error: `${concat(..)}` is not generating a valid identifier
  --> $DIR/concat-trace-errors.rs:17:24
   |
LL |         const _: () = ${concat("hi", $a, "bye")};
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^
...
LL | post_expansion!("!");
   | -------------------- in this macro invocation
   |
   = note: this error originates in the macro `post_expansion` (in Nightly builds, run with -Z macro-backtrace for more info)

error: `${concat(..)}` is not generating a valid identifier
  --> $DIR/concat-trace-errors.rs:26:24
   |
LL |         const _: () = ${concat($a, $b, $c, $d, $e)};
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
LL | post_expansion_many!(a, b, c, ".d", e);
   | -------------------------------------- in this macro invocation
   |
   = note: this error originates in the macro `post_expansion_many` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors