The type signature of the function will also enforce that. You can also use generics to allow you to accept a wider variety of numbers, e.g.: fn takes_a_flo

The type signature of the function will also enforce that. You can also use generics to allow you to accept a wider variety of numbers, e.g.: fn takes_a_float_like_number<T>(number: T) where T: Into<f64> { // Number will now be a 64-bit IEEE floating point object // for this scope let number = number.into(); } This will only compile if your type is small enough to fit into an f64. So takes_a_float_like_number(500_u32) will compile but takes_a_float_like_number(500_i64) will not. The error message will be a wee bit obtuse as it will be something like "trait Into<f64> not implemented for i64".