left_join

In std/collections.

fn left_join<T, U, K, R>(self: Seq<T>, other: Seq<U>, *, left: T -> K, right: U -> K, on: (T, U) -> Bool, into: (T, Option<U>) -> R) : Seq<R>
fn left_join<T, U, K, R>(self: Seq<T>, other: Seq<U>, *, left: T -> K, right: U -> K, into: (T, Option<U>) -> R) : Seq<R>
fn left_join<T, U, R>(self: Seq<T>, other: Seq<U>, *, on: (T, U) -> Bool, into: (T, Option<U>) -> R) : Seq<R>

A join that keeps every element of the input: one with no match in other is built by into with none on the right.