8.11 Inlining Functions

Inlining replaces a function call with the function’s body, avoiding call overhead. Rust’s compiler applies inlining optimizations when it sees fit.

8.11.1 #[inline] Attribute

#[inline]
fn add(a: i32, b: i32) -> i32 {
    a + b
}
  • #[inline(always)]: A stronger hint. However, the compiler may still decline to inline if it deems it inappropriate.
  • Too much inlining can cause code bloat.

8.11.2 Optimizations

Inlining can eliminate function-call overhead and enable specialized optimizations when arguments are known at compile time. For instance, if you mark a function with #[inline(always)] and pass compile-time constants, the compiler may generate a specialized code path. Similar benefits can appear when passing generic closures, allowing the compiler to tailor the generated code. We will see more about closures and optimization in a later chapter.