It occurs to me that I can't ever write a sudoku solver.
Not because I don't know how to (I can both write code and solve sudoku, combining these skills seems obviously possible), but because it would actively make my life worse to have one: I've discovered that doing sudoku in bed is just the right level of mentally engaging to make me sleepy quickly (a few fixed algorithms that take low but nonzero effort, not exciting enough to stay awake for more), and I also know that I can't bear to do something manually when I spent time automating that specific thing.