Rollup merge of #127622 - compiler-errors:builtin-internal, r=lqd

Mark `builtin_syntax` as internal

Tracking issue literally says:

> There will never be a general stabilization.

cc #110680 `@est31`

compiler/rustc_feature/src/unstable.rs

@@ -248,6 +248,8 @@ declare_features! (
     (unstable, auto_traits, "1.50.0", Some(13231)),
     /// Allows using `box` in patterns (RFC 469).
     (unstable, box_patterns, "1.0.0", Some(29641)),
+    /// Allows builtin # foo() syntax
+    (internal, builtin_syntax, "1.71.0", Some(110680)),
     /// Allows `#[doc(notable_trait)]`.
     /// Renamed from `doc_spotlight`.
     (unstable, doc_notable_trait, "1.52.0", Some(45040)),
@@ -361,8 +363,6 @@ declare_features! (
     (unstable, async_fn_track_caller, "1.73.0", Some(110011)),
     /// Allows `for await` loops.
     (unstable, async_for_loop, "1.77.0", Some(118898)),
-    /// Allows builtin # foo() syntax
-    (unstable, builtin_syntax, "1.71.0", Some(110680)),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930)),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.