Auto merge of #53324 - alexreg:self_in_typedefs, r=eddyb

`Self` in type definitions (self_in_typedefs)

This implements the [`self_in_typedefs` feature](https://github.com/rust-lang/rfcs/blob/master/text/2300-self-in-typedefs.md) ([tracking issue 49303](https://github.com/rust-lang/rust/issues/49303)).

r? @eddyb

CC @Centril

1 files changed, 24 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/language-features/self-in-typedefs.md b/src/doc/unstable-book/src/language-features/self-in-typedefs.md
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+# `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
+}
+```