where

Adds trait constraints to a generic function: where Trait<A, B>.

It comes after the return type and is needed for constraints with several type parameters. A single-parameter bound is written on the parameter: fn f<T: Num>, or a separate bounded impl block, impl Cell<T: Num> { … }, for methods that need a bound on the impl’s parameter. A unary where Num<T> is an error.

fn convert<A, B>(A x) -> B where Into<A, B> {
    return into(x);
}

class Cell<T> {
    pub v: T,
}

impl Cell<T: Num> {
    pub fn doubled() -> T {
        return self.v + self.v;
    }
}

See also#

• fn: Declares a function, a method, or (written in an expression) an anonymous function.
• let: Declares a mutable local variable.
• const: Declares a binding that cannot be reassigned: a local, a module constant, or a class field that is set only by new.
• static: Declares a module-level or class-level slot shared by the whole program, or a method without self.
• readonly: A read-only view of a value: nothing reachable through it can be changed.
• if: Runs a block when a bool condition is true.
• else: Introduces the branch of an if that runs when its condition is false (or its pattern does not match).
• while: Repeats a block while a bool condition is true.