Understanding and Fixing Rust Error E0698

Rust Error E0698 in Generators and Async Functions

Rust error E0698 occurs when using generators or async functions and type variables are not properly bound. These unbound type variables make it impossible for the Rust compiler to infer the correct type and construct the generator.

Erroneous code example:

#![allow(unused)]
fn main() {
async fn bar<T>() -> () {}

async fn foo() {
    bar().await; // error: cannot infer type for `T`
}
}

In the above example, the Rust compiler cannot determine the type for `T` in the `bar` function.

Solution for Rust Error E0698

To fix Rust error E0698, you must bind the type variable to a concrete type, such as String, so that the Rust compiler can construct the generator. See the example below:

#![allow(unused)]
fn main() {
async fn bar<T>() -> () {}

async fn foo() {
    bar::<String>().await;
    //   ^^^^^^^^ specify type explicitly
}
}

In this modified example, the type variable `T` is explicitly specified as `String` when calling the `bar` function, so the code can now be successfully compiled.

Generalizing the Solution for Rust Error E0698

The solution for Rust error E0698 depends on the specific code and logic, but the key is to provide enough information for the Rust compiler to infer the correct type. You can do this in several ways, including specifying the type explicitly when calling a function, or by using a function signature that contains the needed type information.

If you encounter Rust error E0698 in your code, always check that your type variables are properly bound and that there is enough information for the Rust compiler to infer the correct type.