1 files changed, 77 insertions, 4 deletions

diff --git a/libbuild2/bin/rule.cxx b/libbuild2/bin/rule.cxx
index 38a3d98..1d33961 100644
--- a/libbuild2/bin/rule.cxx
+++ b/libbuild2/bin/rule.cxx
@@ -35,14 +35,87 @@ namespace build2
     // libul_rule
     //
     bool libul_rule::
-    match (action a, target& t) const
+    match (action a, target& t, const string& h, match_extra&) const
     {
-      fail << diag_doing (a, t) << " target group" <<
-        info << "explicitly select libua{} or libus{} member" << endf;
+      if (!metadata_)
+      {
+        fail << diag_doing (a, t) << " target group" <<
+          info << "explicitly select libua{} or libus{} member" <<
+          info << "or use bin.metadata rule hint if this is metadata library";
+      }
+
+      return (h == "bin.metadata");
     }
 
     recipe libul_rule::
-    apply (action, target&) const {return empty_recipe;}
+    apply (action a, target& t, match_extra&) const
+    {
+      assert (metadata_);
+
+      // Pick one of the members. First looking for the one already matched.
+      //
+      const target* m (nullptr);
+
+      const libus* ls (nullptr);
+      {
+        ls = search_existing<libus> (t.ctx, t.dir, t.out, t.name);
+
+        if (ls != nullptr && ls->matched (a))
+          m = ls;
+      }
+
+      const libua* la (nullptr);
+      if (m == nullptr)
+      {
+        la = search_existing<libua> (t.ctx, t.dir, t.out, t.name);
+
+        if (la != nullptr && la->matched (a))
+          m = la;
+      }
+
+      if (m == nullptr)
+      {
+        const scope& bs (t.base_scope ());
+
+        lmembers lm (link_members (*bs.root_scope ()));
+
+        if (lm.s && lm.a)
+        {
+          // Use the bin.exe.lib order as a heuristics to pick the library
+          // (i.e., the most likely utility library to be built is the one
+          // most likely to be linked).
+          //
+          lorder lo (link_order (bs, otype::e));
+
+          (lo == lorder::s_a || lo == lorder::s ? lm.a : lm.s) = false;
+        }
+
+        if (lm.s)
+          m = ls != nullptr ? ls : &search<libus> (t, t.dir, t.out, t.name);
+        else
+          m = la != nullptr ? la : &search<libua> (t, t.dir, t.out, t.name);
+      }
+
+      // Save the member we picked in case others (e.g., $x.lib_poptions())
+      // need this information.
+      //
+      t.prerequisite_targets[a].push_back (m);
+
+      if (match_sync (a, *m, unmatch::safe).first)
+        return noop_recipe;
+
+      return [] (action a, const target& t)
+      {
+        const target* m (t.prerequisite_targets[a].back ());
+
+        // For update always return unchanged so we are consistent whether we
+        // managed to unmatch or now. Note that for clean we may get postponed
+        // so let's return the actual target state.
+        //
+        target_state r (execute_sync (a, *m));
+        return a == perform_update_id ? target_state::unchanged : r;
+      };
+    }
 
     // lib_rule
     //