Source files have a relative_path() to aide in finding their path from the source root

src/dds/build/plan/compile_file.cpp

 }
 
 fs::path compile_file_plan::calc_object_file_path(const build_env& env) const noexcept {
-    // `relpath` is just the path from the root of the source directory to the source file.
-    auto relpath = fs::relative(_source.path, _source.basis_path);
+    auto relpath = _source.relative_path();
     // The full output directory is prefixed by `_subdir`
     auto ret = env.output_root / _subdir / relpath;
     ret.replace_filename(relpath.filename().string() + env.toolchain.object_suffix());

src/dds/build/plan/library.cpp

         // Pick a subdir based on app/test
         const auto subdir_base = is_test ? params.out_subdir / "test" : params.out_subdir;
         // Put test/app executables in a further subdirectory based on the source file path
-        const auto subdir
-            = subdir_base / fs::relative(source.path.parent_path(), lib.src_source_root().path);
+        const auto subdir = subdir_base / source.relative_path().parent_path();
         // Pick compile rules based on app/test
         auto rules = is_test ? test_rules : compile_rules;
         // Pick input libs based on app/test

src/dds/source/file.hpp

 std::optional<source_kind> infer_source_kind(path_ref) noexcept;
 
 struct source_file {
-    fs::path    path;
-    fs::path    basis_path;
+    /**
+     * The actual path to the file
+     */
+    fs::path path;
+    /**
+     * The path to source root that contains the file in question
+     */
+    fs::path basis_path;
+    /**
+     * The kind of the source file
+     */
     source_kind kind;
 
     static std::optional<source_file> from_path(path_ref path, path_ref base_path) noexcept;
+
+    fs::path relative_path() const noexcept { return fs::relative(path, basis_path); }
 };
 
 using source_list = std::vector<source_file>;