todo!宏

有时你想粗略写点写代码帮助你想象你的项目。例如，想象一个简单的项目，用书籍做一些事情。下面是你写的时候的想法:

struct Book {} // Okay, first I need a book struct.
               // Nothing in there yet - will add later

enum BookType { // A book can be hardcover or softcover, so add an enum
    HardCover,
    SoftCover,
}

fn get_book(book: &Book) -> Option<String> {} // ⚠️ get_book should take a &Book and return an Option<String>

fn delete_book(book: Book) -> Result<(), String> {} // delete_book should take a Book and return a Result...
                                                    // TODO: impl block and make these functions methods...
fn check_book_type(book_type: &BookType) { // Let's make sure the match statement works
    match book_type {
        BookType::HardCover => println!("It's hardcover"),
        BookType::SoftCover => println!("It's softcover"),
    }
}

fn main() {
    let book_type = BookType::HardCover;
    check_book_type(&book_type); // Okay, let's check this function!
}

但Rust对get_book和delete_book不满意。它说

error[E0308]: mismatched types
  --> src\main.rs:32:29
   |
32 | fn get_book(book: &Book) -> Option<String> {}
   |    --------                 ^^^^^^^^^^^^^^ expected enum `std::option::Option`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:   expected enum `std::option::Option<std::string::String>`
           found unit type `()`

error[E0308]: mismatched types
  --> src\main.rs:34:31
   |
34 | fn delete_book(book: Book) -> Result<(), String> {}
   |    -----------                ^^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found `()`
   |    |
   |    implicitly returns `()` as its body has no tail or `return` expression
   |
   = note:   expected enum `std::result::Result<(), std::string::String>`
           found unit type `()`

但是你现在不关心get_book和delete_book。这时你可以使用todo!()。如果你把这个加到函数中，Rust不会抱怨，而且会编译。

struct Book {}

fn get_book(book: &Book) -> Option<String> {
    todo!() // todo means "I will do it later, please be quiet"
}

fn delete_book(book: Book) -> Result<(), String> {
    todo!()
}

fn main() {}

所以现在代码编译，你可以看到check_book_type的结果:It's hardcover。

但是要小心，因为它只是编译--你不能使用函数。如果你调用里面有todo!()的函数，它就会崩溃。

另外，todo!()函数仍然需要真实的输入和输出类型。如果你只写这个，它将无法编译。

struct Book {}

fn get_book(book: &Book) -> WorldsBestType { // ⚠️
    todo!()
}

fn main() {}

它会说

error[E0412]: cannot find type `WorldsBestType` in this scope
  --> src\main.rs:32:29
   |
32 | fn get_book(book: &Book) -> WorldsBestType {
   |                             ^^^^^^^^^^^^^^ not found in this scope

todo!()其实和另一个宏一样：unimplemented!()。程序员们经常使用 unimplemented!()，但打字时太长了，所以他们创建了 todo!()，它比较短。