Rust by Example

While Rust chooses how to capture variables on the fly mostly without type annotation, this ambiguity is not allowed when writing functions. When taking a closure as an input parameter, the closure's complete type must be annotated using one of a few traits. In order of decreasing restriction, they are:

• Fn: the closure captures by reference (&T)

• FnMut: the closure captures by mutable reference (&mut T)

• FnOnce: the closure captures by value (T)

On a variable-by-variable basis, the compiler will capture variables in the least restrictive manner possible.

For instance, consider a parameter annotated as FnOnce. This specifies that the closure may capture by &T, &mut T, or T, but the compiler will ultimately choose based on how the captured variables are used in the closure.

This is because if a move is possible, then any type of borrow should also be possible. Note that the reverse is not true. If the parameter is annotated as Fn, then capturing variables by &mut T or T are not allowed.

In the following example, try swapping the usage of Fn, FnMut, and FnOnce to see what happens:

// A function which takes a closure as an argument and calls it.

//  denotes that F is a "Generic type parameter"

fn apply(f: F) where

// The closure takes no input and returns nothing.

F: FnOnce() {

// ^ TODO: Try changing this to `Fn` or `FnMut`.

f();

}

// A function which takes a closure and returns an `i32`.

fn apply_to_3(f: F) -> i32 where

// The closure takes an `i32` and returns an `i32`.

F: Fn(i32) -> i32 {

f(3)

}

fn main() {

use std::mem;

let greeting = "hello";

// A non-copy type.

// `to_owned` creates owned data from borrowed one

let mut farewell = "goodbye".to_owned();

// Capture 2 variables: `greeting` by reference and

// `farewell` by value.

let diary = || {

// `greeting` is by reference: requires `Fn`.

println!("I said {}.", greeting);

// Mutation forces `farewell` to be captured by

// mutable reference. Now requires `FnMut`.

farewell.push_str("!!!");

println!("Then I screamed {}.", farewell);

println!("Now I can sleep. zzzzz");

// Manually calling drop forces `farewell` to

// be captured by value. Now requires `FnOnce`.

mem::drop(farewell);

};

// Call the function which applies the closure.

apply(diary);

// `double` satisfies `apply_to_3`'s trait bound

let double = |x| 2 * x;

println!("3 doubled: {}", apply_to_3(double));

}

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