StringGuts: Annotate decls with effects(readonly).

Annotating the unmanaged*Views with @effects(readonly) allows
non-inlining callers such as NSContiguousString to avoid having to
conservatively keep self alive for fear of side effects.

Author: milseman

stdlib/public/core/StringGuts.swift

@@ -338,30 +338,35 @@ extension _StringGuts {
   @_inlineable
   internal
   var _unmanagedASCIIView: _UnmanagedString<UInt8> {
-    _sanityCheck(_object.isContiguousASCII)
-    if _object.isUnmanaged {
-      return _asUnmanaged()
-    } else if _object.isNative {
-      return _object.nativeStorage(of: UInt8.self).unmanagedView
-    } else {
-      _sanityCheck(_object.isContiguousCocoa)
-      return _asContiguousCocoa(of: UInt8.self)
+    @effects(readonly)
+    get {
+      _sanityCheck(_object.isContiguousASCII)
+      if _object.isUnmanaged {
+        return _asUnmanaged()
+      } else if _object.isNative {
+        return _object.nativeStorage(of: UInt8.self).unmanagedView
+      } else {
+        _sanityCheck(_object.isContiguousCocoa)
+        return _asContiguousCocoa(of: UInt8.self)
+      }
     }
   }
 
   @_versioned
   @_inlineable
   internal
   var _unmanagedUTF16View: _UnmanagedString<UTF16.CodeUnit> {
-    _sanityCheck(_object.isContiguousUTF16)
-    if _object.isUnmanaged {
-      return _asUnmanaged()
-    } else if _object.isNative {
-      return _object.nativeStorage(of: UTF16.CodeUnit.self).unmanagedView
-    } else if _object.isCocoa {
-      return _asContiguousCocoa(of: UTF16.CodeUnit.self)
-    } else {
-      fatalError("Small strings aren't contiguous")
+    @effects(readonly)
+    get {
+      _sanityCheck(_object.isContiguousUTF16)
+      if _object.isUnmanaged {
+        return _asUnmanaged()
+      } else if _object.isNative {
+        return _object.nativeStorage(of: UTF16.CodeUnit.self).unmanagedView
+      } else {
+        _sanityCheck(_object.isContiguousCocoa)
+        return _asContiguousCocoa(of: UTF16.CodeUnit.self)
+      }
     }
   }
 }