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Comment by jrimbault | Hacker News Reader

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.

  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.