Months ago I was trying to debug why this[0] solver didn't quite work. If someone wants to spend some time debugging it, you're welcome to it.

It's a Rust compile time Sudoku solver. It solves the almost complete puzzle, not the "real" puzzle.

[0]: https://play.rust-lang.org/?version=nightly&mode=debug&editi...

  while grid[index] == 9 || grid[index] != 0 {

is equivalent to

  while grid[index] != 0 {

so when you backtrack, you always backtrack all the way to the beginning.

I think you want

  while grid[index] == 9 || initial_grid[index] != 0 {
    grid[index] = initial_grid[index];

or something to that effect.

I had a hearty chuckle when he refers to "business logic" in the YouTube video linked in the post.

That code is more easily readable than most visual basic business logic running billion dollars in trades I saw in the 00s.