feat: Improve C/CXX language detection

src/cython_cmake/cmake/UseCython.cmake

@@ -16,9 +16,8 @@
 # Options:
 #
 # ``LANGUAGE [C | CXX]``
-#   Force the generation of either a C or C++ file. By default, a C file is
-#   generated, unless the C language is not enabled for the project; in this
-#   case, a C++ file is generated by default.
+#   Force the generation of either a C or C++ file. Recommended; will attempt
+#   to be deduced if not specified, defaults to C unless only CXX is enabled.
 #
 # ``CYTHON_ARGS <args>``
 #   Specify additional arguments for the cythonization process. Will default to
@@ -109,31 +108,15 @@ function(Cython_compile_pyx)
     message(FATAL_ERROR "One and only one input file must be specified, got '${_source_files}'")
   endif()
 
-  # Set target language
-  get_property(_languages GLOBAL PROPERTY ENABLED_LANGUAGES)
-  set(_language ${_args_LANGUAGE})
-  if(NOT _language)
-    if("C" IN_LIST _languages)
-      set(_language "C")
-    elseif("CXX" IN_LIST _languages)
-      set(_language "CXX")
-    endif()
-  else()
-    if(NOT _language MATCHES "^(C|CXX)$")
-      message(FATAL_ERROR "LANGUAGE must be one of C or CXX")
-    endif()
-  endif()
-
-  set(_language_C_arg "")
-  set(_language_C_extension "c")
-  set(_language_CXX_arg "--cplus")
-  set(_language_CXX_extension "cxx")
-
-  set(_language_arg ${_language_${_language}_arg})
-  set(_language_extension ${_language_${_language}_extension})
-
-  function(_compile_pyx _source_file generated_file)
+  function(_compile_pyx _source_file generated_file language)
 
+    if(language STREQUAL "C")
+      set(_language_arg "")
+    elseif(language STREQUAL "CXX")
+      set(_language_arg "--cplus")
+    else()
+      message(FATAL_ERROR "_compile_pyx language must be one of C or CXX")
+    endif()
 
     set_source_files_properties(${generated_file} PROPERTIES GENERATED TRUE)
 
@@ -168,7 +151,15 @@ function(Cython_compile_pyx)
     )
   endfunction()
 
-  function(_set_output _input_file _output_var)
+  function(_set_output _input_file _language _output_var)
+    if(_language STREQUAL "C")
+      set(_language_extension "c")
+    elseif(_language STREQUAL "CXX")
+      set(_language_extension "cxx")
+    else()
+      message(FATAL_ERROR "_set_output language must be one of C or CXX")
+    endif()
+
     # Can use cmake_path for CMake 3.20+
     # cmake_path(GET _input_file STEM basename)
     get_filename_component(_basename "${_input_file}" NAME_WE)
@@ -180,15 +171,37 @@ function(Cython_compile_pyx)
 
   list(GET _source_files 0 _source_file)
 
+  # Set target language
+  set(_language ${_args_LANGUAGE})
+  if(NOT _language)
+    get_property(_languages GLOBAL PROPERTY ENABLED_LANGUAGES)
+    if("C" IN_LIST _languages AND "CXX" IN_LIST _languages)
+      # Try to compute language. Returns falsy if not found.
+      _cython_compute_language(_language ${_source_file})
+    elseif("C" IN_LIST _languages)
+      # If only C is enabled globally, assume C
+      set(_language "C")
+    elseif("CXX" IN_LIST _languages)
+      # Likewise for CXX
+      set(_language "CXX")
+    else()
+      message(FATAL_ERROR "LANGUAGE keyword required if neither C nor CXX enabled globally")
+    endif()
+  endif()
+
+  if(NOT _language MATCHES "^(C|CXX)$")
+    message(FATAL_ERROR "cython_compile_pyx LANGUAGE must be one of C or CXX")
+  endif()
+
   # Place the cython files in the current binary dir if no path given
   if(NOT _args_OUTPUT)
-    _set_output(${_source_file} _args_OUTPUT)
+    _set_output(${_source_file} ${_language} _args_OUTPUT)
   elseif(NOT IS_ABSOLUTE ${_args_OUTPUT})
     set(_args_OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${_args_OUTPUT}")
   endif()
 
   set(generated_file ${_args_OUTPUT})
-  _compile_pyx(${_source_file} ${generated_file})
+  _compile_pyx(${_source_file} ${generated_file} ${_language})
   list(APPEND generated_files ${generated_file})
 
   # Output variable only if set
@@ -198,3 +211,14 @@ function(Cython_compile_pyx)
   endif()
 
 endfunction()
+
+function(_cython_compute_language OUTPUT_VARIABLE FILENAME)
+  file(READ "${FILENAME}" FILE_CONTENT)
+  # Check for compiler directive similar to "# distutils: language = c++"
+  # See https://cython.readthedocs.io/en/latest/src/userguide/wrapping_CPlusPlus.html#declare-a-var-with-the-wrapped-c-class
+  set(REGEX_PATTERN [=[^[[:space:]]*#[[:space:]]*distutils:.*language[[:space:]]*=[[:space:]]*(c\\+\\+|c)]=])
+  string(REGEX MATCH "${REGEX_PATTERN}" MATCH_RESULT "${FILE_CONTENT}")
+  string(TOUPPER "${MATCH_RESULT}" LANGUAGE_NAME)
+  string(REPLACE "+" "X" LANGUAGE_NAME "${LANGUAGE_NAME}")
+  set(${OUTPUT_VARIABLE} ${LANGUAGE_NAME} PARENT_SCOPE)
+endfunction()

tests/test_package.py

@@ -71,3 +71,59 @@ def test_output_argument(monkeypatch, tmp_path, output_arg):
     build_files = {x.name for x in build_dir.iterdir()}
     assert f"{generated_file}.dep" in build_files
     assert generated_file in build_files
+
+
+def test_implicit_cxx(monkeypatch, tmp_path):
+    package_dir = tmp_path / "pkg3"
+    shutil.copytree(DIR / "packages/simple", package_dir)
+    monkeypatch.chdir(package_dir)
+
+    cmakelists = Path("CMakeLists.txt")
+    txt = (
+        cmakelists.read_text()
+        .replace("LANGUAGE C", "")
+        .replace("LANGUAGES C", "LANGUAGES CXX")
+    )
+    cmakelists.write_text(txt)
+
+    wheel = build_wheel(
+        str(tmp_path), {"build-dir": "build", "wheel.license-files": []}
+    )
+
+    with zipfile.ZipFile(tmp_path / wheel) as f:
+        file_names = set(f.namelist())
+    assert len(file_names) == 4
+
+    build_files = {x.name for x in Path("build").iterdir()}
+    assert "simple.cxx.dep" in build_files
+    assert "simple.cxx" in build_files
+
+
+def test_directive_cxx(monkeypatch, tmp_path):
+    package_dir = tmp_path / "pkg4"
+    shutil.copytree(DIR / "packages/simple", package_dir)
+    monkeypatch.chdir(package_dir)
+
+    cmakelists = Path("CMakeLists.txt")
+    txt = (
+        cmakelists.read_text()
+        .replace("LANGUAGE C", "")
+        .replace("LANGUAGES C", "LANGUAGES CXX")
+    )
+    cmakelists.write_text(txt)
+
+    simple = Path("simple.pyx")
+    txt = simple.read_text()
+    simple.write_text(f"# distutils: language=c++\n{txt}")
+
+    wheel = build_wheel(
+        str(tmp_path), {"build-dir": "build", "wheel.license-files": []}
+    )
+
+    with zipfile.ZipFile(tmp_path / wheel) as f:
+        file_names = set(f.namelist())
+    assert len(file_names) == 4
+
+    build_files = {x.name for x in Path("build").iterdir()}
+    assert "simple.cxx.dep" in build_files
+    assert "simple.cxx" in build_files