[RFC][DNM] Add isIdentical Method for Quick Comparisons to Data

[vanvoorden]

Background

#1383

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

• AttributedString
• AttributedSubstring
• Data

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

• AttributedString, AttributedSubstring
• Data

Changes

Most of our clues to check identity equality come from the new "fast path" we landed on Data.¹

extension Data {
  @_alwaysEmitIntoClient
  public func isIdentical(to other: Self) -> Bool {
    // See if both are empty
    ...

    // Unequal length data can never be equal
    ...
    
    if length1 > 0 {
        return self.withUnsafeBytes { (b1: UnsafeRawBufferPointer) in
            return other.withUnsafeBytes { (b2: UnsafeRawBufferPointer) in
                // If they have the same base address and same count, it is equal
                ...
            }
        }
    }
    return true
  }
}

Test Plan

TODO

Benchmarks

TODO

Footnotes

1. https://github.com/swiftlang/swift-foundation/pull/1322 ↩