Textfile-parsing function fails to compile owing to type-mismatch error

问题

I'm trying to parse a simple config text file, which contains one three-word entry per line, laid out as follows:

ITEM name value
ITEM name value
//etc.

I've reproduced the function which does the parsing (and the subsequent compilation error) here (and on the Rust Playpen):

pub fn parse(path: &Path) -> config_struct {

    let file = File::open(&path).unwrap();
    let reader = BufReader::new(&file);
    let line_iterator = reader.lines();
    let mut connection_map = HashMap::new();
    let mut target_map = HashMap::new();

    for line in line_iterator {

        let line_slice = line.unwrap();
        let word_vector: Vec<&str> = line_slice.split_whitespace().collect();

        if word_vector.len() != 3 { continue; }

        match word_vector[0] {
            "CONNECTION" => connection_map.insert(word_vector[1], word_vector[2]),
            "TARGET" => target_map.insert(word_vector[1], word_vector[2]),
            _ => continue,
        }
    }

    config_struct { connections: connection_map, targets: target_map }
}

pub struct config_struct<'a>  {
    // <name, value>
    connections: HashMap<&'a str, &'a str>,
    // <name, value>
    targets: HashMap<&'a str, &'a str>,
}


src/parse_conf_file.rs:23:3: 27:4 error: mismatched types:
 expected `()`,
    found `core::option::Option<&str>`
(expected (),
    found enum `core::option::Option`) [E0308]
src/parse_conf_file.rs:23 match word_vector[0] {
src/parse_conf_file.rs:24   "CONNECTION" => connection_map.insert(word_vector[1], word_vector[2]),
src/parse_conf_file.rs:25   "TARGET" => target_map.insert(word_vector[1], word_vector[2]),
src/parse_conf_file.rs:26   _ => continue,
src/parse_conf_file.rs:27 }

In essence, I seem to have created a match statement that expects an empty tuple, and also finds the contents of a Vec<&str> to be wrapped in an Option!

NB. This post originally contained two questions (that I'd believed were one error manifesting itself differently), but as-per advice in the comments I've split it into two separate posts. The latter post is here.

回答1:

Your original problem is just that you have a non-() expression at the end of your loop body. Your match expression has type Option<&str> (because that is the return type of HashMap::insert), not type (). This problem is solved by simply putting a semicolon after the match expression:

match word_vector[0] {
    "CONNECTION" => connection_map.insert(word_vector[1], word_vector[2]),
    "TARGET" => target_map.insert(word_vector[1], word_vector[2]),
    _ => continue,
};

For the latter, isn't word_vector populated with owned objects that don't point to line_slice?

No, which is precisely the issue. word_vector contains elements of type &str, i.e. borrowed strings. These point into line_slice, which only lives until the end of the current loop iteration. You probably want to convert them to Strings (using String::from) before inserting them into the map.

来源：https://stackoverflow.com/questions/31620888/textfile-parsing-function-fails-to-compile-owing-to-type-mismatch-error

标签

rust

text-parsing

lifetime

type-mismatch

borrow-checker