propagate "const char*" that Py3.7 returns from PyUnicode_AsStringAndSize() to...

propagate "const char*" that Py3.7 returns from PyUnicode_AsStringAndSize() to user code and make the casts to "char*" explicit for each assignment/conversion
see https://bugs.python.org/issue28769

Cython/Compiler/PyrexTypes.py

@@ -2169,15 +2169,17 @@ class CPointerBaseType(CType):
             if base_type.signed == 2:
                 self.to_py_function = "__Pyx_PyObject_FromCString"
                 if self.is_ptr:
-                    self.from_py_function = "__Pyx_PyObject_AsSString"
+                    self.from_py_function = "__Pyx_PyObject_As%sSString"
             elif base_type.signed:
                 self.to_py_function = "__Pyx_PyObject_FromString"
                 if self.is_ptr:
-                    self.from_py_function = "__Pyx_PyObject_AsString"
+                    self.from_py_function = "__Pyx_PyObject_As%sString"
             else:
                 self.to_py_function = "__Pyx_PyObject_FromCString"
                 if self.is_ptr:
-                    self.from_py_function = "__Pyx_PyObject_AsUString"
+                    self.from_py_function = "__Pyx_PyObject_As%sUString"
+            if self.is_ptr:
+                self.from_py_function %= '' if self.base_type.is_const else 'Writable'
             self.exception_value = "NULL"
         elif self.is_pyunicode_ptr and not base_type.is_error:
             self.to_py_function = "__Pyx_PyUnicode_FromUnicode"

Cython/Utility/CppConvert.pyx

@@ -7,12 +7,12 @@ cdef extern from *:
     cdef cppclass string "{{type}}":
         string()
         string(char* c_str, size_t size)
-    cdef char* __Pyx_PyObject_AsStringAndSize(object, Py_ssize_t*) except NULL
+    cdef const char* __Pyx_PyObject_AsStringAndSize(object, Py_ssize_t*) except NULL
 
 @cname("{{cname}}")
 cdef string {{cname}}(object o) except *:
     cdef Py_ssize_t length
-    cdef char* data = __Pyx_PyObject_AsStringAndSize(o, &length)
+    cdef const char* data = __Pyx_PyObject_AsStringAndSize(o, &length)
     return string(data, length)
 
 
@@ -27,7 +27,7 @@ cdef extern from *:
 
 {{for py_type in ['PyObject', 'PyUnicode', 'PyStr', 'PyBytes', 'PyByteArray']}}
 cdef extern from *:
-    cdef object __Pyx_{{py_type}}_FromStringAndSize(char*, size_t)
+    cdef object __Pyx_{{py_type}}_FromStringAndSize(const char*, size_t)
 
 @cname("{{cname.replace("PyObject", py_type, 1)}}")
 cdef inline object {{cname.replace("PyObject", py_type, 1)}}(const string& s):

Cython/Utility/TypeConversion.c

@@ -37,8 +37,8 @@
     #define __Pyx_sst_abs(value) ((value<0) ? -value : value)
 #endif
 
-static CYTHON_INLINE char* __Pyx_PyObject_AsString(PyObject*);
-static CYTHON_INLINE char* __Pyx_PyObject_AsStringAndSize(PyObject*, Py_ssize_t* length);
+static CYTHON_INLINE const char* __Pyx_PyObject_AsString(PyObject*);
+static CYTHON_INLINE const char* __Pyx_PyObject_AsStringAndSize(PyObject*, Py_ssize_t* length);
 
 #define __Pyx_PyByteArray_FromString(s) PyByteArray_FromStringAndSize((const char*)s, strlen((const char*)s))
 #define __Pyx_PyByteArray_FromStringAndSize(s, l) PyByteArray_FromStringAndSize((const char*)s, l)
@@ -54,8 +54,11 @@ static CYTHON_INLINE PyObject* __Pyx_PyUnicode_FromString(const char*);
     #define __Pyx_PyStr_FromStringAndSize __Pyx_PyUnicode_FromStringAndSize
 #endif
 
-#define __Pyx_PyObject_AsSString(s)    ((signed char*) __Pyx_PyObject_AsString(s))
-#define __Pyx_PyObject_AsUString(s)    ((unsigned char*) __Pyx_PyObject_AsString(s))
+#define __Pyx_PyObject_AsWritableString(s)    ((char*) __Pyx_PyObject_AsString(s))
+#define __Pyx_PyObject_AsWritableSString(s)    ((signed char*) __Pyx_PyObject_AsString(s))
+#define __Pyx_PyObject_AsWritableUString(s)    ((unsigned char*) __Pyx_PyObject_AsString(s))
+#define __Pyx_PyObject_AsSString(s)    ((const signed char*) __Pyx_PyObject_AsString(s))
+#define __Pyx_PyObject_AsUString(s)    ((const unsigned char*) __Pyx_PyObject_AsString(s))
 #define __Pyx_PyObject_FromCString(s)  __Pyx_PyObject_FromString((const char*)s)
 #define __Pyx_PyBytes_FromCString(s)   __Pyx_PyBytes_FromString((const char*)s)
 #define __Pyx_PyByteArray_FromCString(s)   __Pyx_PyByteArray_FromString((const char*)s)
@@ -189,12 +192,14 @@ static CYTHON_INLINE PyObject* __Pyx_PyUnicode_FromString(const char* c_str) {
     return __Pyx_PyUnicode_FromStringAndSize(c_str, (Py_ssize_t)strlen(c_str));
 }
 
-static CYTHON_INLINE char* __Pyx_PyObject_AsString(PyObject* o) {
+// Py3.7 returns a "const char*" for unicode strings
+static CYTHON_INLINE const char* __Pyx_PyObject_AsString(PyObject* o) {
     Py_ssize_t ignore;
     return __Pyx_PyObject_AsStringAndSize(o, &ignore);
 }
 
-static CYTHON_INLINE char* __Pyx_PyObject_AsStringAndSize(PyObject* o, Py_ssize_t *length) {
+// Py3.7 returns a "const char*" for unicode strings
+static CYTHON_INLINE const char* __Pyx_PyObject_AsStringAndSize(PyObject* o, Py_ssize_t *length) {
 #if CYTHON_COMPILING_IN_CPYTHON && (__PYX_DEFAULT_STRING_ENCODING_IS_ASCII || __PYX_DEFAULT_STRING_ENCODING_IS_DEFAULT)
     if (
 #if PY_MAJOR_VERSION < 3 && __PYX_DEFAULT_STRING_ENCODING_IS_ASCII
@@ -228,18 +233,14 @@ static CYTHON_INLINE char* __Pyx_PyObject_AsStringAndSize(PyObject* o, Py_ssize_
         if (PyUnicode_IS_ASCII(o)) {
             // cached for the lifetime of the object
             *length = PyUnicode_GET_LENGTH(o);
-            // Py3.7 returns a "const char*", need to cast to "char*" for backwards compatibility
-            // see https://bugs.python.org/issue28769
-            return (char*) PyUnicode_AsUTF8(o);
+            return PyUnicode_AsUTF8(o);
         } else {
             // raise the error
             PyUnicode_AsASCIIString(o);
             return NULL;
         }
 #else /* __PYX_DEFAULT_STRING_ENCODING_IS_ASCII */
-        // Py3.7 returns a "const char*", need to cast to "char*" for backwards compatibility
-        // see https://bugs.python.org/issue28769
-        return (char*) PyUnicode_AsUTF8AndSize(o, length);
+        return PyUnicode_AsUTF8AndSize(o, length);
 #endif /* __PYX_DEFAULT_STRING_ENCODING_IS_ASCII */
 #endif /* PY_VERSION_HEX < 0x03030000 */
     } else