Mark the AtomicString(const String&) constructor as explicit

Source/wtf/text/AtomicString.h

 /*
  * Copyright (C) 2004, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #ifndef AtomicString_h
 #define AtomicString_h
 
 #include "wtf/HashTableDeletedValueType.h"
 #include "wtf/WTFExport.h"
 #include "wtf/text/CString.h"
 #include "wtf/text/WTFString.h"
 
-// Define 'NO_IMPLICIT_ATOMICSTRING' before including this header,
-// to disallow (expensive) implicit String-->AtomicString conversions.
-#ifdef NO_IMPLICIT_ATOMICSTRING
-#define ATOMICSTRING_CONVERSION explicit
-#else
-#define ATOMICSTRING_CONVERSION
-#endif
-
 namespace WTF {
 
 struct AtomicStringHash;
 
 class WTF_EXPORT AtomicString {
 public:
     static void init();
 
     AtomicString() { }
     AtomicString(const LChar* s) : m_string(add(s)) { }
     AtomicString(const char* s) : m_string(add(s)) { }
     AtomicString(const LChar* s, unsigned length) : m_string(add(s, length)) { }
     AtomicString(const UChar* s, unsigned length) : m_string(add(s, length)) { }
     AtomicString(const UChar* s, unsigned length, unsigned existingHash) : m_string(add(s, length, existingHash)) { }
     AtomicString(const UChar* s) : m_string(add(s)) { }
 
     template<size_t inlineCapacity>
     explicit AtomicString(const Vector<UChar, inlineCapacity>& characters)
         : m_string(add(characters.data(), characters.size()))
     {
     }
 
+    // Constructing an AtomicString from a String / StringImpl can be expensive if
+    // the StringImpl is not already atomic.
     explicit AtomicString(StringImpl* imp) : m_string(add(imp)) { }
-    ATOMICSTRING_CONVERSION AtomicString(const String& s) : m_string(add(s.impl())) { }
+    explicit AtomicString(const String& s) : m_string(add(s.impl())) { }
+
     AtomicString(StringImpl* baseString, unsigned start, unsigned length) : m_string(add(baseString, start, length)) { }
 
     enum ConstructFromLiteralTag { ConstructFromLiteral };
     AtomicString(const char* characters, unsigned length, ConstructFromLiteralTag)
         : m_string(addFromLiteralData(characters, length))
     {
     }
 
     template<unsigned charactersCount>
     ALWAYS_INLINE AtomicString(const char (&characters)[charactersCount], ConstructFromLiteralTag)
@@ skipping 196 matching lines @@
 using WTF::nullAtom;
 using WTF::emptyAtom;
 using WTF::starAtom;
 using WTF::xmlAtom;
 using WTF::xmlnsAtom;
 using WTF::xlinkAtom;
 #endif
 
 #include "wtf/text/StringConcatenate.h"
 #endif // AtomicString_h