Add boost::ignore_unused()

doc/core.qbk

@@ -37,6 +37,10 @@ Currently, the Core library contains:
    [[@../../enable_if.html enable_if]]
    [`boost::enable_if`]
  ]
+ [
+   [[link core.ignore_unused ignore_unused]]
+   [`boost::ignore_unused`]
+ ]
  [
    [[link core.lightweight_test lightweight_test]]
    [`BOOST_TEST, BOOST_ERROR, BOOST_TEST_EQ, BOOST_TEST_NE,
@@ -74,6 +78,7 @@ Currently, the Core library contains:
 
 [include:core addressof.qbk]
 [include:core explicit_operator_bool.qbk]
+[include:core ignore_unused.qbk]
 [include:core lightweight_test.qbk]
 [include:core no_exceptions_support.qbk]
 [include:core noncopyable.qbk]

doc/ignore_unused.qbk

@@ -0,0 +1,29 @@
+[section:ignore_unused Header <boost/core/ignore_unused.hpp>]
+
+The header `<boost/core/ignore_unused.hpp>` defines the function template
+`boost::ignore_unused()`. It may be used to suppress the "unused variable" or
+"unused local typedefs" compiler warnings when the variable or typedef
+can't be removed or commented out, e.g. when some blocks of the code are
+conditionally activated. C++11 variadic templates are used if they're supported,
+otherwise they're emulated with overloads.
+
+`boost::ignore_unused()` was contributed by Adam Wulkiewicz.
+
+Usage
+
+    boost::ignore_unused(v1, v2, v3);
+    boost::ignore_unused<T1, T2, T3>();
+
+Example
+
+    int fun( int foo, int bar )
+    {
+       boost::ignore_unused(bar);
+    #ifdef ENABLE_DEBUG_OUTPUT
+       if ( foo < bar )
+          std::cerr << "warning! foo < bar";
+    #endif
+       return foo + 2;
+    }
+
+[endsect]