clarify alignment requirements in Vec::from_raw_parts

[rust.git] / src / liballoc / vec.rs

diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index 4f6b7870e2e8ceca10d51509c2803b9592c97534..fc50d06b33c579c062e5b44b3e48a608bb65cabf 100644 (file)
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -404,7 +404,10 @@ pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
     ///
     /// * `ptr` needs to have been previously allocated via [`String`]/`Vec<T>`
     ///   (at least, it's highly likely to be incorrect if it wasn't).
-    /// * `ptr`'s `T` needs to have the same size and alignment as it was allocated with.
+    /// * `T` needs to have the same size and alignment as what `ptr` was allocated with.
+    ///   (`T` having a less strict alignment is not sufficient, the alignment really
+    ///   needs to be equal to satsify the [`dealloc`] requirement that memory must be
+    ///   allocated and deallocated with the same layout.)
     /// * `length` needs to be less than or equal to `capacity`.
     /// * `capacity` needs to be the capacity that the pointer was allocated with.
     ///
@@ -423,6 +426,7 @@ pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
     /// function.
     ///
     /// [`String`]: ../../std/string/struct.String.html
+    /// [`dealloc`]: ../../alloc/alloc/trait.GlobalAlloc.html#tymethod.dealloc
     ///
     /// # Examples
     ///
@@ -2622,7 +2626,9 @@ fn clone(&self) -> IntoIter<T> {
 unsafe impl<#[may_dangle] T> Drop for IntoIter<T> {
     fn drop(&mut self) {
         // destroy the remaining elements
-        for _x in self.by_ref() {}
+        unsafe {
+            ptr::drop_in_place(self.as_mut_slice());
+        }
 
         // RawVec handles deallocation
         let _ = unsafe { RawVec::from_raw_parts(self.buf.as_ptr(), self.cap) };
@@ -2702,23 +2708,42 @@ fn next_back(&mut self) -> Option<T> {
 #[stable(feature = "drain", since = "1.6.0")]
 impl<T> Drop for Drain<'_, T> {
     fn drop(&mut self) {
-        // exhaust self first
-        self.for_each(drop);
+        /// Continues dropping the remaining elements in the `Drain`, then moves back the
+        /// un-`Drain`ed elements to restore the original `Vec`.
+        struct DropGuard<'r, 'a, T>(&'r mut Drain<'a, T>);
 
-        if self.tail_len > 0 {
-            unsafe {
-                let source_vec = self.vec.as_mut();
-                // memmove back untouched tail, update to new length
-                let start = source_vec.len();
-                let tail = self.tail_start;
-                if tail != start {
-                    let src = source_vec.as_ptr().add(tail);
-                    let dst = source_vec.as_mut_ptr().add(start);
-                    ptr::copy(src, dst, self.tail_len);
+        impl<'r, 'a, T> Drop for DropGuard<'r, 'a, T> {
+            fn drop(&mut self) {
+                // Continue the same loop we have below. If the loop already finished, this does
+                // nothing.
+                self.0.for_each(drop);
+
+                if self.0.tail_len > 0 {
+                    unsafe {
+                        let source_vec = self.0.vec.as_mut();
+                        // memmove back untouched tail, update to new length
+                        let start = source_vec.len();
+                        let tail = self.0.tail_start;
+                        if tail != start {
+                            let src = source_vec.as_ptr().add(tail);
+                            let dst = source_vec.as_mut_ptr().add(start);
+                            ptr::copy(src, dst, self.0.tail_len);
+                        }
+                        source_vec.set_len(start + self.0.tail_len);
+                    }
                 }
-                source_vec.set_len(start + self.tail_len);
             }
         }
+
+        // exhaust self first
+        while let Some(item) = self.next() {
+            let guard = DropGuard(self);
+            drop(item);
+            mem::forget(guard);
+        }
+
+        // Drop a `DropGuard` to move back the non-drained tail of `self`.
+        DropGuard(self);
     }
 }