attributed string fast equality (#1415)

Author: vanvoorden

Benchmarks/Benchmarks/AttributedString/BenchmarkAttributedString.swift

@@ -423,13 +423,21 @@ let benchmarks = {
     let manyAttributesSubstring = manyAttributesString[manyAttributesStringRange]
     let manyAttributes2Substring = manyAttributesString2[manyAttributesStringRange]
 
+    Benchmark("equalityShared") { benchmark in
+        blackHole(manyAttributesString == manyAttributesString)
+    }
+
     Benchmark("equality") { benchmark in
         blackHole(manyAttributesString == manyAttributesString2)
     }
-    
+
     Benchmark("equalityDifferingCharacters") { benchmark in
         blackHole(manyAttributesString == manyAttributesString3)
     }
+
+    Benchmark("substringEqualityShared") { benchmark in
+        blackHole(manyAttributesSubstring == manyAttributesSubstring)
+    }
 
     Benchmark("substringEquality") { benchmark in
         blackHole(manyAttributesSubstring == manyAttributes2Substring)

Sources/FoundationEssentials/AttributedString/AttributedString.swift

@@ -150,7 +150,10 @@ extension AttributedString {
 @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
 extension AttributedString { // Equatable
     public static func == (lhs: Self, rhs: Self) -> Bool {
-        AttributedString.Guts.characterwiseIsEqual(lhs._guts, to: rhs._guts)
+        if lhs._guts === rhs._guts {
+            return true
+        }
+        return AttributedString.Guts.characterwiseIsEqual(lhs._guts, to: rhs._guts)
     }
 }