warn when taking char* from a global Python variable (which is externally modifiable)

Cython/Compiler/ExprNodes.py

@@ -9187,9 +9187,14 @@ class CoerceFromPyTypeNode(CoercionNode):
         if not result_type.create_from_py_utility_code(env):
             error(arg.pos,
                   "Cannot convert Python object to '%s'" % result_type)
-        if self.type.is_string and self.arg.is_ephemeral():
+        if self.type.is_string:
+            if self.arg.is_ephemeral():
                 error(arg.pos,
-                  "Obtaining char * from temporary Python value")
+                      "Obtaining char* from temporary Python value")
+            elif self.arg.is_name and self.arg.entry and self.arg.entry.is_pyglobal:
+                warning(arg.pos,
+                        "Obtaining char* from externally modifiable global Python value",
+                        level=1)
 
     def analyse_types(self, env):
         # The arg is always already analysed

tests/errors/charptr_from_temp.pyx

+# mode: error
+# tag: werror, charptr, conversion, temp
+
+s = b"abc"
+
+cdef char* cptr
+
+cptr = s
+
+cptr = s + b"cba"
+
+_ERRORS = """
+ 8:8: Obtaining char* from externally modifiable global Python value
+10:9: Obtaining char* from temporary Python value
+"""