error: expected string literal
  --> tests/ui/concat-display.rs:8:17
   |
8  |         #[error(concat!("invalid ", $what))]
   |                 ^^^^^^
...
13 | error_type!(Error, "foo");
   | -------------------------- in this macro invocation
   |
   = note: this error originates in the macro `error_type` (in Nightly builds, run with -Z macro-backtrace for more info)