Add feature gate

tests/ui/feature-gates/feature-gate-builtin_syntax.rs

@@ -0,0 +1,7 @@
+struct Foo {
+    v: u8,
+    w: u8,
+}
+fn main() {
+    builtin # offset_of(Foo, v); //~ ERROR `builtin #` syntax is unstable
+}

tests/ui/feature-gates/feature-gate-builtin_syntax.stderr

@@ -0,0 +1,12 @@
+error[E0658]: `builtin #` syntax is unstable
+  --> $DIR/feature-gate-builtin_syntax.rs:6:15
+   |
+LL |     builtin # offset_of(Foo, v);
+   |               ^^^^^^^^^
+   |
+   = note: see issue #110680 <https://github.com/rust-lang/rust/issues/110680> for more information
+   = help: add `#![feature(builtin_syntax)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/offset-of/offset-of-builtin.rs

@@ -1,9 +1,9 @@
+#![feature(builtin_syntax)]
+
 // For the exposed macro we already test these errors in the other files,
 // but this test helps to make sure the builtin construct also errors.
 // This has the same examples as offset-of-arg-count.rs
 
-
-
 fn main() {
     builtin # offset_of(NotEnoughArguments); //~ ERROR expected one of
 }

tests/ui/parser/builtin-syntax.rs

@@ -1,3 +1,5 @@
+#![feature(builtin_syntax)]
+
 fn main() {
     builtin # foobar(); //~ ERROR unknown `builtin #` construct
 }

tests/ui/parser/builtin-syntax.stderr

@@ -1,11 +1,11 @@
 error: unknown `builtin #` construct `foobar`
-  --> $DIR/builtin-syntax.rs:2:5
+  --> $DIR/builtin-syntax.rs:4:5
    |
 LL |     builtin # foobar();
    |     ^^^^^^^^^^^^^^^^
 
 error: expected identifier after `builtin #`
-  --> $DIR/builtin-syntax.rs:6:15
+  --> $DIR/builtin-syntax.rs:8:15
    |
 LL |     builtin # {}();
    |               ^