[Pre-RFC] Add FromIterator impl for HashMap<K, Vec<V>>

[vultix]

Summary

This RFC proposes a new FromIterator<(K, V)> implementation for HashMap<K, Vec<V>>.

Motivation

It's relatively common to want to group a list of values over some specific key. The current method of doing this either requires a third party crate or manually constructing the HashMap in a for loop, making use of the Entry API.

This is common enough that I was surprised not to see an ergonomic approach using collect. This RFC proposes adding an impl to make this possible.

Example

As an example, imagine you had a list of books, and wanted to group the books by the author that wrote them.

Current Implementation

let mut books_by_author: HashMap<&str, Vec<&Book>> = HashMap::new();  
  
for book in &books {  
    books_by_author.entry(book.author).or_default().push(book);  
}

With This RFC

let books_by_author: HashMap<&str, Vec<&Book>> =
    books.iter().map(|book| (book.author, book)).collect();

Future Considerations

• Instead of adding this implementation for just Vec<V>, we could consider adding it for all collections C: Default + Extend<V>, extending this functionality to all collections, such as a HashSet.

Prior Art

• multimap
• itertools::into_group_map