Missing dependency between C++ modules and include files

[redstar]

I recently converted a project to use C++ modules and xmake at the same time. I am using LLVM-style .def "database" files. It seems that the dependency between the C++ module and the .def file is not known/ignored when building.

E.g. without modules:

.
├── src
│   ├── Token.cpp
│   └── Token.def
└── xmake.lua

// Token.def
#ifndef TOKEN
#error "Missing TOKEN"
#endif

TOKEN(dot, ".")
TOKEN(arrow, "->")

#undef TOKEN

// Token.cpp
enum class Token {
#define TOKEN(id, name) id,
#include "Token.def"
};

const char *TokenName[] = {
#define TOKEN(id, name) name,
#include "Token.def"
};

-- xmake.lua
add_rules("mode.release")

set_languages("c++20")

target("libtest")
    set_kind("static")
    add_files("src/*.cpp")

This works as expected:

$ xmake config --toolchain=clang
checking for platform ... linux
checking for architecture ... x86_64
$ xmake build
[ 50%]: cache compiling.release src/Token.cpp
[ 75%]: archiving.release liblibtest.a
[100%]: build ok, spent 1.734s
$ touch src/Token.def
$ xmake build
[ 50%]: cache compiling.release src/Token.cpp
[ 75%]: archiving.release liblibtest.a
[100%]: build ok, spent 0.135s

With the conversion to C++ modules I now have the following:

.
├── src
│   ├── Token.cppm
│   └── Token.def
└── xmake.lua

// Token.cppm
export module token;

export {
enum class Token {
#define TOKEN(id, name) id,
#include "Token.def"
};

const char *TokenName[] = {
#define TOKEN(id, name) name,
#include "Token.def"
};
}

-- xmake.lua
add_rules("mode.release")

set_languages("c++20")

target("libtest")
    set_kind("static")
    add_files("src/*.cppm", {public = true})

Now the object file of the module is not rebuild when the .def file changes:

$ xmake config --toolchain=clang
checking for platform ... linux
checking for architecture ... x86_64
$ xmake build
[  0%]: <libtest> generating.module.deps src/Token.cppm
[ 60%]: <libtest> compiling.module.release token
[ 80%]: archiving.release liblibtest.a
[100%]: build ok, spent 1.763s
warning: std and std.compat modules not found! maybe try to add --sdk=<PATH/TO/LLVM> or install libc++
$ touch src/Token.def
$ xmake build
[100%]: build ok, spent 0.023s
warning: std and std.compat modules not found! maybe try to add --sdk=<PATH/TO/LLVM> or install libc++

What do I need to change that the missing dependency is added?

I am using xmake v2.9.7+dev.b029e65ed, clang 19.1.1, Ubuntu 24.10.

Thanks!

[waruqi]

no any cppm file are compiled. you should use set_kind("moduleonly") instead of static kind if no any cpp files.

[redstar]

I am sorry but I do not fully understand. The variable TokenName is defined in the module, therefore imho it must be compiled into an object file to avoid multiple copies. (However, your hint applies to another problem I still have not tackled, so thank you!) Maybe my example is to small. Same happens when I add a function to the module, which certainly must be compiled. There is no change in behaviour when Token.def is changed.

export module token;

export {
enum class Token {
#define TOKEN(id, name) id,
#include "Token.def"
};

const char *getTokenName(Token);
} // of export

const char *TokenName[] = {
#define TOKEN(id, name) name,
#include "Token.def"
};

const char *getTokenName(Token T) {
  return TokenName[static_cast<unsigned>(T)];
}

Original files:
Module (plus other files in the same directory)
def file
xmake.lue

[waruqi]

clang-scan-deps does not provide any dependency information about Token.def.

So xmake can't do anything. It's clang-scan-deps issue, not xmake.

[  0%]: <libtest> generating.module.deps src/test.cppm
/usr/local/Cellar/llvm/19.1.2/bin/clang-scan-deps --format=p1689 -- /usr/local/Cellar/llvm/19.1.2/bin/clang -x c++ -c src/test.c
ppm -o build/.objs/libtest/macosx/x86_64/release/src/test.cppm.o -Qunused-arguments -m64 -isysroot /Applications/Xcode.app/Conte
nts/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.1.sdk -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c
++20 -DNDEBUG

{
  "revision": 0,
  "rules": [
    {
      "primary-output": "build/.objs/libtest/macosx/x86_64/release/src/test.cppm.o",
      "provides": [
        {
          "is-interface": true,
          "logical-name": "token",
          "source-path": "src/test.cppm"
        }
      ]
    }
  ],
  "version": 1
}