Auto merge of #104571 - clubby789:remove-vec-rc-opt, r=the8472

Revert Vec/Rc storage reuse opt

Remove the optimization for using storage added by #104205.
The perf wins were pretty small, and it relies on non-guarenteed behaviour. On platforms that don't implement shrinking in place, the performance will be significantly worse.

While it could be gated to platforms that do this (such as GNU), I don't think it's worth the overhead of maintaining it for very small gains. (#104565, #104563)

cc `@RalfJung` `@matthiaskrgr`

Fixes #104565
Fixes #104563

library/alloc/src/rc.rs

@@ -1441,48 +1441,6 @@ impl<T> Rc<[T]> {
         }
     }
 
-    /// Create an `Rc<[T]>` by reusing the underlying memory
-    /// of a `Vec<T>`. This will return the vector if the existing allocation
-    /// is not large enough.
-    #[cfg(not(no_global_oom_handling))]
-    fn try_from_vec_in_place(mut v: Vec<T>) -> Result<Rc<[T]>, Vec<T>> {
-        let layout_elements = Layout::array::<T>(v.len()).unwrap();
-        let layout_allocation = Layout::array::<T>(v.capacity()).unwrap();
-        let layout_rcbox = rcbox_layout_for_value_layout(layout_elements);
-        let mut ptr = NonNull::new(v.as_mut_ptr()).expect("`Vec<T>` stores `NonNull<T>`");
-        if layout_rcbox.size() > layout_allocation.size()
-            || layout_rcbox.align() > layout_allocation.align()
-        {
-            // Can't fit - calling `grow` would involve `realloc`
-            // (which copies the elements), followed by copying again.
-            return Err(v);
-        }
-        if layout_rcbox.size() < layout_allocation.size()
-            || layout_rcbox.align() < layout_allocation.align()
-        {
-            // We need to shrink the allocation so that it fits
-            // https://doc.rust-lang.org/nightly/std/alloc/trait.Allocator.html#memory-fitting
-            // SAFETY:
-            // - Vec allocates by requesting `Layout::array::<T>(capacity)`, so this capacity matches
-            // - `layout_rcbox` is smaller
-            // If this fails, the ownership has not been transferred
-            if let Ok(p) = unsafe { Global.shrink(ptr.cast(), layout_allocation, layout_rcbox) } {
-                ptr = p.cast();
-            } else {
-                return Err(v);
-            }
-        }
-        // Make sure the vec's memory isn't deallocated now
-        let v = mem::ManuallyDrop::new(v);
-        let ptr: *mut RcBox<[T]> = ptr::slice_from_raw_parts_mut(ptr.as_ptr(), v.len()) as _;
-        unsafe {
-            ptr::copy(ptr.cast::<T>(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
-            ptr::write(&mut (*ptr).strong, Cell::new(1));
-            ptr::write(&mut (*ptr).weak, Cell::new(1));
-            Ok(Self::from_ptr(ptr))
-        }
-    }
-
     /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
     ///
     /// Behavior is undefined should the size be wrong.
@@ -2008,17 +1966,12 @@ impl<T> From<Vec<T>> for Rc<[T]> {
     /// assert_eq!(vec![1, 2, 3], *shared);
     /// ```
     #[inline]
-    fn from(v: Vec<T>) -> Rc<[T]> {
-        match Rc::try_from_vec_in_place(v) {
-            Ok(rc) => rc,
-            Err(mut v) => {
-                unsafe {
-                    let rc = Rc::copy_from_slice(&v);
-                    // Allow the Vec to free its memory, but not destroy its contents
-                    v.set_len(0);
-                    rc
-                }
-            }
+    fn from(mut v: Vec<T>) -> Rc<[T]> {
+        unsafe {
+            let rc = Rc::copy_from_slice(&v);
+            // Allow the Vec to free its memory, but not destroy its contents
+            v.set_len(0);
+            rc
         }
     }
 }

library/alloc/src/sync.rs

@@ -1261,49 +1261,6 @@ impl<T> Arc<[T]> {
         }
     }
 
-    /// Create an `Arc<[T]>` by reusing the underlying memory
-    /// of a `Vec<T>`. This will return the vector if the existing allocation
-    /// is not large enough.
-    #[cfg(not(no_global_oom_handling))]
-    fn try_from_vec_in_place(mut v: Vec<T>) -> Result<Arc<[T]>, Vec<T>> {
-        let layout_elements = Layout::array::<T>(v.len()).unwrap();
-        let layout_allocation = Layout::array::<T>(v.capacity()).unwrap();
-        let layout_arcinner = arcinner_layout_for_value_layout(layout_elements);
-        let mut ptr = NonNull::new(v.as_mut_ptr()).expect("`Vec<T>` stores `NonNull<T>`");
-        if layout_arcinner.size() > layout_allocation.size()
-            || layout_arcinner.align() > layout_allocation.align()
-        {
-            // Can't fit - calling `grow` would involve `realloc`
-            // (which copies the elements), followed by copying again.
-            return Err(v);
-        }
-        if layout_arcinner.size() < layout_allocation.size()
-            || layout_arcinner.align() < layout_allocation.align()
-        {
-            // We need to shrink the allocation so that it fits
-            // https://doc.rust-lang.org/nightly/std/alloc/trait.Allocator.html#memory-fitting
-            // SAFETY:
-            // - Vec allocates by requesting `Layout::array::<T>(capacity)`, so this capacity matches
-            // - `layout_arcinner` is smaller
-            // If this fails, the ownership has not been transferred
-            if let Ok(p) = unsafe { Global.shrink(ptr.cast(), layout_allocation, layout_arcinner) }
-            {
-                ptr = p.cast();
-            } else {
-                return Err(v);
-            }
-        }
-        // Make sure the vec's memory isn't deallocated now
-        let v = mem::ManuallyDrop::new(v);
-        let ptr: *mut ArcInner<[T]> = ptr::slice_from_raw_parts_mut(ptr.as_ptr(), v.len()) as _;
-        unsafe {
-            ptr::copy(ptr.cast::<T>(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
-            ptr::write(&mut (*ptr).strong, atomic::AtomicUsize::new(1));
-            ptr::write(&mut (*ptr).weak, atomic::AtomicUsize::new(1));
-            Ok(Self::from_ptr(ptr))
-        }
-    }
-
     /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
     ///
     /// Behavior is undefined should the size be wrong.
@@ -2615,17 +2572,12 @@ impl<T> From<Vec<T>> for Arc<[T]> {
     /// assert_eq!(&[1, 2, 3], &shared[..]);
     /// ```
     #[inline]
-    fn from(v: Vec<T>) -> Arc<[T]> {
-        match Arc::try_from_vec_in_place(v) {
-            Ok(rc) => rc,
-            Err(mut v) => {
-                unsafe {
-                    let rc = Arc::copy_from_slice(&v);
-                    // Allow the Vec to free its memory, but not destroy its contents
-                    v.set_len(0);
-                    rc
-                }
-            }
+    fn from(mut v: Vec<T>) -> Arc<[T]> {
+        unsafe {
+            let rc = Arc::copy_from_slice(&v);
+            // Allow the Vec to free its memory, but not destroy its contents
+            v.set_len(0);
+            rc
         }
     }
 }

library/alloc/tests/arc.rs

@@ -210,18 +210,3 @@ fn weak_may_dangle() {
     // `val` dropped here while still borrowed
     // borrow might be used here, when `val` is dropped and runs the `Drop` code for type `std::sync::Weak`
 }
-
-#[test]
-fn arc_from_vec_opt() {
-    let mut v = Vec::with_capacity(64);
-    v.push(0usize);
-    let addr = v.as_ptr().cast::<u8>();
-    let arc: Arc<[_]> = v.into();
-    unsafe {
-        assert_eq!(
-            arc.as_ptr().cast::<u8>().offset_from(addr),
-            (std::mem::size_of::<usize>() * 2) as isize,
-            "Vector allocation not reused"
-        );
-    }
-}

library/alloc/tests/rc.rs

@@ -206,18 +206,3 @@ fn weak_may_dangle() {
     // `val` dropped here while still borrowed
     // borrow might be used here, when `val` is dropped and runs the `Drop` code for type `std::rc::Weak`
 }
-
-#[test]
-fn rc_from_vec_opt() {
-    let mut v = Vec::with_capacity(64);
-    v.push(0usize);
-    let addr = v.as_ptr().cast::<u8>();
-    let rc: Rc<[_]> = v.into();
-    unsafe {
-        assert_eq!(
-            rc.as_ptr().cast::<u8>().offset_from(addr),
-            (std::mem::size_of::<usize>() * 2) as isize,
-            "Vector allocation not reused"
-        );
-    }
-}