Added page for feature to unstable book.

Author: Alexander Regueiro

src/doc/unstable-book/src/language-features/self-in-typedefs.md

@@ -0,0 +1,24 @@
+# `self_in_typedefs`
+
+The tracking issue for this feature is: [#49303]
+
+[#49303]: https://github.com/rust-lang/rust/issues/49303
+
+------------------------
+
+The `self_in_typedefs` feature gate lets you use the special `Self` identifier
+in `struct`, `enum`, and `union` type definitions.
+
+A simple example is:
+
+```rust
+#![feature(self_in_typedefs)]
+
+enum List<T>
+where
+    Self: PartialOrd<Self> // can write `Self` instead of `List<T>`
+{
+    Nil,
+    Cons(T, Box<Self>) // likewise here
+}
+```

src/librustc_resolve/lib.rs

@@ -9,8 +9,8 @@
 // except according to those terms.
 
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
-      html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
-      html_root_url = "https://doc.rust-lang.org/nightly/")]
+       html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
+       html_root_url = "https://doc.rust-lang.org/nightly/")]
 
 #![feature(crate_visibility_modifier)]
 #![cfg_attr(not(stage0), feature(nll))]