Understanding Rust Error E0518

Overview of Rust Error E0518

Rust Error E0518 occurs when the #[inline(..)] attribute is incorrectly placed on something other than a function or method. The #[inline] attribute provides hints to the compiler about whether or not to attempt inlining a method or function. By default, the Rust compiler does a good job of determining this on its own. However, when necessary, you can use #[inline(always)] and #[inline(never)] to override or enforce the compiler's decision.

Erroneous Code Example

#![allow(unused)]
fn main() {
    #[inline(always)]
    struct Foo;

    #[inline(never)]
    impl Foo {
        // ...
    }
}

In this example, both the struct Foo and the impl Foo blocks have incorrectly placed #[inline] attributes. These attributes should only be applied to functions and methods.

Correct Usage of #[inline] Attributes

To fix the Rust Error E0518, place the #[inline] attributes on functions or methods within the implementation block, rather than applying them to the entire impl or struct. Here's a corrected version of the previous example:

#![allow(unused)]
fn main() {
    struct Foo;

    impl Foo {
        #[inline(always)]
        fn example_method(&self) {
            // ...
        }

        #[inline(never)]
        fn another_method(&self) {
            // ...
        }
    }
}

In this corrected example, the #[inline(always)] attribute is applied to the example_method() function, and the #[inline(never)] attribute is applied to the another_method() function within the Foo implementation block.

Takeaways

To avoid Rust Error E0518, ensure that #[inline(..)] attributes are placed only on functions or methods, and not on other elements like structs or the entire implementation blocks. Although such fine-grained control over inlining is often unnecessary due to the Rust compiler's own optimizations, using these attributes correctly can help you further tailor your code's performance.