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That signifies a macro. It's meant to visually distinguish macros from ordinary function calls because macros may have arbitrary semantics (they can expand into any code at all). The stdlib defines `println` as a macro because it allows for a neat compile-time parsing of format strings.

(See [1] and the linked source if curious.)

[1] https://doc.rust-lang.org/std/macro.println!.html

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Sounds like what Rust needs is compile macros [1]. That way, printf can still be a function, but it would be optimized at compile time if the formatting string is constant.

[1] https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node101.html

Compile macros wouldn't be sufficient for Rust's println! because the type signature (even the number of arguments) depends on the format string. The format string has to be constant and has to be interpreted at compile time so that the variables being printed can be type-checked.

Thanks, that makes a lot of sense as a design decision, should be a big step forwards for security.

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