Auto merge of #56225 - alexreg:type_alias_enum_variants, r=petrochenkov

Implement RFC 2338, "Type alias enum variants"

This PR implements [RFC 2338](https://github.com/rust-lang/rfcs/pull/2338), allowing one to write code like the following.

```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 } => {}
    }
}
```

Since `Self` can be considered a type alias in this context, it also enables using `Self::Variant` as both a constructor and pattern.

Fixes issues #56199 and #56611.

N.B., after discussing the syntax for type arguments on enum variants with @petrochenkov and @eddyb (there are also a few comments on the [tracking issue](https://github.com/rust-lang/rust/issues/49683)), the consensus seems to be treat the syntax as follows, which ought to be backwards-compatible.

```rust
Option::<u8>::None; // OK
Option::None::<u8>; // OK, but lint in near future (hard error next edition?)
Alias::<u8>::None; // OK
Alias::None::<u8>; // Error
```

I do not know if this will need an FCP, but let's start one if so.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index be49c55a54c..3f2122e24a6 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -471,11 +471,14 @@ declare_features! (
     // Allows `const _: TYPE = VALUE`.
     (active, underscore_const_names, "1.31.0", Some(54912), None),
 
-    // `reason = ` in lint attributes and `expect` lint attribute
+    // Adds `reason` and `expect` lint attributes.
     (active, lint_reasons, "1.31.0", Some(54503), None),
 
     // `extern crate self as foo;` puts local crate root into extern prelude under name `foo`.
     (active, extern_crate_self, "1.31.0", Some(56409), None),
+
+    // Allows paths to enum variants on type aliases.
+    (active, type_alias_enum_variants, "1.31.0", Some(49683), None),
 );
 
 declare_features! (