don't apply formatting to builtin type ascription syntax

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

src/expr.rs

@@ -253,14 +253,6 @@ 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)
         }
@@ -402,6 +394,7 @@ pub(crate) fn format_expr(
         }
         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)
+}