Bounds and Bounds Errors

Overview

In this guide, you can learn about bounds and how to resolve a common bounds error.

What are Bounds?

Trait bounds allow methods to restrict which types they accept as parameters and what functionality those types must implement. For example, if you define a method that accepts a generic type parameter and prints its value, the parameter must implement the Display trait for printing purposes. The following example defines the printer() method and specifies that its parameter must implement Display:

fn printer<T: Display>(t: T) {
   println!("{}", t);
}

Common Bounds Error

When calling a method on a data type, you might encounter an error stating that the method's trait bounds are not satisfied. For example, the driver might raise the following error message when you call the try_next() method on a Cursor instance:

error[E0599]: the method `try_next` exists for struct `mongodb::Cursor<T>`,
but its trait bounds were not satisfied

The Cursor<T> type only implements the Stream trait, which is required to access the try_next() method, if the trait bounds for T are satisfied. Namely, T must implement the DeserializeOwned trait, as specified in the Cursor API documentation. The following example reproduces the preceding error message by defining a custom Actor struct that models documents in the actors collection. However, this struct does not implement the DeserializeOwned trait and iterating over Actor by using the try_next() method causes an error:

// This code example produces a trait bounds error because
// the Actor struct does not implement the DeserializeOwned trait.
struct Actor {
   name: String,
}
// Add setup code here
let my_coll: Collection<Actor> = client.database("db").collection("actors");
let mut cursor = my_coll.find(doc! {}).await?;
while let Some(result) = cursor.try_next().await? {
     println!("{:?}", result);
 };

To resolve the trait bounds error, identify the data type over which the cursor iterates and ensure that this data type implements the DeserializeOwned trait. You can use the derive attribute to apply the required trait bound.

The following example adjusts the Actor struct definition to implement Deserialize:

#[derive(Deserialize)]
struct Actor {
   name: String,
}

Additional Resources

For more information about trait bounds, see the following resources:

• Bounds in the Rust language documentation

• Derive in the Rust language documentation