Understanding Rust Error E0329

Rust Error E0329: NotSupportedException on Associated Constants

Rust Error E0329 occurs when you try to access an associated constant through a generic type parameter or Self. However, this behavior is not yet supported in Rust. Below is an example code that'll trigger this error:

trait Foo {
    const BAR: f64;
}

struct MyStruct;

impl Foo for MyStruct {
    const BAR: f64 = 0f64;
}

fn get_bar_bad<F: Foo>(t: F) -> f64 {
    F::BAR
}

Resolving Rust Error E0329

To resolve Rust Error E0329, you need to access the associated constant through a concrete type instead of a generic type parameter or Self. Modify your code like this example to fix the error:

trait Foo {
    const BAR: f64;
}

struct MyStruct;

impl Foo for MyStruct {
    const BAR: f64 = 0f64;
}

fn get_bar_good() -> f64 {
    <MyStruct as Foo>::BAR
}

In this modified example, the associated constant is accessed through a concrete type instead of a generic type parameter.

Understanding Associated Constants

Associated constants are a feature in Rust that allows you to define constants within the scope of a trait. They provide a convenient way to declare constant values while maintaining an association with the trait. This is useful when you want to define constants that will be used in conjunction with the trait's methods.

Traits and Associated Types

In some cases, you may want to use associated types with traits. These are a way to add an additional level of indirection to your trait. This allows you to have methods on your trait that return a specific associated type.

For example, you could use associated types in conjunction with associated constants to implement a trait that describes a mathematical operation with specific constants unique to that type, such as pi for circles or e for logarithms.

Conclusion