Pushing to a vector with a lifetime

[heady bloom]

I have the following code:

    value: &'a str,
    children: Option<Vec<Parent<'a>>>,
}

impl<'a> Parent<'a> {
    pub fn new(value: &'a str, children: Option<Vec<Parent<'a>>>) -> Self {
        Self { value, children }
    }

    pub fn add_child(&'a mut self, parent: Parent<'a>) {
        self.children.as_mut().unwrap().push(parent);
    }
}

And I can't figure out how to call add_child() twice without getting the error parent was mutably borrowed here in the previous iteration of the loop. When I use &'static it works, but I would like to know the proper way to fix this

[frozen nimbus]

my advice would be to remove 'a from &'a mut self

also Option<Vec> is a weird and inconvenient type

an empty Vec is good enough (it doesn't allocate)

[heady bloom]

i see, thank you for the help! I think i sort of get why removing the 'a works, but i can't quite put it into words haha

[heady bloom]

thanks for the advice

[heady bloom]

oh, i didn't know this

[frozen nimbus]

creating a &'a mut Whatever<'a> is an immediate red flag, it's a nearly useless type that constricts what you can borrow considerably

although that's almost folklore, it's not really explained anywhere

[heady bloom]

can you correct me if my explanation is wrong? let's say i have this code:

fn main() {
    let mut parent = Parent::new("", Some(vec![]));
    parent.add_child(Parent::new("", None));
    parent.add_child(Parent::new("", None));
}

is the lifetime of parent the same as that of main? if it is, then when add_child() is called for the first time, it holds onto a mutable reference of parent for the lifetime of main, which is why the second add_child() cannot have another mutable reference to parent

[frozen nimbus]

yep, that's more or less it

although there's no explanation why it has to hold it for the whole main

that involves variance, which is a complicated topic

one more thing is that it's possible that a &'a self could work fine

that also has to do with variance

[heady bloom]

i see, is it something that would change my view on this drastically?

[frozen nimbus]

I'm not sure 😅

[heady bloom]

haha, maybe i should go and read up on it

but thanks so much for this, i managed to learn a lot