12.6 Performance Considerations
Closures in Rust can be very efficient, often inlined like regular functions. In most cases, they do not require heap allocation unless you store them as trait objects (Box<dyn Fn(...)> or similar) or otherwise need dynamic dispatch.
12.6.1 Heap Allocation
Closures typically live on the stack if their size is known at compile time. However, when you store a closure behind a trait object (like dyn Fn), the closure is accessed via dynamic dispatch, which can involve a heap allocation.
In many performance-critical contexts, you can rely on generics (impl Fn(...)) to keep things monomorphized and inlineable.
12.6.2 Dynamic Dispatch vs. Static Dispatch
- Static dispatch (generics): allows the compiler to inline and optimize the closure, yielding performance similar to a regular function call.
- Dynamic dispatch (
Box<dyn Fn(...)>): offers flexibility at the cost of a small runtime overhead and potential heap allocation.