INHTWAMA

[jesse99]

The current borrow checker can be quite difficult to work around. imagine-never-hearing-the-phrase-aliasable has some ideas for improvements.

[jesse99]

I ran into an annoying problem with the borrow checker for a very common sort of operation: maintaining a cache with a LinearMap. Here is an example:

extern mod std;
use core::send_map::linear::{LinearMap};

pub type Cache = LinearMap<~str, ~str>;

fn animal_noise(cache: &mut Cache, animal: ~str) -> ~str
{
    fn find_noise(animal: ~str) -> ~str
    {
        let noises = ~[(~"dog", ~"The dog barks."), (~"cat", ~"The cat meows."), (~"cow", ~"The cow moos.")];
        match do noises.position |x| {x.first_ref() == &animal}
        {
            option::Some(index) => copy *noises[index].second_ref(),
            option::None => fmt!("Don't know what %s does.", animal)
        }
    }

    match cache.find_ref(&animal)   // currently can't call this because cache is mutable
    {
            Some(noise) => copy *noise,
            None =>
            {
                let noise = find_noise(copy animal);
                cache.insert(animal, copy noise);
                noise
            }
    }
}

#[test]
fn test_noises()
{
    let mut cache = LinearMap();
    assert animal_noise(&mut cache, ~"dog") == ~"The dog barks.";
    assert animal_noise(&mut cache, ~"dog") == ~"The dog barks.";
    assert animal_noise(&mut cache, ~"cat") == ~"The cat meows.";
}

Best fix I have found is to move the cache in and out of the function and introduce a helper function that works like LinearMap::find_ref but returns a copy instead. Bu INHTWAMA would allow &mut pointers to be temporarily frozen which should allow the above to be written in a much more natural manner.

[sanxiyn]

@nikomatsakis, this is done?

[nikomatsakis]

It's done enough. #5074 is still pending, but that's a more concrete issue.