error: invalid format string: expected `'}'` but string was terminated
  --> $DIR/issue-91556.rs:2:19
   |
LL |   let _ = format!(concat!("{0}𝖳𝖾𝗌𝗍{"), i);
   |                   ^^^^^^^^^^^^^^^^^^^ expected `'}'` in format string
   |
   = note: if you intended to print `{`, you can escape it using `{{`
   = note: this error originates in the macro `concat` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error