into newly allocated Arc<\[T\]>
  ///
  /// Unsafe because the caller must either take ownership or bind `T: Copy`.
@@ -1653,7 +1639,7 @@ struct WeakInner<'a> {
  strong: &'a atomic::AtomicUsize,
 }
 
-impl<T> Weak<T> {
+impl<T: ?Sized> 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,
@@ -1683,15 +1669,15 @@ impl<T> Weak<T> {
  pub fn as_ptr(&self) -> *const T {
  let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
 
- // SAFETY: we must offset the pointer manually, and said pointer may be
- // a dangling weak (usize::MAX) if T is sized. data_offset is safe to call,
- // because we know that a pointer to unsized T was derived from a real
- // unsized T, as dangling weaks are only created for sized T. wrapping_offset
- // is used so that we can use the same code path for the non-dangling
- // unsized case and the potentially dangling sized case.
- unsafe {
- let offset = data_offset(ptr as *mut T);
- set_data_ptr(ptr as *mut T, (ptr as *mut u8).wrapping_offset(offset))
+ if is_dangling(ptr) {
+  // If the pointer is dangling, we return the sentinel directly. This cannot be
+  // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
+  ptr as *const T
+ } else {
+  // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
+  // The payload may be dropped at this point, and we have to maintain provenance,
+ // so use raw pointer manipulation.
+ unsafe { &raw mut (*ptr).data }
  }
  }
 
@@ -1773,18 +1759,22 @@ impl<T> Weak<T> {
  /// [`forget`]: std::mem::forget
  #[stable(feature = "weak_into_raw", since = "1.45.0")]
  pub unsafe fn from_raw(ptr: *const T) -> Self {
- // 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 ArcInner.
- // SAFETY: we use wrapping_offset here because the pointer may be dangling (but only if T: Sized)
- let ptr = unsafe {
- set_data_ptr(ptr as *mut ArcInner<T>, (ptr as *mut u8).wrapping_offset(-offset))
+ let ptr = if is_dangling(ptr as *mut T) {
+ // This is a dangling Weak.
+ ptr as *mut ArcInner<T>
+ } else {
+ // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
+ // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
+ let offset = unsafe { data_offset(ptr) };
+ // Thus, we reverse the offset to get the whole RcBox.
+ // SAFETY: the pointer originated from a Weak, so this offset is safe.
+ unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
  };
 
  // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
- unsafe { Weak { ptr: NonNull::new_unchecked(ptr) } }
+ Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
  }
 }
 
@@ -1889,7 +1879,7 @@ impl<T: ?Sized> Weak<T> {
  /// (i.e., when this `Weak` was created by `Weak::new`).
  #[inline]
  fn inner(&self) -> Option<WeakInner<'_>> {
- if is_dangling(self.ptr) {
+ if is_dangling(self.ptr.as_ptr()) {
  None
  } else {
  // We are careful to *not* create a reference covering the "data" field, as
@@ -2469,21 +2459,19 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
 #[stable(feature = "pin", since = "1.33.0")]
 impl<T: ?Sized> Unpin for Arc<T> {}
 
-/// Get the offset within an `ArcInner` for
-/// a payload of type described by a pointer.
+/// Get the offset within an `ArcInner` for the payload behind 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
-/// acquired from the real instance that you are getting this offset for.
+/// The pointer must point to (and have valid metadata for) a previously
+/// valid instance of T, but the T is allowed to be dropped.
 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.
- // Note: This is a detail of the current implementation of the compiler,
- // and is not a guaranteed language detail. Do not rely on it outside of std.
+ // Align the unsized value to the end of the ArcInner.
+ // Because RcBox is repr(C), it will always be the last field in memory.
+ // SAFETY: since the only unsized types possible are slices, trait objects,
+ // and extern types, the input safety requirement is currently enough to
+ // satisfy the requirements of align_of_val_raw; this is an implementation
+ // detail of the language that may not be relied upon outside of std.
  unsafe { data_offset_align(align_of_val_raw(ptr)) }
 }