Operator Traits

Start with the idea

a + b calls Add::add, {} calls Display::fmt, and .into() calls Into, which you get for free by implementing From. Because they are normal traits, your own types participate in the same syntax as the built-in ones, and the compiler checks the result type at every use.

Operator syntax in Rust is backed by trait implementations. a + b invokes Add::add, and the implementation selects an associated Output type. In the Money example, adding two money values yields another Money; the newtype prevents an ordinary integer from being treated as money without an explicit conversion. This is useful because a small wrapper can make invalid operations harder to write while keeping familiar arithmetic syntax.

Do not overload an operator merely because you can. Readers expect + to combine values predictably; surprising side effects or hidden I/O make code harder to trust. Decide whether addition consumes operands, borrows them, or needs multiple implementations.