Centralize path handling logic around generated header roots

src/dds/build/builder.cpp

@@ -153,9 +153,9 @@ prepare_ureqs(const build_plan& plan, const toolchain& toolchain, path_ref out_r
             if (const auto& arc = lib.archive_plan()) {
                 lib_reqs.linkable_path = out_root / arc->calc_archive_file_path(toolchain);
             }
-            if (lib.has_generated_headers()) {
-                lib_reqs.include_paths.push_back(out_root / "__dds/gen"
-                                                 / lib.output_subdirectory());
+            auto gen_incdir_opt = lib.generated_include_dir();
+            if (gen_incdir_opt) {
+                lib_reqs.include_paths.push_back(out_root / *gen_incdir_opt);
             }
         }
     }

src/dds/build/plan/library.cpp

@@ -7,9 +7,24 @@
 #include <range/v3/view/transform.hpp>
 
 #include <cassert>
+#include <string>
 
 using namespace dds;
 
+namespace {
+
+const std::string gen_dir_qual = "__dds/gen";
+
+fs::path rebase_gen_incdir(path_ref subdir) { return gen_dir_qual / subdir; }
+}  // namespace
+
+std::optional<fs::path> library_plan::generated_include_dir() const noexcept {
+    if (_templates.empty()) {
+        return std::nullopt;
+    }
+    return rebase_gen_incdir(output_subdirectory());
+}
+
 library_plan library_plan::create(const library_root&             lib,
                                   const library_build_params&     params,
                                   std::optional<std::string_view> qual_name_) {
@@ -48,8 +63,10 @@ library_plan library_plan::create(const library_root&             lib,
     compile_rules.enable_warnings() = params.enable_warnings;
     compile_rules.uses()            = lib.manifest().uses;
 
+    const auto codegen_subdir = rebase_gen_incdir(params.out_subdir);
+
     if (!template_sources.empty()) {
-        compile_rules.include_dirs().push_back("__dds/gen" / params.out_subdir);
+        compile_rules.include_dirs().push_back(codegen_subdir);
     }
 
     // Convert the library sources into their respective file compilation plans.
@@ -113,7 +130,7 @@ library_plan library_plan::create(const library_root&             lib,
 
     std::vector<render_template_plan> render_templates;
     for (const auto& sf : template_sources) {
-        render_templates.emplace_back(sf, "__dds/gen" / params.out_subdir);
+        render_templates.emplace_back(sf, codegen_subdir);
     }
 
     // Done!

src/dds/build/plan/library.hpp

@@ -128,9 +128,11 @@ public:
      */
     auto& links() const noexcept { return _lib.manifest().links; }
     /**
-     * Return `true` if this object has generated header files
+     * The path to the directory that should be added for the #include search
+     * path for this library, relative to the build root. Returns `nullopt` if
+     * this library has no generated headers.
      */
-    bool has_generated_headers() const noexcept { return !templates().empty(); }
+    std::optional<fs::path> generated_include_dir() const noexcept;
 
     /**
      * Named constructor: Create a new `library_plan` automatically from some build-time parameters.