如何在递归函数中累加值？

Question

我正在尝试了解借用检查器在递归函数调用情况下的工作原理：

fn visit_dirs_rec(dir: &Path, mut acc: Vec<PathBuf>) -> Result<Vec<PathBuf>, io::Error> {
    if dir.is_dir() {
        for entry in std::fs::read_dir(dir)? {
            let entry = entry?;
            let path = entry.path();
            if path.is_dir() {
                visit_dirs_rec(&path, &acc)?;
            } else if path.is_file() {
                acc.push(path)
            } else {
                error!("{:?}", path);
            }
        }
    }
    Ok(acc)
}

fn visit_dirs(dir: &Path) -> Result<Vec<PathBuf>, io::Error> {
    visit_dirs_rec(dir, vec![])
}

error[E0382]: use of moved value: `acc`
  --> src/main.rs:16:39
   |
10 | fn visit_dirs_rec(dir: &Path, mut acc: Vec<PathBuf>) -> Result<Vec<PathBuf>, io::Error> {
   |                               ------- move occurs because `acc` has type `Vec<PathBuf>`, which does not implement the `Copy` trait
11 |     if dir.is_dir() {
12 |         for entry in std::fs::read_dir(dir)? {
   |         ------------------------------------ inside of this loop
...
16 |                 visit_dirs_rec(&path, acc)?;
   |                                       ^^^ value moved here, in previous iteration of loop
   |
note: consider changing this parameter type in function `visit_dirs_rec` to borrow instead if owning the value isn't necessary
  --> src/main.rs:10:40
   |
10 | fn visit_dirs_rec(dir: &Path, mut acc: Vec<PathBuf>) -> Result<Vec<PathBuf>, io::Error> {
   |    -------------- in this function     ^^^^^^^^^^^^ this parameter takes ownership of the value
help: consider cloning the value if the performance cost is acceptable
   |
16 |                 visit_dirs_rec(&path, acc.clone())?;
   |                                          ++++++++

For more information about this error, try `rustc --explain E0382`.
warning: `api` (bin "api") generated 1 warning
error: could not compile `api` (bin "api") due to previous error; 1 warning emitted

有没有办法让借阅检查员满意而无需.clone？

Answer 1

您可以向函数传递可变引用，因此所有递归调用都将使用相同的数组：

// You need to explicitly mark the function as accepting a mutable reference
fn visit_dirs_rec(dir: &Path, acc: &mut Vec<PathBuf>) -> Result<(), io::Error> {
    if dir.is_dir() {
        for entry in std::fs::read_dir(dir)? {
            let entry = entry?;
            let path = entry.path();
            if path.is_dir() {
                visit_dirs_rec(&path, acc)?;
            } else if path.is_file() {
                acc.push(path)
            } else {
                error!("{:?}", path);
            }
        }
    }
    // Value is just added to the input variable, so no need to return anything
    Ok(())
}

fn visit_dirs(dir: &Path) -> Result<Vec<PathBuf>, io::Error> {
    // create a variable for the values to be outputted into
    let mut out = vec![]
    visit_dirs_rec(dir, &mut out)?;
    // return the output
    return Ok(out);
}