Pass const target& to recipes

37 files changed, 465 insertions, 356 deletions

diff --git a/build2/algorithm b/build2/algorithm
index 09e0c4d..5bfa1e0 100644
--- a/build2/algorithm
+++ b/build2/algorithm
@@ -71,7 +71,7 @@ namespace build2
   // Note that currently we return NULL for project-qualified names and
   // unknown target types.
   //
-  target*
+  const target*
   search_existing (const name&,
                    const scope&,
                    const dir_path& out = dir_path ());
@@ -152,7 +152,7 @@ namespace build2
   // executor implementation. Decrements the dependents count.
   //
   target_state
-  execute (action, target&);
+  execute (action, const target&);
 
   // Execute the recipe obtained with match_delegate(). Note that
   // the target's state is neither checked nor updated by this
@@ -161,7 +161,7 @@ namespace build2
   // state into the one returned.
   //
   target_state
-  execute_delegate (const recipe&, action, target&);
+  execute_delegate (const recipe&, action, const target&);
 
   // A special version of the above that should be used for "direct"
   // and "now" execution, that is, side-stepping the normal target-
@@ -171,7 +171,7 @@ namespace build2
   // is busy.
   //
   target_state
-  execute_direct (action, target&);
+  execute_direct (action, const target&);
 
   // The default prerequisite execute implementation. It calls execute()
   // on each non-ignored (non-NULL) prerequisite target in a loop. If this
@@ -181,12 +181,12 @@ namespace build2
   // function can be used as a recipe.
   //
   target_state
-  execute_prerequisites (action, target&);
+  execute_prerequisites (action, const target&);
 
   // As above but iterates over the prerequisites in reverse.
   //
   target_state
-  reverse_execute_prerequisites (action, target&);
+  reverse_execute_prerequisites (action, const target&);
 
   // A version of the above that also determines whether the action needs to
   // be executed on the target based on the passed timestamp and filter.
@@ -207,10 +207,10 @@ namespace build2
   // Note that because we use mtime, this function should normally only be
   // used in the perform_update action.
   //
-  using prerequisite_filter = function<bool (target&)>;
+  using prerequisite_filter = function<bool (const target&)>;
 
   pair<bool, target_state>
-  execute_prerequisites (action, target&,
+  execute_prerequisites (action, const target&,
                          const timestamp&,
                          const prerequisite_filter& = nullptr);
 
@@ -222,45 +222,45 @@ namespace build2
   // prerequisites of the same type may get injected).
   //
   template <typename T>
-  pair<T*, target_state>
-  execute_prerequisites (action, target&,
+  pair<const T*, target_state>
+  execute_prerequisites (action, const target&,
                          const timestamp&,
                          const prerequisite_filter& = nullptr);
 
-  pair<target*, target_state>
+  pair<const target*, target_state>
   execute_prerequisites (const target_type&,
-                         action, target&,
+                         action, const target&,
                          const timestamp&,
                          const prerequisite_filter& = nullptr);
 
   template <typename T>
-  pair<T*, target_state>
+  pair<const T*, target_state>
   execute_prerequisites (const target_type&,
-                         action, target&,
+                         action, const target&,
                          const timestamp&,
                          const prerequisite_filter& = nullptr);
 
   // Return noop_recipe instead of using this function directly.
   //
   target_state
-  noop_action (action, target&);
+  noop_action (action, const target&);
 
   // Default action implementation which forwards to the prerequisites.
   // Use default_recipe instead of using this function directly.
   //
   target_state
-  default_action (action, target&);
+  default_action (action, const target&);
 
   // Standard perform(clean) action implementation for the file target
   // (or derived).
   //
   target_state
-  perform_clean (action, target&);
+  perform_clean (action, const target&);
 
   // As above, but also removes the auxiliary dependency database (.d file).
   //
   target_state
-  perform_clean_depdb (action, target&);
+  perform_clean_depdb (action, const target&);
 
   // Helper for custom perform(clean) implementations that cleans extra files
   // and directories (recursively) specified as a list of either absolute
@@ -281,11 +281,11 @@ namespace build2
   // You can also clean extra files derived from adhoc group members.
   //
   target_state
-  clean_extra (action, file&,
+  clean_extra (action, const file&,
                initializer_list<initializer_list<const char*>> extra);
 
   inline target_state
-  clean_extra (action a, file& f, initializer_list<const char*> extra)
+  clean_extra (action a, const file& f, initializer_list<const char*> extra)
   {
     return clean_extra (a, f, {extra});
   }
diff --git a/build2/algorithm.cxx b/build2/algorithm.cxx
index f173af6..74e7d7d 100644
--- a/build2/algorithm.cxx
+++ b/build2/algorithm.cxx
@@ -63,7 +63,7 @@ namespace build2
                    n.proj);
   }
 
-  target*
+  const target*
   search_existing (const name& cn, const scope& s, const dir_path& out)
   {
     assert (phase == run_phase::search_match || phase == run_phase::execute);
@@ -422,8 +422,10 @@ namespace build2
   }
 
   target_state
-  execute (action a, target& t)
+  execute (action a, const target& ct)
   {
+    target& t (const_cast<target&> (ct)); // MT-aware.
+
     // text << "E " << t << ": " << t.dependents << " " << dependency_count;
 
     size_t d (0);
@@ -494,8 +496,10 @@ namespace build2
   }
 
   target_state
-  execute_direct (action a, target& t)
+  execute_direct (action a, const target& ct)
   {
+    target& t (const_cast<target&> (ct)); // MT-aware.
+
     size_t tc (target::count_unexecuted);
     if (t.task_count.compare_exchange_strong (tc, target::count_executing))
       return execute_impl (a, t);
@@ -515,11 +519,11 @@ namespace build2
   }
 
   target_state
-  execute_prerequisites (action a, target& t)
+  execute_prerequisites (action a, const target& t)
   {
     target_state r (target_state::unchanged);
 
-    for (target* pt: t.prerequisite_targets)
+    for (const target* pt: t.prerequisite_targets)
     {
       if (pt == nullptr) // Skipped.
         continue;
@@ -531,11 +535,11 @@ namespace build2
   }
 
   target_state
-  reverse_execute_prerequisites (action a, target& t)
+  reverse_execute_prerequisites (action a, const target& t)
   {
     target_state r (target_state::unchanged);
 
-    for (target* pt: reverse_iterate (t.prerequisite_targets))
+    for (const target* pt: reverse_iterate (t.prerequisite_targets))
     {
       if (pt == nullptr) // Skipped.
         continue;
@@ -546,17 +550,17 @@ namespace build2
     return r;
   }
 
-  pair<target*, target_state>
+  pair<const target*, target_state>
   execute_prerequisites (const target_type* tt,
-                         action a, target& t,
+                         action a, const target& t,
                          const timestamp& mt, const prerequisite_filter& pf)
   {
     bool e (mt == timestamp_nonexistent);
 
-    target* rt (tt != nullptr ? nullptr : &t);
+    const target* rt (tt != nullptr ? nullptr : &t);
     target_state rs (target_state::unchanged);
 
-    for (target* pt: t.prerequisite_targets)
+    for (const target* pt: t.prerequisite_targets)
     {
       if (pt == nullptr) // Skip ignored.
         continue;
@@ -598,7 +602,7 @@ namespace build2
   }
 
   target_state
-  noop_action (action a, target& t)
+  noop_action (action a, const target& t)
   {
     text << "noop action triggered for " << diag_doing (a, t);
     assert (false); // We shouldn't be called, see target::recipe().
@@ -606,12 +610,13 @@ namespace build2
   }
 
   target_state
-  group_action (action a, target& t)
+  group_action (action a, const target& t)
   {
     // If the group is busy, we wait, similar to prerequisites.
     //
-    if (execute (a, *t.group) == target_state::busy)
-      sched.wait (target::count_executed, t.group->task_count);
+    const target& g (*t.group);
+    if (execute (a, g) == target_state::busy)
+      sched.wait (target::count_executed, g.task_count);
 
     // Indicate to execute() that this target's state comes from the group.
     //
@@ -619,7 +624,7 @@ namespace build2
   }
 
   target_state
-  default_action (action a, target& t)
+  default_action (action a, const target& t)
   {
     return current_mode == execution_mode::first
       ? execute_prerequisites (a, t)
@@ -628,7 +633,7 @@ namespace build2
 
   target_state
   clean_extra (action a,
-               file& ft,
+               const file& ft,
                initializer_list<initializer_list<const char*>> extra)
   {
     // Clean the extras first and don't print the commands at verbosity level
@@ -639,7 +644,8 @@ namespace build2
     bool ed (false);
     path ep;
 
-    auto clean = [&er, &ed, &ep] (file& f, initializer_list<const char*> es)
+    auto clean = [&er, &ed, &ep] (const file& f,
+                                  initializer_list<const char*> es)
     {
       for (const char* e: es)
       {
@@ -713,9 +719,9 @@ namespace build2
 
     // Now clean the ad hoc group file members, if any.
     //
-    for (target* m (ft.member); m != nullptr; m = m->member)
+    for (const target* m (ft.member); m != nullptr; m = m->member)
     {
-      file* fm (dynamic_cast<file*> (m));
+      const file* fm (dynamic_cast<const file*> (m));
 
       if (fm == nullptr || fm->path ().empty ())
         continue;
@@ -776,14 +782,14 @@ namespace build2
   }
 
   target_state
-  perform_clean (action a, target& t)
+  perform_clean (action a, const target& t)
   {
-    return clean_extra (a, dynamic_cast<file&> (t), {nullptr});
+    return clean_extra (a, dynamic_cast<const file&> (t), {nullptr});
   }
 
   target_state
-  perform_clean_depdb (action a, target& t)
+  perform_clean_depdb (action a, const target& t)
   {
-    return clean_extra (a, dynamic_cast<file&> (t), {".d"});
+    return clean_extra (a, dynamic_cast<const file&> (t), {".d"});
   }
 }
diff --git a/build2/algorithm.ixx b/build2/algorithm.ixx
index cf234d8..0c171fd 100644
--- a/build2/algorithm.ixx
+++ b/build2/algorithm.ixx
@@ -173,7 +173,7 @@ namespace build2
   }
 
   inline target_state
-  execute_delegate (const recipe& r, action a, target& t)
+  execute_delegate (const recipe& r, action a, const target& t)
   {
     return r (a, t);
   }
@@ -181,13 +181,13 @@ namespace build2
   // If the first argument is NULL, then the result is treated as a boolean
   // value.
   //
-  pair<target*, target_state>
+  pair<const target*, target_state>
   execute_prerequisites (const target_type*,
-                         action, target&,
+                         action, const target&,
                          const timestamp&, const prerequisite_filter&);
 
   inline pair<bool, target_state>
-  execute_prerequisites (action a, target& t,
+  execute_prerequisites (action a, const target& t,
                          const timestamp& mt, const prerequisite_filter& pf)
   {
     auto p (execute_prerequisites (nullptr, a, t, mt, pf));
@@ -195,29 +195,29 @@ namespace build2
   }
 
   template <typename T>
-  inline pair<T*, target_state>
-  execute_prerequisites (action a, target& t,
+  inline pair<const T*, target_state>
+  execute_prerequisites (action a, const target& t,
                          const timestamp& mt, const prerequisite_filter& pf)
   {
     auto p (execute_prerequisites (T::static_type, a, t, mt, pf));
-    return make_pair (static_cast<T*> (p.first), p.second);
+    return make_pair (static_cast<const T*> (p.first), p.second);
   }
 
-  inline pair<target*, target_state>
+  inline pair<const target*, target_state>
   execute_prerequisites (const target_type& tt,
-                         action a, target& t,
+                         action a, const target& t,
                          const timestamp& mt, const prerequisite_filter& pf)
   {
     return execute_prerequisites (&tt, a, t, mt, pf);
   }
 
   template <typename T>
-  inline pair<T*, target_state>
+  inline pair<const T*, target_state>
   execute_prerequisites (const target_type& tt,
-                         action a, target& t,
+                         action a, const target& t,
                          const timestamp& mt, const prerequisite_filter& pf)
   {
     auto p (execute_prerequisites (tt, a, t, mt, pf));
-    return make_pair (static_cast<T*> (p.first), p.second);
+    return make_pair (static_cast<const T*> (p.first), p.second);
   }
 }
diff --git a/build2/bin/rule b/build2/bin/rule
index c903990..5fe9069 100644
--- a/build2/bin/rule
+++ b/build2/bin/rule
@@ -38,7 +38,7 @@ namespace build2
       apply (slock&, action, target&) const override;
 
       static target_state
-      perform (action, target&);
+      perform (action, const target&);
     };
   }
 }
diff --git a/build2/bin/rule.cxx b/build2/bin/rule.cxx
index c9e8b3e..a73ad3d 100644
--- a/build2/bin/rule.cxx
+++ b/build2/bin/rule.cxx
@@ -124,9 +124,9 @@ namespace build2
     }
 
     target_state lib_rule::
-    perform (action act, target& xt)
+    perform (action act, const target& xt)
     {
-      lib& t (static_cast<lib&> (xt));
+      const lib& t (static_cast<const lib&> (xt));
 
       const match_data& md (t.data<match_data> ());
       const string& type (md.type);
diff --git a/build2/bin/target.cxx b/build2/bin/target.cxx
index ba15c7d..caef8e0 100644
--- a/build2/bin/target.cxx
+++ b/build2/bin/target.cxx
@@ -22,7 +22,7 @@ namespace build2
       obj* o (targets.find<obj> (dir, out, n));
       obje* e (new obje (move (dir), move (out), move (n)));
 
-      if ((e->group = o))
+      if ((e->group = o) != nullptr)
         o->e = e;
 
       return make_pair (e, move (ext));
@@ -49,7 +49,7 @@ namespace build2
       obj* o (targets.find<obj> (dir, out, n));
       obja* a (new obja (move (dir), move (out), move (n)));
 
-      if ((a->group = o))
+      if ((a->group = o) != nullptr)
         o->a = a;
 
       return make_pair (a, move (ext));
@@ -76,7 +76,7 @@ namespace build2
       obj* o (targets.find<obj> (dir, out, n));
       objs* s (new objs (move (dir), move (out), move (n)));
 
-      if ((s->group = o))
+      if ((s->group = o) != nullptr)
         o->s = s;
 
       return make_pair (s, move (ext));
@@ -106,13 +106,13 @@ namespace build2
 
       obj* o (new obj (move (dir), move (out), move (n)));
 
-      if ((o->e = e))
+      if ((o->e = e) != nullptr)
         e->group = o;
 
-      if ((o->a = a))
+      if ((o->a = a)!= nullptr)
         a->group = o;
 
-      if ((o->s = s))
+      if ((o->s = s)!= nullptr)
         s->group = o;
 
       return make_pair (o, move (ext));
@@ -141,7 +141,7 @@ namespace build2
       lib* l (t == liba::static_type ? targets.find<lib> (d, o, n) : nullptr);
       liba* a (new liba (move (d), move (o), move (n)));
 
-      if ((a->group = l))
+      if ((a->group = l) != nullptr)
         l->a = a;
 
       return make_pair (a, move (ext));
@@ -182,7 +182,7 @@ namespace build2
       lib* l (t == libs::static_type ? targets.find<lib> (d, o, n) : nullptr);
       libs* s (new libs (move (d), move (o), move (n)));
 
-      if ((s->group = l))
+      if ((s->group = l) != nullptr)
         l->s = s;
 
       return make_pair (s, move (ext));
@@ -222,10 +222,10 @@ namespace build2
 
       lib* l (new lib (move (d), move (o), move (n)));
 
-      if ((l->a = a))
+      if ((l->a = a) != nullptr)
         a->group = l;
 
-      if ((l->s = s))
+      if ((l->s = s) != nullptr)
         s->group = l;
 
       return make_pair (l, move (ext));
diff --git a/build2/cc/common b/build2/cc/common
index 06df4c3..860913e 100644
--- a/build2/cc/common
+++ b/build2/cc/common
@@ -184,18 +184,16 @@ namespace build2
       //
     public:
       void
-      process_libraries (const scope&,
-                         lorder,
-                         const dir_paths&,
-                         file&,
-                         bool,
-                         const function<bool (file&, bool)>&,
-                         const function<void (file*, const string&, bool)>&,
-                         const function<void (file&,
-                                              const string&,
-                                              bool,
-                                              bool)>&,
-                         bool = false) const;
+      process_libraries (
+        const scope&,
+        lorder,
+        const dir_paths&,
+        const file&,
+        bool,
+        const function<bool (const file&, bool)>&,
+        const function<void (const file*, const string&, bool)>&,
+        const function<void (const file&, const string&, bool, bool)>&,
+        bool = false) const;
 
       target*
       search_library (const dir_paths& sysd,
@@ -209,7 +207,7 @@ namespace build2
       }
 
     private:
-      file&
+      const file&
       resolve_library (const scope&,
                        name,
                        lorder,
diff --git a/build2/cc/common.cxx b/build2/cc/common.cxx
index e7b26a1..62a4ab2 100644
--- a/build2/cc/common.cxx
+++ b/build2/cc/common.cxx
@@ -49,14 +49,14 @@ namespace build2
       const scope& top_bs,
       lorder top_lo,
       const dir_paths& top_sysd,
-      file& l,
+      const file& l,
       bool la,
-      const function<bool (file&,
+      const function<bool (const file&,
                            bool la)>& proc_impl, // Implementation?
-      const function<void (file*,                // Can be NULL.
+      const function<void (const file*,          // Can be NULL.
                            const string& path,   // Library path.
                            bool sys)>& proc_lib, // True if system library.
-      const function<void (file&,
+      const function<void (const file&,
                            const string& type,   // cc.type
                            bool com,             // cc. or x.
                            bool exp)>& proc_opt, // *.export.
@@ -213,10 +213,10 @@ namespace build2
       //
       if (impl && !c_e_libs.defined () && !x_e_libs.defined ())
       {
-        for (target* p: l.prerequisite_targets)
+        for (const target* p: l.prerequisite_targets)
         {
           bool a;
-          file* f;
+          const file* f;
 
           if ((a = (f = p->is_a<liba> ()) != nullptr)
               ||   (f = p->is_a<libs> ()) != nullptr)
@@ -287,7 +287,7 @@ namespace build2
             if (sysd == nullptr) find_sysd ();
             if (!lo) find_lo ();
 
-            file& t (resolve_library (bs, n, *lo, *sysd, usrd));
+            const file& t (resolve_library (bs, n, *lo, *sysd, usrd));
 
             if (proc_lib)
             {
@@ -385,7 +385,7 @@ namespace build2
     // will select exactly the same target as the library's matched rule and
     // that's the only way to guarantee it will be up-to-date.
     //
-    file& common::
+    const file& common::
     resolve_library (const scope& s,
                      name n,
                      lorder lo,
@@ -395,7 +395,7 @@ namespace build2
       if (n.type != "lib" && n.type != "liba" && n.type != "libs")
         fail << "target name " << n << " is not a library";
 
-      target* xt (nullptr);
+      const target* xt (nullptr);
 
       if (n.dir.absolute () && !n.qualified ())
       {
@@ -422,12 +422,12 @@ namespace build2
         //
         dir_path out;
         prerequisite_key pk {n.proj, {tt, &n.dir, &out, &n.value, ext}, &s};
-        xt = search_library (sysd, usrd, pk);
+        xt = search_library (sysd, usrd, pk); //@@ TM const
 
         if (xt == nullptr)
         {
           if (n.qualified ())
-            xt = &import (pk);
+            xt = &import (pk); //@@ TM const
           else
             fail << "unable to find library " << pk;
         }
@@ -435,10 +435,10 @@ namespace build2
 
       // If this is lib{}, pick appropriate member.
       //
-      if (lib* l = xt->is_a<lib> ())
+      if (const lib* l = xt->is_a<lib> ())
         xt = &link_member (*l, lo); // Pick liba{} or libs{}.
 
-      return static_cast<file&> (*xt);
+      return static_cast<const file&> (*xt);
     }
 
     // Note that pk's scope should not be NULL (even if dir is absolute). If
diff --git a/build2/cc/compile b/build2/cc/compile
index b6fc7d6..63ce286 100644
--- a/build2/cc/compile
+++ b/build2/cc/compile
@@ -33,17 +33,23 @@ namespace build2
       apply (slock&, action, target&) const override;
 
       target_state
-      perform_update (action, target&) const;
+      perform_update (action, const target&) const;
 
       target_state
-      perform_clean (action, target&) const;
+      perform_clean (action, const target&) const;
 
     private:
       void
-      append_lib_options (const scope&, cstrings&, target&, lorder) const;
+      append_lib_options (const scope&,
+                          cstrings&,
+                          const target&,
+                          lorder) const;
 
       void
-      hash_lib_options (const scope&, sha256&, target&, lorder) const;
+      hash_lib_options (const scope&,
+                        sha256&,
+                        const target&,
+                        lorder) const;
 
       // Mapping of include prefixes (e.g., foo in <foo/bar>) for auto-
       // generated headers to directories where they will be generated.
@@ -58,7 +64,7 @@ namespace build2
       using prefix_map = butl::dir_path_map<dir_path>;
 
       void
-      append_prefixes (prefix_map&, target&, const variable&) const;
+      append_prefixes (prefix_map&, const target&, const variable&) const;
 
       void
       append_lib_prefixes (const scope&, prefix_map&, target&, lorder) const;
diff --git a/build2/cc/compile.cxx b/build2/cc/compile.cxx
index 66ea9bd..98d1218 100644
--- a/build2/cc/compile.cxx
+++ b/build2/cc/compile.cxx
@@ -78,10 +78,11 @@ namespace build2
     void compile::
     append_lib_options (const scope& bs,
                         cstrings& args,
-                        target& t,
+                        const target& t,
                         lorder lo) const
     {
-      auto opt = [&args, this] (file& l, const string& t, bool com, bool exp)
+      auto opt = [&args, this] (
+        const file& l, const string& t, bool com, bool exp)
       {
         // Note that in our model *.export.poptions are always "interface",
         // even if set on liba{}/libs{}, unlike loptions.
@@ -98,31 +99,35 @@ namespace build2
 
       // In case we don't have the "small function object" optimization.
       //
-      const function<void (file&, const string&, bool, bool)> optf (opt);
+      const function<void (const file&, const string&, bool, bool)> optf (opt);
 
       // Note that here we don't need to see group members (see apply()).
       //
-      for (prerequisite& p: group_prerequisites (t))
+      for (const prerequisite& p: const_group_prerequisites (t))
       {
-        target* pt (p.target); // Already searched and matched.
+        const target* pt (p.target); // Already searched and matched.
 
         bool a;
 
-        if (lib* l = pt->is_a<lib> ())
+        if (const lib* l = pt->is_a<lib> ())
           a = (pt = &link_member (*l, lo))->is_a<liba> ();
         else if (!(a = pt->is_a<liba> ()) && !pt->is_a<libs> ())
           continue;
 
         process_libraries (bs, lo, sys_lib_dirs,
-                           static_cast<file&> (*pt), a,
+                           static_cast<const file&> (*pt), a,
                            nullptr, nullptr, optf);
       }
     }
 
     void compile::
-    hash_lib_options (const scope& bs, sha256& cs, target& t, lorder lo) const
+    hash_lib_options (const scope& bs,
+                      sha256& cs,
+                      const target& t,
+                      lorder lo) const
     {
-      auto opt = [&cs, this] (file& l, const string& t, bool com, bool exp)
+      auto opt = [&cs, this] (
+        const file& l, const string& t, bool com, bool exp)
       {
         assert (exp);
 
@@ -136,21 +141,21 @@ namespace build2
 
       // In case we don't have the "small function object" optimization.
       //
-      const function<void (file&, const string&, bool, bool)> optf (opt);
+      const function<void (const file&, const string&, bool, bool)> optf (opt);
 
-      for (prerequisite& p: group_prerequisites (t))
+      for (const prerequisite& p: const_group_prerequisites (t))
       {
-        target* pt (p.target); // Already searched and matched.
+        const target* pt (p.target); // Already searched and matched.
 
         bool a;
 
-        if (lib* l = pt->is_a<lib> ())
+        if (const lib* l = pt->is_a<lib> ())
           a = (pt = &link_member (*l, lo))->is_a<liba> ();
         else if (!(a = pt->is_a<liba> ()) && !pt->is_a<libs> ())
           continue;
 
         process_libraries (bs, lo, sys_lib_dirs,
-                           static_cast<file&> (*pt), a,
+                           static_cast<const file&> (*pt), a,
                            nullptr, nullptr, optf);
       }
     }
@@ -164,7 +169,8 @@ namespace build2
                          target& t,
                          lorder lo) const
     {
-      auto opt = [&m, this] (file& l, const string& t, bool com, bool exp)
+      auto opt = [&m, this] (
+        const file& l, const string& t, bool com, bool exp)
       {
         assert (exp);
 
@@ -178,7 +184,7 @@ namespace build2
 
       // In case we don't have the "small function object" optimization.
       //
-      const function<void (file&, const string&, bool, bool)> optf (opt);
+      const function<void (const file&, const string&, bool, bool)> optf (opt);
 
       for (prerequisite& p: group_prerequisites (t))
       {
@@ -192,7 +198,7 @@ namespace build2
           continue;
 
         process_libraries (bs, lo, sys_lib_dirs,
-                           static_cast<file&> (*pt), a,
+                           static_cast<const file&> (*pt), a,
                            nullptr, nullptr, optf);
       }
     }
@@ -399,12 +405,15 @@ namespace build2
 
       switch (a)
       {
-      case perform_update_id:
-        return [this] (action a, target& t) {return perform_update (a, t);};
-      case perform_clean_id:
-        return [this] (action a, target& t) {return perform_clean (a, t);};
-      default:
-        return noop_recipe; // Configure update.
+      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);
+        };
+      default: return noop_recipe; // Configure update.
       }
     }
 
@@ -440,7 +449,7 @@ namespace build2
     }
 
     void compile::
-    append_prefixes (prefix_map& m, target& t, const variable& var) const
+    append_prefixes (prefix_map& m, const target& t, const variable& var) const
     {
       tracer trace (x, "append_prefixes");
 
@@ -1394,15 +1403,15 @@ namespace build2
     msvc_filter_cl (ifdstream&, const path& src);
 
     target_state compile::
-    perform_update (action a, target& xt) const
+    perform_update (action a, const target& xt) const
     {
-      file& t (static_cast<file&> (xt));
+      const file& t (static_cast<const file&> (xt));
 
       // Update prerequisites and determine if any relevant ones render us
       // out-of-date. Note that currently we treat all the prerequisites
       // as potentially affecting the result (for simplicity/performance).
       //
-      file* s;
+      const file* s;
       {
         auto p (execute_prerequisites<file> (x_src, a, t, t.mtime ()));
 
@@ -1621,9 +1630,9 @@ namespace build2
     }
 
     target_state compile::
-    perform_clean (action a, target& xt) const
+    perform_clean (action a, const target& xt) const
     {
-      file& t (static_cast<file&> (xt));
+      const file& t (static_cast<const file&> (xt));
 
       if (cid == "msvc")
         return clean_extra (a, t, {".d", ".idb", ".pdb"});
diff --git a/build2/cc/install b/build2/cc/install
index ff7af4d..f676d72 100644
--- a/build2/cc/install
+++ b/build2/cc/install
@@ -30,11 +30,14 @@ namespace build2
       virtual match_result
       match (slock&, action, target&, const string&) const override;
 
+      virtual recipe
+      apply (slock&, action, target&) const override;
+
       virtual void
-      install_extra (file&, const install_dir&) const override;
+      install_extra (const file&, const install_dir&) const override;
 
       virtual bool
-      uninstall_extra (file&, const install_dir&) const override;
+      uninstall_extra (const file&, const install_dir&) const override;
 
     private:
       const link& link_;
diff --git a/build2/cc/install.cxx b/build2/cc/install.cxx
index 074654b..f022a92 100644
--- a/build2/cc/install.cxx
+++ b/build2/cc/install.cxx
@@ -66,17 +66,35 @@ namespace build2
       // ones building this target. So first run link's match().
       //
       match_result r (link_.match (ml, a, t, hint));
-      return r ? install::file_rule::match (ml, a, t, "") : r;
+      return r ? file_rule::match (ml, a, t, "") : r;
+    }
+
+    recipe install::
+    apply (slock& s, action a, target& t) const
+    {
+      recipe r (file_rule::apply (s, a, t));
+
+      // Derive shared library paths and cache them in the target's aux
+      // storage if we are (un)installing (used in *_extra() functions below).
+      //
+      if (a.operation () == install_id || a.operation () == uninstall_id)
+      {
+        file* f;
+        if ((f = t.is_a<libs> ()) != nullptr && tclass != "windows")
+          t.data (link_.derive_libs_paths (*f));
+      }
+
+      return r;
     }
 
     void install::
-    install_extra (file& t, const install_dir& id) const
+    install_extra (const file& t, const install_dir& id) const
     {
       if (t.is_a<libs> () && tclass != "windows")
       {
         // Here we may have a bunch of symlinks that we need to install.
         //
-        link::libs_paths lp (link_.derive_libs_paths (t));
+        auto& lp (t.data<link::libs_paths> ());
 
         auto ln = [&id, this] (const path& f, const path& l)
         {
@@ -87,7 +105,7 @@ namespace build2
         const path& so (lp.soname);
         const path& in (lp.interm);
 
-        const path* f (lp.real);
+        const path* f (&lp.real);
 
         if (!in.empty ()) {ln (*f, in); f = &in;}
         if (!so.empty ()) {ln (*f, so); f = &so;}
@@ -96,7 +114,7 @@ namespace build2
     }
 
     bool install::
-    uninstall_extra (file& t, const install_dir& id) const
+    uninstall_extra (const file& t, const install_dir& id) const
     {
       bool r (false);
 
@@ -104,7 +122,7 @@ namespace build2
       {
         // Here we may have a bunch of symlinks that we need to uninstall.
         //
-        link::libs_paths lp (link_.derive_libs_paths (t));
+        auto& lp (t.data<link::libs_paths> ());
 
         auto rm = [&id, this] (const path& l)
         {
diff --git a/build2/cc/link b/build2/cc/link
index d1b0e7a..c787015 100644
--- a/build2/cc/link
+++ b/build2/cc/link
@@ -31,10 +31,10 @@ namespace build2
       apply (slock&, action, target&) const override;
 
       target_state
-      perform_update (action, target&) const;
+      perform_update (action, const target&) const;
 
       target_state
-      perform_clean (action, target&) const;
+      perform_clean (action, const target&) const;
 
     private:
       friend class install;
@@ -50,19 +50,16 @@ namespace build2
         // The libs{} path is always the real path. On Windows the link path
         // is the import library.
         //
-        // @@ TODO: change real to reference, make other const once cache the
-        // object.
-        //
-              path  link;   // What we link: libfoo.so
-              path  soname; // SONAME:       libfoo-1.so, libfoo.so.1
-              path  interm; // Intermediate: libfoo.so.1.2
-        const path* real;   // Real:         libfoo.so.1.2.3
+        const path  link;   // What we link: libfoo.so
+        const path  soname; // SONAME:       libfoo-1.so, libfoo.so.1
+        const path  interm; // Intermediate: libfoo.so.1.2
+        const path& real;   // Real:         libfoo.so.1.2.3
 
         inline const path&
         effect_link () const {return link.empty () ? effect_soname () : link;}
 
         inline const path&
-        effect_soname () const {return soname.empty () ? *real : soname;}
+        effect_soname () const {return soname.empty () ? real : soname;}
       };
 
       libs_paths
@@ -71,13 +68,21 @@ namespace build2
       // Library handling.
       //
       void
-      append_libraries (strings&, file&, bool, const scope&, lorder) const;
+      append_libraries (strings&,
+                        const file&, bool,
+                        const scope&, lorder) const;
 
       void
-      hash_libraries (sha256&, file&, bool, const scope&, lorder) const;
+      hash_libraries (sha256&,
+                      const file&, bool,
+                      const scope&, lorder) const;
 
       void
-      rpath_libraries (strings&, target&, const scope&, lorder, bool) const;
+      rpath_libraries (strings&,
+                       const target&,
+                       const scope&,
+                       lorder,
+                       bool) const;
 
       // Windows rpath emulation (windows-rpath.cxx).
       //
@@ -93,13 +98,13 @@ namespace build2
       using windows_dlls = std::set<windows_dll>;
 
       timestamp
-      windows_rpath_timestamp (file&, const scope&, lorder) const;
+      windows_rpath_timestamp (const file&, const scope&, lorder) const;
 
       windows_dlls
-      windows_rpath_dlls (file&, const scope&, lorder) const;
+      windows_rpath_dlls (const file&, const scope&, lorder) const;
 
       void
-      windows_rpath_assembly (file&, const scope&, lorder,
+      windows_rpath_assembly (const file&, const scope&, lorder,
                               const string&,
                               timestamp,
                               bool) const;
@@ -107,7 +112,7 @@ namespace build2
       // Windows-specific (windows-manifest.cxx).
       //
       path
-      windows_manifest (file&, bool rpath_assembly) const;
+      windows_manifest (const file&, bool rpath_assembly) const;
 
     private:
       const string rule_id;
diff --git a/build2/cc/link.cxx b/build2/cc/link.cxx
index 83a7c0d..7c946aa 100644
--- a/build2/cc/link.cxx
+++ b/build2/cc/link.cxx
@@ -266,7 +266,6 @@ namespace build2
       // Now determine the paths.
       //
       path lk, so, in;
-      const path* re (nullptr);
 
       // We start with the basic path.
       //
@@ -310,7 +309,7 @@ namespace build2
       if (!v.empty ())
         b += v;
 
-      re = &ls.derive_path (move (b));
+      const path& re (ls.derive_path (move (b)));
 
       return libs_paths {move (lk), move (so), move (in), re};
     }
@@ -318,6 +317,9 @@ namespace build2
     recipe link::
     apply (slock& ml, action a, target& xt) const
     {
+      static_assert (sizeof (link::libs_paths) <= target::data_size,
+                     "insufficient space");
+
       tracer trace (x, "link::apply");
 
       file& t (static_cast<file&> (xt));
@@ -388,7 +390,7 @@ namespace build2
             if (tclass == "windows")
               add_adhoc (t, "libi");
 
-            derive_libs_paths (t);
+            t.data (derive_libs_paths (t)); // Cache in target.
             break;
           }
         }
@@ -645,18 +647,21 @@ namespace build2
 
       switch (a)
       {
-      case perform_update_id:
-        return [this] (action a, target& t) {return perform_update (a, t);};
-      case perform_clean_id:
-        return [this] (action a, target& t) {return perform_clean (a, t);};
-      default:
-        return noop_recipe; // Configure update.
+      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);
+        };
+      default: return noop_recipe; // Configure update.
       }
     }
 
     void link::
     append_libraries (strings& args,
-                      file& l, bool la,
+                      const file& l, bool la,
                       const scope& bs, lorder lo) const
     {
       // Note: lack of the "small function object" optimization will really
@@ -664,9 +669,9 @@ namespace build2
       //
       bool win (tclass == "windows");
 
-      auto imp = [] (file&, bool la) {return la;};
+      auto imp = [] (const file&, bool la) {return la;};
 
-      auto lib = [&args, win] (file* f, const string& p, bool)
+      auto lib = [&args, win] (const file* f, const string& p, bool)
       {
         if (f != nullptr)
         {
@@ -675,7 +680,7 @@ namespace build2
           // (see search_library() for details).
           //
           if (win && f->member != nullptr && f->is_a<libs> ())
-            f = static_cast<file*> (f->member);
+            f = static_cast<const file*> (f->member.get ());
 
           args.push_back (relative (f->path ()).string ());
         }
@@ -683,11 +688,12 @@ namespace build2
           args.push_back (p);
       };
 
-      auto opt = [&args, this] (file& l, const string& t, bool com, bool exp)
+      auto opt = [&args, this] (
+        const file& l, const string& t, bool com, bool exp)
       {
         // If we need an interface value, then use the group (lib{}).
         //
-        if (target* g = exp && l.is_a<libs> () ? l.group : &l)
+        if (const target* g = exp && l.is_a<libs> () ? l.group : &l)
         {
           const variable& var (
             com
@@ -705,16 +711,14 @@ namespace build2
 
     void link::
     hash_libraries (sha256& cs,
-                    file& l,
-                    bool la,
-                    const scope& bs,
-                    lorder lo) const
+                    const file& l, bool la,
+                    const scope& bs, lorder lo) const
     {
       bool win (tclass == "windows");
 
-      auto imp = [] (file&, bool la) {return la;};
+      auto imp = [] (const file&, bool la) {return la;};
 
-      auto lib = [&cs, win] (file* f, const string& p, bool)
+      auto lib = [&cs, win] (const file* f, const string& p, bool)
       {
         if (f != nullptr)
         {
@@ -723,7 +727,7 @@ namespace build2
           // (see search_library() for details).
           //
           if (win && f->member != nullptr && f->is_a<libs> ())
-            f = static_cast<file*> (f->member);
+            f = static_cast<const file*> (f->member.get ());
 
           cs.append (f->path ().string ());
         }
@@ -731,9 +735,10 @@ namespace build2
           cs.append (p);
       };
 
-      auto opt = [&cs, this] (file& l, const string& t, bool com, bool exp)
+      auto opt = [&cs, this] (
+        const file& l, const string& t, bool com, bool exp)
       {
-        if (target* g = exp && l.is_a<libs> () ? l.group : &l)
+        if (const target* g = exp && l.is_a<libs> () ? l.group : &l)
         {
           const variable& var (
             com
@@ -751,7 +756,7 @@ namespace build2
 
     void link::
     rpath_libraries (strings& args,
-                     target& t,
+                     const target& t,
                      const scope& bs,
                      lorder lo,
                      bool for_install) const
@@ -765,7 +770,7 @@ namespace build2
           return;
       }
 
-      auto imp = [for_install] (file&, bool la)
+      auto imp = [for_install] (const file&, bool la)
       {
         // If we are not installing, then we only need to rpath interface
         // libraries (they will include rpath's for their implementations).
@@ -791,7 +796,7 @@ namespace build2
         bool for_install;
       } d {args, for_install};
 
-      auto lib = [&d, this] (file* l, const string& f, bool sys)
+      auto lib = [&d, this] (const file* l, const string& f, bool sys)
       {
         // We don't rpath system libraries. Why, you may ask? There are many
         // good reasons and I have them written on an napkin somewhere...
@@ -845,13 +850,13 @@ namespace build2
 
       // In case we don't have the "small function object" optimization.
       //
-      const function<bool (file&, bool)> impf (imp);
-      const function<void (file*, const string&, bool)> libf (lib);
+      const function<bool (const file&, bool)> impf (imp);
+      const function<void (const file*, const string&, bool)> libf (lib);
 
-      for (target* pt: t.prerequisite_targets)
+      for (const target* pt: t.prerequisite_targets)
       {
-        file* f;
-        liba* a;
+        const file* f;
+        const liba* a;
 
         if ((f = a = pt->is_a<liba> ()) ||
             (f =     pt->is_a<libs> ()))
@@ -884,14 +889,14 @@ namespace build2
     msvc_machine (const string& cpu); // msvc.cxx
 
     target_state link::
-    perform_update (action a, target& xt) const
+    perform_update (action a, const target& xt) const
     {
       tracer trace (x, "link::perform_update");
 
       auto oop (a.outer_operation ());
       bool for_install (oop == install_id || oop == uninstall_id);
 
-      file& t (static_cast<file&> (xt));
+      const file& t (static_cast<const file&> (xt));
 
       const scope& bs (t.base_scope ());
       const scope& rs (*bs.root_scope ());
@@ -1071,10 +1076,6 @@ namespace build2
       //
       cstrings args {nullptr}; // Reserve one for config.bin.ar/config.x.
 
-      libs_paths paths;
-      if (lt == otype::s)
-        paths = derive_libs_paths (t);
-
       // Storage.
       //
       string soname1, soname2;
@@ -1127,6 +1128,7 @@ namespace build2
           //
           if (lt == otype::s)
           {
+            const libs_paths& paths (t.data<libs_paths> ());
             const string& leaf (paths.effect_soname ().leaf ().string ());
 
             if (tclass == "macosx")
@@ -1195,11 +1197,11 @@ namespace build2
       {
         sha256 cs;
 
-        for (target* pt: t.prerequisite_targets)
+        for (const target* pt: t.prerequisite_targets)
         {
-          file* f;
-          liba* a (nullptr);
-          libs* s (nullptr);
+          const file* f;
+          const liba* a (nullptr);
+          const libs* s (nullptr);
 
           if ((f = pt->is_a<obje> ()) ||
               (f = pt->is_a<obja> ()) ||
@@ -1367,8 +1369,8 @@ namespace build2
               // derived from the import library by changing the extension.
               // Lucky for us -- there is no option to name it.
               //
-              auto imp (static_cast<file*> (t.member));
-              out2 = "/IMPLIB:" + relative (imp->path ()).string ();
+              auto& imp (static_cast<const file&> (*t.member));
+              out2 = "/IMPLIB:" + relative (imp.path ()).string ();
               args.push_back (out2.c_str ());
             }
 
@@ -1377,9 +1379,10 @@ namespace build2
             //
             if (find_option ("/DEBUG", args, true))
             {
-              auto pdb (static_cast<file*> (
-                          lt == otype::e ? t.member : t.member->member));
-              out1 = "/PDB:" + relative (pdb->path ()).string ();
+              auto& pdb (
+                static_cast<const file&> (
+                  lt == otype::e ? *t.member : *t.member->member));
+              out1 = "/PDB:" + relative (pdb.path ()).string ();
               args.push_back (out1.c_str ());
             }
 
@@ -1412,8 +1415,8 @@ namespace build2
                 // On Windows libs{} is the DLL and its first ad hoc group
                 // member is the import library.
                 //
-                auto imp (static_cast<file*> (t.member));
-                out = "-Wl,--out-implib=" + relative (imp->path ()).string ();
+                auto& imp (static_cast<const file&> (*t.member));
+                out = "-Wl,--out-implib=" + relative (imp.path ()).string ();
                 args.push_back (out.c_str ());
               }
             }
@@ -1428,11 +1431,11 @@ namespace build2
 
       args[0] = ld->recall_string ();
 
-      for (target* pt: t.prerequisite_targets)
+      for (const target* pt: t.prerequisite_targets)
       {
-        file* f;
-        liba* a (nullptr);
-        libs* s (nullptr);
+        const file* f;
+        const liba* a (nullptr);
+        const libs* s (nullptr);
 
         if ((f = pt->is_a<obje> ()) ||
             (f = pt->is_a<obja> ()) ||
@@ -1600,11 +1603,13 @@ namespace build2
           }
         };
 
+        const libs_paths& paths (t.data<libs_paths> ());
+
         const path& lk (paths.link);
         const path& so (paths.soname);
         const path& in (paths.interm);
 
-        const path* f (paths.real);
+        const path* f (&paths.real);
 
         if (!in.empty ()) {ln (f->leaf (), in); f = &in;}
         if (!so.empty ()) {ln (f->leaf (), so); f = &so;}
@@ -1622,11 +1627,9 @@ namespace build2
     }
 
     target_state link::
-    perform_clean (action a, target& xt) const
+    perform_clean (action a, const target& xt) const
     {
-      file& t (static_cast<file&> (xt));
-
-      libs_paths paths;
+      const file& t (static_cast<const file&> (xt));
 
       switch (link_type (t))
       {
@@ -1666,7 +1669,8 @@ namespace build2
             // Here we can have a bunch of symlinks that we need to remove. If
             // the paths are empty, then they will be ignored.
             //
-            paths = derive_libs_paths (t);
+            const libs_paths& paths (t.data<libs_paths> ());
+
             return clean_extra (a, t, {".d",
                                        paths.link.string ().c_str (),
                                        paths.soname.string ().c_str (),
diff --git a/build2/cc/msvc.cxx b/build2/cc/msvc.cxx
index 4ac82a0..fc2979c 100644
--- a/build2/cc/msvc.cxx
+++ b/build2/cc/msvc.cxx
@@ -69,7 +69,7 @@ namespace build2
         //
         if (lt == otype::s && l.compare (0, 3, "   ") == 0)
         {
-          path imp (static_cast<file*> (t.member)->path ().leaf ());
+          path imp (static_cast<const file&> (*t.member).path ().leaf ());
 
           if (l.find (imp.string ()) != string::npos &&
               l.find (imp.base ().string () + ".exp") != string::npos)
diff --git a/build2/cc/utility b/build2/cc/utility
index 050b645..bb7ed34 100644
--- a/build2/cc/utility
+++ b/build2/cc/utility
@@ -22,10 +22,10 @@ namespace build2
     // Compile/link output type.
     //
     otype
-    compile_type (target&);
+    compile_type (const target&);
 
     otype
-    link_type (target&);
+    link_type (const target&);
 
     // Library link order.
     //
@@ -41,8 +41,13 @@ namespace build2
 
     // Given the link order return the library member (liba or libs) to link.
     //
+    // Note that the const version assumes you have already called non-const.
+    //
     target&
     link_member (bin::lib&, lorder);
+
+    const target&
+    link_member (const bin::lib&, lorder);
   }
 }
 
diff --git a/build2/cc/utility.cxx b/build2/cc/utility.cxx
index 5c5d1fe..4ccdcaf 100644
--- a/build2/cc/utility.cxx
+++ b/build2/cc/utility.cxx
@@ -38,6 +38,37 @@ namespace build2
         : v.size () > 1 && v[1] == "shared" ? lorder::a_s : lorder::a;
     }
 
+    const target&
+    link_member (const bin::lib& l, lorder lo)
+    {
+      bool ls (true);
+      const string& at (cast<string> (l["bin.lib"])); // Available members.
+
+      switch (lo)
+      {
+      case lorder::a:
+      case lorder::a_s:
+        ls = false; // Fall through.
+      case lorder::s:
+      case lorder::s_a:
+        {
+          if (ls ? at == "static" : at == "shared")
+          {
+            if (lo == lorder::a_s || lo == lorder::s_a)
+              ls = !ls;
+            else
+              assert (false);
+          }
+        }
+      }
+
+      target* r (ls ? static_cast<target*> (l.s) : l.a);
+
+      assert (r != nullptr);
+
+      return *r;
+    }
+
     target&
     link_member (bin::lib& l, lorder lo)
     {
diff --git a/build2/cc/utility.ixx b/build2/cc/utility.ixx
index 8695a8e..bc9cd05 100644
--- a/build2/cc/utility.ixx
+++ b/build2/cc/utility.ixx
@@ -7,7 +7,7 @@ namespace build2
   namespace cc
   {
     inline otype
-    compile_type (target& t)
+    compile_type (const target& t)
     {
       return
         t.is_a<bin::obje> () ? otype::e :
@@ -16,7 +16,7 @@ namespace build2
     }
 
     inline otype
-    link_type (target& t)
+    link_type (const target& t)
     {
       return
         t.is_a<exe> ()       ? otype::e :
diff --git a/build2/cc/windows-manifest.cxx b/build2/cc/windows-manifest.cxx
index b0b0feb..0e38e7d 100644
--- a/build2/cc/windows-manifest.cxx
+++ b/build2/cc/windows-manifest.cxx
@@ -39,7 +39,7 @@ namespace build2
     // file corresponding to the exe{} target. Return the manifest file path.
     //
     path link::
-    windows_manifest (file& t, bool rpath_assembly) const
+    windows_manifest (const file& t, bool rpath_assembly) const
     {
       tracer trace (x, "windows_manifest");
 
diff --git a/build2/cc/windows-rpath.cxx b/build2/cc/windows-rpath.cxx
index 00911d4..94b1145 100644
--- a/build2/cc/windows-rpath.cxx
+++ b/build2/cc/windows-rpath.cxx
@@ -46,16 +46,16 @@ namespace build2
     // adding to the assembly or timestamp_nonexistent if there aren't any.
     //
     timestamp link::
-    windows_rpath_timestamp (file& t, const scope& bs, lorder lo) const
+    windows_rpath_timestamp (const file& t, const scope& bs, lorder lo) const
     {
       timestamp r (timestamp_nonexistent);
 
       // We need to collect all the DLLs, so go into implementation of both
       // shared and static (in case they depend on shared).
       //
-      auto imp = [] (file&, bool) {return true;};
+      auto imp = [] (const file&, bool) {return true;};
 
-      auto lib = [&r] (file* l, const string& f, bool sys)
+      auto lib = [&r] (const file* l, const string& f, bool sys)
       {
         // We don't rpath system libraries.
         //
@@ -97,10 +97,10 @@ namespace build2
           r = t;
       };
 
-      for (target* pt: t.prerequisite_targets)
+      for (const target* pt: t.prerequisite_targets)
       {
-        file* f;
-        liba* a;
+        const file* f;
+        const liba* a;
 
         if ((f = a = pt->is_a<liba> ()) ||
             (f =     pt->is_a<libs> ()))
@@ -116,15 +116,15 @@ namespace build2
     // duplicates).
     //
     auto link::
-    windows_rpath_dlls (file& t,
+    windows_rpath_dlls (const file& t,
                         const scope& bs,
                         lorder lo) const -> windows_dlls
     {
       windows_dlls r;
 
-      auto imp = [] (file&, bool) {return true;};
+      auto imp = [] (const file&, bool) {return true;};
 
-      auto lib = [&r] (file* l, const string& f, bool sys)
+      auto lib = [&r] (const file* l, const string& f, bool sys)
       {
         if (sys)
           return;
@@ -137,7 +137,8 @@ namespace build2
             //
             const string* pdb (
               l->member != nullptr && l->member->member != nullptr
-              ? &static_cast<file&> (*l->member->member).path ().string ()
+              ? &static_cast<const file&> (
+                  *l->member->member).path ().string ()
               : nullptr);
 
             r.insert (windows_dll {f, pdb, string ()});
@@ -178,10 +179,10 @@ namespace build2
         }
       };
 
-      for (target* pt: t.prerequisite_targets)
+      for (const target* pt: t.prerequisite_targets)
       {
-        file* f;
-        liba* a;
+        const file* f;
+        const liba* a;
 
         if ((f = a = pt->is_a<liba> ()) ||
             (f =     pt->is_a<libs> ()))
@@ -205,7 +206,7 @@ namespace build2
     // manifest file.
     //
     void link::
-    windows_rpath_assembly (file& t,
+    windows_rpath_assembly (const file& t,
                             const scope& bs,
                             lorder lo,
                             const string& tcpu,
diff --git a/build2/cli/rule b/build2/cli/rule
index fbacacf..1fbbde4 100644
--- a/build2/cli/rule
+++ b/build2/cli/rule
@@ -26,10 +26,10 @@ namespace build2
       apply (slock&, action, target&) const override;
 
       static target_state
-      perform_update (action, target&);
+      perform_update (action, const target&);
 
       static target_state
-      perform_clean (action, target&);
+      perform_clean (action, const target&);
     };
   }
 }
diff --git a/build2/cli/rule.cxx b/build2/cli/rule.cxx
index 9f54950..5dcec99 100644
--- a/build2/cli/rule.cxx
+++ b/build2/cli/rule.cxx
@@ -199,7 +199,7 @@ namespace build2
       }
       else
       {
-        cli_cxx& g (*static_cast<cli_cxx*> (xt.group));
+        cli_cxx& g (static_cast<cli_cxx&> (*xt.group));
         build2::match (ml, a, g);
         return group_recipe; // Execute the group's recipe.
       }
@@ -228,16 +228,16 @@ namespace build2
     }
 
     target_state compile::
-    perform_update (action a, target& xt)
+    perform_update (action a, const target& xt)
     {
-      cli_cxx& t (static_cast<cli_cxx&> (xt));
+      const cli_cxx& t (static_cast<const cli_cxx&> (xt));
 
       // Update prerequisites and determine if any relevant ones render us
       // out-of-date. Note that currently we treat all the prerequisites
       // as potentially affecting the result (think prologues/epilogues,
       // etc).
       //
-      cli* s;
+      const cli* s;
       {
         auto p (execute_prerequisites<cli> (a, t, t.mtime ()));
 
@@ -320,9 +320,9 @@ namespace build2
     }
 
     target_state compile::
-    perform_clean (action a, target& xt)
+    perform_clean (action a, const target& xt)
     {
-      cli_cxx& t (static_cast<cli_cxx&> (xt));
+      const cli_cxx& t (static_cast<const cli_cxx&> (xt));
 
       // The reverse order of update: first delete the files, then clean
       // prerequisites. Also update timestamp in case there are operations
diff --git a/build2/install/rule b/build2/install/rule
index 169c43a..24111ce 100644
--- a/build2/install/rule
+++ b/build2/install/rule
@@ -53,12 +53,12 @@ namespace build2
       using install_dir = install::install_dir;
 
       virtual void
-      install_extra (file&, const install_dir&) const;
+      install_extra (const file&, const install_dir&) const;
 
       // Return true if anything was uninstalled.
       //
       virtual bool
-      uninstall_extra (file&, const install_dir&) const;
+      uninstall_extra (const file&, const install_dir&) const;
 
       // Installation "commands".
       //
@@ -84,16 +84,16 @@ namespace build2
       //
       static bool
       uninstall_f (const install_dir& base,
-                   file* t,
+                   const file* t,
                    const path& name,
                    bool verbose);
 
     private:
       target_state
-      perform_install (action, target&) const;
+      perform_install (action, const target&) const;
 
       target_state
-      perform_uninstall (action, target&) const;
+      perform_uninstall (action, const target&) const;
     };
   }
 }
diff --git a/build2/install/rule.cxx b/build2/install/rule.cxx
index 8c21dc1..3be7eef 100644
--- a/build2/install/rule.cxx
+++ b/build2/install/rule.cxx
@@ -129,7 +129,8 @@ namespace build2
     apply (slock& ml, action a, target& t) const
     {
       match_data md (move (t.data<match_data> ()));
-      t.clear_data (); // In case delegated-to rule also uses aux storage.
+      t.clear_data (); // In case delegated-to rule (or the rule that overrides
+                       // us; see cc/install) also uses aux storage.
 
       if (!md.install) // Not installable.
         return noop_recipe;
@@ -220,8 +221,8 @@ namespace build2
         // Ok, the worst case scenario: we need to cause update of
         // prerequisite targets and also delegate to the real update.
         //
-        return [pt = move (p), dr = move (d)]
-          (action a, target& t) mutable -> target_state
+        return [pt = move (p), dr = move (d)] (
+          action a, const target& t) mutable -> target_state
         {
           // Do the target update first.
           //
@@ -237,16 +238,22 @@ namespace build2
         };
       }
       else if (a.operation () == install_id)
-        return [this] (action a, target& t) {return perform_install (a, t);};
+        return [this] (action a, const target& t)
+        {
+          return perform_install (a, t);
+        };
       else
-        return [this] (action a, target& t) {return perform_uninstall (a, t);};
+        return [this] (action a, const target& t)
+        {
+          return perform_uninstall (a, t);
+        };
     }
 
     void file_rule::
-    install_extra (file&, const install_dir&) const {}
+    install_extra (const file&, const install_dir&) const {}
 
     bool file_rule::
-    uninstall_extra (file&, const install_dir&) const {return false;}
+    uninstall_extra (const file&, const install_dir&) const {return false;}
 
     struct install_dir
     {
@@ -279,7 +286,7 @@ namespace build2
     //
     static install_dir&
     resolve_subdir (install_dirs& rs,
-                    target& t,
+                    const target& t,
                     const scope& s,
                     const lookup& l)
     {
@@ -310,7 +317,7 @@ namespace build2
     // all the super-directories leading up to the destination (last).
     //
     static install_dirs
-    resolve (target& t, dir_path d, const string* var = nullptr)
+    resolve (const target& t, dir_path d, const string* var = nullptr)
     {
       install_dirs rs;
 
@@ -498,7 +505,7 @@ namespace build2
     static void
     install_f (const install_dir& base,
                const path& name,
-               file& t,
+               const file& t,
                bool verbose)
     {
       path relf (relative (t.path ()));
@@ -605,12 +612,12 @@ namespace build2
     }
 
     target_state file_rule::
-    perform_install (action a, target& xt) const
+    perform_install (action a, const target& xt) const
     {
-      file& t (static_cast<file&> (xt));
+      const file& t (static_cast<const file&> (xt));
       assert (!t.path ().empty ()); // Should have been assigned by update.
 
-      auto install_target = [this] (file& t, const path& p, bool verbose)
+      auto install_target = [this] (const file& t, const path& p, bool verbose)
       {
         bool n (!p.to_directory ());
         dir_path d (n ? p.directory () : path_cast<dir_path> (p));
@@ -653,10 +660,10 @@ namespace build2
 
       // Then installable ad hoc group members, if any.
       //
-      for (target* m (t.member); m != nullptr; m = m->member)
+      for (const target* m (t.member); m != nullptr; m = m->member)
       {
         if (const path* p = lookup_install<path> (*m, "install"))
-          install_target (static_cast<file&> (*m), *p, false);
+          install_target (static_cast<const file&> (*m), *p, false);
       }
 
       // Finally install the target itself (since we got here we know the
@@ -777,7 +784,7 @@ namespace build2
 
     bool file_rule::
     uninstall_f (const install_dir& base,
-                 file* t,
+                 const file* t,
                  const path& name,
                  bool verbose)
     {
@@ -863,13 +870,13 @@ namespace build2
     }
 
     target_state file_rule::
-    perform_uninstall (action a, target& xt) const
+    perform_uninstall (action a, const target& xt) const
     {
-      file& t (static_cast<file&> (xt));
+      const file& t (static_cast<const file&> (xt));
       assert (!t.path ().empty ()); // Should have been assigned by update.
 
-      auto uninstall_target = [this] (file& t, const path& p, bool verbose)
-        -> target_state
+      auto uninstall_target = [this] (
+        const file& t, const path& p, bool verbose) -> target_state
       {
         bool n (!p.to_directory ());
         dir_path d (n ? p.directory () : path_cast<dir_path> (p));
@@ -920,10 +927,10 @@ namespace build2
       // we would have to do it in reverse, but that's not easy (it's a
       // single-linked list).
       //
-      for (target* m (t.member); m != nullptr; m = m->member)
+      for (const target* m (t.member); m != nullptr; m = m->member)
       {
         if (const path* p = lookup_install<path> (*m, "install"))
-          r |= uninstall_target (static_cast<file&> (*m),
+          r |= uninstall_target (static_cast<const file&> (*m),
                                  *p,
                                  r != target_state::changed);
       }
diff --git a/build2/operation.cxx b/build2/operation.cxx
index 19f1ff7..01f469e 100644
--- a/build2/operation.cxx
+++ b/build2/operation.cxx
@@ -133,11 +133,11 @@ namespace build2
     // Execute collecting postponed targets (to be re-examined later).
     // Do it in reverse order if the execution mode is 'last'.
     //
-    vector<reference_wrapper<target>> psp;
+    vector<reference_wrapper<const target>> psp;
 
-    auto body = [a, quiet, &psp, &trace] (void* v)
+    auto body = [a, quiet, &psp, &trace] (const void* v)
     {
-      target& t (*static_cast<target*> (v));
+      const target& t (*static_cast<const target*> (v));
 
       l5 ([&]{trace << diag_doing (a, t);});
 
@@ -162,9 +162,9 @@ namespace build2
     };
 
     if (current_mode == execution_mode::first)
-      for (void* v: ts) body (v);
+      for (const void* v: ts) body (v);
     else
-      for (void* v: reverse_iterate (ts)) body (v);
+      for (const void* v: reverse_iterate (ts)) body (v);
 
     // We should have executed every target that we matched.
     //
@@ -175,7 +175,7 @@ namespace build2
     //
     // Note: must be serial.
     //
-    for (target& t: psp)
+    for (const target& t: psp)
     {
       switch (execute (a, t))
       {
diff --git a/build2/prerequisite b/build2/prerequisite
index a6d3632..9e1dbfa 100644
--- a/build2/prerequisite
+++ b/build2/prerequisite
@@ -71,10 +71,9 @@ namespace build2
     const optional<string> ext; // Absent if unspecified.
     const scope_type& scope;
 
-    target_type* target;        // NULL if not yet resolved. Note that this
-                                // should always be the "primary target", not
-                                // a member of a target group.
-
+    const_ptr<target_type> target; // NULL if not yet resolved. Note that this
+                                   // should always be the "primary target",
+                                   // not a member of a target group.
   public:
     prerequisite (optional<string> p,
                   const target_type_type& t,
diff --git a/build2/rule b/build2/rule
index cc98bfa..fad4316 100644
--- a/build2/rule
+++ b/build2/rule
@@ -85,10 +85,10 @@ namespace build2
     apply (slock&, action, target&) const override;
 
     static target_state
-    perform_update (action, target&);
+    perform_update (action, const target&);
 
     static target_state
-    perform_clean (action, target&);
+    perform_clean (action, const target&);
 
     static const fsdir_rule instance;
   };
diff --git a/build2/rule.cxx b/build2/rule.cxx
index 88c7941..922a4f7 100644
--- a/build2/rule.cxx
+++ b/build2/rule.cxx
@@ -163,7 +163,7 @@ namespace build2
   }
 
   target_state fsdir_rule::
-  perform_update (action a, target& t)
+  perform_update (action a, const target& t)
   {
     target_state ts (target_state::unchanged);
 
@@ -203,7 +203,7 @@ namespace build2
   }
 
   target_state fsdir_rule::
-  perform_clean (action a, target& t)
+  perform_clean (action a, const target& t)
   {
     // The reverse order of update: first delete this directory,
     // then clean prerequisites (e.g., delete parent directories).
diff --git a/build2/target b/build2/target
index fb8fd08..7653da6 100644
--- a/build2/target
+++ b/build2/target
@@ -75,7 +75,7 @@ namespace build2
   // 14u2). With the size ranging (in bytes for 64-bit target) from 32 (GCC)
   // to 64 (VC).
   //
-  using recipe_function = target_state (action, target&);
+  using recipe_function = target_state (action, const target&);
   using recipe = function<recipe_function>;
 
   // Commonly-used recipes. The default recipe executes the action on
@@ -92,10 +92,10 @@ namespace build2
   extern const recipe group_recipe;
 
   target_state
-  noop_action (action, target&); // Defined in <build2/algorithm>.
+  noop_action (action, const target&); // Defined in <build2/algorithm>.
 
   target_state
-  group_action (action, target&); // Defined in <build2/algorithm>.
+  group_action (action, const target&); // Defined in <build2/algorithm>.
 
   // A view of target group members.
   //
@@ -179,7 +179,8 @@ namespace build2
     // special target_state::group state. You would normally also use the
     // group_recipe for group members.
     //
-    target* group = nullptr;
+    const_ptr<target> group = nullptr;
+
 
     // What has been described above is a "normal" group. That is, there is
     // a dedicated target type that explicitly serves as a group and there
@@ -221,7 +222,7 @@ namespace build2
     // - Member variable lookup skips the ad hoc group (since the group is
     //   the first member, this is normally what we want).
     //
-    target* member = nullptr;
+    const_ptr<target> member = nullptr;
 
     bool
     adhoc_group () const
@@ -307,8 +308,10 @@ namespace build2
     // track of the action here since the targets will be updated
     // if the recipe is updated, normally as part of rule::apply().
     //
-    typedef vector<target*> prerequisite_targets_type;
-    prerequisite_targets_type prerequisite_targets;
+    // Note that the recipe may modify (mutable) this list.
+    //
+    using prerequisite_targets_type = vector<const target*>;
+    mutable prerequisite_targets_type prerequisite_targets;
 
     // Check if there are any prerequisites, taking into account
     // group prerequisites.
@@ -477,13 +480,10 @@ namespace build2
     //
     // Currenly the data is not destroyed until the next match.
     //
-    std::aligned_storage<sizeof (void*) * 4>::type data_pad;
+    static constexpr size_t               data_size = sizeof (string) * 4;
+    std::aligned_storage<data_size>::type data_pad;
     void (*data_dtor) (void*) = nullptr;
 
-    // VC 14 needs decltype.
-    //
-    static const size_t data_size = sizeof (decltype (data_pad));
-
     template <typename R,
               typename T = typename std::remove_cv<
                 typename std::remove_reference<R>::type>::type>
@@ -611,32 +611,30 @@ namespace build2
   // also be traversed in reverse, but that's what you usually want,
   // anyway.
   //
-  class group_prerequisites
+  // For constant iteration use const_group_prerequisites().
+  //
+  template <typename T, typename P, typename I>
+  class group_prerequisites_impl
   {
   public:
-    typedef target::prerequisites_type prerequisites_type;
-
     explicit
-    group_prerequisites (target& t)
+    group_prerequisites_impl (T& t)
         : t_ (t),
-          g_ (t_.group == nullptr ||
-              t_.group->member != nullptr || // Ad hoc group member.
+          g_ (t_.group == nullptr              ||
+              t_.group->member != nullptr      || // Ad hoc group member.
               t_.group->prerequisites.empty ()
               ? nullptr : t_.group) {}
 
     struct iterator
     {
-      typedef prerequisites_type::iterator base_iterator;
-
-      typedef base_iterator::value_type value_type;
-      typedef base_iterator::pointer pointer;
-      typedef base_iterator::reference reference;
-      typedef base_iterator::difference_type difference_type;
-      typedef std::bidirectional_iterator_tag iterator_category;
+      using value_type        = typename I::value_type;
+      using pointer           = typename I::pointer;
+      using reference         = typename I::reference;
+      using difference_type   = typename I::difference_type;
+      using iterator_category = std::bidirectional_iterator_tag;
 
       iterator () {}
-      iterator (target* t, target* g, prerequisites_type* c, base_iterator i)
-          : t_ (t), g_ (g), c_ (c), i_ (i) {}
+      iterator (T* t, T* g, P* c, I i): t_ (t), g_ (g), c_ (c), i_ (i) {}
 
       iterator&
       operator++ ()
@@ -681,25 +679,25 @@ namespace build2
       operator!= (const iterator& x, const iterator& y) {return !(x == y);}
 
     private:
-      target* t_ = nullptr;
-      target* g_ = nullptr;
-      prerequisites_type* c_ = nullptr;
-      base_iterator i_;
+      T* t_ = nullptr;
+      T* g_ = nullptr;
+      P* c_ = nullptr;
+      I  i_;
     };
 
-    typedef std::reverse_iterator<iterator> reverse_iterator;
+    using reverse_iterator = std::reverse_iterator<iterator>;
 
     iterator
     begin () const
     {
-      auto& c ((g_ != nullptr ? *g_ : t_).prerequisites);
+      P& c ((g_ != nullptr ? *g_ : t_).prerequisites);
       return iterator (&t_, g_, &c, c.begin ());
     }
 
     iterator
     end () const
     {
-      auto& c (t_.prerequisites);
+      P& c (t_.prerequisites);
       return iterator (&t_, g_, &c, c.end ());
     }
 
@@ -717,10 +715,20 @@ namespace build2
     }
 
   private:
-    target& t_;
-    target* g_;
+    T& t_;
+    T* g_;
   };
 
+  using group_prerequisites = group_prerequisites_impl<
+    target,
+    target::prerequisites_type,
+    target::prerequisites_type::iterator>;
+
+  using const_group_prerequisites = group_prerequisites_impl<
+    const target,
+    const target::prerequisites_type,
+    target::prerequisites_type::const_iterator>;
+
   // A member of a prerequisite. If 'target' is NULL, then this is the
   // prerequisite itself. Otherwise, it is its member. In this case
   // 'prerequisite' still refers to the prerequisite.
@@ -1128,21 +1136,21 @@ namespace build2
     timestamp
     mtime (bool load = true) const
     {
-      const mtime_target* t (state_ == target_state::group
-                             ? static_cast<const mtime_target*> (group)
-                             : this);
+      const mtime_target& t (state_ == target_state::group
+                             ? static_cast<const mtime_target&> (*group)
+                             : *this);
 
-      if (load && t->mtime_ == timestamp_unknown)
-        t->mtime_ = t->load_mtime ();
+      if (load && t.mtime_ == timestamp_unknown)
+        t.mtime_ = t.load_mtime ();
 
-      return t->mtime_;
+      return t.mtime_;
     }
 
     // Note that while we can cache the mtime at any time, it may be ignored
     // if the target state is group (see the mtime() accessor).
     //
     void
-    mtime (timestamp mt)
+    mtime (timestamp mt) const
     {
       mtime_ = mt;
     }
@@ -1150,7 +1158,7 @@ namespace build2
     // Return true if this target is newer than the specified timestamp.
     //
     bool
-    newer (timestamp mt)
+    newer (timestamp mt) const
     {
       timestamp mp (mtime ());
 
diff --git a/build2/target.cxx b/build2/target.cxx
index f7387c7..c032028 100644
--- a/build2/target.cxx
+++ b/build2/target.cxx
@@ -173,7 +173,7 @@ namespace build2
     if (auto p = vars.find (var))
       r.first = lookup (p, &vars);
 
-    target* g (nullptr);
+    const target* g (nullptr);
 
     if (!r.first)
     {
diff --git a/build2/test/common b/build2/test/common
index d3678c8..44c7bf8 100644
--- a/build2/test/common
+++ b/build2/test/common
@@ -25,18 +25,18 @@ namespace build2
       // prerequisites.
       //
       bool
-      pass (target& alias_target) const;
+      pass (const target& alias_target) const;
 
       // Return true if the specified target should be tested.
       //
       bool
-      test (target& test_target) const;
+      test (const target& test_target) const;
 
       // Return true if the specified target should be tested with the
       // specified testscript test (or group).
       //
       bool
-      test (target& test_target, const path& id_path) const;
+      test (const target& test_target, const path& id_path) const;
     };
   }
 }
diff --git a/build2/test/common.cxx b/build2/test/common.cxx
index 1b4c194..be0e690 100644
--- a/build2/test/common.cxx
+++ b/build2/test/common.cxx
@@ -55,7 +55,7 @@ namespace build2
     }
 
     bool common::
-    pass (target& a) const
+    pass (const target& a) const
     {
       if (test_ == nullptr)
         return true;
@@ -95,7 +95,7 @@ namespace build2
     }
 
     bool common::
-    test (target& t) const
+    test (const target& t) const
     {
       if (test_ == nullptr)
         return true;
@@ -159,7 +159,7 @@ namespace build2
     }
 
     bool common::
-    test (target& t, const path& id) const
+    test (const target& t, const path& id) const
     {
       if (test_ == nullptr)
         return true;
diff --git a/build2/test/rule b/build2/test/rule
index 0e208ff..da55173 100644
--- a/build2/test/rule
+++ b/build2/test/rule
@@ -24,7 +24,7 @@ namespace build2
       match (slock&, action, target&, const string&) const override;
 
       target_state
-      perform_script (action, target&) const;
+      perform_script (action, const target&) const;
     };
 
     class rule: public rule_common
@@ -34,7 +34,7 @@ namespace build2
       apply (slock&, action, target&) const override;
 
       static target_state
-      perform_test (action, target&);
+      perform_test (action, const target&);
     };
 
     class alias_rule: public rule_common
@@ -44,7 +44,7 @@ namespace build2
       apply (slock&, action, target&) const override;
 
       target_state
-      perform_test (action, target&) const;
+      perform_test (action, const target&) const;
     };
   }
 }
diff --git a/build2/test/rule.cxx b/build2/test/rule.cxx
index 05d5eac..38c24f9 100644
--- a/build2/test/rule.cxx
+++ b/build2/test/rule.cxx
@@ -186,7 +186,7 @@ namespace build2
       // If not a test then also redirect to the alias rule.
       //
       return md.test
-        ? [this] (action a, target& t) {return perform_test (a, t);}
+        ? [this] (action a, const target& t) {return perform_test (a, t);}
         : default_recipe;
     }
 
@@ -218,7 +218,10 @@ namespace build2
             t.prerequisite_targets.push_back (&p.search ());
         }
 
-        return [this] (action a, target& t) {return perform_script (a, t);};
+        return [this] (action a, const target& t)
+        {
+          return perform_script (a, t);
+        };
       }
       else
       {
@@ -327,7 +330,8 @@ namespace build2
           // update of input/output targets and also delegate to the real
           // update.
           //
-          return [it, ot, dr = move (d)] (action a, target& t) -> target_state
+          return [it, ot, dr = move (d)] (
+            action a, const target& t) -> target_state
           {
             // Do the general update first.
             //
@@ -363,7 +367,7 @@ namespace build2
     }
 
     target_state rule_common::
-    perform_script (action, target& t) const
+    perform_script (action, const target& t) const
     {
       // Figure out whether the testscript file is called 'testscript', in
       // which case it should be the only one.
@@ -371,11 +375,11 @@ namespace build2
       bool one;
       {
         optional<bool> o;
-        for (target* pt: t.prerequisite_targets)
+        for (const target* pt: t.prerequisite_targets)
         {
           // In case we are using the alias rule's list (see above).
           //
-          if (testscript* ts = pt->is_a<testscript> ())
+          if (const testscript* ts = pt->is_a<testscript> ())
           {
             bool r (ts->name == "testscript");
 
@@ -435,9 +439,9 @@ namespace build2
 
       // Run all the testscripts.
       //
-      for (target* pt: t.prerequisite_targets)
+      for (const target* pt: t.prerequisite_targets)
       {
-        if (testscript* ts = pt->is_a<testscript> ())
+        if (const testscript* ts = pt->is_a<testscript> ())
         {
           // If this is just the testscript, then its id path is empty (and
           // it can only be ignored by ignoring the test target, which makes
@@ -489,7 +493,7 @@ namespace build2
     // nameN arg arg ... nullptr nullptr
     //
     static bool
-    run_test (target& t,
+    run_test (const target& t,
               diag_record& dr,
               char const** args,
               process* prev = nullptr)
@@ -547,7 +551,7 @@ namespace build2
     }
 
     target_state rule::
-    perform_test (action, target& tt)
+    perform_test (action, const target& tt)
     {
       // @@ Would be nice to print what signal/core was dumped.
       //
@@ -562,7 +566,7 @@ namespace build2
 
         // Note that we have similar code for scripted tests.
         //
-        target* t (nullptr);
+        const target* t (nullptr);
 
         if (l.defined ())
         {
@@ -636,7 +640,7 @@ namespace build2
       auto& pts (tt.prerequisite_targets);
       if (pts.size () != 0 && pts[0] != nullptr)
       {
-        file& it (static_cast<file&> (*pts[0]));
+        const file& it (static_cast<const file&> (*pts[0]));
         assert (!it.path ().empty ()); // Should have been assigned by update.
         args.push_back (it.path ().string ().c_str ());
       }
@@ -656,7 +660,7 @@ namespace build2
       process_path dpp;
       if (pts.size () != 0 && pts[1] != nullptr)
       {
-        file& ot (static_cast<file&> (*pts[1]));
+        const file& ot (static_cast<const file&> (*pts[1]));
         assert (!ot.path ().empty ()); // Should have been assigned by update.
 
         dpp = run_search (dp, true);
@@ -693,7 +697,7 @@ namespace build2
     }
 
     target_state alias_rule::
-    perform_test (action a, target& t) const
+    perform_test (action a, const target& t) const
     {
       // Run the alias recipe first then the test.
       //
diff --git a/build2/test/script/script b/build2/test/script/script
index e528cdd..2438fa5 100644
--- a/build2/test/script/script
+++ b/build2/test/script/script
@@ -511,8 +511,8 @@ namespace build2
       class script: public script_base, public group
       {
       public:
-        script (target& test_target,
-                testscript& script_target,
+        script (const target& test_target,
+                const testscript& script_target,
                 const dir_path& root_wd);
 
         script (script&&) = delete;
@@ -521,8 +521,8 @@ namespace build2
         script& operator= (const script&) = delete;
 
       public:
-        target& test_target;       // Target we are testing.
-        testscript& script_target; // Target of the testscript file.
+        const target& test_target;       // Target we are testing.
+        const testscript& script_target; // Target of the testscript file.
 
         // Pre-parse data.
         //
diff --git a/build2/test/script/script.cxx b/build2/test/script/script.cxx
index bd13845..bc816d7 100644
--- a/build2/test/script/script.cxx
+++ b/build2/test/script/script.cxx
@@ -500,7 +500,7 @@ namespace build2
       // script
       //
       script::
-      script (target& tt, testscript& st, const dir_path& rwd)
+      script (const target& tt, const testscript& st, const dir_path& rwd)
           : group (st.name == "testscript" ? string () : st.name),
             test_target (tt),
             script_target (st)
@@ -522,7 +522,7 @@ namespace build2
 
           // Note that we have similar code for simple tests.
           //
-          target* t (nullptr);
+          const target* t (nullptr);
 
           if (l.defined ())
           {
diff --git a/build2/types b/build2/types
index 7afc24e..0e6012b 100644
--- a/build2/types
+++ b/build2/types
@@ -37,6 +37,7 @@
 #include <butl/process>
 #include <butl/fdstream>
 #include <butl/optional>
+#include <butl/const-ptr>
 #include <butl/timestamp>
 #include <butl/vector-view>
 #include <butl/small-vector>
@@ -116,6 +117,10 @@ namespace build2
   using butl::optional;
   using butl::nullopt;
 
+  // <butl/const-ptr>
+  //
+  using butl::const_ptr;
+
   // <butl/path>
   //
   using butl::path;