Added chapter to Unstable Book.

Author: Alexander Regueiro

src/doc/unstable-book/src/language-features/type-alias-enum-variants.md

@@ -0,0 +1,36 @@
+# `type_alias_enum_variants`
+
+The tracking issue for this feature is: [#49683]
+
+[#49683]: https://github.com/rust-lang/rust/issues/49683
+
+------------------------
+
+The `type_alias_enum_variants` feature enables the use of variants on type
+aliases that refer to enums, as both a constructor and a pattern. That is,
+it allows for the syntax `EnumAlias::Variant`, which behaves exactly the same
+as `Enum::Variant` (assuming that `EnumAlias` is an alias for some enum type
+`Enum`).
+
+Note that since `Self` exists as a type alias, this feature also enables the
+use of the syntax `Self::Variant` within an impl block for an enum type.
+
+```rust
+#![feature(type_alias_enum_variants)]
+
+enum Foo {
+    Bar(i32),
+    Baz { i: i32 },
+}
+
+type Alias = Foo;
+
+fn main() {
+    let t = Alias::Bar(0);
+    let t = Alias::Baz { i: 0 };
+    match t {
+        Alias::Bar(_i) => {}
+        Alias::Baz { i: _i } => {}
+    }
+}
+```