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Understanding and Fixing Rust Error E0549

What is Rust Error E0549?

Rust Error E0549 occurs when a deprecated attribute is not paired with either a stable or unstable attribute while using the #[feature(staged_api)] directive. It indicates that the stability status of the function needs to be specified for proper deprecation.

Erroneous code example:

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

fn main() {
#[deprecated(
    since = "1.0.1",
    note = "explanation for deprecation"
)] // invalid
fn _deprecated_fn() {}
}

How to fix Rust Error E0549?

To resolve this error, include either a #[stable] or #[unstable] attribute alongside the deprecated attribute. This will provide explicit information about the function's stability status.

Corrected code example:

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

fn main() {
#[stable(since = "1.0.0", feature = "test")]
#[deprecated(
    since = "1.0.1",
    note = "explanation for deprecation"
)] // ok!
fn _deprecated_fn() {}
}

Understanding Stable and Unstable Attributes

In Rust, the stability of a function or feature can be annotated using stable and unstable attributes. The #[stable] attribute indicates that the function or feature is considered stable and can be used in stable Rust releases without problems. On the other hand, the #[unstable] attribute suggests that the functionality may change or be removed in the future and is only available in nightly Rust builds. Including these attributes along with the deprecated attribute helps provide better information about the function's stability and deprecation status.

Further Resources

For more information on Rust's stability attributes and stages, check out the Rustc Dev Guide's chapter on Stability Attributes and The Rust Programming Language Book's Nightly Rust appendix.

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