[swift-evolution] (Draft) Add last(where:) and lastIndex(where:) methods

[Nate Cook]

I've needed these in the past and used them in other languages—any feedback on this  idea?

Add last(where:) and lastIndex(where:) Methods to Bidirectional Collections
The standard library should include methods for finding the last element of a bidirectional collection that matches a predicate, along with the index of that element.

Motivation
The standard library currently has (or will soon have) methods that perform a linear search from the beginning of a collection to find an element that matches a predicate:

let a = [20, 30, 10, 40, 20, 30, 10, 40, 20]
a.first(where: { $0 > 25 })         // 30
a.index(of: 10)                     // 2
a.index(where: { $0 > 25 })         // 1
Unfortunately, there is no such method that searches from the end of a bidirectional collection. Finding the last of particular kind of element has multiple applications, particularly with text, such as wrapping a long string into lines of a maximum length or trimming whitespace from the beginning and end of a string.

This limitation can be worked around by using the methods above on the reversed collection, but the resulting code is truly dreadful. For example, to find the corresponding last index to a.index(where: { $0 > 25 }), this unholy incantation is required:

(a.reversed().index(where: { $0 > 25 })?.base).flatMap({ a.index(before: $0) })
Wat.

Proposed solution
Bidirectional collections should include three new methods for symmetry with the existing forward-searching APIs: last(where:), lastIndex(where:), and lastIndex(of:), specifically for collections of Equatable elements.

These additions would remove the need for searching in a reversed collection and allow code like the following:

a.last(where: { $0 > 25 })          // 40
a.lastIndex(of: 10)                 // 6
a.lastIndex(where: { $0 > 25 })     // 7
Much better!
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