Understanding Rust Error E0401

Rust Error E0401 Explained

Rust error E0401 occurs when inner items do not inherit type or const parameters from the functions they are embedded in. Inner items are bound by the same scoping rules as top-level items but can only be accessed within the parent function.

Error Example

#![allow(unused)]
fn main() {
fn foo<T>(x: T) {
    fn bar(y: T) { // T is defined in the "outer" function
        // ..
    }
    bar(x);
}
}

In this example, the 'bar' function tries to use the generic type 'T' from its parent function 'foo', but this is not allowed

Possible Solutions

1. Use a closure for inner functions: By defining the inner function as a closure, it can capture the parent function's environment and use the type parameters.

Example with Closure

#![allow(unused)]
fn main() {
fn foo<T>(x: T) {
    let bar = |y: T| { // explicit type annotation may not be necessary
        // ..
    };
    bar(x);
}
}

2. Copy over type parameters and bounds: For a generic item, you can manually copy over the type parameters and any bounds from the parent function.

Example with Copied Type Parameters

#![allow(unused)]
fn main() {
fn foo<T>(x: T) {
    fn bar<T>(y: T) {
        // ..
    }
    bar(x);
}
}

3. Use private helper functions with impl blocks: In case the inner item is a function inside an 'impl' block, you can define a private helper function which has access to the type parameters.

Example with Private Helper Function

#![allow(unused)]
fn main() {
struct Foo<T>(T);
impl<T> Foo<T> {
    pub fn foo(&self, x: T) {
        self.bar(x);
    }

    fn bar(&self, y: T) {
        // ..
    }
}
}

For default impls in traits, the private helper solution won't work, however closures or copying the parameters should still work.

Recommended Reading

• Lifetime Syntax