Fix some bugs on Windows (GH-4331)

* Avoid usage of `const` or `volatile` in buffer struct field declarations.
* Avoid newline replacement in the test helper functions `write_file()` and `write_newer_file()`.

See https://github.com/cython/cython/pull/4324

Cython/Compiler/Buffer.py

@@ -678,9 +678,17 @@ def get_type_information_cname(code, dtype, maxdepth=None):
             types = [get_type_information_cname(code, f.type, maxdepth - 1)
                      for f in fields]
             typecode.putln("static __Pyx_StructField %s[] = {" % structinfo_name, safe=True)
+
+            if dtype.is_cv_qualified:
+                # roughly speaking, remove "const" from struct_type
+                struct_type = dtype.cv_base_type.empty_declaration_code()
+            else:
+                struct_type = dtype.empty_declaration_code()
+
             for f, typeinfo in zip(fields, types):
                 typecode.putln('  {&%s, "%s", offsetof(%s, %s)},' %
-                           (typeinfo, f.name, dtype.empty_declaration_code(), f.cname), safe=True)
+                               (typeinfo, f.name, struct_type, f.cname), safe=True)
+
             typecode.putln('  {NULL, NULL, 0}', safe=True)
             typecode.putln("};", safe=True)
         else:

Cython/TestUtils.py

@@ -228,23 +228,44 @@ def unpack_source_tree(tree_file, workdir, cython_root):
     return workdir, header
 
 
-def write_file(file_path, content, dedent=False):
-    """
-    Write some content (text or bytes) to the file at "file_path".
+def write_file(file_path, content, dedent=False, encoding=None):
+    r"""Write some content (text or bytes) to the file
+    at `file_path` without translating `'\n'` into `os.linesep`.
+
+    The default encoding is `'utf-8'`.
     """
-    mode = 'wb' if isinstance(content, bytes) else 'w'
+    if isinstance(content, bytes):
+        mode = "wb"
+
+        # binary mode doesn't take an encoding and newline arguments
+        newline = None
+        default_encoding = None
+    else:
+        mode = "w"
+
+        # any "\n" characters written are not translated
+        # to the system default line separator, os.linesep
+        newline = "\n"
+        default_encoding = "utf-8"
+
+    if encoding is None:
+        encoding = default_encoding
+
     if dedent:
         content = textwrap.dedent(content)
 
-    with open(file_path, mode=mode) as f:
+    with open(file_path, mode=mode, encoding=encoding, newline=newline) as f:
         f.write(content)
 
 
-def write_newer_file(file_path, newer_than, content, dedent=False):
-    """
-    Write 'content' to the file 'file_path' and make sure it is newer than the file 'newer_than'.
+def write_newer_file(file_path, newer_than, content, dedent=False, encoding=None):
+    r"""
+    Write `content` to the file `file_path` without translating `'\n'`
+    into `os.linesep` and make sure it is newer than the file `newer_than`.
+
+    The default encoding is `'utf-8'` (same as for `write_file`).
     """
-    write_file(file_path, content, dedent=dedent)
+    write_file(file_path, content, dedent=dedent, encoding=encoding)
 
     try:
         other_time = os.path.getmtime(newer_than)
@@ -253,4 +274,4 @@ def write_newer_file(file_path, newer_than, content, dedent=False):
         other_time = None
 
     while other_time is None or other_time >= os.path.getmtime(file_path):
-        write_file(file_path, content, dedent=dedent)
+        write_file(file_path, content, dedent=dedent, encoding=encoding)