Add ignore to doc code

[rust.git] / src / doc / unstable-book / src / library-features / fnbox.md

diff --git a/src/doc/unstable-book/src/library-features/fnbox.md b/src/doc/unstable-book/src/library-features/fnbox.md
index a9b74d4f00470a16aaa18a5d149782ffec60df45..cb3386b7152112a5953d457b12c36c181f061618 100644 (file)
--- a/src/doc/unstable-book/src/library-features/fnbox.md
+++ b/src/doc/unstable-book/src/library-features/fnbox.md
@@ -1,7 +1,32 @@
 # `fnbox`
 
-The tracking issue for this feature is: [#28796]
+The tracking issue for this feature is [#28796]
 
 [#28796]: https://github.com/rust-lang/rust/issues/28796
 
 ------------------------
+
+This had been a temporary alternative to the following impls:
+
+```rust,ignore
+impl<A, F> FnOnce for Box<F> where F: FnOnce<A> + ?Sized {}
+impl<A, F> FnMut for Box<F> where F: FnMut<A> + ?Sized {}
+impl<A, F> Fn for Box<F> where F: Fn<A> + ?Sized {}
+```
+
+The impls are parallel to these (relatively old) impls:
+
+```rust,ignore
+impl<A, F> FnOnce for &mut F where F: FnMut<A> + ?Sized {}
+impl<A, F> FnMut for &mut F where F: FnMut<A> + ?Sized {}
+impl<A, F> Fn for &mut F where F: Fn<A> + ?Sized {}
+impl<A, F> FnOnce for &F where F: Fn<A> + ?Sized {}
+impl<A, F> FnMut for &F where F: Fn<A> + ?Sized {}
+impl<A, F> Fn for &F where F: Fn<A> + ?Sized {}
+```
+
+Before the introduction of [`unsized_locals`][unsized_locals], we had been unable to provide the former impls. That means, unlike `&dyn Fn()` or `&mut dyn FnMut()` we could not use `Box<dyn FnOnce()>` at that time.
+
+[unsized_locals]: language-features/unsized-locals.html
+
+`FnBox()` is an alternative approach to `Box<dyn FnBox()>` is delegated to `FnBox::call_box` which doesn't need unsized locals. As we now have `Box<dyn FnOnce()>` working, the `fnbox` feature is going to be removed.