Borrow checker doesn't realize that `clear` drops reference to local variable

Question

The following code reads space-delimited records from stdin, and writes comma-delimited records to stdout. Even with optimized builds it\'s rather slow (about twice as slow

Answer 1

Another approach is to refrain from storing references altogether, and to store indices instead. This trick can also be useful in other data structure contexts, so this might be a nice opportunity to try it out.

use std::io::BufRead;

fn main() {
    let stdin = std::io::stdin();
    let mut cache = Vec::new();
    for line in stdin.lock().lines().map(|x| x.unwrap()) {
        cache.push(0);
        cache.extend(line.match_indices(' ').map(|x| x.0 + 1));
        // cache now contains the indices where new words start

        // do something with this information
        for i in 0..(cache.len() - 1) {
            print!("{},", &line[cache[i]..(cache[i + 1] - 1)]);
        }
        println!("{}", &line[*cache.last().unwrap()..]);
        cache.clear();
    }
}

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Though you made the remark yourself in the question, I feel the need to point out that there are more elegant methods to do this using iterators, that might avoid the allocation of a vector altogether.

The approach above was inspired by a similar question here, and becomes more useful if you need to do something more complicated than printing.