Error code E0061

An invalid number of arguments was passed when calling a function.

Erroneous code example:

#![allow(unused)] fn main() { fn f(u: i32) {} f(); // error! }

The number of arguments passed to a function must match the number of arguments specified in the function signature.

For example, a function like:

#![allow(unused)] fn main() { fn f(a: u16, b: &str) {} }

Must always be called with exactly two arguments, e.g., f(2, "test").

Note that Rust does not have a notion of optional function arguments or variadic functions (except for its C-FFI).