[OrderedSet] forward ordered set equality to elements property

[vanvoorden]

Background

#335

Similar to OrderedDictionary, we can forward our OrderedSet equality checks down to the ContiguousArray instance to take advantage of early return when both arrays point to the same identity (no need to perform a linear comparison over all elements).

A side-effect of this optimization is we also optimize OrderedDictionary.==, which performs an equality check on the keys property (which is an OrderedSet).[1]

[1] https://github.com/apple/swift-collections/blob/main/Sources/OrderedCollections/OrderedDictionary/OrderedDictionary%2BEquatable.swift#L20-L22

---

Changes

From:

public static func ==(left: Self, right: Self) -> Bool {
  left.elementsEqual(right)
}

To:

public static func ==(left: Self, right: Self) -> Bool {
  left._elements == right._elements
}

We can also migrate OrderedSet.SubSequence. From:

public static func ==(left: Self, right: Self) -> Bool {
  left.elementsEqual(right)
}

To:

public static func ==(left: Self, right: Self) -> Bool {
  left._base._elements[left._bounds] == right._base._elements[right._bounds]
}

---

Test Plan

Five new tests are added:

• OrderedSetTests.test_equal
• OrderedSetTests.test_not_equal
• OrderedSetTests.test_equal_elements
• OrderedSetTests.test_subsequence_equality
• OrderedSetTests.test_subsequence_not_equality

---

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[vanvoorden]

Here's what the benchmarks look like running locally.

[vanvoorden]

Here are more benchmarks running locally.

[lorentey]

Great catch again -- we'll definitely want to have this change. 👍

I think I found a correctness issue, though; it also indicates that we should do another pass on the test additions, to improve coverage. 🤔 I commented some suggested changes. (Beware that I haven't tried them, though. 😛)

[vanvoorden]

@lorentey I confirmed that this test "broke" on the previous equality check with the bug.

[lorentey]

This looks ready! 👍

[lorentey]

@swift-ci test