閉包

閉包*在Rust 是一個稍微專業的語法，可以捕捉到封閉的環境函數。 這種語法和能力使它們在運行使用非常方便。一些特性包括：

• 使用 || 替代 () 圍繞輸入變量。
• 輸入和返回類型可以推斷出。
• 輸入變量名稱必須指定。
• 主體定界 ({}) 是可選的一個表達式。強製性其他。
• 外環境變量可能被捕獲。
• 調用閉包和函數與 call(var)是完全一樣的

fn main() {
    // Increment via closures and functions.
    fn  function            (i: i32) -> i32 { i + 1 }

    // Annotation is identical to function annotation but is optional
    // as are the `{}` wrapping the body. These nameless functions
    // are assigned to appropriately named variables.
    let closure_annotated = |i: i32| -> i32 { i + 1 };
    let closure_inferred  = |i     |          i + 1  ;

    let i = 1;
    // Call the function and closures.
    println!("function: {}", function(i));
    println!("annotated closure: {}", closure_annotated(i));
    println!("inferred closure: {}", closure_inferred(i));

    // A closure taking no arguments which returns an `i32`.
    // The return type is inferred.
    let one = || 1;
    println!("closure returning one: {}", one());

    // It is possible to capture variables from the enclosing
    // environment; something which is impossible with functions.
    let professor_x = "Charles Xavier";

    // A closure which takes no argument, returning nothing, prints
    // a variable from the enclosing scope.
    let print = || println!("Professor X's name is: {}", professor_x);

    // Call the closure.
    print();
}

*.也被稱為lambda表達式或匿名函數。