[StdLib][RFC][DNM] Add isIdentical Methods for Quick Comparisons to String and Substring

[vanvoorden]

Background

swiftlang/swift-evolution#2875

We propose new isIdentical instance methods to the following concrete types for determining in constant-time if two instances must be equal by-value:

• String
• Substring
• Array
• ArraySlice
• ContiguousArray
• Dictionary
• Set

Instead of “one big diff”… we can try and keep the diffs grouped together by similar functionality:

• String, Substring
• Array, ArraySlice, ContiguousArray
• Dictionary, Set

Let’s start with a diff to add isIdentical to String and Substring.

Changes

String

We already expose a public-but-underscored method on String to return identity equality.¹ We can add a new public-and-supported method:

extension String {
  @_alwaysEmitIntoClient
  public func isIdentical(to other: Self) -> Bool {
    self._guts.rawBits == other._guts.rawBits
  }
}

What happens to the existing underscored method?

• Delete it?
• Deprecate it?
• Leave it?

For now… we don't need to have a strong opinion about the underscored method. The underscored method is alwaysEmitIntoClient and already deploys back to earlier releases. Deleting the underscored method would then mean a breaking change for apps that already depend on the underscored method.

These implementation diffs will unblock a formal proposal review. My understanding is we don't need to block a proposal review on deciding the fate of the underscored method. We can keep that conversation going and either update this diff with a new strategy or land some cleanup in a follow up diff before the next branch cut.

Substring

We do not currently expose identity equality on Substring. We can follow a similar pattern to String:

extension Substring {
  @_alwaysEmitIntoClient
  public func isIdentical(to other: Self) -> Bool {
    self._wholeGuts.rawBits == other._wholeGuts.rawBits &&
    self._offsetRange == other._offsetRange
  }
}

Test Plan

New tests were added for String and Substring.

Benchmarks

New benchmarks were added for String and Substring.

Footnotes

1. https://github.com/swiftlang/swift/blob/swift-6.1.2-RELEASE/stdlib/public/core/String.swift#L397-L415 ↩

[vanvoorden]

@lorentey No big rush… but if you had any advice about these implementations that would be a big help. I'm planning to post another pitch to swift forums on monday. It's just a pitch thread… so it's not blocked on implementation diffs.

I would expect at least one more week before I would have some more implementations for the rest of the types to unblock a proposal review thread.