Rust Error E0636: Duplicate Feature Attribute Declaration

Understanding Rust Error E0636

Rust Error E0636 occurs when a #![feature] attribute is declared multiple times in the same scope. In Rust, each unique feature attribute should be declared only once to avoid confusion and redundancy.

Consider the following erroneous code example:

#![allow(unused)]
#![allow(stable_features)]
#![feature(rust1)]
#![feature(rust1)] // error: the feature `rust1` has already been declared
fn main() {
}

In this example, the `rust1` feature is declared twice, causing the Rust compiler to throw Error E0636.

How to Resolve Rust Error E0636

To resolve Rust Error E0636, simply remove the duplicate #![feature] attribute declaration. Ensure that each unique feature attribute is declared only once within the same scope.

Here's the corrected code:

#![allow(unused)]
#![allow(stable_features)]
#![feature(rust1)]

fn main() {
}

By removing the duplicate #![feature(rust1)] attribute declaration, the code now compiles without generating Rust Error E0636.

Note on Feature Attributes

It's important to remember that feature attributes in Rust are unstable and subject to change. Using a feature attribute may cause your code to be incompatible with future versions of the Rust compiler. Be cautious and keep your dependencies up-to-date to minimize the impact of breaking changes.