Rust by Example

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  MyTrait for YourType {}

// Expressing bounds with a `where` clause

impl  MyTrait 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 PrintInOption for T where

Option: Debug {

// We want `Option: 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();

}

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