don't apply formatting to builtin type ascription syntax

The syntax changed from `expr: ty` -> `builtin # type_ascribe(expr, ty)`
For now, rustfmt will just emit the contents of the span.

CHANGELOG.md

@@ -8,6 +8,13 @@
 [bytecount#92]: https://github.com/llogiq/bytecount/pull/92
 [bytecount#93]: https://github.com/llogiq/bytecount/pull/93
 
+- Output correct syntax for type ascription builtin [#6159](https://github.com/rust-lang/rustfmt/issues/6159)
+  ```rust
+  fn main() {
+      builtin # type_ascribe(10, usize)
+  }
+  ```
+
 ## [1.7.0] 2023-10-22
 
 ### Fixed

src/expr.rs

@@ -263,14 +263,6 @@ pub(crate) fn format_expr(
             shape,
             SeparatorPlace::Front,
         ),
-        ast::ExprKind::Type(ref expr, ref ty) => rewrite_pair(
-            &**expr,
-            &**ty,
-            PairParts::infix(": "),
-            context,
-            shape,
-            SeparatorPlace::Back,
-        ),
         ast::ExprKind::Index(ref expr, ref index, _) => {
             rewrite_index(&**expr, &**index, context, shape)
         }
@@ -412,6 +404,7 @@ fn needs_space_after_range(rhs: &ast::Expr) -> bool {
         }
         ast::ExprKind::Underscore => Some("_".to_owned()),
         ast::ExprKind::FormatArgs(..)
+        | ast::ExprKind::Type(..)
         | ast::ExprKind::IncludedBytes(..)
         | ast::ExprKind::OffsetOf(..) => {
             // These don't normally occur in the AST because macros aren't expanded. However,

tests/target/issue_6159.rs

@@ -0,0 +1,3 @@
+fn main() {
+    builtin # type_ascribe(10, usize)
+}