What does String.iter() do in Rust?

问题:

I am getting confused with some code from Rust but I am managing it. But I have problem in understanding how the function iter() work in Rust. What is the possible result for a String?

Edit

impl Selector {
    pub fn specificity(&self) -> Specificity {
        // http://www.w3.org/TR/selectors/#specificity
        let Selector::Simple(ref simple) = *self;
        let a = simple.id.iter().len();
        let b = simple.class.len();
        let c = simple.tag_name.iter().len();
        (a, b, c)
    }
}

Where simple.id is string.

回答1:

You seem to be referencing code from Robinson, specifically this file.

Note that SimpleSelector is defined as:

pub struct SimpleSelector {
    pub tag_name: Option<String>,
    pub id: Option<String>,
    pub class: Vec<String>,
}

So id is not a String, but an Option<String>.

iter on Option is defined as:

Returns an iterator over the possibly contained value.

This allows you to write code that will happen if the value is Some and will not happen if it is None.