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

benchmark/single-source/StringTests.swift

@@ -40,6 +40,10 @@ public var benchmarks: [BenchmarkInfo] {
       runFunction: run_StringHasSuffixUnicode,
       tags: [.validation, .api, .String],
       legacyFactor: 1000),
+    BenchmarkInfo(
+      name: "StringIdentical",
+      runFunction: run_StringIdentical,
+      tags: [.validation, .api, .String]),
   ]
 
   if #available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *) {
@@ -1676,3 +1680,13 @@ public func run_iterateWords(_ n: Int) {
     blackHole(swiftOrgHTML._words)
   }
 }
+
+public func run_StringIdentical(_ n: Int) {
+  let str1 = "The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. "
+  let str2 = str1
+  for _ in 0 ..< n {
+    for _ in 0 ..< 100_000 {
+      check(str1.isIdentical(to: str2))
+    }
+  }
+}

benchmark/single-source/SubstringTest.swift

@@ -30,6 +30,7 @@ public let benchmarks = [
   BenchmarkInfo(name: "SubstringFromLongString2", runFunction: run_SubstringFromLongString, tags: [.validation, .api, .String]),
   BenchmarkInfo(name: "SubstringFromLongStringGeneric2", runFunction: run_SubstringFromLongStringGeneric, tags: [.validation, .api, .String]),
   BenchmarkInfo(name: "SubstringTrimmingASCIIWhitespace", runFunction: run_SubstringTrimmingASCIIWhitespace, tags: [.validation, .api, .String]),
+  BenchmarkInfo(name: "SubstringIdentical", runFunction: run_SubstringIdentical, tags: [.validation, .String]),
 ]
 
 // A string that doesn't fit in small string storage and doesn't fit in Latin-1
@@ -332,3 +333,11 @@ public func run _LessSubstringSubstringGenericStringProtocol(_ n: Int) {
   }
 }
 */
+
+@inline(never)
+public func run_SubstringIdentical(_ n: Int) {
+  let (a, b) = (ss1, ss1)
+  for _ in 1...n*500 {
+    blackHole(a.isIdentical(to: b))
+  }
+}

stdlib/public/core/String.swift

@@ -1112,4 +1112,22 @@ extension String {
   }
 }
 
-
+extension String {
+  /// Returns a boolean value indicating whether this string is identical to
+  /// `other`.
+  ///
+  /// Two string values are identical if there is no way to distinguish between
+  /// them.
+  ///
+  /// Comparing strings this way includes comparing (normally) hidden
+  /// implementation details such as the memory location of any underlying
+  /// string storage object. Therefore, identical strings are guaranteed to
+  /// compare equal with `==`, but not all equal strings are considered
+  /// identical.
+  ///
+  /// - Performance: O(1)
+  @_alwaysEmitIntoClient
+  public func isIdentical(to other: Self) -> Bool {
+    self._guts.rawBits == other._guts.rawBits
+  }
+}

stdlib/public/core/Substring.swift

@@ -1307,3 +1307,24 @@ extension Substring {
     return Substring(_unchecked: Slice(base: base, bounds: r))
   }
 }
+
+extension Substring {
+  /// Returns a boolean value indicating whether this substring is identical to
+  /// `other`.
+  ///
+  /// Two substring values are identical if there is no way to distinguish
+  /// between them.
+  ///
+  /// Comparing substrings this way includes comparing (normally) hidden
+  /// implementation details such as the memory location of any underlying
+  /// substring storage object. Therefore, identical substrings are guaranteed
+  /// to compare equal with `==`, but not all equal substrings are considered
+  /// identical.
+  ///
+  /// - Performance: O(1)
+  @_alwaysEmitIntoClient
+  public func isIdentical(to other: Self) -> Bool {
+    self._wholeGuts.rawBits == other._wholeGuts.rawBits &&
+    self._offsetRange == other._offsetRange
+  }
+}

test/stdlib/StringAPI.swift

@@ -533,4 +533,41 @@ StringTests.test("hasPrefix/hasSuffix vs Character boundaries") {
   expectFalse(s2.hasSuffix("\n"))
 }
 
+StringTests.test("isIdentical(to:) small ascii") {
+  let a = "Hello"
+  let b = "Hello"
+
+  precondition(a == b)
+
+  expectTrue(a.isIdentical(to: a))
+  expectTrue(b.isIdentical(to: b))
+  expectTrue(a.isIdentical(to: b)) // Both small ASCII strings
+  expectTrue(b.isIdentical(to: a))
+}
+
+StringTests.test("isIdentical(to:) small unicode") {
+  let a = "Cafe\u{301}"
+  let b = "Cafe\u{301}"
+  let c = "Café"
+
+  precondition(a == b)
+  precondition(b == c)
+
+  expectTrue(a.isIdentical(to: b))
+  expectTrue(b.isIdentical(to: a))
+  expectFalse(a.isIdentical(to: c))
+  expectFalse(b.isIdentical(to: c))
+}
+
+StringTests.test("isIdentical(to:) large ascii") {
+  let a = String(repeating: "foo", count: 1000)
+  let b = String(repeating: "foo", count: 1000)
+
+  precondition(a == b)
+
+  expectFalse(a.isIdentical(to: b)) // Two large, distinct native strings
+  expectTrue(a.isIdentical(to: a))
+  expectTrue(b.isIdentical(to: b))
+}
+
 runAllTests()

test/stdlib/subString.swift

@@ -31,6 +31,41 @@ func checkHasContiguousStorageSubstring(_ x: Substring.UTF8View) {
   expectTrue(hasStorage)
 }
 
+fileprivate func slices(
+  _ s: String,
+  from: Int,
+  to: Int
+) -> (
+  Substring,
+  Substring,
+  Substring
+) {
+  let s1 = s[s.index(s.startIndex, offsetBy: from) ..<
+    s.index(s.startIndex, offsetBy: to)]
+  let s2 = s1[s1.startIndex..<s1.endIndex]
+  let s3 = s2[s1.startIndex..<s1.endIndex]
+  return (s1, s2, s3)
+}
+
+fileprivate func allNotEmpty(
+  _ s: Substring...
+) -> Bool {
+  s.allSatisfy { $0.isEmpty == false }
+}
+
+fileprivate func allEqual(
+  _ s: Substring...
+) -> Bool {
+  for i in 0..<s.count {
+    for j in (i + 1)..<s.count {
+      if s[i] != s[j] {
+        return false
+      }
+    }
+  }
+  return true
+}
+
 SubstringTests.test("Equality") {
   let s = "abcdefg"
   let s1 = s[s.index(s.startIndex, offsetBy: 2) ..<
@@ -282,4 +317,101 @@ SubstringTests.test("Substring.base") {
   }
 }
 
+SubstringTests.test("isIdentical(to:) small ascii") {
+  let (a1, a2, a3) = slices("Hello", from: 2, to: 4)
+  let (b1, b2, b3) = slices("Hello", from: 2, to: 4)
+
+  precondition(allNotEmpty(a1, a2, a3, b1, b2, b3))
+  precondition(allEqual(a1, a2, a3, b1, b2, b3))
+
+  expectTrue(a1.isIdentical(to: a1))
+  expectTrue(a1.isIdentical(to: a2))
+  expectTrue(a1.isIdentical(to: a3))
+  expectTrue(a1.isIdentical(to: b1))
+  expectTrue(a1.isIdentical(to: b2))
+  expectTrue(a1.isIdentical(to: b3))
+
+  expectTrue(a2.isIdentical(to: a1))
+  expectTrue(a2.isIdentical(to: a2))
+  expectTrue(a2.isIdentical(to: a3))
+  expectTrue(a2.isIdentical(to: b1))
+  expectTrue(a2.isIdentical(to: b2))
+  expectTrue(a2.isIdentical(to: b3))
+
+  expectTrue(a3.isIdentical(to: a1))
+  expectTrue(a3.isIdentical(to: a2))
+  expectTrue(a3.isIdentical(to: a3))
+  expectTrue(a3.isIdentical(to: b1))
+  expectTrue(a3.isIdentical(to: b2))
+  expectTrue(a3.isIdentical(to: b3))
+}
+
+SubstringTests.test("isIdentical(to:) small unicode") {
+  let (a1, a2, a3) = slices("Hello Cafe\u{301}", from: 2, to: 4)
+  let (b1, b2, b3) = slices("Hello Cafe\u{301}", from: 2, to: 4)
+  let (c1, c2, c3) = slices("Hello Café", from: 2, to: 4)
+
+  precondition(allNotEmpty(a1, a2, a3, b1, b2, b3, c1, c2, c3))
+  precondition(allEqual(a1, a2, a3, b1, b2, b3, c1, c2, c3))
+
+  expectTrue(a1.isIdentical(to: a1))
+  expectTrue(a1.isIdentical(to: a2))
+  expectTrue(a1.isIdentical(to: a3))
+  expectTrue(a1.isIdentical(to: b1))
+  expectTrue(a1.isIdentical(to: b2))
+  expectTrue(a1.isIdentical(to: b3))
+  expectFalse(a1.isIdentical(to: c1))
+  expectFalse(a1.isIdentical(to: c2))
+  expectFalse(a1.isIdentical(to: c3))
+
+  expectTrue(a2.isIdentical(to: a1))
+  expectTrue(a2.isIdentical(to: a2))
+  expectTrue(a2.isIdentical(to: a3))
+  expectTrue(a2.isIdentical(to: b1))
+  expectTrue(a2.isIdentical(to: b2))
+  expectTrue(a2.isIdentical(to: b3))
+  expectFalse(a2.isIdentical(to: c1))
+  expectFalse(a2.isIdentical(to: c2))
+  expectFalse(a2.isIdentical(to: c3))
+
+  expectTrue(a3.isIdentical(to: a1))
+  expectTrue(a3.isIdentical(to: a2))
+  expectTrue(a3.isIdentical(to: a3))
+  expectTrue(a3.isIdentical(to: b1))
+  expectTrue(a3.isIdentical(to: b2))
+  expectTrue(a3.isIdentical(to: b3))
+  expectFalse(a3.isIdentical(to: c1))
+  expectFalse(a3.isIdentical(to: c2))
+  expectFalse(a3.isIdentical(to: c3))
+}
+
+SubstringTests.test("isIdentical(to:) large ascii") {
+  let (a1, a2, a3) = slices(String(repeating: "Hello", count: 1000), from: 2, to: 4)
+  let (b1, b2, b3) = slices(String(repeating: "Hello", count: 1000), from: 2, to: 4)
+
+  precondition(allNotEmpty(a1, a2, a3, b1, b2, b3))
+  precondition(allEqual(a1, a2, a3, b1, b2, b3))
+
+  expectTrue(a1.isIdentical(to: a1))
+  expectTrue(a1.isIdentical(to: a2))
+  expectTrue(a1.isIdentical(to: a3))
+  expectFalse(a1.isIdentical(to: b1))
+  expectFalse(a1.isIdentical(to: b2))
+  expectFalse(a1.isIdentical(to: b3))
+
+  expectTrue(a2.isIdentical(to: a1))
+  expectTrue(a2.isIdentical(to: a2))
+  expectTrue(a2.isIdentical(to: a3))
+  expectFalse(a2.isIdentical(to: b1))
+  expectFalse(a2.isIdentical(to: b2))
+  expectFalse(a2.isIdentical(to: b3))
+
+  expectTrue(a3.isIdentical(to: a1))
+  expectTrue(a3.isIdentical(to: a2))
+  expectTrue(a3.isIdentical(to: a3))
+  expectFalse(a3.isIdentical(to: b1))
+  expectFalse(a3.isIdentical(to: b2))
+  expectFalse(a3.isIdentical(to: b3))
+}
+
 runAllTests()