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Understanding Rust Error E0379 - Trait Method Declared Const

Introduction to Rust Error E0379

When working with Rust, you might encounter Rust Error E0379, which occurs when a method inside a trait is declared as const. According to Rust language design and RFC 911, trait methods cannot be declared const.

Example of Erroneous Code

#![allow(unused)]
fn main() {
  trait Foo {
    const fn bar() -> u32; // error!
  }
}

In the code above, we have a trait named 'Foo' that has declared a const method named 'bar'. This will result in Rust Error E0379.

Understanding Const Trait Methods

The const keyword in Rust allows us to run functions and evaluate expressions during the compilation process. However, Rust's design prohibits const trait methods in order to maintain the flexibility, clarity, and runtime properties that traits provide. This means that you cannot use const in conjunction with trait methods.

How to Resolve Rust Error E0379

To fix Rust Error E0379, you will need to remove the const keyword from the problematic trait method. Since trait methods cannot be declared const, doing this will resolve the error, and your code will compile.

Example of Corrected Code

#![allow(unused)]
fn main() {
  trait Foo {
    fn bar() -> u32; // no longer an error
  }
}

In the corrected code above, we have removed the const keyword from the 'bar' method in the 'Foo' trait. Now the code will compile without triggering the E0379 error.

Conclusion

Rust Error E0379 is triggered when a trait method is declared const, which is disallowed by design. To resolve this error, simply remove the const keyword from the problematic trait method. For more information on const functions and Rust language design, refer to RFC 911.

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