Why does the implementation of `FromIterator` use an `IntoIterator` instead of an `Iterator`?

Question

The example implementation of the FromIterator trait in the Rust docs is:

impl FromIterator<i32> for MyCollection {
    fn from_iter<I: IntoIterator<Item=i32>>(iter: I) -> Self {
        let mut c = MyCollection::new();

        for i in iter {
            c.add(i);
        }

        c
    }
}

FromIterator defines how a type will be created from an iterator. The signature of from_iter requires a type that implements IntoIterator, which defines how a type may be converted into an Iterator.

Is from_iter defined this way because IntoIterator is not as strict a requirement as Iterator?

Shepmaster · Accepted Answer · 2019-05-24 19:10:39Z

3

Is from_iter defined this way because IntoIterator is not as strict a requirement as Iterator?

Yes.

IntoIterator is automatically implemented for Iterator, therefore the set of types implementing IntoIterator is a superset of those implementing Iterator.

When crafting a generic function, it's good to minimize its requirements, i.e., make it as generic as possible.

Of course, there's a trade-off between:

ease of use: works with as many types as possible,
ease of implementation.

For a lone function in your isolated codebase, it may not be worth optimizing for ease-of-use much; for a trait/function in the standard library, since the number of users far outweigh the number of developers (of this particular piece), the trade-off is strongly in favor of ease-of-use.

And since going from IntoIterator to Iterator is so simple, and thus doesn't weigh much on the implementation, it's an easy call to make.

edited May 24, 2019 at 19:10

Shepmaster

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answered Feb 25, 2018 at 19:08

Matthieu M.

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