Trait Bounds

When working with generics, you often want to require the types to implement some trait, so that you can call this trait's methods.

You can do this with T: Trait:

# // Copyright 2022 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn duplicate<T: Clone>(a: T) -> (T, T) {
    (a.clone(), a.clone())
}

struct NotCloneable;

fn main() {
    let foo = String::from("foo");
    let pair = duplicate(foo);
    println!("{pair:?}");
}

• Try making a NotCloneable and passing it to duplicate.

• When multiple traits are necessary, use + to join them.

• Show a where clause, students will encounter it when reading code.

  rust,ignoreCopy

  # // Copyright 2022 Google LLC
  # // SPDX-License-Identifier: Apache-2.0
  #
  fn duplicate<T>(a: T) -> (T, T)
  where
      T: Clone,
  {
      (a.clone(), a.clone())
  }
  

  • It declutters the function signature if you have many parameters.
  • It has additional features making it more powerful.
    • If someone asks, the extra feature is that the type on the left of ":" can be arbitrary, like Option<T>.

• Note that Rust does not (yet) support specialization. For example, given the original duplicate, it is invalid to add a specialized duplicate(a: u32).