Closure: FnOnce, FnMut, and Fn, why are there so many types? #

Hello, I’m Chen Tian.

In modern programming languages, closures are a very important tool that allow us to conveniently write code in a functional programming style. Closures can be passed as arguments to functions, returned by functions, and implement certain traits to exhibit behaviors other than just being invoked as functions.

How are all these achieved? This has to do with the nature of closures in Rust, and today we will learn the last basic concept: closures.

Definition of Closures #

We previously introduced the basic concept of closures and a very simple example:

A closure is a data structure that stores both the function, or the code, and its environment together. Free variables in the context referenced by the closure will be captured into the structure of the closure, becoming part of the closure type ([Lecture 2]).

Closures will capture free variables from the environment based on internal usage. In Rust, a closure can be expressed as |args| {code}. The closure c in the figure captures a and b from the context, using these two free variables through references:

Besides capturing free variables by reference, there’s another method that uses the move keyword, move |args| {code}.

In previous lessons, the creation of a new thread with thread::spawn, which accepts a closure as a parameter, has been seen several times:

pub fn spawn<F, T>(f: F) -> JoinHandle<T>
where
    F: FnOnce() -> T,
    F: Send + 'static,
    T: Send + 'static,

Look closely at this interface:

1. F: FnOnce() → T, indicates that F is a closure that accepts 0 parameters and returns T. We will talk about FnOnce in a moment.
2. F: Send + ‘static, suggests that the closure F needs either a static lifetime or ownership, and it can be sent to another thread.
3. T: Send + ‘static, suggests that the structure T returned by closure F needs either a static lifetime or ownership, and it can be sent to another thread.

1 and 3 are easy to understand, but 2 is somewhat confusing. Isn’t a closure just a piece of code plus captured variables? What does it mean to require static lifetime or ownership?

Let’s break it down. Naturally, the code has a static lifetime, so does it mean that the captured variables need a static lifetime or ownership?

Indeed, this is the case. When using thread::spawn, we need to use the move keyword to transfer ownership of the variables from the current scope to the closure’s scope, allowing thread::spawn to compile successfully:

use std::thread;

fn main() {
    let s = String::from("hello world");

    let handle = thread::spawn(move || {
        println!("moved: {:?}", s);
    });

    handle.join().unwrap();
}

Have you ever wondered what the essential difference is between a closure with move and without move? What kind of data type is a closure after all that allows the compiler to determine whether it satisfies Send + ‘static? Let’s try to answer these two questions starting from the nature of closures.

What is the essence of a closure? #

In the official Rust reference, there is such a definition:

A closure expression produces a closure value with a unique, anonymous type that cannot be written out. A closure type is approximately equivalent to a struct which contains the captured variables.

A closure is an anonymous type, once declared, a new type is produced, but this type cannot be used elsewhere. This type is like a struct, which includes all captured variables.

Is a closure similar to a special struct?

To understand this, we need to write some code to explore. I suggest that you follow along and type it out thoughtfully (code):

// Example code here.

Five closures were generated:

• c1 has no parameters and captures no variables. The output shows that the length of c1 is 0.
• c2 has an i32 as a parameter and captures no variables, its length is also 0, indicating parameters do not affect the closure size.
• c3 captures a reference to the variable name, which is a &String, with a length of 8. So is c3’s length 8.
• c4 captures name1 and table. As move is used, their ownership is moved to c4. The length of c4 is 72, exactly equal to 24 bytes of String plus 48 bytes of HashMap.
• c5 captures name2, with name2’s ownership moved to c5. Despite local variables in c5, its size is unrelated to them; c5’s size is equal to 24 bytes of String.

From here, the first question is solved; we see that closures without move capture references to their respective free variables; with move, the respective free variables’ ownership is moved into the closure structure.

Continuing to analyze the results of this code execution.

We also know that, the size of a closure is irrelevant to parameters and local variables, but is only related to the captured variables. If you recall [Lecture 1] about function calls, how parameters and local variables are stored in the stack, it’s clear: because they are allocated in memory only at the moment of the call, which is fundamentally unrelated to the closure type itself, so the closure size is naturally unrelated to them.

Then, how is a closure type organized in memory, and how is it different from a struct? We need to explore again using rust-gdb and see what’s inside a few non-zero-sized closures’ memory before the above code completes execution:

As you can see, c3 is indeed a reference. Printing the 24 bytes of memory address it points to, you see the standard layout of (ptr | cap | len). If you print the three bytes of the heap memory represented by ptr, they are ’t’ ‘y’ ‘r’.

The captured name and table in c4 have a memory structure exactly like the following struct:

struct Closure4 {
    name: String,  // 24 bytes (ptr|cap|len)
    table: HashMap<&str, &str> // 48 bytes (RandomState(16)|mask|ctrl|left|len)
}

However, for the closure type, the compiler knows that it’s legal to call the closure c4() like a function, and knows where to jump to execute when c4() is called. During execution, if it encounters name and table, it can obtain them from its data structure.

So, think further, is the order of the variables captured by a closure the same as the memory layout? Indeed it is. If we change the usage order of name1 and table within the closure:

let c4 = move || println!("hello: {:?}, {}", table, name1);

Their positions in data are reversed, similar to:

struct Closure4 {
    table: HashMap<&str, &str> // 48 bytes (RandomState(16)|mask|ctrl|left|len)
    name: String,  // 24 bytes (ptr|cap|len)
}

From gdb, we can also see the same result:

However, this is only the logical position. If you remember [Lecture 11] about the memory layout of structs, the Rust compiler may rearrange memory so that data can be aligned with the least overhead, so in some cases

The content provided was a comprehensive examination of closures in the Rust programming language, describing how they work, their memory layout, and the differences between the three main closure traits in Rust: FnOnce, FnMut, and Fn. Furthermore, it discussed the efficiency of Rust closures compared to other languages, situations where closures are commonly used, and provided exercises to reinforce understanding.

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