Rust Error E0534: Malformed Inline Attribute

Understanding Rust Error E0534

Rust Error E0534 occurs when the inline attribute is malformed. The inline attribute is used to give the compiler hints about inlining opportunities. This error is encountered when the parameter within the parenthesized inline attribute is missing. Below is an example of code that triggers this error:

#[inline()]
pub fn something() {}

fn main() {}

Correct Usage of Inline Attribute

The inline attribute requires the parameter 'always' or 'never' to be specified within the parenthesis, as shown in the examples below:

#![allow(unused)]
fn main() {
#[inline(always)]
fn something() {}
}

#![allow(unused)]
fn main() {
#[inline(never)]
fn something() {}
}

Alternatively, a paren-less version of the attribute can be used without any parameters to hint the compiler about inlining opportunities:

#![allow(unused)]
fn main() {
#[inline]
fn something() {}
}

Inlining Guidelines

Inlining is a performance optimization technique that can improve the speed of your Rust code. However, it is essential to understand when and how to use the inline attribute correctly. The compiler automatically inlines small functions, so it's usually not necessary to use the inline attribute explicitly unless you have a particular purpose or reason.

It is generally a good idea to use the #[inline(always)] attribute for small, frequently called functions, while #[inline(never)] can be used to prevent the inlining of larger functions. Using the paren-less inline attribute simply provides a hint to the compiler that the function might benefit from inlining, but won't force the behavior.