Macro

Still using println! for quick debugging? Try dbg! instead — it prints the expression, its value, and the file/line number to stderr. And it returns the value, so you can wrap it around anything.

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let a = 2;
let b = dbg!(a * 2) + 1; // prints: [src/main.rs:3] a * 2 = 4
assert_eq!(b, 5);

// works with multiple values too
dbg!(a, b, a + b); // prints each as a tuple

Unlike println!, dbg! takes ownership (or copies for Copy types). If you need to keep the value, pass a reference:

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let name = String::from("Ferris");
dbg!(&name); // borrows, doesn't move
println!("{name}"); // still works!

Bonus: dbg! works the same in release builds, and outputs to stderr so it won’t pollute your stdout.

Need to check if an expression matches certain pattern?

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let foo = vec!['1','2'];
assert!(matches!(foo.len(), 0..=1));

The example above checks if len is 0 or 1.

Or

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let bar = Some(4);

assert!(matches!(bar, Some(x) if x > 2));