如何从同一类型的另一个方法内调用带有 &mut self 的方法？

Question

我有一个名为Scanner的结构体，它实现了一个scan(&mut self)方法。代码如下：

pub struct Scanner {
    input: String,
    output: Vec<String>,
    state: ScannerState,
}

impl Scanner {
    pub fn scan(&mut self) {
        self.state = ScannerState::CharMode;
        for character in self.input.chars() {
            match character {
                i @ '0'...'9'   => self.output.push(format!("Integer digit: {}", i)),
                '+'             => self.output.push("Addition operator: +".to_string()),
                '-'             => self.output.push("Subtraction operator: -".to_string()),
                '*'             => self.output.push("Multiplication operator: *".to_string()),
                '/'             => self.output.push("Division operator: /".to_string()),
                '%'             => self.output.push("Modulus operator: %".to_string()),
                '^'             => self.output.push("Exponent operator: ^".to_string()),
                '='             => self.output.push("Assignment operator: =".to_string()),
                ';'             => self.output.push("Statement terminator: ;".to_string()),
                c @ 'a'...'z'| c @ 'A'...'Z'
                                => self.output.push(format!("Variable name: {}", c)),
                ' '             => self.output.push("Space, ignoring.".to_string()),
                z @ _           => self.output.push(format!("Unrecognized token: {}", z))
            }
        }
    }        
}

然而，正如您所猜测的那样，Scanner的状态将会改变，并且在不同状态下需要对标记进行不同的处理。因此，在scan()方法内部调用Scanner的私有方法将是很有用的，就像这样:

pub fn scan(&mut self) {
    self.state = ScannerState::CharMode;

    while self.state == ScannerState::CharMode {
        for character in self.input.chars() {
            self.char_match(character);
        }
        self.state = ScannerState::Done;
    }
}

fn char_match(&mut self, c: char) {
    match c {
        '+'             => self.output.push("Addition operator: +".to_string()),
        '-'             => self.output.push("Subtraction operator: -".to_string()),
        '*'             => self.output.push("Multiplication operator: *".to_string()),
        '/'             => self.output.push("Division operator: /".to_string()),
        '%'             => self.output.push("Modulus operator: %".to_string()),
        '^'             => self.output.push("Exponent operator: ^".to_string()),
        '='             => self.output.push("Assignment operator: =".to_string()),
        ';'             => self.output.push("Statement terminator: ;".to_string()),
        ' '             => self.output.push("Space, ignoring.".to_string()),
        'q'             => self.state = ScannerState::QuitMode,
        i @ '0'...'9'   => self.output.push(format!("Integer digit: {}", i)),
        c @ 'a'...'z'   => self.output.push(format!("Variable name: {}", c)),
        z @ _           => self.output.push(format!("Unrecognized token: {}", z))
    }
}

但是等等！我们不能那样做，Rust告诉我们：

src/scanner.rs:34:17: 34:21 error: cannot borrow `*self` as mutable because `self.input` is also borrowed as immutable
src/scanner.rs:34                 self.char_match(character);
                                  ^~~~

然而，我们的char_match()方法需要对self进行可变引用，因为它需要推送(push)，而在Vec上进行push()需要可变性。那么我的问题是，在了解了前面的知识后，有什么理想的方法来解决这个问题？

我是否只需要将scan()编写成一个长方法？

- bretmattingly

Answer 1

char_match 不需要对 self 进行可变访问。对 self.output 和 self.state 的可变访问就足够了。如果你不给它访问 self.input 的权限，编译器将不再报错。

- FlyingFoX

Answer 2

我的建议是将char_match设置为无状态：

pub fn scan(&mut self) {
    self.state = ScannerState::CharMode;

    while self.state == ScannerState::CharMode {
        for character in self.input.chars() {
            match char_match(character) {
                Some(string) => self.output.push(string),
                None => self.state = ScannerState::QuitMode
            }
        }
        self.state = ScannerState::Done;
    }
}

fn char_match(c: char) -> Option<String> {
    Some(match c {
        '+' => "Addition operator: +".into(),
        '-' => "Subtraction operator: -".into(),
        '*' => "Multiplication operator: *".into(),
        '/' => "Division operator: /".into(),
        '%' => "Modulus operator: %".into(),
        '^' => "Exponent operator: ^".into(),
        '=' => "Assignment operator: =".into(),
        ';' => "Statement terminator: ;".into(),
        ' ' => "Space, ignoring.".into(),

        'q' => return None,

        i @ '0'...'9' => format!("Integer digit: {}", i),
        c @ 'a'...'z' => format!("Variable name: {}", c),
        z @ _         => format!("Unrecognized token: {}", z)
    })
}

这样做完全避免了借用，并且更具可组合性。

- Veedrac

---

（给未来读者）为了避免在'q'之前对所有静态字符串进行分配和复制，可以考虑使用并返回Option<std::borrow::Cow>。 - vallentin