Rust Error E0538: Duplicate Meta Items in Attributes

Understanding Rust Error E0538

Rust Error E0538 occurs when an attribute contains the same meta item key more than once. Meta items are the key-value pairs inside an attribute, and each key may only be used once per attribute. The Rust compiler will display this error when it encounters such duplicate meta item keys.

Example of Erroneous Code

#![allow(unused)]
fn main() {
#[deprecated(
    since="1.0.0",
    note="First deprecation note.",
    note="Second deprecation note." // error: multiple same meta item
)]
fn deprecated_function() {}
}

In the example above, the 'note' meta item key is used twice in the same attribute, resulting in Rust Error E0538.

Fixing Rust Error E0538

To resolve Rust Error E0538, remove all but one of the meta items with the same key from the attribute. Combining the information from duplicate meta items into a single meta item value is also an option.

Corrected Code Example

#![allow(unused)]
fn main() {
#[deprecated(
    since="1.0.0",
    note="First deprecation note."
)]
fn deprecated_function() {}
}

In the corrected code above, we removed the duplicate 'note' meta item key, resolving Rust Error E0538.