[String] String-from-whole-Substring fast-path.

Add in a fast-path for Strings created from Substring which covers the
entire String. Put String-from-Substring behind a non-inlinable
resilience barrier for future flexibility.

Author: milseman

stdlib/public/core/StringCreate.swift

@@ -169,5 +169,16 @@ extension String {
   ) -> String {
     return String._fromCodeUnits(utf16, encoding: UTF16.self, repair: true)!.0
   }
+
+  @usableFromInline
+  internal static func _fromSubstring(
+    _ substring: __shared Substring
+  ) -> String {
+    if substring._offsetRange == substring._wholeString._offsetRange {
+      return substring._wholeString
+    }
+
+    return substring._withUTF8 { return String._uncheckedFromUTF8($0) }
+  }
 }
 

stdlib/public/core/Substring.swift

@@ -20,7 +20,7 @@ extension String {
   /// - Complexity: O(*n*), where *n* is the length of `substring`.
   @inlinable
   public init(_ substring: __shared Substring) {
-    self = substring._withUTF8 { return String._uncheckedFromUTF8($0) }
+    self = String._fromSubstring(substring)
   }
 }