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E0042: Wrong Generic Arity

You used a generic type with the wrong number of type arguments.

Example​

fn test() {
    let x: Option = Option::Some(42);           // Error: expects 1 generic argument, found 0
    let y: Option<i64, bool> = Option::Some(42); // Error: expects 1 generic argument, found 2
    let z: Result<i64> = Result::Ok(42);          // Error: expects 2 generic arguments, found 1
}

How to fix​

Provide the correct number of type arguments:

fn test() {
    let x: Option<i64> = Option::Some(42);             // OK
    let y: Result<i64, bool> = Result::Ok(42);          // OK
}

Option<T> takes exactly 1 type argument. Result<T, E> takes exactly 2.