tests/ui/macros/issue-39388.rs - toolchain/rustc - Git at Google

 #![allow(unused_macros)]

 macro_rules! assign {
     (($($a:tt)*) = ($($b:tt))*) => { //~ ERROR expected one of: `*`, `+`, or `?`
         $($a)* = $($b)*
     }
 }

 fn main() {}
	#![allow(unused_macros)]

	macro_rules! assign {
	(($($a:tt)) = ($($b:tt))) => { //~ ERROR expected one of: `*`, `+`, or `?`
	$($a)* = $($b)*
	}
	}

	fn main() {}