The trait Extend is not implemented for Vec when partitioning an iterator

Question

I am running into the error when calling .partition() on a vector iterator:

error[E0277]: the trait bound `std::vec::Vec<std::result::Result<std::collections::HashSet<&std::string::String>, std::boxed::Box<dyn std::error::Error>>>: std::iter::Extend<&std::result::Result<std::collections::HashSet<std::string::String>, std::boxed::Box<dyn std::error::Error>>>` is not satisfied
 --> src/main.rs:9:24
  |
9 |         results.iter().partition(|r| r.is_ok());
  |                        ^^^^^^^^^ the trait `std::iter::Extend<&std::result::Result<std::collections::HashSet<std::string::String>, std::boxed::Box<dyn std::error::Error>>>` is not implemented for `std::vec::Vec<std::result::Result<std::collections::HashSet<&std::string::String>, std::boxed::Box<dyn std::error::Error>>>`
  |
  = help: the following implementations were found:
            <std::vec::Vec<T> as std::iter::Extend<&'a T>>
            <std::vec::Vec<T> as std::iter::Extend<T>>

When running the following code:

use std::collections::HashSet;

type Result<T> = std::result::Result<T, Box<dyn std::error::Error>>;

fn main() {
    let mut results: Vec<Result<HashSet<String>>> = Default::default();

    let (okays, errors): (Vec<Result<HashSet<&String>>>, Vec<_>) =
        results.iter().partition(|r| r.is_ok());
}

See playground for example.

Answer 1

As the error message states (with namespacing removed):

the trait Extend<&Result<HashSet<String>, Box<dyn Error>>> is not implemented for Vec<Result<HashSet<&String>, Box<dyn Error>>>

You can't extend a Vec<T> with elements of type &T because they aren't the same type.

Instead, you can do one of these:

1. Change the type of the destination collection to Vec<&Result<HashSet<String>>> (or just Vec<_>, like your second destination type, to allow the compiler to infer the inner type).

2. Convert the reference to an owned value, perhaps via clone or to_owned.

3. Don't iterate over references to start with, using into_iter or drain instead.

However, your current type will be very hard or expensive to achieve, as you state that you want an owned Result with an owned HashMap but a reference the String.

I think the best thing is to use Itertools::partition_map and into_iter:

use itertools::Itertools; // 0.9.0
use std::collections::HashSet;

type Error = Box<dyn std::error::Error>;
type Result<T, E = Error> = std::result::Result<T, E>;

fn main() {
    let mut results: Vec<Result<HashSet<String>>> = Default::default();

    let (errors, okays): (Vec<_>, Vec<_>) = results.into_iter().partition_map(Into::into);
    // let (errors, okays): (Vec<Error>, Vec<HashSet<String>>)
}

See also:

• What is the difference between iter and into_iter?