feat: user defined metafunctions

examples/user-defined-metafunction/build.cpp2

@@ -0,0 +1,5 @@
+import cpp2b.build;
+
+build: (inout b: cpp2b::build) -> void = {
+	b.cppfront_reflect_inject("metafunction.h2");
+}

share/cpp2b.build.cppm.tpl

@@ -15,6 +15,9 @@ CPP2B_BUILD_API void (*cpp2b_detail_build_binary_name)(
     cpp2b_detail_build_impl *, std::filesystem::path,
     std::string_view) = nullptr;
 
+CPP2B_BUILD_API void (*cpp2b_detail_build_cppfront_reflect_inject)(
+    cpp2b_detail_build_impl *, std::filesystem::path) = nullptr;
+
 export namespace cpp2b {
 class build {
   cpp2b_detail_build_impl *impl;
@@ -34,6 +37,24 @@ public:
     return (*cpp2b_detail_build_binary_name)(impl, binary_source_path,
                                              new_binary_name);
   }
+
+  /**
+   * Parses the .h2 header looking for functions with any of these signatures:
+   *    
+   *    (inout _: cpp2::meta::declaration) -> void
+   *    (inout _: cpp2::meta::type_declaration) -> void
+   *    (inout _: cpp2::meta::function_declaration) -> void
+   *    (inout _: cpp2::meta::object_declaration) -> void
+   *    (inout _: cpp2::meta::alias_declaration) -> void
+   *
+   * Every function with one of these signatures are injected into cppfront's
+   * reflect.h2 `apply_metafunctions`, making them available as user-defined
+   * metafunctions. Additionally the header_path is injected as an #include at
+   * the top of the reflect.h2 header.
+   */
+  inline auto cppfront_reflect_inject(std::filesystem::path header_path) -> void {
+    return (*cpp2b_detail_build_cppfront_reflect_inject)(header_path);
+  }
 };
 } // namespace cpp2b
 

src/main.cpp2

@@ -635,10 +635,15 @@ build_binary: (info: cpp2b_source_binary_info) -> build_binary_result = {
 	return result;
 }
 
+cpp2b_reflect_header_info: @struct type = {
+	public header_paths: std::vector<fs::path>;
+	public metafunction_names: std::vector<std::string>;
+}
 
 cpp2b_detail_build_impl: @struct type = {
 	public build_cpp2_dir: fs::path;
 	public bins: *std::vector<cpp2b_source_binary_info>;
+	public reflect_inject: cpp2b_reflect_header_info;
 }
 cpp2b_detail_build_binary_name: (
 	copy impl: *cpp2b_detail_build_impl,
@@ -660,6 +665,21 @@ cpp2b_detail_build_binary_name: (
 	bin*.preferred_name = (build_cpp2_dir / new_binary_name).generic_string();
 }
 
+
+cpp2b_detail_build_cppfront_reflect_inject: (
+	copy impl: *cpp2b_detail_build_impl,
+	copy header_path: std::filesystem::path,
+) = {
+	build_cpp2_dir: fs::path = impl*.build_cpp2_dir;
+	p := build_cpp2_dir / header_path;
+
+	if !fs::exists(header_path) {
+		return;
+	}
+
+	impl*.reflect_inject.header_paths.emplace_back(header_path);
+}
+
 cpp2b_detail_build: (copy _impl: *cpp2b_detail_build_impl) -> void = {
 	// empty. this is just so we can decltype the signature
 }
@@ -922,8 +942,10 @@ do_build: (targets: std::vector<std::string>) -> (stuff: full_build_info, exit_c
 
 		build_dylib.assert_symbol("cpp2b_detail_build");
 		build_dylib.assert_symbol("cpp2b_detail_build_binary_name");
+		build_dylib.assert_symbol("cpp2b_detail_build_cppfront_reflect_inject");
 
 		build_dylib.get_variable<decltype(cpp2b_detail_build_binary_name&)>("cpp2b_detail_build_binary_name") = cpp2b_detail_build_binary_name&;
+		build_dylib.get_variable<decltype(cpp2b_detail_build_cppfront_reflect_inject&)>("cpp2b_detail_build_binary_name") = cpp2b_detail_build_cppfront_reflect_inject&;
 
 		b: cpp2b_detail_build_impl = (build_script.src.parent_path(), stuff.bins&);
 		build_dylib.get_function<decltype(cpp2b_detail_build)>("cpp2b_detail_build")(b&);