How do I return local data with a recursive function in Rust?

Question

I'm struggling to write a recursive algorithm in Rust. With the following code:

use std::collections::HashMap;

enum Error {
    Bad,
    ReallyBad,
}

fn expand_symbols<'a, T: AsRef<str>>(
    symbols: &'a [T],
    ops: &HashMap<String, String>,
    user_ops: &'a HashMap<String, String>,
) -> std::result::Result<Vec<&'a str>, Error> {
    if symbols.iter().all(|x| ops.get(x.as_ref()).is_some()) {
        let symbols = symbols.iter().map(|x| x.as_ref()).collect();
        return Ok(symbols);
    }

    let mut expanded: Vec<&str> = vec![];
    for s in symbols {
        let s = s.as_ref();
        if ops.contains_key(s) || s.parse::<i32>().is_ok() {
            expanded.push(s);
        } else {
            let mut resolved = user_ops
                .get(s)
                .ok_or(Error::Bad)?
                .split_ascii_whitespace()
                .collect::<Vec<_>>();
            expanded.append(&mut resolved);
        }
    }
    expand_symbols(&expanded, ops, user_ops)
}

I get:

error[E0515]: cannot return value referencing local variable `expanded`
  --> src/main.rs:32:5
   |
32 |     expand_symbols(&expanded, ops, user_ops)
   |     ^^^^^^^^^^^^^^^---------^^^^^^^^^^^^^^^^
   |     |              |
   |     |              `expanded` is borrowed here
   |     returns a value referencing data owned by the current function

For more information about this error, try `rustc --explain E0515`.

However, if I change the last statement to:

Ok(expanded)

it works, but it's not longer recursive.

I understand the idea that I'm trying to return a value borrowed from a local frame, but I think this is safe based on the second example. How can I tell the compiler that?

Note: I'm using AsRef because I want to be able to pass both a Vec<String> and Vec<&str> to expand_symbols() . Maybe I need to forget about that?

Answer 1

With Ok(expanded) the variable expanded is moved out of the function, meaning no reference to it existing after the function returned. So, the second sample, if you meant by Ok(expanded) , is not same as the original one.

To address the issue I think you can pass a mutable reference to symbols as the first parameter of the function, and do in-place edit on it instead of creating a new local vector, explanded .

fn expand_symbols<'a>(
    symbols: &'a mut Vec<&'a str>,
    ops: &HashMap<String, String>,
    user_ops: &'a HashMap<String, String>,
) -> std::result::Result<&'a Vec<&'a str>, Error> {
    if symbols.is_empty() || symbols.iter().all(|x| ops.get(*x).is_some()) {
         return Ok(symbols);
    }

    let mut unresolved: Vec<&str> = vec![]; 
    let mut i = 0;
    while i < symbols.len() {
        let s = symbols[i];
        if ops.contains_key(s) || s.parse::<i32>().is_ok() {
            i += 1;
        } else {
            unresolved.push(symbols.remove(i));
        }
    }

    for s in unresolved.iter() {
        let mut resolved = user_ops
            .get(*s)
            .ok_or(Error::Bad)?
            .split_ascii_whitespace()
            .collect::<Vec<_>>();
        symbols.append(&mut resolved);
    };

    expand_symbols(symbols, ops, user_ops)
}

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