Why do I get “type annotations needed” when using Iterator::collect?

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On an iterator, the collect method can produce many types of collections:

fn collect<B>(self) -> B
where
    B: FromIterator<Self::Item>,

Types that implement FromIterator include Vec, String and many more. Because there are so many possibilities, something needs to constrain the result type. You can specify the type with something like .collect::<Vec<_>>() or let something: Vec<_> = some_iter.collect().

Until the type is known, you cannot call the method len() because it’s impossible to know if an unknown type has a specific method.

If you’re purely wanting to find out how many items there are in an iterator, use Iterator.count(); creating a vector for the purpose is rather inefficient.

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