Understanding Rust Error E0719
Error E0719: Associated Type Specified Multiple Times
Rust error E0719 occurs when an associated type value is specified multiple times in a trait or trait bound constraint. This can lead to ambiguity and is not allowed.
Example of Erroneous Code
Consider the following example, where the error E0719 occurs:
#![allow(unused)]
#![feature(associated_type_bounds)]
fn main() {
trait FooTrait {}
trait BarTrait {}
// error: associated type `Item` in trait `Iterator` is specified twice
struct Foo<T: Iterator<Item: FooTrait, Item: BarTrait>> { f: T }
}In this example, the associated type `Item` in the trait `Iterator` is specified twice for the `struct Foo`. This is not allowed.
Solution to Rust Error E0719
To fix error E0719, you need to create a new trait that combines the desired traits and specify the associated type with the new combined trait. Here's the corrected version of the previous example:
#![allow(unused)]
#![feature(associated_type_bounds)]
fn main() {
trait FooTrait {}
trait BarTrait {}
trait FooBarTrait: FooTrait + BarTrait {}
struct Foo<T: Iterator<Item: FooBarTrait>> { f: T } // ok!
}In the corrected code, a new trait `FooBarTrait` is created that combines `FooTrait` and `BarTrait`. The struct `Foo` now specifies the associated type `Item` with the combined new trait `FooBarTrait`.
Additional Resources
For more information on associated types in Rust, refer to the Rust book's chapter on Traits: Defining Shared Behavior. For more information on associated type bounds, see Rust RFC 2289.