CODEWITHSUNDEEP
rusttraits
Ray Toal
Ray Toal

Reputation: 88398

How is `last` allowed to be called for an Args value?

Here is a small Rust script (yes, I know unwrap is discouraged but it works for this short example):

fn main() {
    let args = std::env::args();
    let word = args.last().unwrap();
    println!("Last argument is {}", word);
}

I see from the Rust documentation that

However, neither Args nor Iterator nor ExactSizeIterator, as far as I can tell from the documentation, ever defines a method called last! Args defines nothing on its own, Iterator just defines next and size_hint, and ExactSizeIterator defines len.

So why does this script work? How is last allowed to be called for an Args value? Is the documentation lacking or am I missing something?

Upvotes: 1

Views: 108

Answers (2)

Shepmaster
Shepmaster

Reputation: 431279

Sometimes you need to work both directions in the documentation. Doing a search for last will lead you to IteratorExt::last:

fn last(self) -> Option<<Self as Iterator>::Item>

IteratorExt is defined as:

pub trait IteratorExt where Self: Iterator {
    // ...
}

This is an extension trait - a collection of methods that are all applied to another trait. It's done this way to help preserve object safety, as some of these methods require consuming the argument, which means that the argument has to have a known size. The trait has a blanket implementation to get it to apply:

impl<I> IteratorExt for I where I: Iterator

However, there's been some work recently to reduce the number of extension traits, as it was realized that you can simply add a bound of Sized to certain methods. It's possible some of that might apply to IteratorExt and they will move back to Iterator proper.

Upvotes: 2

Vaelden
Vaelden

Reputation: 2249

The last method comes from the IteratorExt trait, which defines methods for all iterators by having a impl<I> IteratorExt for I where I: Iterator blanket implementation.

These methods are in scope, because the trait IteratorExt is imported in the prelude.

Upvotes: 4

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