Issue errors for nogil buffer acquisition

Cython/Compiler/Nodes.py

@@ -1807,8 +1807,14 @@ class CFuncDefNode(FuncDefNode):
             formal_arg.type = type_arg.type
             formal_arg.name = type_arg.name
             formal_arg.cname = type_arg.cname
-            if type_arg.type.is_buffer and 'inline' in self.modifiers:
+            if type_arg.type.is_buffer:
+                if self.type.nogil:
+                    error(formal_arg.pos,
+                          "Buffer may not be acquired without the GIL. "
+                          "Consider using memoryview slices instead.")
+                elif 'inline' in self.modifiers:
                     warning(formal_arg.pos, "Buffer unpacking not optimized away.", 1)
+
         name = name_declarator.name
         cname = name_declarator.cname
         self.entry = env.declare_cfunction(

tests/errors/nogil_buffer_acquisition.pyx

+# mode: error
+
+cimport numpy as np
+
+cdef void func(np.ndarray[np.double_t, ndim=1] myarray) nogil:
+    pass
+
+_ERRORS = u"""
+5:15: Buffer may not be acquired without the GIL. Consider using memoryview slices instead.
+"""