Use target recipe for auxiliary data storage during match-apply

In particular, we now have separate auxiliary data storage for inner
and outer operations.

19 files changed, 288 insertions, 147 deletions

diff --git a/libbuild2/action.hxx b/libbuild2/action.hxx
index dce3a1a..85012ba 100644
--- a/libbuild2/action.hxx
+++ b/libbuild2/action.hxx
@@ -45,16 +45,17 @@ namespace build2
   // inner rule. In particular, it should not replace or override the inner's
   // logic.
   //
-  // While most of the relevant target state is duplicated, certain things are
-  // shared among the inner/outer rules, such as the target data pad and the
-  // group state. In particular, it is assumed the group state is always
-  // determined by the inner rule (see resolve_members()).
+  // While most of the action-specific target state is duplicated (see
+  // target::opstate), certain things are shared among the inner/outer rules,
+  // such as the path, mtime, and group state. In particular, it is assumed
+  // the group state is always determined by the inner rule (see
+  // resolve_members()).
   //
   // Normally, an outer rule will be responsible for any additional, outer
   // operation-specific work. Sometimes, however, the inner rule needs to
   // customize its behavior. In this case the outer and inner rules must
-  // communicate this explicitly (normally via the target's data pad) and
-  // there is a number of restrictions to this approach. See
+  // communicate this explicitly (normally via the target's auxiliary data
+  // storage) and there is a number of restrictions to this approach. See
   // cc::{link,install}_rule for details.
   //
   struct action
diff --git a/libbuild2/adhoc-rule-buildscript.cxx b/libbuild2/adhoc-rule-buildscript.cxx
index aa30552..03e810d 100644
--- a/libbuild2/adhoc-rule-buildscript.cxx
+++ b/libbuild2/adhoc-rule-buildscript.cxx
@@ -470,10 +470,22 @@ namespace build2
     //
     if (a != perform_update_id || !xt.is_a<file> ())
     {
-      return [d, this] (action a, const target& t)
+      // Make sure we get small object optimization.
+      //
+      if (d)
       {
-        return default_action (a, t, d);
-      };
+        return [dv = *d, this] (action a, const target& t)
+        {
+          return default_action (a, t, dv);
+        };
+      }
+      else
+      {
+        return [this] (action a, const target& t)
+        {
+          return default_action (a, t, nullopt);
+        };
+      }
     }
 
     // See if this is the simple case with only static dependencies.
@@ -589,6 +601,10 @@ namespace build2
       }
     }
 
+    // Note that while it's tempting to turn match_data* into recipes, some of
+    // their members are not movable. And in the end we will have the same
+    // result: one dynamic memory allocation.
+    //
     unique_ptr<match_data> md;
     unique_ptr<match_data_byproduct> mdb;
 
diff --git a/libbuild2/adhoc-rule-regex-pattern.cxx b/libbuild2/adhoc-rule-regex-pattern.cxx
index 3bd3bb1..98acfb6 100644
--- a/libbuild2/adhoc-rule-regex-pattern.cxx
+++ b/libbuild2/adhoc-rule-regex-pattern.cxx
@@ -272,7 +272,7 @@ namespace build2
       return false;
     }
 
-    t.data (move (mr));
+    t.data (a, move (mr));
     return true;
   }
 
@@ -297,9 +297,9 @@ namespace build2
   }
 
   void adhoc_rule_regex_pattern::
-  apply_adhoc_members (action, target& t, const scope&, match_extra&) const
+  apply_adhoc_members (action a, target& t, const scope&, match_extra&) const
   {
-    const auto& mr (t.data<regex_match_results> ());
+    const auto& mr (t.data<regex_match_results> (a));
 
     for (auto i (targets_.begin () + 1); i != targets_.end (); ++i)
     {
@@ -342,7 +342,7 @@ namespace build2
                        const scope& bs,
                        match_extra&) const
   {
-    const auto& mr (t.data<regex_match_results> ());
+    const auto& mr (t.data<regex_match_results> (a));
 
     // Re-create the same clean semantics as in match_prerequisite_members().
     //
diff --git a/libbuild2/algorithm.cxx b/libbuild2/algorithm.cxx
index 7116b8b..355e633 100644
--- a/libbuild2/algorithm.cxx
+++ b/libbuild2/algorithm.cxx
@@ -821,9 +821,9 @@ namespace build2
           //
 
           // Clear the rule-specific variables, resolved targets list, and the
-          // data pad before calling match(). The rule is free to modify these
-          // in its match() (provided that it matches) in order to, for
-          // example, convey some information to apply().
+          // auxiliary data storage before calling match(). The rule is free
+          // to modify these in its match() (provided that it matches) in
+          // order to, for example, convey some information to apply().
           //
           clear_target (a, t);
 
@@ -1920,7 +1920,8 @@ namespace build2
     // group_state()) and should retain its value.
     //
     //
-    s.recipe = nullptr;
+    if (!s.recipe_keep)
+      s.recipe = nullptr;
 
     // Decrement the target count (see set_recipe() for details).
     //
diff --git a/libbuild2/algorithm.hxx b/libbuild2/algorithm.hxx
index 439af78..75976bf 100644
--- a/libbuild2/algorithm.hxx
+++ b/libbuild2/algorithm.hxx
@@ -396,9 +396,8 @@ namespace build2
   // Match a "delegate rule" from withing another rules' apply() function
   // avoiding recursive matches (thus the third argument). Unless try_match is
   // true, fail if no rule is found. Otherwise return empty recipe. Note that
-  // unlike match(), this function does not increment the dependents count and
-  // the two rules must coordinate who is using the target's data pad and/or
-  // prerequisite_targets. See also the companion execute_delegate().
+  // unlike match(), this function does not increment the dependents count.
+  // See also the companion execute_delegate().
   //
   recipe
   match_delegate (action, target&, const rule&, bool try_match = false);
diff --git a/libbuild2/algorithm.ixx b/libbuild2/algorithm.ixx
index 02c430b..2637349 100644
--- a/libbuild2/algorithm.ixx
+++ b/libbuild2/algorithm.ixx
@@ -495,8 +495,7 @@ namespace build2
   {
     t[a].vars.clear ();
     t.prerequisite_targets[a].clear ();
-    if (a.inner ())
-      t.clear_data ();
+    t.clear_data (a);
   }
 
   inline void
diff --git a/libbuild2/bash/rule.cxx b/libbuild2/bash/rule.cxx
index fec85be..55f3cf0 100644
--- a/libbuild2/bash/rule.cxx
+++ b/libbuild2/bash/rule.cxx
@@ -26,6 +26,9 @@ namespace build2
 
     struct match_data
     {
+      explicit
+      match_data (const in_rule& r): rule (r) {}
+
       // The "for install" condition is signalled to us by install_rule when
       // it is matched for the update operation. It also verifies that if we
       // have already been executed, then it was for install.
@@ -33,6 +36,25 @@ namespace build2
       // See cc::link_rule for a discussion of some subtleties in this logic.
       //
       optional<bool> for_install;
+
+      const in_rule& rule;
+
+      target_state
+      operator() (action a, const target& t)
+      {
+        // Unless the outer install rule signalled that this is update for
+        // install, signal back that we've performed plain update.
+        //
+        if (!for_install)
+          for_install = false;
+
+        //@@ TODO: need to verify all the modules we depend on are compatible
+        //         with our for_install value, similar to cc::link_rule's
+        //         append_libraries() (and which is the other half of the check
+        //         in install_rule).
+
+        return rule.perform_update (a, t);
+      }
     };
 
     static_assert (sizeof (match_data) <= target::small_data_size,
@@ -66,37 +88,23 @@ namespace build2
       if (!fm)
         l4 ([&]{trace << "no bash module prerequisite for target " << t;});
 
-      return (fi && fm);
+      return fi && fm;
     }
 
     recipe in_rule::
     apply (action a, target& t) const
     {
-      // Note that for-install is signalled by install_rule and therefore
-      // can only be relied upon during execute.
-      //
-      t.data (match_data ());
+      recipe r (rule::apply (a, t));
 
-      return rule::apply (a, t);
-    }
-
-    target_state in_rule::
-    perform_update (action a, const target& t) const
-    {
-      // Unless the outer install rule signalled that this is update for
-      // install, signal back that we've performed plain update.
-      //
-      match_data& md (t.data<match_data> ());
-
-      if (!md.for_install)
-        md.for_install = false;
-
-      //@@ TODO: need to verify all the modules we depend on are compatible
-      //         with our for_install value, similar to cc::link_rule's
-      //         append_libraries() (and which is the other half of the check
-      //         in install_rule).
+      if (a == perform_update_id)
+      {
+        // Note that for-install is signalled by install_rule and therefore
+        // can only be relied upon during execute.
+        //
+        return match_data (*this);
+      }
 
-      return rule::perform_update (a, t);
+      return r;
     }
 
     prerequisite_target in_rule::
@@ -338,7 +346,7 @@ namespace build2
       if (ap == nullptr)
         fail (l) << "unable to resolve import path " << ip;
 
-      match_data& md (t.data<match_data> ());
+      match_data& md (t.data<match_data> (a));
       assert (md.for_install);
 
       if (*md.for_install)
@@ -449,7 +457,7 @@ namespace build2
         // Signal to the in rule that this is update for install. And if the
         // update has already been executed, verify it was done for install.
         //
-        auto& md (t.data<match_data> ());
+        auto& md (t.data<match_data> (a.inner_action ()));
 
         if (md.for_install)
         {
diff --git a/libbuild2/bash/rule.hxx b/libbuild2/bash/rule.hxx
index 1a0cb36..3da0c73 100644
--- a/libbuild2/bash/rule.hxx
+++ b/libbuild2/bash/rule.hxx
@@ -37,9 +37,6 @@ namespace build2
       virtual recipe
       apply (action, target&) const override;
 
-      virtual target_state
-      perform_update (action, const target&) const override;
-
       virtual prerequisite_target
       search (action,
               const target&,
diff --git a/libbuild2/cc/compile-rule.cxx b/libbuild2/cc/compile-rule.cxx
index f680001..ada0cc6 100644
--- a/libbuild2/cc/compile-rule.cxx
+++ b/libbuild2/cc/compile-rule.cxx
@@ -229,10 +229,16 @@ namespace build2
       return nullopt;
     }
 
+    // Note that we don't really need this for clean (where we only need
+    // unrefined unit type) so we could make this update-only. But let's keep
+    // it simple for now.
+    //
     struct compile_rule::match_data
     {
-      match_data (unit_type t, const prerequisite_member& s)
-          : type (t), src (s) {}
+      match_data (const compile_rule& r,
+                  unit_type t,
+                  const prerequisite_member& s)
+          : type (t), src (s), rule (r) {}
 
       unit_type type;
       preprocessed pp = preprocessed::none;
@@ -245,6 +251,14 @@ namespace build2
       path dd;                              // Dependency database path.
       size_t header_units = 0;              // Number of imported header units.
       module_positions modules = {0, 0, 0}; // Positions of imported modules.
+
+      const compile_rule& rule;
+
+      target_state
+      operator() (action a, const target& t)
+      {
+        return rule.perform_update (a, t, *this);
+      }
     };
 
     compile_rule::
@@ -464,7 +478,7 @@ namespace build2
         {
           // Save in the target's auxiliary storage.
           //
-          t.data (match_data (ut, p));
+          t.data (a, match_data (*this, ut, p));
           return true;
         }
       }
@@ -809,7 +823,7 @@ namespace build2
 
       file& t (xt.as<file> ()); // Either obj*{} or bmi*{}.
 
-      match_data& md (t.data<match_data> ());
+      match_data& md (t.data<match_data> (a));
 
       context& ctx (t.ctx);
 
@@ -1497,10 +1511,7 @@ namespace build2
 
       switch (a)
       {
-      case perform_update_id: return [this] (action a, const target& t)
-        {
-          return perform_update (a, t);
-        };
+      case perform_update_id: return move (md);
       case perform_clean_id: return [this] (action a, const target& t)
         {
           return perform_clean (a, t);
@@ -5439,10 +5450,11 @@ namespace build2
       // 1. There is no good place in prerequisite_targets to store the
       //    exported flag (no, using the marking facility across match/execute
       //    is a bad idea). So what we are going to do is put re-exported
-      //    bmi{}s at the back and store (in the target's data pad) the start
-      //    position. One bad aspect about this part is that we assume those
-      //    bmi{}s have been matched by the same rule. But let's not kid
-      //    ourselves, there will be no other rule that matches bmi{}s.
+      //    bmi{}s at the back and store (in the target's auxiliary data
+      //    storage) the start position. One bad aspect about this part is
+      //    that we assume those bmi{}s have been matched by the same
+      //    rule. But let's not kid ourselves, there will be no other rule
+      //    that matches bmi{}s.
       //
       //    @@ I think now we could use prerequisite_targets::data for this?
       //
@@ -5833,7 +5845,7 @@ namespace build2
 
         // Copy over bmi{}s from our prerequisites weeding out duplicates.
         //
-        if (size_t j = bt->data<match_data> ().modules.start)
+        if (size_t j = bt->data<match_data> (a).modules.start)
         {
           // Hard to say whether we should reserve or not. We will probably
           // get quite a bit of duplications.
@@ -6578,12 +6590,11 @@ namespace build2
     }
 
     target_state compile_rule::
-    perform_update (action a, const target& xt) const
+    perform_update (action a, const target& xt, match_data& md) const
     {
       const file& t (xt.as<file> ());
       const path& tp (t.path ());
 
-      match_data md (move (t.data<match_data> ()));
       unit_type ut (md.type);
 
       context& ctx (t.ctx);
diff --git a/libbuild2/cc/compile-rule.hxx b/libbuild2/cc/compile-rule.hxx
index 16b33fa..49d33eb 100644
--- a/libbuild2/cc/compile-rule.hxx
+++ b/libbuild2/cc/compile-rule.hxx
@@ -44,6 +44,8 @@ namespace build2
                                                dyndep_rule
     {
     public:
+      struct match_data;
+
       compile_rule (data&&, const scope&);
 
       virtual bool
@@ -53,7 +55,7 @@ namespace build2
       apply (action, target&) const override;
 
       target_state
-      perform_update (action, const target&) const;
+      perform_update (action, const target&, match_data&) const;
 
       target_state
       perform_clean (action, const target&) const;
@@ -74,7 +76,6 @@ namespace build2
       functions (function_family&, const char*); // functions.cxx
 
     private:
-      struct match_data;
       using environment = small_vector<const char*, 2>;
 
       template <typename T>
diff --git a/libbuild2/cc/install-rule.cxx b/libbuild2/cc/install-rule.cxx
index d83bf70..0b4d1e1 100644
--- a/libbuild2/cc/install-rule.cxx
+++ b/libbuild2/cc/install-rule.cxx
@@ -160,6 +160,20 @@ namespace build2
         file_rule::match (a, t);
     }
 
+    // Wrap the file_rule's recipe into a data-carrying recipe.
+    //
+    struct install_match_data
+    {
+      build2::recipe        recipe;
+      link_rule::libs_paths libs_paths;
+
+      target_state
+      operator() (action a, const target& t)
+      {
+        return recipe (a, t);
+      }
+    };
+
     recipe install_rule::
     apply (action a, target& t) const
     {
@@ -173,7 +187,7 @@ namespace build2
         // Signal to the link rule that this is update for install. And if the
         // update has already been executed, verify it was done for install.
         //
-        auto& md (t.data<link_rule::match_data> ());
+        auto& md (t.data<link_rule::match_data> (a.inner_action ()));
 
         if (md.for_install)
         {
@@ -198,10 +212,12 @@ namespace build2
           {
             const string* p (cast_null<string> (t["bin.lib.prefix"]));
             const string* s (cast_null<string> (t["bin.lib.suffix"]));
-            t.data (
+
+            return install_match_data {
+              move (r),
               link_.derive_libs_paths (*f,
                                        p != nullptr ? p->c_str (): nullptr,
-                                       s != nullptr ? s->c_str (): nullptr));
+                                       s != nullptr ? s->c_str (): nullptr)};
           }
         }
       }
@@ -219,7 +235,7 @@ namespace build2
         // Here we may have a bunch of symlinks that we need to install.
         //
         const scope& rs (t.root_scope ());
-        auto& lp (t.data<link_rule::libs_paths> ());
+        auto& lp (t.data<install_match_data> (perform_install_id).libs_paths);
 
         auto ln = [&rs, &id] (const path& f, const path& l)
         {
@@ -253,7 +269,7 @@ namespace build2
         // Here we may have a bunch of symlinks that we need to uninstall.
         //
         const scope& rs (t.root_scope ());
-        auto& lp (t.data<link_rule::libs_paths> ());
+        auto& lp (t.data<install_match_data> (perform_uninstall_id).libs_paths);
 
         auto rm = [&rs, &id] (const path& l)
         {
diff --git a/libbuild2/cc/link-rule.cxx b/libbuild2/cc/link-rule.cxx
index f2e3775..de47822 100644
--- a/libbuild2/cc/link-rule.cxx
+++ b/libbuild2/cc/link-rule.cxx
@@ -875,8 +875,11 @@ namespace build2
       // Note that for_install is signalled by install_rule and therefore
       // can only be relied upon during execute.
       //
-      t.data (match_data ());
-      match_data& md (t.data<match_data> ());
+      // Note that we don't really need to set it as target data: while there
+      // are calls to get it, they should only happen after the target has
+      // been matched.
+      //
+      match_data md (*this);
 
       const scope& bs (t.base_scope ());
       const scope& rs (*bs.root_scope ());
@@ -2008,14 +2011,11 @@ namespace build2
 
       switch (a)
       {
-      case perform_update_id: return [this] (action a, const target& t)
-        {
-          return perform_update (a, t);
-        };
-      case perform_clean_id: return [this] (action a, const target& t)
-        {
-          return perform_clean (a, t);
-        };
+        // Keep the recipe (which is match_data) after execution to allow the
+        // install rule to examine it.
+        //
+      case perform_update_id: t.keep_data (a); // Fall through.
+      case perform_clean_id: return move (md);
       default: return noop_recipe; // Configure update.
       }
     }
@@ -2158,7 +2158,7 @@ namespace build2
                    *type != "cc" &&
                    type->compare (0, 3, "cc,") != 0)
           {
-            auto& md (l->data<link_rule::match_data> ());
+            auto& md (l->data<link_rule::match_data> (d.a));
             assert (md.for_install); // Must have been executed.
 
             // The user will get the target name from the context info.
@@ -2556,7 +2556,7 @@ namespace build2
     msvc_machine (const string& cpu); // msvc.cxx
 
     target_state link_rule::
-    perform_update (action a, const target& xt) const
+    perform_update (action a, const target& xt, match_data& md) const
     {
       tracer trace (x, "link_rule::perform_update");
 
@@ -2568,8 +2568,6 @@ namespace build2
       const scope& bs (t.base_scope ());
       const scope& rs (*bs.root_scope ());
 
-      match_data& md (t.data<match_data> ());
-
       // Unless the outer install rule signalled that this is update for
       // install, signal back that we've performed plain update.
       //
@@ -4078,12 +4076,11 @@ namespace build2
     }
 
     target_state link_rule::
-    perform_clean (action a, const target& xt) const
+    perform_clean (action a, const target& xt, match_data& md) const
     {
       const file& t (xt.as<file> ());
 
       ltype lt (link_type (t));
-      const match_data& md (t.data<match_data> ());
 
       clean_extras extras;
       clean_adhoc_extras adhoc_extras;
diff --git a/libbuild2/cc/link-rule.hxx b/libbuild2/cc/link-rule.hxx
index f0052e9..14a70d2 100644
--- a/libbuild2/cc/link-rule.hxx
+++ b/libbuild2/cc/link-rule.hxx
@@ -23,6 +23,8 @@ namespace build2
     public:
       link_rule (data&&);
 
+      struct match_data;
+
       struct match_result
       {
         bool seen_x   = false;
@@ -52,10 +54,10 @@ namespace build2
       apply (action, target&, match_extra&) const override;
 
       target_state
-      perform_update (action, const target&) const;
+      perform_update (action, const target&, match_data&) const;
 
       target_state
-      perform_clean (action, const target&) const;
+      perform_clean (action, const target&, match_data&) const;
 
     public:
       // Library handling.
@@ -226,9 +228,9 @@ namespace build2
       static void
       functions (function_family&, const char*); // functions.cxx
 
-    private:
-      friend class install_rule;
-      friend class libux_install_rule;
+      // Implementation details.
+      //
+    public:
 
       // Shared library paths.
       //
@@ -271,6 +273,9 @@ namespace build2
 
       struct match_data
       {
+        explicit
+        match_data (const link_rule& r): rule (r) {}
+
         // The "for install" condition is signalled to us by install_rule when
         // it is matched for the update operation. It also verifies that if we
         // have already been executed, then it was for install.
@@ -305,10 +310,21 @@ namespace build2
         size_t start; // Parallel prerequisites/prerequisite_targets start.
 
         link_rule::libs_paths libs_paths;
+
+        const link_rule& rule;
+
+        target_state
+        operator() (action a, const target& t)
+        {
+          return a == perform_update_id
+            ? rule.perform_update (a, t, *this)
+            : rule.perform_clean (a, t, *this);
+        }
       };
 
       // Windows rpath emulation (windows-rpath.cxx).
       //
+     private:
       struct windows_dll
       {
         reference_wrapper<const string> dll;
diff --git a/libbuild2/in/rule.hxx b/libbuild2/in/rule.hxx
index 98ab3b4..bd0d15a 100644
--- a/libbuild2/in/rule.hxx
+++ b/libbuild2/in/rule.hxx
@@ -18,8 +18,9 @@ namespace build2
   {
     // Preprocess an .in file.
     //
-    // Note that a derived rule can use the target data pad to cache data
-    // (e.g., in match() or apply()) to be used in substitute/lookup() calls.
+    // Note that a derived rule can use the target auxiliary data storage to
+    // cache data (e.g., in match() or apply()) to be used in substitute() and
+    // lookup() calls.
     //
     // Note also that currently this rule ignores the dry-run mode (see
     // perform_update() for the rationale).
diff --git a/libbuild2/target.cxx b/libbuild2/target.cxx
index 4f11b54..78bc5ac 100644
--- a/libbuild2/target.cxx
+++ b/libbuild2/target.cxx
@@ -79,7 +79,6 @@ namespace build2
   target::
   ~target ()
   {
-    clear_data ();
   }
 
   const string& target::
diff --git a/libbuild2/target.hxx b/libbuild2/target.hxx
index e3a64ca..563c264 100644
--- a/libbuild2/target.hxx
+++ b/libbuild2/target.hxx
@@ -625,8 +625,13 @@ namespace build2
 
       // Applied recipe.
       //
-      build2::recipe recipe;
-      bool           recipe_group_action; // Recipe is group_action.
+      // Note: also used as the auxiliary data storage during match, which is
+      //       why mutable (see the target::data() API below for details). The
+      //       default recipe_keep value is set by clear_target().
+      //
+      mutable build2::recipe recipe;
+      mutable bool           recipe_keep;         // Keep after execution.
+      bool                   recipe_group_action; // Recipe is group_action.
 
       // Target state for this operation. Note that it is undetermined until
       // a rule is matched and recipe applied (see set_recipe()).
@@ -640,8 +645,8 @@ namespace build2
       // no iffy modifications of the group's variables by member's rules).
       //
       // They are also automatically cleared before another rule is matched,
-      // similar to the data pad. In other words, rule-specific variables are
-      // only valid for this match-execute phase.
+      // similar to the auxiliary data storage. In other words, rule-specific
+      // variables are only valid for this match-execute phase.
       //
       variable_map vars;
 
@@ -785,30 +790,63 @@ namespace build2
     //
     mutable action_state<build2::prerequisite_targets> prerequisite_targets;
 
-    // Auxilary data storage.
+    // Auxiliary data storage.
     //
     // A rule that matches (i.e., returns true from its match() function) may
-    // use this pad to pass data between its match and apply functions as well
-    // as the recipe. After the recipe is executed, the data is destroyed. The
-    // rule may static assert that the small size of the pad (which doesn't
-    // require dynamic memory allocation) is sufficient for its needs.
-    //
-    // Note also that normally at least 2 extra pointers may be stored without
-    // a dynamic allocation in the returned recipe (small object optimization
-    // in std::function). So if you need to pass data only between apply() and
-    // the recipe, then this might be a more convenient way. @@ TMP
-    //
-    // Note also that a rule that delegates to another rule may not be able to
-    // use this mechanism fully since the delegated-to rule may also need the
-    // data pad.
-    //
-    // The data is not destroyed until the next match, which is relied upon to
-    // communicate between rules for inner/outer operations. @@ TMP
-    //
-    // Note that the recipe may modify the data. Currently reserved for the
-    // inner part of the action. @@ TMP
-    //
-    // See also match_extra::buffer.
+    // use this facility to pass data between its match and apply functions as
+    // well as the recipe. Specifically, between match() and apply() the data
+    // is stored in the recipe member (which is std::move_only_function-like).
+    // If the data needs to be passed on to the recipe, then it must become
+    // the recipe itself. Here is a typical arrangement:
+    //
+    // class compile_rule
+    // {
+    //   struct match_data
+    //   {
+    //     ... // Data.
+    //
+    //     const compile_rule& rule;
+    //
+    //     target_state
+    //     operator() (action a, const target& t)
+    //     {
+    //       return rule.perform_update (a, t, this);
+    //     }
+    //   };
+    //
+    //   virtual bool
+    //   match (action a, const target& t)
+    //   {
+    //     ... // Determine if matching.
+    //
+    //     t.data (a, match_data {..., *this});
+    //     return true;
+    //   }
+    //
+    //   virtual bool
+    //   apply (action a, target& t)
+    //   {
+    //     match_data& md (t.data (a));
+    //
+    //     ... // Match prerequisites, etc.
+    //
+    //     return move (md); // Data becomes the recipe.
+    //   }
+    //
+    //   target_state
+    //   perform_update (action a, const target& t, match_data& md) const
+    //   {
+    //     ... // Access data (also available as t.data<match_data> (a)).
+    //   }
+    // };
+    //
+    // After the recipe is executed, the recipe/data is destroyed, unless
+    // explicitly requested not to (see below). The rule may static assert
+    // that the small size of the storage (which doesn't require dynamic
+    // memory allocation) is sufficient for its needs.
+    //
+    // Note also that a rule that delegates to another rule may need to store
+    // the base rule's data/recipe in its own data/recipe.
 
     // Provide the small object optimization size for the common compilers
     // (see recipe.hxx for details) in case a rule wants to make sure its data
@@ -825,14 +863,10 @@ namespace build2
 #elif defined(_MSC_VER)
       sizeof (void*) * 6
 #else
-      // Assume at least 2 pointers.
-      //
-      sizeof (void*) * 2
+      sizeof (void*) * 2 // Assume at least 2 pointers.
 #endif
       ;
 
-    mutable recipe data_pad;
-
     template <typename T>
     struct data_wrapper
     {
@@ -857,43 +891,60 @@ namespace build2
 
     template <typename T>
     typename std::enable_if<!data_invocable<T>::value, void>::type
-    data (T&& d) const
+    data (action a, T&& d) const
     {
       using V = typename std::remove_cv<
         typename std::remove_reference<T>::type>::type;
 
-      data_pad = data_wrapper<V> {forward<T> (d)};
+      const opstate& s (state[a]);
+      s.recipe = data_wrapper<V> {forward<T> (d)};
+      s.recipe_keep = false; // Can't keep non-recipe data.
     }
 
     template <typename T>
     typename std::enable_if<!data_invocable<T>::value, T&>::type&
-    data () const
+    data (action a) const
     {
       using V = typename std::remove_cv<T>::type;
-      return data_pad.target<data_wrapper<V>> ()->d;
+      return state[a].recipe.target<data_wrapper<V>> ()->d;
     }
 
     // Note that in this case we don't strip const (the expectation is that we
     // move the recipe in/out of data).
     //
+    // If keep is true, then keep the recipe as data after execution. In
+    // particular, this can be used to communicate between inner/outer rules
+    // (see cc::install_rule for an example).
+    //
+    //
     template <typename T>
     typename std::enable_if<data_invocable<T>::value, void>::type
-    data (T&& d) const
+    data (action a, T&& d, bool keep = false) const
+    {
+      const opstate& s (state[a]);
+      s.recipe = forward<T> (d);
+      s.recipe_keep = keep;
+    }
+
+    void
+    keep_data (action a, bool keep = true) const
     {
-      data_pad = forward<T> (d);
+      state[a].recipe_keep = keep;
     }
 
     template <typename T>
     typename std::enable_if<data_invocable<T>::value, T&>::type&
-    data () const
+    data (action a) const
     {
-      return *data_pad.target<T> ();
+      return *state[a].recipe.target<T> ();
     }
 
     void
-    clear_data () const
+    clear_data (action a) const
     {
-      data_pad = nullptr;
+      const opstate& s (state[a]);
+      s.recipe = nullptr;
+      s.recipe_keep = false;
     }
 
     // Target type info and casting.
diff --git a/libbuild2/version/rule.cxx b/libbuild2/version/rule.cxx
index ad26da4..1799666 100644
--- a/libbuild2/version/rule.cxx
+++ b/libbuild2/version/rule.cxx
@@ -46,6 +46,25 @@ namespace build2
 
     // in_rule
     //
+
+    // Wrap the in::rule's perform_update recipe into a data-carrying recipe.
+    //
+    // To optimize this a bit further (i.e., to avoid the dynamic memory
+    // allocation) we are going to call in::rule::perform_update() directly
+    // (after all it's virtual and thus part of the in_rule's interface).
+    //
+    struct match_data
+    {
+      const module& mod;
+      const in_rule& rule;
+
+      target_state
+      operator() (action a, const target& t)
+      {
+        return rule.perform_update (a, t);
+      }
+    };
+
     bool in_rule::
     match (action a, target& xt) const
     {
@@ -74,14 +93,20 @@ namespace build2
       if (!fi)
         l5 ([&]{trace << "no in file prerequisite for target " << t;});
 
-      bool r (fm && fi);
+      return fm && fi;
+    }
+
+    recipe in_rule::
+    apply (action a, target& t) const
+    {
+      recipe r (rule::apply (a, t));
 
-      // If we match, lookup and cache the module for the update operation.
+      // Lookup and cache the module for the update operation.
       //
-      if (r && a == perform_update_id)
-        t.data (rs.find_module<module> (module::name));
-
-      return r;
+      return a == perform_update_id
+        ? match_data {*t.root_scope ().find_module<module> (module::name),
+                      *this}
+        : move (r);
     }
 
     string in_rule::
@@ -97,7 +122,7 @@ namespace build2
       // Note that this code will be executed during up-to-date check for each
       // substitution so let's try not to do anything overly sub-optimal here.
       //
-      const module& m (*t.data<const module*> ());
+      const module& m (t.data<match_data> (a).mod);
 
       // Split it into the package name and the variable/condition name.
       //
diff --git a/libbuild2/version/rule.hxx b/libbuild2/version/rule.hxx
index 55b4aee..ba673e5 100644
--- a/libbuild2/version/rule.hxx
+++ b/libbuild2/version/rule.hxx
@@ -25,6 +25,9 @@ namespace build2
       virtual bool
       match (action, target&) const override;
 
+      virtual recipe
+      apply (action, target&) const override;
+
       virtual string
       lookup (const location&,
               action,
diff --git a/tests/recipe/cxx/testscript b/tests/recipe/cxx/testscript
index c94148e..214468f 100644
--- a/tests/recipe/cxx/testscript
+++ b/tests/recipe/cxx/testscript
@@ -180,9 +180,9 @@ if (!$static && $test.target == $build.host)
         --
 
         recipe
-        apply (action, target& t) const override
+        apply (action a, target& t) const override
         {
-          const auto& mrs (t.data<regex_match_results> ());
+          const auto& mrs (t.data<regex_match_results> (a));
 
           return [this, mr = mrs.str (1)] (action a, const target& t)
           {