Implement len(memoryview) and allow its usage in nogil sections.

Closes #1733

Cython/Compiler/Optimize.py

@@ -2429,6 +2429,14 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
                 node.pos, "__Pyx_Py_UNICODE_strlen", self.Pyx_Py_UNICODE_strlen_func_type,
                 args = [arg],
                 is_temp = node.is_temp)
+        elif arg.type.is_memoryviewslice:
+            func_type = PyrexTypes.CFuncType(
+                PyrexTypes.c_size_t_type, [
+                    PyrexTypes.CFuncTypeArg("memoryviewslice", arg.type, None)
+                ], nogil=True)
+            new_node = ExprNodes.PythonCapiCallNode(
+                node.pos, "__Pyx_MemoryView_Len", func_type,
+                args=[arg], is_temp=node.is_temp)
         elif arg.type.is_pyobject:
             cfunc_name = self._map_to_capi_len_function(arg.type)
             if cfunc_name is None:
@@ -2442,8 +2450,7 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
                 "object of type 'NoneType' has no len()")
             new_node = ExprNodes.PythonCapiCallNode(
                 node.pos, cfunc_name, self.PyObject_Size_func_type,
-                args = [arg],
-                is_temp = node.is_temp)
+                args=[arg], is_temp=node.is_temp)
         elif arg.type.is_unicode_char:
             return ExprNodes.IntNode(node.pos, value='1', constant_result=1,
                                      type=node.type)

Cython/Utility/MemoryView_C.c

@@ -11,6 +11,9 @@ typedef struct {
   Py_ssize_t suboffsets[{{max_dims}}];
 } {{memviewslice_name}};
 
+// used for "len(memviewslice)"
+#define __Pyx_MemoryView_Len(m)  (m.shape[0])
+
 
 /////////// Atomics.proto /////////////
 

tests/memoryview/memoryviewattrs.pyx

@@ -279,6 +279,13 @@ def two_dee():
 
     assert len(arr) == 2
 
+    try:
+        _ = len(mv1)
+    except UnboundLocalError:
+        pass
+    else:
+        assert False, "UnboundLocalError not raised for uninitialised memory view"
+
     cdef long *arr_data
     arr_data = <long*>arr.data
 

tests/memoryview/memslice.pyx

@@ -1677,7 +1677,7 @@ cdef int cdef_nogil(int[:, :] a) nogil except 0:
         for j in range(b.shape[1]):
             b[i, j] = -b[i, j]
 
-    return 1
+    return len(a)
 
 @testcase
 def test_nogil():
@@ -1690,10 +1690,15 @@ def test_nogil():
     released A
     """
     _a = IntMockBuffer("A", range(4 * 9), shape=(4, 9))
-    cdef_nogil(_a)
+    assert cdef_nogil(_a) == 4
     cdef int[:, :] a = _a
     print a[2, 7]
 
+    cdef int length
+    with nogil:
+        length = cdef_nogil(a)
+    assert length == 4
+
 @testcase
 def test_convert_slicenode_to_indexnode():
     """