GNU core utils is 134 lines of code, not 50, so the Rust version is even slightly shorter. You can make yes a lot shorter in both C and Rust, but this size goes into speed. For reference, OpenBSD's yes is just 17 lines of code[2]. It essentially boils down to this:

  int main(int argc, char *argv[])
  {
    if (pledge("stdio", NULL) == -1)
      err(1, "pledge");
    if (argc > 1)
      for (;;)
        puts(argv[1]);
    else
      for (;;)
        puts("y");
  }

This is as simple as it gets, but the joke yes-rs implementation is right about one thing: "blazing fast" speed often comes at the cost of greatly increased complexity. The BSD implementation of yes is almost 10 times shorter than the GNU implementation, but the GNU implementation is 100 times faster[3].

[1] https://github.com/coreutils/coreutils/blob/master/src/yes.c

[2] https://github.com/openbsd/src/blob/master/usr.bin/yes/yes.c

[3] https://www.reddit.com/r/unix/comments/6gxduc/how_is_gnu_yes...