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13.6 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