Rollup merge of #97397 - JohnTitor:stabilize-box-into-pin, r=Mark-Simulacrum

Stabilize `box_into_pin`

FCP has been completed: https://github.com/rust-lang/rust/issues/62370#issuecomment-1012162279
Also, adds notes as per https://github.com/rust-lang/rust/issues/62370#issuecomment-1004108116
Closes #62370

library/alloc/src/boxed.rs

@@ -1175,14 +1175,33 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// This conversion does not allocate on the heap and happens in place.
     ///
     /// This is also available via [`From`].
-    #[unstable(feature = "box_into_pin", issue = "62370")]
+    ///
+    /// # Notes
+    ///
+    /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
+    /// as it'll introduce an ambiguity when calling `Pin::from`.
+    /// A demonstration of such a poor impl is shown below.
+    ///
+    /// ```compile_fail
+    /// # use std::pin::Pin;
+    /// struct Foo; // A type defined in this crate.
+    /// impl From<Box<()>> for Pin<Foo> {
+    ///     fn from(_: Box<()>) -> Pin<Foo> {
+    ///         Pin::new(Foo)
+    ///     }
+    /// }
+    ///
+    /// let foo = Box::new(());
+    /// let bar = Pin::from(foo);
+    /// ```
+    #[stable(feature = "box_into_pin", since = "1.63.0")]
     #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     pub const fn into_pin(boxed: Self) -> Pin<Self>
     where
         A: 'static,
     {
         // It's not possible to move or replace the insides of a `Pin<Box<T>>`
-        // when `T: !Unpin`,  so it's safe to pin it directly without any
+        // when `T: !Unpin`, so it's safe to pin it directly without any
         // additional requirements.
         unsafe { Pin::new_unchecked(boxed) }
     }