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

The safe solution is to use .drain(..) instead of .clear() where .. is a "full range". It returns an iterator, so drained elements can be processed in a loop. It is also available for other collections (String, HashMap, etc.)

fn main() {
    let mut cache = Vec::<&str>::new();
    for line in ["first line allocates for", "second"].iter() {
        println!("Size and capacity: {}/{}", cache.len(), cache.capacity());
        cache.extend(line.split(' '));
        println!("    {}", cache.join(","));
        cache.drain(..);
    }
}