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The {:?} formatter uses the Debug trait. dbg! prints the expression, source location, and value to stderr, then returns the value unchanged. Borrow with dbg!(&value) when you want to inspect an owned value without moving it. Remove noisy probes or replace them with deliberate logging before shipping.
A useful debug probe answers a specific question. Before running the program, predict the value at a boundary: after parsing, before a branch, or after a collection update. Put dbg! there, run the smallest input that exposes the issue, and compare the printed value with the prediction. If the value is already wrong, move the probe earlier; if it is right, move it later. This narrows the search instead of filling the terminal with unrelated output.
println! writes to standard output, which users or another program may be reading as the result. dbg!
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