为什么 IntoIterator 特征需要显式指定关联类型 Item？

Question

Since the associated type IntoIter of the IntoIterator trait, implements the Iterator trait, isn't that sufficient to infer the associated type Item ?

Answer 1

Why does the IntoIterator trait require explicit type Item declaration?

It doesn't . You are correct that when you impl IntoIterator for... then Item is redundant , and could be obtained through IntoIter .

---

This was introduced in PR #22313 . In short, the reason is was introduced was to simplify where clauses. If you have to specify IntoIter then that can quickly become cumbersome.

where I: IntoIterator<IntoIter = ...>

In that case it is much easier to do:

where I: IntoIterator<Item = ...>

---

Let's consider a random example, like print_strings . Then before you needed to do something like this:

fn print_strings<I, T>(iter: I)
where
    I: IntoIterator<IntoIter = T>,
    T: Iterator<Item = &'static str>,
{
    for s in iter {
        println!("{}", s);
    }
}

Whereas now that can be simplified to just:

fn print_strings<I>(iter: I)
where
    I: IntoIterator<Item = &'static str>,
{
    for s in iter {
        println!("{}", s);
    }
}

fn main() {
    print_strings(vec!["foo", "bar", "baz"]);
}