Add option to retain #include directives in preprocessor

tests/test_preprocessor.py

@@ -1,9 +1,16 @@
 import os
 import pathlib
+import re
 
 from cxxheaderparser.options import ParserOptions
 from cxxheaderparser.preprocessor import make_pcpp_preprocessor
-from cxxheaderparser.simple import NamespaceScope, ParsedData, parse_file, parse_string
+from cxxheaderparser.simple import (
+    NamespaceScope,
+    ParsedData,
+    parse_file,
+    parse_string,
+    Include,
+)
 from cxxheaderparser.types import (
     FundamentalSpecifier,
     NameSpecifier,
@@ -135,3 +142,23 @@ def test_preprocessor_encoding(tmp_path: pathlib.Path) -> None:
             ]
         )
     )
+
+
+def test_preprocessor_passthru_includes(tmp_path: pathlib.Path) -> None:
+    """Ensure that all #include pass through"""
+    h_content = '#include "t2.h"\n'
+
+    with open(tmp_path / "t1.h", "w") as fp:
+        fp.write(h_content)
+
+    with open(tmp_path / "t2.h", "w") as fp:
+        fp.write("")
+
+    options = ParserOptions(
+        preprocessor=make_pcpp_preprocessor(passthru_includes=re.compile(".+"))
+    )
+    data = parse_file(tmp_path / "t1.h", options=options)
+
+    assert data == ParsedData(
+        namespace=NamespaceScope(), includes=[Include(filename='"t2.h"')]
+    )