Allow direct conversion from char* to std::string.

Previously this was only possible through Python (requring the GIL),
sometimes implicitly (e.g. for string literals).

Cython/Compiler/PyrexTypes.py

@@ -3730,6 +3730,8 @@ class CppClassType(CType):
             return True
         elif other_type.is_cpp_class:
             return other_type.is_subclass(self)
+        elif other_type.is_string and self.cname in cpp_string_conversions:
+            return True
 
     def attributes_known(self):
         return self.scope is not None

tests/run/cpp_stl_conversion.pyx

@@ -72,6 +72,16 @@ def test_string_call(a, b):
     """
     return add_strings(a, b)
 
+def test_c_string_convert(char *c_string):
+    """
+    >>> normalize(test_c_string_convert("abc"))
+    'abc'
+    """
+    cdef string s
+    with nogil:
+        s = c_string
+    return s
+
 def test_int_vector(o):
     """
     >>> test_int_vector([1, 2, 3])