Answer.

Higher-order functions are functions that take other functions as parameters or return them as a result. From the very beginning of its development, Rust has emphasized type safety and performance, which is reflected in its handling of such functions.

Background:

In functional languages, higher-order functions are considered standard; however, in many systems languages, they often led to performance leaks (due to allocations or the inability to inline code). In Rust, this functionality is implemented through a strict type system, static dispatch, or traits (Fn, FnMut, FnOnce), which allows avoiding overhead in most cases.

Problem:

The main problem lies in the need to pass a function or closure while maintaining type safety, the capacity to capture variables (the ease of lambda expressions), and performance without allocations or virtual calls.

Solution:

In Rust, higher-order functions are implemented through generic parameters and trait wrappers for functions/closures. The standard traits Fn, FnMut, and FnOnce allow very clear declarations of requirements for the passed function (whether it can mutate or consume the environment). Passing through generics allows for inlining calls at compile time. There is also dynamic dispatch through Box<dyn Fn...> when the type is unknown in advance.

Example code:

fn apply_to_vec<F: Fn(i32) -> i32>(v: Vec<i32>, f: F) -> Vec<i32> {
    v.into_iter().map(f).collect()
}

let nums = vec![1, 2, 3];
let doubled = apply_to_vec(nums, |x| x * 2);
// doubled == [2, 4, 6]

Key features:

Type safety is guaranteed at compile time.
Support for both static and dynamic dispatch (at the developer's choice).
The closure mechanism is compatible with Rust's borrowing and ownership model.

Tricky questions.

What are the differences between Fn, FnMut, and FnOnce?

Many believe that they differ only in syntax or that Fn and FnMut can replace each other. In fact:

FnOnce can be called only once (for example, if the lambda moves a captured value inside).
FnMut can change the state of the captured environment but can be called multiple times.
Fn does not change its environment.

Example:

let mut sum = 0;
let mut add = |x| { sum += x; };
// add implements FnMut, but not Fn

Can a function be passed as a value without boxing?

It is often thought that any function arguments must necessarily be boxed (Box<dyn Fn...>). In reality, boxing is needed ONLY for dynamic dispatch when the type number is unknown until execution. Through generic parameters, the function can be fully statically typed, without allocations and boxing.

When does a closure stop being Copy?

Some believe that a simple closure is always Copy or Clone if the variable inside is Copy. In reality, closures are not Copy by default, even if captured variables are Copy. You need to explicitly implement the trait or stick to simple functions.

Common mistakes and anti-patterns

Always using Box<dyn Fn> even without necessity, which hampers performance.
Misunderstanding the differences between Fn/FnMut/FnOnce, leading to unnecessary clones or borrow conflicts.
Expecting that closures are automatically Copy if they capture Copy data.

Real-life examples

Negative case

In a project, only Box<dyn Fn()> was used for all callbacks in collections, without considering inlining and allocations. As a result, performance gains were not achieved, and frequent allocations led to delays.

Pros:

Simplified API interfaces.

Cons:

Significant performance drop in loops and on large input data.

Positive case

Event handlers were configured through generic functions with the FnMut trait constraint, completely avoiding allocations.