Understanding Rust Error E0539

Introduction to Rust Error E0539

Rust Error E0539 occurs when an invalid meta-item is used inside an attribute. Meta-items are the key-value pairs found inside attributes. To resolve this error, you need to provide the required key-value pairs for the attribute.

Erroneous Code Example

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

fn main() {
#[deprecated(note)] // error!
#[unstable(feature = "deprecated_fn", issue = "123")]
fn deprecated() {}

#[unstable(feature = "unstable_struct", issue)] // error!
struct Unstable;

#[rustc_const_unstable(feature)] // error!
const fn unstable_fn() {}

#[stable(feature = "stable_struct", since)] // error!
struct Stable;

#[rustc_const_stable(feature)] // error!
const fn stable_fn() {}
}

In the code above, there are several cases where the required key-value pairs are not provided, leading to Rust Error E0539. Let's go through each error and its solution.

Fixing the First Error - Deprecated Function

The deprecated attribute requires both 'since' and 'note' key-value pairs. In the provided code, only 'note' was given. To fix it, add missing 'since' key-value pair.

Correct Code Example

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

fn main() {
#[deprecated(since = "1.39.0", note = "reason")] // ok!
#[unstable(feature = "deprecated_fn", issue = "123")]
fn deprecated() {}
}

Fixing the Second Error - Unstable Struct

The unstable struct attribute in the original code is missing the 'issue' key-value pair. Add the required 'issue' key-value pair to fix the error.

Correct Code Example

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

fn main() {
#[unstable(feature = "unstable_struct", issue = "123")] // ok!
struct Unstable;
}

Fixing the Third Error - Unstable Constant Function

The unstable constant function attribute in the original code is missing both 'feature' and 'issue' key-value pairs. Add the required key-value pairs to fix the error.

Correct Code Example

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

fn main() {
#[rustc_const_unstable(feature = "unstable_fn", issue = "124")] // ok!
const fn unstable_fn() {}
}

Fixing the Fourth Error - Stable Struct

The stable struct attribute in the original code is missing the 'since' key-value pair. Add the required 'since' key-value pair to fix the error.

Correct Code Example

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

fn main() {
#[stable(feature = "stable_struct", since = "1.39.0")] // ok!
struct Stable;
}

Fixing the Fifth Error - Stable Constant Function

The stable constant function attribute in the original code is missing both 'feature' and 'since' key-value pairs. Add the required key-value pairs to fix the error.

Correct Code Example

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

fn main() {
#[rustc_const_stable(feature = "stable_fn", since = "1.39.0")] // ok!
const fn stable_fn() {}
}

Summary

Rust Error E0539 is caused by using invalid meta-items inside an attribute. To fix the error, ensure that the required key-value pairs are provided for the attribute.