Suggest that the compiler inline a function at its call sites.

Inlining replaces a call with a copy of the called function’s body, which can remove the overhead of the call. The inline attribute is only a hint: the compiler may ignore it, and it already inlines functions on its own when that looks worthwhile. Poor choices about what to inline can make a program larger or slower.

Where it does matter is inlining across crate boundaries. A non-generic function is not normally inlined into another crate, since the calling crate compiles against only its signature. Marking it #[inline] makes the body available to other crates so they can inline it too:

#[inline]
pub fn square(x: i32) -> i32 {
    x * x
}

Generic functions do not need this. They are instantiated in each crate that uses them, so their bodies are already available to inline.

The attribute applies to functions and has three forms:

• #[inline] suggests inlining the function.
• #[inline(always)] suggests inlining it at every call site.
• #[inline(never)] suggests never inlining it.

You should almost never need #[inline(always)]: prefer to let the compiler decide unless profiling shows a small, hot function that benefits from it. #[inline(never)] is useful to keep a rarely used path, such as a function that only reports an error, out of its caller.

For more information, see the Reference on the inline attribute.