Comment by GregBuchholz

I tried:

  #lang racket/base
  (let ((a '#0=(0 . #0#)))
    (display (car a)))

...over at

https://onecompiler.com/racket

...and it didn't compile, complaining:

  read-syntax: `#...=` forms not  enabled for `read-syntax` mode

...maybe there is a switch needed to enable it?