add #[must_use] to Box::from_raw

Author: rhysd

library/alloc/src/boxed.rs

@@ -949,6 +949,7 @@ impl<T: ?Sized> Box<T> {
     /// [`Layout`]: crate::Layout
     #[stable(feature = "box_raw", since = "1.4.0")]
     #[inline]
+    #[must_use = "call `drop(from_raw(ptr))` if you intend to drop the `Box`"]
     pub unsafe fn from_raw(raw: *mut T) -> Self {
         unsafe { Self::from_raw_in(raw, Global) }
     }

src/test/ui/lint/unused/must-use-box-from-raw.rs

@@ -0,0 +1,11 @@
+// #99269
+
+// check-pass
+
+#![warn(unused_must_use)]
+
+unsafe fn free<T>(ptr: *mut T) {
+    Box::from_raw(ptr); //~ WARNING unused return value
+}
+
+fn main() {}

src/test/ui/lint/unused/must-use-box-from-raw.stderr

@@ -0,0 +1,15 @@
+warning: unused return value of `Box::<T>::from_raw` that must be used
+  --> $DIR/must-use-box-from-raw.rs:8:5
+   |
+LL |     Box::from_raw(ptr);
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/must-use-box-from-raw.rs:5:9
+   |
+LL | #![warn(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+   = note: call `drop(from_raw(ptr))` if you intend to drop the `Box`
+
+warning: 1 warning emitted
+