Clarify/add must_use messages for more into_raw* functions of alloc types.

library/alloc/src/boxed.rs

@@ -1097,6 +1097,7 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// ```
     ///
     /// [memory layout]: self#memory-layout
+    #[must_use = "losing the pointer will leak memory"]
     #[stable(feature = "box_raw", since = "1.4.0")]
     #[inline]
     pub fn into_raw(b: Self) -> *mut T {
@@ -1150,6 +1151,7 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// ```
     ///
     /// [memory layout]: self#memory-layout
+    #[must_use = "losing the pointer will leak memory"]
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline]
     pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {

library/alloc/src/rc.rs

@@ -1372,6 +1372,7 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     /// let x = unsafe { Rc::from_raw_in(ptr, alloc) };
     /// assert_eq!(&*x, "hello");
     /// ```
+    #[must_use = "losing the pointer will leak memory"]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
         let this = mem::ManuallyDrop::new(this);
@@ -3100,6 +3101,7 @@ impl<T: ?Sized, A: Allocator> Weak<T, A> {
     ///
     /// [`from_raw_in`]: Weak::from_raw_in
     /// [`as_ptr`]: Weak::as_ptr
+    #[must_use = "losing the pointer will leak memory"]
     #[inline]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn into_raw_with_allocator(self) -> (*const T, A) {

library/alloc/src/string.rs

@@ -903,7 +903,7 @@ impl String {
     /// let rebuilt = unsafe { String::from_raw_parts(ptr, len, cap) };
     /// assert_eq!(rebuilt, "hello");
     /// ```
-    #[must_use = "`self` will be dropped if the result is not used"]
+    #[must_use = "losing the pointer will leak memory"]
     #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
     pub fn into_raw_parts(self) -> (*mut u8, usize, usize) {
         self.vec.into_raw_parts()

library/alloc/src/vec/mod.rs

@@ -879,6 +879,7 @@ impl<T, A: Allocator> Vec<T, A> {
     /// };
     /// assert_eq!(rebuilt, [4294967295, 0, 1]);
     /// ```
+    #[must_use = "losing the pointer will leak memory"]
     #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
     pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
         let mut me = ManuallyDrop::new(self);
@@ -922,6 +923,7 @@ impl<T, A: Allocator> Vec<T, A> {
     /// };
     /// assert_eq!(rebuilt, [4294967295, 0, 1]);
     /// ```
+    #[must_use = "losing the pointer will leak memory"]
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
     pub fn into_raw_parts_with_alloc(self) -> (*mut T, usize, usize, A) {