Where clauses - Rust By Example
Description: A description

# Where clauses

A bound can also be expressed using a `where` clause immediately before the opening `{`, rather than at the type's first mention. Additionally, `where` clauses can apply bounds to arbitrary types, rather than just to type parameters.

Some cases that a `where` clause is useful:

- When specifying generic types and bounds separately is clearer:

    impl <A: TraitB + TraitC, D: TraitE + TraitF> MyTrait<A, D> for YourType {}

    // Expressing bounds with a `where` clause
    impl <A, D> MyTrait<A, D> for YourType where
        A: TraitB + TraitC,
        D: TraitE + TraitF {}

- When using a `where` clause is more expressive than using normal syntax. The `impl` in this example cannot be directly expressed without a `where` clause:

    use std::fmt::Debug;

    trait PrintInOption {
        fn print_in_option(self);
    }

    // Because we would otherwise have to express this as `T: Debug` or
    // use another method of indirect approach, this requires a `where` clause:
    impl<T> PrintInOption for T where
        Option<T>: Debug {
        // We want `Option<T>: Debug` as our bound because that is what's
        // being printed. Doing otherwise would be using the wrong bound.
        fn print_in_option(self) {
            println!("{:?}", Some(self));
        }
    }

    fn main() {
        let vec = vec![1, 2, 3];

        vec.print_in_option();
    }

### See also:

RFC, `struct`, and `trait`