error: expected a literal
  --> $DIR/bad-concat.rs:5:21
   |
LL |     let _ = concat!(x, y, z, "bar");
   |                     ^  ^  ^
   |
   = note: only literals (like `"foo"`, `-42` and `3.14`) can be passed to `concat!()`

error: aborting due to 1 previous error