UTF-8 repairing ASCII fast-path

Author: milseman

stdlib/public/core/Character.swift

@@ -205,13 +205,15 @@ extension String {
 
 extension Character : Equatable {
   @inlinable @inline(__always)
+  @_effects(readonly)
   public static func == (lhs: Character, rhs: Character) -> Bool {
     return lhs._str == rhs._str
   }
 }
 
 extension Character : Comparable {
   @inlinable @inline(__always)
+  @_effects(readonly)
   public static func < (lhs: Character, rhs: Character) -> Bool {
     return lhs._str < rhs._str
   }

stdlib/public/core/StringCreate.swift

@@ -65,7 +65,11 @@ extension String {
   @usableFromInline
   internal static func _fromUTF8Repairing(
     _ input: UnsafeBufferPointer<UInt8>
-  ) -> (String, Bool) {
+  ) -> (result: String, repairsMade: Bool) {
+    if _allASCII(input) {
+      return (String._uncheckedFromUTF8(input, asciiPreScanResult: true), false)
+    }
+
     // TODO(UTF8 perf): More efficient validation
 
     // TODO(UTF8 perf): Skip intermediary array
@@ -95,6 +99,21 @@ extension String {
     return storage.asString
   }
 
+  // If we've already pre-scanned for ASCII, just supply the result
+  @usableFromInline
+  internal static func _uncheckedFromUTF8(
+    _ input: UnsafeBufferPointer<UInt8>, asciiPreScanResult: Bool
+  ) -> String {
+    if let smol = _SmallString(input) {
+      return String(_StringGuts(smol))
+    }
+
+    let isASCII = asciiPreScanResult
+    let storage = _StringStorage.create(
+      initializingFrom: input, isASCII: isASCII)
+    return storage.asString
+  }
+
   @usableFromInline
   internal static func _uncheckedFromUTF16(
     _ input: UnsafeBufferPointer<UInt16>