[string] Minor simplifications

stdlib/public/core/StringGuts.swift

@@ -500,12 +500,7 @@ extension _StringGuts {
       return _taggedCocoaObject
     }
     _sanityCheck(_object.isUnmanaged)
-    if _object.isSingleByte {
-      return _NSContiguousString(_StringGuts(_asUnmanaged(of: UInt8.self)))
-    }
-
-    return _NSContiguousString(
-      _StringGuts(_asUnmanaged(of: UTF16.CodeUnit.self)))
+    return _NSContiguousString(_unmanaged: self)
   }
 
   /// Return an NSString instance containing a slice of this string.

stdlib/public/core/StringLegacy.swift

@@ -48,11 +48,11 @@ extension String {
   ///     string.
   @_inlineable // FIXME(sil-serialize-all)
   public init(repeating repeatedValue: String, count: Int) {
-    if count == 0 {
-      self = ""
-    } else if count == 1 {
-      self = repeatedValue
-    } else {
+    guard count > 1 else {
+      self = count == 0 ? "" : repeatedValue
+      return
+    }
+
     precondition(count > 0, "Negative count not allowed")
     defer { _fixLifetime(repeatedValue) }
     if _slowPath(repeatedValue._guts._isOpaque) {
@@ -66,7 +66,6 @@ extension String {
       self.init(_StringGuts(utf16._repeated(count)))
     }
   }
-  }
 
   /// A Boolean value indicating whether a string has no characters.
   @_inlineable // FIXME(sil-serialize-all)

stdlib/public/core/StringObject.swift

@@ -735,7 +735,7 @@ extension _StringObject {
     } else {
       fatalError("Unimplemented string form")
     }
-#endif
+#endif // INTERNAL_CHECKS_ENABLED
   }
 }
 
@@ -754,22 +754,28 @@ extension _StringObject {
     isOpaque: Bool,
     isTwoByte: Bool
   ) {
+#if INTERNAL_CHECKS_ENABLED
+    defer {
+      _sanityCheck(isSmall == (isValue && isSmallOrObjC))
+      _sanityCheck(isUnmanaged == (isValue && !isSmallOrObjC))
+      _sanityCheck(isCocoa == (!isValue && isSmallOrObjC))
+      _sanityCheck(isNative == (!isValue && !isSmallOrObjC))
+    }
+#endif
+
 #if arch(i386) || arch(arm)
-    var variant: _Variant
-    var bits: UInt
     if isValue {
       if isSmallOrObjC {
         _sanityCheck(isOpaque)
-        self.init(
-          isTwoByte ? .smallDoubleByte : .smallSingleByte,
-          _payloadBits)
+        self.init(isTwoByte ? .smallDoubleByte : .smallSingleByte, _payloadBits)
       } else {
         _sanityCheck(!isOpaque)
         self.init(
-          isTwoByte ? .unmanagedDoubleByte : .unmanagedSingleByte,
-          _payloadBits)
+          isTwoByte ? .unmanagedDoubleByte : .unmanagedSingleByte, _payloadBits)
       }
-    } else {
+      return
+    }
+
     var bits: UInt = 0
     if isSmallOrObjC {
       bits |= _StringObject._isCocoaBit
@@ -781,7 +787,6 @@ extension _StringObject {
       bits |= _StringObject._twoByteBit
     }
     self.init(.strong(Builtin.reinterpretCast(_payloadBits)), bits)
-    }
 #else
     _sanityCheck(_payloadBits & ~_StringObject._payloadMask == 0)
     var rawBits = _payloadBits & _StringObject._payloadMask
@@ -815,10 +820,6 @@ extension _StringObject {
       self.init(nonTaggedRawBits: rawBits)
     }
 #endif
-    _sanityCheck(isSmall == (isValue && isSmallOrObjC))
-    _sanityCheck(isUnmanaged == (isValue && !isSmallOrObjC))
-    _sanityCheck(isCocoa == (!isValue && isSmallOrObjC))
-    _sanityCheck(isNative == (!isValue && !isSmallOrObjC))
   }
 
   @_versioned