[stdlib] Some cleanup enabled by _alwaysEmitIntoClient.

Refactor some copy-pasted code into a helper computed variable and
outline some cold paths.

Author: milseman

stdlib/public/core/Character.swift

@@ -82,9 +82,7 @@ extension Character {
     _internalInvariant(_str.count == 1)
     _internalInvariant(_str._guts.isFastUTF8)
 
-    // TODO(@eject): Switch to a helper property on StringObject/StringGuts.
-    _internalInvariant(
-      _str._guts.isSmall || _str._guts._object._countAndFlags.isTailAllocated)
+    _internalInvariant(_str._guts._object.isPreferredRepresentation)
   }
   #endif // INTERNAL_CHECKS_ENABLED
 }
@@ -178,16 +176,11 @@ extension Character :
     _debugPrecondition(s.index(after: s.startIndex) == s.endIndex,
       "Can't form a Character from a String containing more than one extended grapheme cluster")
 
-    // TODO(@eject): Switch to a helper property on StringObject/StringGuts.
-    if _fastPath(
-      s._guts.isSmall || s._guts._object._countAndFlags.isTailAllocated
-    ) {
+    if _fastPath(s._guts._object.isPreferredRepresentation) {
       self.init(unchecked: s)
       return
     }
-
-    // TODO(@eject): Outline this
-    self.init(unchecked: s._withUTF8 { String._uncheckedFromUTF8($0) })
+    self.init(unchecked: String._copying(s))
   }
 }
 

stdlib/public/core/ContiguouslyStored.swift

@@ -88,10 +88,7 @@ extension String: _HasContiguousBytes {
         try body($0)
       }
     }
-
-    return try ContiguousArray(self.utf8).withUnsafeBufferPointer {
-      try body($0)
-    }
+    return try String._copying(self)._guts.withFastUTF8 { try body($0) }
   }
 
   @inlinable @inline(__always)
@@ -116,10 +113,7 @@ extension Substring: _HasContiguousBytes {
         return try body($0)
       }
     }
-
-    return try ContiguousArray(self.utf8).withUnsafeBufferPointer {
-      try body($0)
-    }
+    return try String._copying(self)._guts.withFastUTF8 { try body($0) }
   }
 
   @inlinable @inline(__always)

stdlib/public/core/StringCreate.swift

@@ -197,7 +197,25 @@ extension String {
       return substring._wholeString
     }
 
-    return substring._withUTF8 { return String._uncheckedFromUTF8($0) }
+    return String._copying(substring)
+  }
+
+  @_alwaysEmitIntoClient
+  @inline(never) // slow-path
+  internal static func _copying(_ str: String) -> String {
+    return String._copying(str[...])
+  }
+  @_alwaysEmitIntoClient
+  @inline(never) // slow-path
+  internal static func _copying(_ str: Substring) -> String {
+    if _fastPath(str._wholeGuts.isFastUTF8) {
+      return str._wholeGuts.withFastUTF8(range: str._offsetRange) {
+        String._uncheckedFromUTF8($0)
+      }
+    }
+    return Array(str.utf8).withUnsafeBufferPointer {
+      String._uncheckedFromUTF8($0)
+    }
   }
 }
 

stdlib/public/core/StringObject.swift

@@ -485,8 +485,15 @@ extension _StringObject {
     _internalInvariant(isLarge)
     return (discriminatedObjectRawBits & 0x4000_0000_0000_0000) != 0
   }
-}
 
+  // Whether this string is in one of our fastest representations:
+  // small or tail-allocated (i.e. mortal/immortal native)
+  @_alwaysEmitIntoClient
+  @inline(__always)
+  internal var isPreferredRepresentation: Bool {
+    return _fastPath(isSmall || _countAndFlags.isTailAllocated)
+  }
+}
 
 /*
 

stdlib/public/core/StringStorage.swift

@@ -491,6 +491,8 @@ extension __StringStorage {
     _internalInvariant(rawSelf + Int(_StringObject.nativeBias) == rawStart)
     _internalInvariant(self._realCapacity > self.count, "no room for nul-terminator")
     _internalInvariant(self.terminator.pointee == 0, "not nul terminated")
+    let str = asString
+    _internalInvariant(str._guts._object.isPreferredRepresentation)
 
     _countAndFlags._invariantCheck()
     if isASCII {
@@ -803,6 +805,8 @@ extension __SharedStringStorage {
     _countAndFlags._invariantCheck()
     _internalInvariant(!_countAndFlags.isNativelyStored)
     _internalInvariant(!_countAndFlags.isTailAllocated)
+    let str = asString
+    _internalInvariant(!str._guts._object.isPreferredRepresentation)
   }
 #endif // INTERNAL_CHECKS_ENABLED
 }