[jrimbault]

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...

[yorwba]

  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.