Rollup merge of #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 #80422 and re-establishes #74160. T-libs [previously signed off](https://github.com/rust-lang/rust/pull/74160#issuecomment-660539373) on this stable API change in #74160.

5 files changed, 153 insertions, 95 deletions

diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index cfad111aa54..d0bfa038aa1 100644
--- a/library/alloc/src/lib.rs
+++ b/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)]
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 0973a6e362b..0e3bab6b20a 100644
--- a/library/alloc/src/rc.rs
+++ b/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)) }
 }
 
diff --git a/library/alloc/src/rc/tests.rs b/library/alloc/src/rc/tests.rs
index 2d183a8c88c..843a9b07fa9 100644
--- a/library/alloc/src/rc/tests.rs
+++ b/library/alloc/src/rc/tests.rs
@@ -209,6 +209,30 @@ 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);
     *Rc::get_mut(&mut x).unwrap() = 4;
@@ -295,6 +319,23 @@ fn test_unsized() {
 }
 
 #[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;
     let foo_rc = Rc::from(foo);
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 05bfeccbda1..c0d684fbb45 100644
--- a/library/alloc/src/sync.rs
+++ b/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)) }
 }
 
diff --git a/library/alloc/src/sync/tests.rs b/library/alloc/src/sync/tests.rs
index 5067af1d4ff..4ccb32fbbf6 100644
--- a/library/alloc/src/sync/tests.rs
+++ b/library/alloc/src/sync/tests.rs
@@ -159,6 +159,30 @@ fn into_from_weak_raw() {
 }
 
 #[test]
+fn test_into_from_weak_raw_unsized() {
+    use std::fmt::Display;
+    use std::string::ToString;
+
+    let arc: Arc<str> = Arc::from("foo");
+    let weak: Weak<str> = Arc::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: Arc<dyn Display> = Arc::new(123);
+    let weak: Weak<dyn Display> = Arc::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 test_cowarc_clone_make_mut() {
     let mut cow0 = Arc::new(75);
     let mut cow1 = cow0.clone();
@@ -330,6 +354,23 @@ fn test_unsized() {
 }
 
 #[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: Arc<CStr> = Arc::from(CString::new("swordfish").unwrap().into_boxed_c_str());
+    assert_eq!(format!("{:?}", x), "\"swordfish\"");
+    let y: Weak<CStr> = Arc::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;
     let foo_arc = Arc::from(foo);