From 5745a2fe944dca5612b31c099101914d6c60b223 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Thu, 14 Dec 2023 07:42:44 +0200
Subject: Allow enabling C++ modules for C++20 and later std.cxx values

---
 libbuild2/cc/compile-rule.cxx | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

(limited to 'libbuild2/cc')

diff --git a/libbuild2/cc/compile-rule.cxx b/libbuild2/cc/compile-rule.cxx
index be5a680..b4d405b 100644
--- a/libbuild2/cc/compile-rule.cxx
+++ b/libbuild2/cc/compile-rule.cxx
@@ -5393,7 +5393,9 @@ namespace build2
             if (!modules)
             {
               if (ut != unit_type::non_modular || !mi.imports.empty ())
-                fail << "modules support required by " << src;
+                fail << "modules support required by " << src <<
+                  info << "consider enabling modules with "
+                       << x << ".features.modules=true in root.build";
             }
             else
             {
@@ -6574,6 +6576,10 @@ namespace build2
     // Find or create a modules sidebuild subproject returning its root
     // directory.
     //
+    // @@ Could we omit creating a subproject if the sidebuild scope is the
+    //    project scope itself? This would speed up simple examples (and
+    //    potentially direct compilation that we may support).
+    //
     pair<dir_path, const scope&> compile_rule::
     find_modules_sidebuild (const scope& rs) const
     {
-- 
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