Understanding Rust Error E0667: impl Trait in Path Parameters
Rust Error E0667: Using impl Trait in Path Parameters
Rust error E0667 occurs when you attempt to use impl Trait in path parameters. This error is raised if you define a function with an incorrect use of impl Trait, as shown in the erroneous code example provided by the official documentation:
#![allow(unused)]
fn main() {
fn some_fn(mut x: impl Iterator) -> <impl Iterator>::Item { // error!
x.next().unwrap()
}
}
The Solution: Use Trait Bounds
To fix Rust error E0667, you should use trait bounds in the function signature instead. In this case, replace impl Iterator with T: Iterator where T is a generic type that implements the Iterator trait. The corrected code should look like this:
#![allow(unused)]
fn main() {
fn some_fn<T: Iterator>(mut x: T) -> T::Item { // ok!
x.next().unwrap()
}
}
Why impl Trait is Not Allowed in Path Parameters
impl Trait is a syntax feature introduced in Rust for making it easier to return complex types resulting from closures and iterator combinator methods. However, it is not allowed in path parameters because it can lead to ambiguity. In contrast, using trait bounds with generic types explicitly defines the required traits and establishes a concrete relationship between the input parameter and the output type.
Additional Examples of Correctly Using Trait Bounds
When working with multiple traits, you can use the + operator within the angle brackets to specify that the generic type must implement each of the listed traits. For example:
#![allow(unused)]
fn main() {
fn some_fn<T: Iterator + Clone>(mut x: T) -> T::Item { // ok!
x.next().unwrap()
}
}
You can also use where clauses for more complex scenarios, like specifying trait bounds for associated types:
#![allow(unused)]
fn main() {
fn some_fn<T>(mut x: T) -> T::Item
where T: Iterator, T::Item: std::fmt::Debug { // ok!
x.next().unwrap()
}
}