Understanding Rust Error E0543: Missing Note Value in Stability Attribute

Overview of Rust Error E0543

Rust Error E0543 occurs when the note value is missing in a stability attribute within Rust code. In this article, we will explore the erroneous code, how to fix it, and provide examples to give a better understanding of this specific error.

Erroneous Code Example

#![allow(unused)]
#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "test")]

fn main() {
#[stable(since = "0.1.0", feature = "_deprecated_fn")]
#[deprecated(
    since = "1.0.0"
)] // invalid
fn _deprecated_fn() {}
}

In the code above, the error occurs because the #[deprecated] attribute lacks the required note value, which should explain the reason for deprecation.

Fixing Rust Error E0543

To fix Rust Error E0543, you need to provide the missing note field within the stability attribute. Below is the corrected code example:

#![allow(unused)]
#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "test")]

fn main() {
#[stable(since = "0.1.0", feature = "_deprecated_fn")]
#[deprecated(
    since = "1.0.0",
    note = "explanation for deprecation"
)] // ok!
fn _deprecated_fn() {}
}

With the added note field, the #[deprecated] attribute is now valid, and the error E0543 is resolved.

Additional Resources

For more information on Rust stability attributes and how they work, refer to the How Rust is Made and the “Nightly Rust” appendix of the Rust Book. You can also consult the Stability Attributes section of the Rustc Dev Guide for further details.