[String] Speed up UTF-8 initialization from non-stdlib types

Avoid generic code paths if initializing for UTF-8 (the vast majority
of the time) from a non-stdlib type (such as Data).

Author: milseman

stdlib/public/core/String.swift

@@ -401,8 +401,13 @@ extension String {
   public init<C: Collection, Encoding: Unicode.Encoding>(
     decoding codeUnits: C, as sourceEncoding: Encoding.Type
   ) where C.Iterator.Element == Encoding.CodeUnit {
+    guard _fastPath(sourceEncoding == UTF8.self) else {
+      self = String._fromCodeUnits(
+        codeUnits, encoding: sourceEncoding, repair: true)!.0
+      return
+    }
+
     if let contigBytes = codeUnits as? _HasContiguousBytes,
-       sourceEncoding == UTF8.self,
        contigBytes._providesContiguousBytesNoCopy
     {
       self = contigBytes.withUnsafeBytes { rawBufPtr in
@@ -414,8 +419,13 @@ extension String {
       return
     }
 
-    self = String._fromCodeUnits(
-      codeUnits, encoding: sourceEncoding, repair: true)!.0
+    // Just copying to an Array is significantly faster than performing
+    // generic operations
+    self = Array(codeUnits).withUnsafeBufferPointer {
+      let raw = UnsafeRawBufferPointer($0)
+      return String._fromUTF8Repairing(raw.bindMemory(to: UInt8.self)).0
+    }
+    return
   }
 
   /// Calls the given closure with a pointer to the contents of the string,