Understanding Rust Error E0666

What is Rust Error E0666?

Rust Error E0666 occurs when impl Trait types are nested within the generic arguments of other impl Trait types. For a better understanding, let's first look at a code example that produces this error:

trait MyGenericTrait<T> {}
trait MyInnerTrait {}

fn foo(
    bar: impl MyGenericTrait<impl MyInnerTrait>, // error!
) {}

Solution: Explicitly Define Named Generic Parameters

To fix Rust Error E0666, you need to explicitly define the type parameters for impl Trait types as named generic parameters. Here's a revised version of the erroneous code example that resolves the error:

trait MyGenericTrait<T> {}
trait MyInnerTrait {}

fn foo<T: MyInnerTrait>(
    bar: impl MyGenericTrait<T>, // ok!
) {}

Why Named Generic Parameters Matter

By specifying a named generic parameter (e.g., T: MyInnerTrait), the Rust compiler can determine the required trait bounds for the generic argument. This makes your code more explicit and helps to avoid potential issues or ambiguity arising from the nested impl Trait syntax.

Additional Examples

Here are a few more examples to further illustrate correct usage of generic parameters with impl Trait in Rust:

// Example 1
trait MyTrait {}
trait AnotherTrait {}

fn sample_function<A: MyTrait, B: AnotherTrait>(
    param_a: impl MyTrait,
    param_b: impl AnotherTrait,
) {}

// Example 2
trait PrintTrait {}
trait MathTrait {}

fn print_and_calculate<T: PrintTrait + MathTrait>(
    value: T,
) {}