Attribute

#[inline]

a hint to the compiler to inline the function. Inlining is an optimization technique where the compiler replaces a function call with the actual body of the function at the call site. It can eliminates the overhead of a function call. #[inline] do not gurantee that a function is inlined or not inlined, it's just a suggestion, the compiler decides whether to inline based on its own heuristics, such as function size, call frequency, and optimization level(--release mode).

Why need inline

The key is function call. use compiler explorer to understand the underling code.

When to use #[inline]

small but called repeatedly. - new