Get rid of extension_pool

33 files changed, 187 insertions, 198 deletions

diff --git a/build2/algorithm b/build2/algorithm
index 9a91781..ad52fe1 100644
--- a/build2/algorithm
+++ b/build2/algorithm
@@ -44,7 +44,7 @@ namespace build2
           const dir_path& dir,
           const dir_path& out,
           const string& name,
-          const string* ext,
+          const optional<string>& ext,
           scope*,
           const optional<string>& proj = nullopt);
 
@@ -55,7 +55,7 @@ namespace build2
   search (const dir_path& dir,
           const dir_path& out,
           const string& name,
-          const string* ext,
+          const optional<string>& ext,
           scope*);
 
   // Search for a target identified by the name. The semantics is "as if" we
diff --git a/build2/algorithm.cxx b/build2/algorithm.cxx
index 78af195..22633bb 100644
--- a/build2/algorithm.cxx
+++ b/build2/algorithm.cxx
@@ -37,8 +37,8 @@ namespace build2
   target&
   search (name n, scope& s)
   {
-    const string* e;
-    const target_type* tt (s.find_target_type (n, e));
+    optional<string> ext;
+    const target_type* tt (s.find_target_type (n, ext));
 
     if (tt == nullptr)
       fail << "unknown target type " << n.type << " in name " << n;
@@ -49,7 +49,7 @@ namespace build2
     // @@ OUT: for now we assume the prerequisite's out is undetermined.
     //         Would need to pass a pair of names.
     //
-    return search (*tt, n.dir, dir_path (), n.value, e, &s, n.proj);
+    return search (*tt, n.dir, dir_path (), n.value, ext, &s, n.proj);
   }
 
   pair<const rule*, match_result>
@@ -333,7 +333,7 @@ namespace build2
 
     // Target is in the out tree, so out directory is empty.
     //
-    fsdir* r (&search<fsdir> (d, dir_path (), string (), nullptr, nullptr));
+    fsdir* r (&search<fsdir> (d, dir_path (), string (), nullopt, nullptr));
     match (a, *r);
     t.prerequisite_targets.emplace_back (r);
     return r;
diff --git a/build2/algorithm.ixx b/build2/algorithm.ixx
index 84b0ee7..90a0789 100644
--- a/build2/algorithm.ixx
+++ b/build2/algorithm.ixx
@@ -30,7 +30,7 @@ namespace build2
           const dir_path& dir,
           const dir_path& out,
           const string& name,
-          const string* ext,
+          const optional<string>& ext,
           scope* scope,
           const optional<string>& proj)
   {
@@ -43,7 +43,7 @@ namespace build2
   search (const dir_path& dir,
           const dir_path& out,
           const string& name,
-          const string* ext,
+          const optional<string>& ext,
           scope* scope)
   {
     return static_cast<T&> (
diff --git a/build2/b.cxx b/build2/b.cxx
index bd77a02..ca8c15b 100644
--- a/build2/b.cxx
+++ b/build2/b.cxx
@@ -967,7 +967,9 @@ main (int argc, char* argv[])
           //
           scope& bs (scopes.find (ts.out_base));
 
-          const string* e;
+          // Find the target type and extract the extension.
+          //
+          optional<string> e;
           const target_type* ti (bs.find_target_type (tn, e));
 
           if (ti == nullptr)
diff --git a/build2/bin/rule.cxx b/build2/bin/rule.cxx
index 70ad7df..ede2c85 100644
--- a/build2/bin/rule.cxx
+++ b/build2/bin/rule.cxx
@@ -75,7 +75,7 @@ namespace build2
       if (a)
       {
         if (t.a == nullptr)
-          t.a = &search<liba> (t.dir, t.out, t.name, nullptr, nullptr);
+          t.a = &search<liba> (t.dir, t.out, t.name, nullopt, nullptr);
 
         match_only (act, *t.a);
       }
@@ -83,7 +83,7 @@ namespace build2
       if (s)
       {
         if (t.s == nullptr)
-          t.s = &search<libs> (t.dir, t.out, t.name, nullptr, nullptr);
+          t.s = &search<libs> (t.dir, t.out, t.name, nullopt, nullptr);
 
         match_only (act, *t.s);
       }
diff --git a/build2/bin/target.cxx b/build2/bin/target.cxx
index 9086369..e9ef81e 100644
--- a/build2/bin/target.cxx
+++ b/build2/bin/target.cxx
@@ -17,10 +17,10 @@ namespace build2
                   dir_path dir,
                   dir_path out,
                   string n,
-                  const string* ext)
+                  optional<string> ext)
     {
       obj* o (targets.find<obj> (dir, out, n));
-      obje* e (new obje (move (dir), move (out), move (n), ext));
+      obje* e (new obje (move (dir), move (out), move (n), move (ext)));
 
       if ((e->group = o))
         o->e = e;
@@ -44,10 +44,10 @@ namespace build2
                   dir_path dir,
                   dir_path out,
                   string n,
-                  const string* ext)
+                  optional<string> ext)
     {
       obj* o (targets.find<obj> (dir, out, n));
-      obja* a (new obja (move (dir), move (out), move (n), ext));
+      obja* a (new obja (move (dir), move (out), move (n), move (ext)));
 
       if ((a->group = o))
         o->a = a;
@@ -71,10 +71,10 @@ namespace build2
                    dir_path dir,
                    dir_path out,
                    string n,
-                   const string* ext)
+                   optional<string> ext)
     {
       obj* o (targets.find<obj> (dir, out, n));
-      objs* s (new objs (move (dir), move (out), move (n), ext));
+      objs* s (new objs (move (dir), move (out), move (n), move (ext)));
 
       if ((s->group = o))
         o->s = s;
@@ -98,13 +98,13 @@ namespace build2
                  dir_path dir,
                  dir_path out,
                  string n,
-                 const string* ext)
+                 optional<string> ext)
     {
       obje* e (targets.find<obje> (dir, out, n));
       obja* a (targets.find<obja> (dir, out, n));
       objs* s (targets.find<objs> (dir, out, n));
 
-      obj* o (new obj (move (dir), move (out), move (n), ext));
+      obj* o (new obj (move (dir), move (out), move (n), move (ext)));
 
       if ((o->e = e))
         e->group = o;
@@ -134,12 +134,12 @@ namespace build2
                   dir_path d,
                   dir_path o,
                   string n,
-                  const string* ext)
+                  optional<string> ext)
     {
       // Only link-up to the group if the types match exactly.
       //
       lib* l (t == liba::static_type ? targets.find<lib> (d, o, n) : nullptr);
-      liba* a (new liba (move (d), move (o), move (n), ext));
+      liba* a (new liba (move (d), move (o), move (n), move (ext)));
 
       if ((a->group = l))
         l->a = a;
@@ -175,12 +175,12 @@ namespace build2
                   dir_path d,
                   dir_path o,
                   string n,
-                  const string* ext)
+                  optional<string> ext)
     {
       // Only link-up to the group if the types match exactly.
       //
       lib* l (t == libs::static_type ? targets.find<lib> (d, o, n) : nullptr);
-      libs* s (new libs (move (d), move (o), move (n), ext));
+      libs* s (new libs (move (d), move (o), move (n), move (ext)));
 
       if ((s->group = l))
         l->s = s;
@@ -215,12 +215,12 @@ namespace build2
                  dir_path d,
                  dir_path o,
                  string n,
-                 const string* ext)
+                 optional<string> ext)
     {
       liba* a (targets.find<liba> (d, o, n));
       libs* s (targets.find<libs> (d, o, n));
 
-      lib* l (new lib (move (d), move (o), move (n), ext));
+      lib* l (new lib (move (d), move (o), move (n), move (ext)));
 
       if ((l->a = a))
         a->group = l;
diff --git a/build2/cc/common.cxx b/build2/cc/common.cxx
index 86d998e..6429a38 100644
--- a/build2/cc/common.cxx
+++ b/build2/cc/common.cxx
@@ -408,7 +408,7 @@ namespace build2
       {
         // This is import.
         //
-        const string* ext;
+        optional<string> ext;
         const target_type* tt (s.find_target_type (n, ext)); // Changes name.
 
         if (tt == nullptr)
@@ -455,7 +455,7 @@ namespace build2
       //    Windows.
       //
       bool l (p.is_a<lib> ());
-      const string* ext (l ? nullptr : p.tk.ext); // Only for liba/libs.
+      const optional<string>& ext (l ? nullopt : p.tk.ext); // Only liba/libs.
 
       // Then figure out what we need to search for.
       //
@@ -464,7 +464,7 @@ namespace build2
       // liba
       //
       path an;
-      const string* ae (nullptr);
+      optional<string> ae;
 
       if (l || p.is_a<liba> ())
       {
@@ -494,10 +494,7 @@ namespace build2
           e = "a";
         }
 
-        ae = ext == nullptr
-          ? &extension_pool.find (e)
-          : ext;
-
+        ae = ext ? ext : string (e);
         if (!ae->empty ())
         {
           an += '.';
@@ -508,7 +505,7 @@ namespace build2
       // libs
       //
       path sn;
-      const string* se (nullptr);
+      optional<string> se;
 
       if (l || p.is_a<libs> ())
       {
@@ -528,10 +525,7 @@ namespace build2
           else                        e = "so";
         }
 
-        se = ext == nullptr
-          ? &extension_pool.find (e)
-          : ext;
-
+        se = ext ? ext : string (e);
         if (!se->empty ())
         {
           sn += '.';
@@ -574,7 +568,7 @@ namespace build2
             //
             if (tclass == "windows")
             {
-              s = &targets.insert<libs> (d, dir_path (), name, nullptr, trace);
+              s = &targets.insert<libs> (d, dir_path (), name, nullopt, trace);
 
               if (s->member == nullptr)
               {
@@ -606,7 +600,7 @@ namespace build2
               s->mtime (mt);
             }
           }
-          else if (ext == nullptr && tsys == "mingw32")
+          else if (!ext && tsys == "mingw32")
           {
             // Above we searched for the import library (.dll.a) but if it's
             // not found, then we also search for the .dll (unless the
@@ -614,7 +608,7 @@ namespace build2
             // directly. Note also that the resulting libs{} would end up
             // being the .dll.
             //
-            se = &extension_pool.find ("dll");
+            se = string ("dll");
             f = f.base (); // Remove .a from .dll.a.
             mt = file_mtime (f);
 
@@ -762,7 +756,7 @@ namespace build2
       //
       lib& lt (
         targets.insert<lib> (
-          *pd, dir_path (), name, l ? p.tk.ext : nullptr, trace));
+          *pd, dir_path (), name, l ? p.tk.ext : nullopt, trace));
 
       // It should automatically link-up to the members we have found.
       //
diff --git a/build2/cc/compile.cxx b/build2/cc/compile.cxx
index 23499ad..f837765 100644
--- a/build2/cc/compile.cxx
+++ b/build2/cc/compile.cxx
@@ -415,12 +415,11 @@ namespace build2
         // only use the target type and name from the target key so we can
         // pass bogus values for the rest.
         //
-        const string* dummy (nullptr);
-        target_key tk {&tt, nullptr, nullptr, &n, dummy};
+        target_key tk {&tt, nullptr, nullptr, &n, target_key::nullext};
 
         // This is like prerequisite search.
         //
-        if (const string* de = tt.extension (tk, s, true))
+        if (optional<string> de = tt.extension (tk, s, true))
           if (*de == e)
             return true;
 
@@ -913,7 +912,7 @@ namespace build2
           //
           dir_path d (f.directory ());
           string n (f.leaf ().base ().string ());
-          const string* e (&extension_pool.find (f.extension ()));
+          string e (f.extension ());
 
           // Determine the target type.
           //
@@ -935,7 +934,7 @@ namespace build2
           scope& bs (scopes.find (d));
           if (scope* rs = bs.root_scope ())
           {
-            tt = map_extension (bs, n, *e);
+            tt = map_extension (bs, n, e);
 
             if (bs.out_path () != bs.src_path () && d.sub (bs.src_path ()))
               out = out_src (d, *rs);
diff --git a/build2/cc/link.cxx b/build2/cc/link.cxx
index 8010598..e1dac16 100644
--- a/build2/cc/link.cxx
+++ b/build2/cc/link.cxx
@@ -337,7 +337,7 @@ namespace build2
         if (t.member != nullptr) // Might already be there.
           assert (t.member->type () == tt);
         else
-          t.member = &search (tt, t.dir, t.out, t.name, nullptr, nullptr);
+          t.member = &search (tt, t.dir, t.out, t.name, nullopt, nullptr);
 
         file& r (static_cast<file&> (*t.member));
         r.recipe (a, group_recipe);
@@ -519,7 +519,7 @@ namespace build2
         // obj*{} is always in the out tree.
         //
         target& ot (
-          search (tt, d, dir_path (), *cp.tk.name, nullptr, cp.scope));
+          search (tt, d, dir_path (), *cp.tk.name, nullopt, cp.scope));
 
         // If we are cleaning, check that this target is in the same or
         // a subdirectory of our project root.
diff --git a/build2/cc/msvc.cxx b/build2/cc/msvc.cxx
index 5ce2f2a..4ac82a0 100644
--- a/build2/cc/msvc.cxx
+++ b/build2/cc/msvc.cxx
@@ -231,7 +231,7 @@ namespace build2
       //
       tracer trace (mod, "msvc_search_library");
 
-      const string* ext (p.tk.ext);
+      const optional<string>& ext (p.tk.ext);
       const string& name (*p.tk.name);
 
       // Assemble the file path.
@@ -249,10 +249,9 @@ namespace build2
       if (*sfx != '\0')
         f += sfx;
 
-      const string& e (
-        ext == nullptr || p.is_a<lib> () // Only for liba/libs.
-        ? extension_pool.find ("lib")
-        : *ext);
+      const string& e (!ext || p.is_a<lib> () // Only for liba/libs.
+                       ? string ("lib")
+                       : *ext);
 
       if (!e.empty ())
       {
@@ -268,7 +267,7 @@ namespace build2
       {
         // Enter the target.
         //
-        T& t (targets.insert<T> (d, dir_path (), name, &e, trace));
+        T& t (targets.insert<T> (d, dir_path (), name, e, trace));
 
         if (t.path ().empty ())
           t.path (move (f));
@@ -323,7 +322,7 @@ namespace build2
               msvc_search_library<libi> (x, ld, d, p, otype::s, pf, sf))
         {
           r = &targets.insert<libs> (
-            d, dir_path (), *p.tk.name, nullptr, trace);
+            d, dir_path (), *p.tk.name, nullopt, trace);
 
           if (r->member == nullptr)
           {
diff --git a/build2/cc/pkgconfig.cxx b/build2/cc/pkgconfig.cxx
index f8d35af..6c8159f 100644
--- a/build2/cc/pkgconfig.cxx
+++ b/build2/cc/pkgconfig.cxx
@@ -417,10 +417,11 @@ namespace build2
           //
           dir_path out;
           string name (l, 2); // Sans -l.
-          const string* ext (nullptr);
 
           prerequisite_key pk {
-            nullopt, {&lib::static_type, &out, &out, &name, ext}, &s};
+            nullopt,
+            {&lib::static_type, &out, &out, &name, target_key::nullext},
+            &s};
 
           if (lib* lt = static_cast<lib*> (search_library (sysd, usrd, pk)))
           {
diff --git a/build2/cli/rule.cxx b/build2/cli/rule.cxx
index b97bea2..d457e6d 100644
--- a/build2/cli/rule.cxx
+++ b/build2/cli/rule.cxx
@@ -88,15 +88,15 @@ namespace build2
         //
         if (t.h == nullptr)
         {
-          t.h = &search<cxx::hxx> (t.dir, t.out, t.name, nullptr, nullptr);
+          t.h = &search<cxx::hxx> (t.dir, t.out, t.name, nullopt, nullptr);
           t.h->group = &t;
 
-          t.c = &search<cxx::cxx> (t.dir, t.out, t.name, nullptr, nullptr);
+          t.c = &search<cxx::cxx> (t.dir, t.out, t.name, nullopt, nullptr);
           t.c->group = &t;
 
           if (!find_option ("--suppress-inline", t, "cli.options"))
           {
-            t.i = &search<cxx::ixx> (t.dir, t.out, t.name, nullptr, nullptr);
+            t.i = &search<cxx::ixx> (t.dir, t.out, t.name, nullopt, nullptr);
             t.i->group = &t;
           }
         }
@@ -211,7 +211,7 @@ namespace build2
                       const char* option,
                       const char* default_extension)
     {
-      assert (t.ext != nullptr); // Should have been figured out in apply().
+      assert (t.ext); // Should have been figured out in apply().
 
       if (*t.ext != default_extension)
       {
diff --git a/build2/cli/target b/build2/cli/target
index 0160ef3..0cc448d 100644
--- a/build2/cli/target
+++ b/build2/cli/target
@@ -49,8 +49,8 @@ namespace build2
 
       //using mtime_target::mtime_target; // @@ GCC 4.8
 
-      cli_cxx (dir_path d, dir_path o, string n, const string* e)
-          : mtime_target (move (d), move (o), move (n), e)
+      cli_cxx (dir_path d, dir_path o, string n, optional<string> e)
+          : mtime_target (move (d), move (o), move (n), move (e))
       {
         m[0] = m[1] = m[2] = nullptr;
       }
diff --git a/build2/cli/target.cxx b/build2/cli/target.cxx
index 5e12a04..3f09d30 100644
--- a/build2/cli/target.cxx
+++ b/build2/cli/target.cxx
@@ -53,7 +53,7 @@ namespace build2
                      dir_path d,
                      dir_path o,
                      string n,
-                     const string* e)
+                     optional<string> e)
     {
       tracer trace ("cli::cli_cxx_factory");
 
@@ -66,7 +66,7 @@ namespace build2
       targets.insert<cxx::cxx> (d, o, n, trace);
       targets.insert<cxx::ixx> (d, o, n, trace);
 
-      return new cli_cxx (move (d), move (o), move (n), e);
+      return new cli_cxx (move (d), move (o), move (n), move (e));
     }
 
     const target_type cli_cxx::static_type
diff --git a/build2/config/operation.cxx b/build2/config/operation.cxx
index 3370c2c..68b61c3 100644
--- a/build2/config/operation.cxx
+++ b/build2/config/operation.cxx
@@ -570,7 +570,7 @@ namespace build2
                             root.out_path (),
                             dir_path (), // Out tree.
                             "",
-                            nullptr,
+                            nullopt,
                             trace).first);
 
           if (!quiet)
diff --git a/build2/context b/build2/context
index cb83105..1dd9daf 100644
--- a/build2/context
+++ b/build2/context
@@ -16,8 +16,6 @@ namespace build2
 {
   class file;
 
-  extern string_pool extension_pool;
-
   // Current action (meta/operation).
   //
   // The names unlike info are available during boot but may not yet be
diff --git a/build2/context.cxx b/build2/context.cxx
index a21de2a..ba6629c 100644
--- a/build2/context.cxx
+++ b/build2/context.cxx
@@ -23,8 +23,6 @@ using namespace butl;
 
 namespace build2
 {
-  string_pool extension_pool;
-
   const string* current_mname;
   const string* current_oname;
 
@@ -56,8 +54,6 @@ namespace build2
     scopes.clear ();
     var_pool.clear ();
 
-    extension_pool.clear ();
-
     // Reset meta/operation tables. Note that the order should match the id
     // constants in <build2/operation>.
     //
diff --git a/build2/dist/operation.cxx b/build2/dist/operation.cxx
index 47e188c..9535566 100644
--- a/build2/dist/operation.cxx
+++ b/build2/dist/operation.cxx
@@ -160,7 +160,7 @@ namespace build2
             move (d),
             move (out),
             p.leaf ().base ().string (),
-            &extension_pool.find (p.extension ()), // Specified.
+            p.extension (),              // Specified.
             trace);
         }
       };
diff --git a/build2/file.cxx b/build2/file.cxx
index 9e957a9..2889cf7 100644
--- a/build2/file.cxx
+++ b/build2/file.cxx
@@ -1112,8 +1112,11 @@ namespace build2
     {
       path n (*tk.dir);
       n /= *tk.name;
-      if (tk.ext != nullptr)
+      if (tk.ext)
+      {
+        n += '.';
         n += *tk.ext;
+      }
 
       process_path pp (process::try_path_search (n, true));
 
@@ -1126,9 +1129,9 @@ namespace build2
           targets.insert<exe> (
             tt,
             p.directory (),
-            dir_path (), // No out (out of project).
+            dir_path (),                 // No out (out of project).
             p.leaf ().base ().string (),
-            &extension_pool.find (p.extension ()), // Specified.
+            p.extension (),              // Always specified.
             trace));
 
         if (t.path ().empty ())
diff --git a/build2/parser.cxx b/build2/parser.cxx
index 6025082..9ce916e 100644
--- a/build2/parser.cxx
+++ b/build2/parser.cxx
@@ -91,7 +91,7 @@ namespace build2
                   tracer& tr)
         : p_ (&p), t_ (p.target_)
     {
-      const string* e;
+      optional<string> e;
       const target_type* ti (p.scope_->find_target_type (n, e));
 
       if (ti == nullptr)
@@ -131,7 +131,7 @@ namespace build2
       // Find or insert.
       //
       p.target_ = &targets.insert (
-        *ti, move (d), move (out), move (n.value), e, tr).first;
+        *ti, move (d), move (out), move (n.value), move (e), tr).first;
     }
 
     ~enter_target ()
@@ -642,7 +642,7 @@ namespace build2
 
             for (auto& pn: pns)
             {
-              const string* e;
+              optional<string> e;
               const target_type* ti (scope_->find_target_type (pn, e));
 
               if (ti == nullptr)
@@ -672,7 +672,7 @@ namespace build2
                   move (pn.dir),
                   dir_path (),
                   move (pn.value),
-                  e,
+                  move (e),
                   *scope_,
                   trace).first);
 
@@ -3560,7 +3560,7 @@ namespace build2
                       scope_->out_path (),
                       dir_path (),         // Out tree target.
                       "",
-                      nullptr,
+                      nullopt,
                       trace) != targets.end ())
       return;
 
@@ -3573,7 +3573,7 @@ namespace build2
                       scope_->out_path (),
                       dir_path (),
                       "",
-                      nullptr,
+                      nullopt,
                       trace).first);
 
     prerequisite& p (
@@ -3608,7 +3608,7 @@ namespace build2
       move (d),
       move (out),
       p.leaf ().base ().string (),
-      &extension_pool.find (p.extension ()), // Always specified.
+      p.extension (),               // Always specified.
       trace);
   }
 
diff --git a/build2/prerequisite b/build2/prerequisite
index b259557..5dfd1aa 100644
--- a/build2/prerequisite
+++ b/build2/prerequisite
@@ -31,11 +31,11 @@ namespace build2
     target_key tk;                // .dir and .out can be relative.
     mutable scope_type* scope;    // Can be NULL if tk.dir is absolute.
 
+    static const optional<string> nullproj;
+
     template <typename T>
     bool is_a () const {return tk.is_a<T> ();}
     bool is_a (const target_type& tt) const {return tk.is_a (tt);}
-
-    static const optional<string> nullproj;
   };
 
   inline bool
@@ -90,14 +90,14 @@ namespace build2
     //
     const optional<string>  proj;
     const target_type_type& type;
-    const dir_path dir;      // Normalized absolute or relative (to scope).
-    const dir_path out;      // Empty, normalized absolute, or relative.
+    const dir_path dir;         // Normalized absolute or relative (to scope).
+    const dir_path out;         // Empty, normalized absolute, or relative.
     const string name;
-    const string* ext;       // NULL if unspecified.
+    const optional<string> ext; // Absent if unspecified.
     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.
+    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,
@@ -105,14 +105,14 @@ namespace build2
                   dir_path d,
                   dir_path o,
                   string n,
-                  const string* e,
+                  optional<string> e,
                   scope_type& s)
         : proj (move (p)),
           type (t),
           dir (move (d)),
           out (move (o)),
           name (move (n)),
-          ext (e),
+          ext (move (e)),
           scope (s),
           target (nullptr) {}
 
@@ -188,7 +188,7 @@ namespace build2
             dir_path dir,
             dir_path out,
             string name,
-            const string* ext,
+            optional<string> ext,
             scope&,
             tracer&);
 
diff --git a/build2/prerequisite.cxx b/build2/prerequisite.cxx
index 82d6ea2..bcd4123 100644
--- a/build2/prerequisite.cxx
+++ b/build2/prerequisite.cxx
@@ -56,7 +56,7 @@ namespace build2
           dir_path dir,
           dir_path out,
           string name,
-          const string* ext,
+          optional<string> ext,
           scope& s,
           tracer& trace) -> pair<prerequisite&, bool>
   {
@@ -66,8 +66,13 @@ namespace build2
 
     // Find or insert.
     //
-    auto r (
-      emplace (move (proj), tt, move (dir), move (out), move (name), ext, s));
+    auto r (emplace (move (proj),
+                     tt,
+                     move (dir),
+                     move (out),
+                     move (name),
+                     ext, // Note: cannot move.
+                     s));
     prerequisite& p (const_cast<prerequisite&> (*r.first));
 
     // Update extension if the existing prerequisite has it unspecified.
@@ -78,7 +83,7 @@ namespace build2
           diag_record r (trace);
           r << "assuming prerequisite " << p << " is the same as the "
             << "one with ";
-          if (ext == nullptr)
+          if (!ext)
             r << "unspecified extension";
           else if (ext->empty ())
             r << "no extension";
@@ -86,8 +91,8 @@ namespace build2
             r << "extension " << *ext;
         });
 
-      if (ext != nullptr)
-        p.ext = ext;
+      if (ext)
+        const_cast<optional<string>&> (p.ext) = move (ext);
     }
 
     return pair<prerequisite&, bool> (p, r.second);
diff --git a/build2/scope b/build2/scope
index 6b5af22..f22383a 100644
--- a/build2/scope
+++ b/build2/scope
@@ -218,7 +218,7 @@ namespace build2
     // necessarily normalized). Return NULL if not found.
     //
     const target_type*
-    find_target_type (name&, const string*& ext) const;
+    find_target_type (name&, optional<string>& ext) const;
 
     // Dynamically derive a new target type from an existing one. Return the
     // reference to the target type and an indicator of whether it was
diff --git a/build2/scope.cxx b/build2/scope.cxx
index 9950042..8d0841b 100644
--- a/build2/scope.cxx
+++ b/build2/scope.cxx
@@ -547,9 +547,9 @@ namespace build2
   static const string file_tt ("file");
 
   const target_type* scope::
-  find_target_type (name& n, const string*& ext) const
+  find_target_type (name& n, optional<string>& ext) const
   {
-    ext = nullptr;
+    ext = nullopt;
 
     string& v (n.value);
 
@@ -613,7 +613,7 @@ namespace build2
 
       if (j != string::npos)
       {
-        ext = &extension_pool.find (v.c_str () + j + 1);
+        ext = string (v.c_str () + j + 1);
         v.resize (j);
       }
     }
@@ -626,7 +626,7 @@ namespace build2
                       dir_path d,
                       dir_path o,
                       string n,
-                      const string* e)
+                      optional<string> e)
   {
     // Pass our type to the base factory so that it can detect that it is
     // being called to construct a derived target. This can be used, for
@@ -639,7 +639,7 @@ namespace build2
     const target_type* bt (t.base);
     for (; bt->factory == &derived_tt_factory; bt = bt->base) ;
 
-    target* r (bt->factory (t, move (d), move (o), move (n), e));
+    target* r (bt->factory (t, move (d), move (o), move (n), move (e)));
     r->derived_type = &t;
     return r;
   }
diff --git a/build2/search.cxx b/build2/search.cxx
index 75d94a0..50318a7 100644
--- a/build2/search.cxx
+++ b/build2/search.cxx
@@ -8,6 +8,7 @@
 
 #include <build2/scope>
 #include <build2/target>
+#include <build2/context>
 #include <build2/prerequisite>
 #include <build2/diagnostics>
 
@@ -76,14 +77,14 @@ namespace build2
 
     // Figure out the extension. Pretty similar logic to file::derive_path().
     //
-    const string* ext (ctk.ext);
+    optional<string> ext (ctk.ext);
 
-    if (ext == nullptr)
+    if (!ext)
     {
       if (auto f = ctk.type->extension)
-        ext = f (ctk, *cpk.scope, true); // Already from the pool.
+        ext = f (ctk, *cpk.scope, true);
 
-      if (ext == nullptr)
+      if (!ext)
       {
         // What should we do here, fail or say we didn't find anything?
         // Current think is that if the target type couldn't find the default
diff --git a/build2/target b/build2/target
index 48c5ebc..421c20a 100644
--- a/build2/target
+++ b/build2/target
@@ -144,10 +144,10 @@ namespace build2
     // when src == out). We also treat out of project targets as being in the
     // out tree.
     //
-    const dir_path dir; // Absolute and normalized.
-    const dir_path out; // Empty or absolute and normalized.
-    const string name;
-    const string* ext;  // Extension. NULL - unspecified, empty - no extension.
+    const dir_path   dir;  // Absolute and normalized.
+    const dir_path   out;  // Empty or absolute and normalized.
+    const string     name;
+    optional<string> ext;  // Absent - unspecified, empty - no extension.
 
     const dir_path&
     out_dir () const {return out.empty () ? dir : out;}
@@ -259,8 +259,8 @@ namespace build2
     target (const target&) = delete;
     target& operator= (const target&) = delete;
 
-    target (dir_path d, dir_path o, string n, const string* e)
-        : dir (move (d)), out (move (o)), name (move (n)), ext (e) {}
+    target (dir_path d, dir_path o, string n, optional<string> e)
+        : dir (move (d)), out (move (o)), name (move (n)), ext (move (e)) {}
 
     // Reset the target before matching it to a rule. The default
     // implementation clears the auxilary data and prerequisite_targets.
@@ -986,7 +986,7 @@ namespace build2
           const dir_path& dir,
           const dir_path& out,
           const string& name,
-          const string* ext,
+          const optional<string>& ext,
           tracer& trace) const
     {
       return find (target_key {&type, &dir, &out, &name, ext}, trace);
@@ -1000,7 +1000,7 @@ namespace build2
     find (const dir_path& dir, const dir_path& out, const string& name) const
     {
       auto i (map_.find (
-                target_key {&T::static_type, &dir, &out, &name, nullptr}));
+                target_key {&T::static_type, &dir, &out, &name, nullopt}));
       return i != map_.end () ? static_cast<T*> (i->second.get ()) : nullptr;
     }
 
@@ -1012,7 +1012,7 @@ namespace build2
             dir_path dir,
             dir_path out,
             string name,
-            const string* ext,
+            optional<string> ext,
             tracer&);
 
     template <typename T>
@@ -1021,11 +1021,11 @@ namespace build2
             dir_path dir,
             dir_path out,
             string name,
-            const string* ext,
+            optional<string> ext,
             tracer& t)
     {
       return static_cast<T&> (
-        insert (tt, move (dir), move (out), move (name), ext, t).first);
+        insert (tt, move (dir), move (out), move (name), move (ext), t).first);
     }
 
     template <typename T>
@@ -1033,7 +1033,7 @@ namespace build2
     insert (const dir_path& dir,
             const dir_path& out,
             const string& name,
-            const string* ext,
+            const optional<string>& ext,
             tracer& t)
     {
       return static_cast<T&> (
@@ -1048,7 +1048,7 @@ namespace build2
             tracer& t)
     {
       return static_cast<T&> (
-        insert (T::static_type, dir, out, name, nullptr, t).first);
+        insert (T::static_type, dir, out, name, nullopt, t).first);
     }
 
     void
@@ -1344,32 +1344,32 @@ namespace build2
                   dir_path d,
                   dir_path o,
                   string n,
-                  const string* e)
+                  optional<string> e)
   {
-    return new T (move (d), move (o), move (n), e);
+    return new T (move (d), move (o), move (n), move (e));
   }
 
   // Return fixed target extension.
   //
   template <const char* ext>
-  const string*
+  optional<string>
   target_extension_fix (const target_key&, scope&, bool);
 
   // Get the extension from the variable or use the default if none set. If
   // the default is NULL, then return NULL.
   //
   template <const char* var, const char* def>
-  const string*
+  optional<string>
   target_extension_var (const target_key&, scope&, bool);
 
   // Always return NULL extension.
   //
-  const string*
+  optional<string>
   target_extension_null (const target_key&, scope&, bool);
 
   // Assert if called.
   //
-  const string*
+  optional<string>
   target_extension_assert (const target_key&, scope&, bool);
 
   // Target print functions.
diff --git a/build2/target-key b/build2/target-key
index f0ea80b..7905f8a 100644
--- a/build2/target-key
+++ b/build2/target-key
@@ -26,7 +26,9 @@ namespace build2
     const dir_path* const dir; // Can be relative if part of prerequisite_key.
     const dir_path* const out; // Can be relative if part of prerequisite_key.
     const string* const name;
-    const string* const& ext;
+    const optional<string>& ext;
+
+    static const optional<string> nullext;
 
     template <typename T>
     bool is_a () const {return type->is_a<T> ();}
@@ -47,15 +49,14 @@ namespace build2
   inline bool
   operator== (const target_key& x, const target_key& y)
   {
-    // Unspecified and specified extension are assumed equal. The extension
-    // strings are from the pool, so we can just compare pointers.
+    // Unspecified and specified extension are assumed equal.
     //
     return
       x.type == y.type &&
       *x.dir == *y.dir &&
       *x.out == *y.out &&
       *x.name == *y.name &&
-      (x.ext == nullptr || y.ext == nullptr || x.ext == y.ext);
+      (!x.ext || !y.ext || *x.ext == *y.ext);
   }
 
   inline bool
diff --git a/build2/target-type b/build2/target-type
index 3bbc73c..cddb9e2 100644
--- a/build2/target-type
+++ b/build2/target-type
@@ -41,8 +41,8 @@ namespace build2
     const char* name;
     const target_type* base;
     target* (*factory) (
-      const target_type&, dir_path, dir_path, string, const string*);
-    const string* (*extension) (const target_key&, scope&, bool search);
+      const target_type&, dir_path, dir_path, string, optional<string>);
+    optional<string> (*extension) (const target_key&, scope&, bool search);
     void (*print) (ostream&, const target_key&);
     target* (*search) (const prerequisite_key&);
     bool see_through; // A group with the default "see through" semantics.
diff --git a/build2/target.cxx b/build2/target.cxx
index d083f22..40967c3 100644
--- a/build2/target.cxx
+++ b/build2/target.cxx
@@ -15,6 +15,10 @@ using namespace std;
 
 namespace build2
 {
+  // target_key
+  //
+  const optional<string> target_key::nullext = nullopt;
+
   // target_type
   //
   bool target_type::
@@ -207,13 +211,13 @@ namespace build2
 
       // Update the extension if the existing target has it unspecified.
       //
-      const string* ext (k.ext);
+      const optional<string>& ext (k.ext);
       if (t.ext != ext)
       {
         l5 ([&]{
             diag_record r (trace);
             r << "assuming target " << t << " is the same as the one with ";
-            if (ext == nullptr)
+            if (!ext)
               r << "unspecified extension";
             else if (ext->empty ())
               r << "no extension";
@@ -221,7 +225,7 @@ namespace build2
               r << "extension " << *ext;
           });
 
-        if (ext != nullptr)
+        if (ext)
           t.ext = ext;
       }
     }
@@ -234,7 +238,7 @@ namespace build2
           dir_path dir,
           dir_path out,
           string name,
-          const string* ext,
+          optional<string> ext,
           tracer& trace)
   {
     iterator i (find (target_key {&tt, &dir, &out, &name, ext}, trace));
@@ -243,7 +247,7 @@ namespace build2
     if (r)
     {
       unique_ptr<target> pt (
-        tt.factory (tt, move (dir), move (out), move (name), ext));
+        tt.factory (tt, move (dir), move (out), move (name), move (ext)));
       i = map_.emplace (
         make_pair (target_key {&tt, &pt->dir, &pt->out, &pt->name, pt->ext},
                    move (pt))).first;
@@ -285,13 +289,13 @@ namespace build2
         // it if there is one. For 2 we print 'foo.?' if it hasn't yet been
         // assigned and 'foo.' if it is assigned as "no extension" (empty).
         //
-        if (ev > 0 && (ev > 1 || (k.ext != nullptr && !k.ext->empty ())))
+        if (ev > 0 && (ev > 1 || (k.ext && !k.ext->empty ())))
         {
-          os << '.' << (k.ext != nullptr ? *k.ext : "?");
+          os << '.' << (k.ext ? *k.ext : "?");
         }
       }
       else
-        assert (k.ext == nullptr);
+        assert (!k.ext);
     }
     else
       os << *k.dir;
@@ -338,7 +342,7 @@ namespace build2
     //
     assert (de == nullptr || type ().extension != nullptr);
 
-    if (ext == nullptr)
+    if (!ext)
     {
       // If the target type has the extension function then try that first.
       // The reason for preferring it over what's been provided by the caller
@@ -346,12 +350,12 @@ namespace build2
       // the user can use to override extensions.
       //
       if (auto f = type ().extension)
-        ext = f (key (), base_scope (), search); // Already from the pool.
+        ext = f (key (), base_scope (), search);
 
-      if (ext == nullptr)
+      if (!ext)
       {
         if (de != nullptr)
-          ext = &extension_pool.find (de);
+          ext = de;
         else
           fail << "no default extension for target " << *this;
       }
@@ -452,13 +456,13 @@ namespace build2
     return t;
   }
 
-  const string*
+  optional<string>
   target_extension_null (const target_key&, scope&, bool)
   {
-    return nullptr;
+    return nullopt;
   }
 
-  const string*
+  optional<string>
   target_extension_assert (const target_key&, scope&, bool)
   {
     assert (false); // Attempt to obtain the default extension.
@@ -521,7 +525,7 @@ namespace build2
                 dir_path d,
                 dir_path o,
                 string n,
-                const string* e)
+                optional<string> e)
   {
     // A generic file target type doesn't imply any extension while a very
     // specific one (say man1) may have a fixed extension. So if one wasn't
@@ -533,9 +537,9 @@ namespace build2
       move (d),
       move (o),
       move (n),
-      (e != nullptr || ext == nullptr || tt.factory != &file_factory<T, ext>
-       ? e
-       : &extension_pool.find (ext)));
+      (e || ext == nullptr || tt.factory != &file_factory<T, ext>
+       ? move (e)
+       : string (ext)));
   }
 
   extern const char file_ext_var[] = "extension"; // VC14 rejects constexpr.
@@ -585,7 +589,7 @@ namespace build2
     false
   };
 
-  static const string*
+  static optional<string>
   exe_extension (const target_key&, scope&, bool search)
   {
     // If we are searching for an executable that is not a target, then
@@ -593,14 +597,14 @@ namespace build2
     // a target, then we expect the rule to use target machine extension.
     //
     return search
-      ? &extension_pool.find (
+      ? optional<string> (
 #ifdef _WIN32
         "exe"
 #else
         ""
 #endif
       )
-      : nullptr;
+      : nullopt;
   }
 
   const target_type exe::static_type
@@ -619,21 +623,21 @@ namespace build2
                      dir_path d,
                      dir_path o,
                      string n,
-                     const string* e)
+                     optional<string> e)
   {
-    if (e == nullptr)
-      e = &extension_pool.find (n == "buildfile" ? "" : "build");
+    if (!e)
+      e = (n == "buildfile" ? string () : "build");
 
-    return new buildfile (move (d), move (o), move (n), e);
+    return new buildfile (move (d), move (o), move (n), move (e));
   }
 
-  static const string*
+  static optional<string>
   buildfile_target_extension (const target_key& tk, scope&, bool)
   {
     // If the name is special 'buildfile', then there is no extension,
     // otherwise it is .build.
     //
-    return &extension_pool.find (*tk.name == "buildfile" ? "" : "build");
+    return *tk.name == "buildfile" ? string () : "build";
   }
 
   const target_type buildfile::static_type
@@ -663,12 +667,12 @@ namespace build2
                dir_path d,
                dir_path o,
                string n,
-               const string* e)
+               optional<string> e)
   {
-    if (e == nullptr)
+    if (!e)
       fail << "man target '" << n << "' must include extension (man section)";
 
-    return new man (move (d), move (o), move (n), e);
+    return new man (move (d), move (o), move (n), move (e));
   }
 
   const target_type man::static_type
diff --git a/build2/target.txx b/build2/target.txx
index bc63c75..66acfb8 100644
--- a/build2/target.txx
+++ b/build2/target.txx
@@ -3,7 +3,6 @@
 // license   : MIT; see accompanying LICENSE file
 
 #include <build2/scope>
-#include <build2/context>     // extension_pool
 #include <build2/diagnostics>
 #include <build2/prerequisite>
 
@@ -34,14 +33,14 @@ namespace build2
   //
   //
   template <const char* ext>
-  const string*
+  optional<string>
   target_extension_fix (const target_key&, scope&, bool)
   {
-    return &extension_pool.find (ext);
+    return string (ext);
   }
 
   template <const char* var, const char* def>
-  const string*
+  optional<string>
   target_extension_var (const target_key& tk, scope& s, bool)
   {
     // Include target type/pattern-specific variables.
@@ -51,10 +50,9 @@ namespace build2
       // Help the user here and strip leading '.' from the extension.
       //
       const string& e (cast<string> (l));
-      return &extension_pool.find (
-        !e.empty () && e.front () == '.' ? string (e, 1) : e);
+      return !e.empty () && e.front () == '.' ? string (e, 1) : e;
     }
 
-    return def != nullptr ? &extension_pool.find (def) : nullptr;
+    return def != nullptr ? optional<string> (def) : nullopt;
   }
 }
diff --git a/build2/test/target.cxx b/build2/test/target.cxx
index af3c89b..44f65aa 100644
--- a/build2/test/target.cxx
+++ b/build2/test/target.cxx
@@ -16,21 +16,21 @@ namespace build2
                         dir_path d,
                         dir_path o,
                         string n,
-                        const string* e)
+                        optional<string> e)
     {
-      if (e == nullptr)
-        e = &extension_pool.find (n == "testscript" ? "" : "test");
+      if (!e)
+        e = (n == "testscript" ? string () : "test");
 
-      return new testscript (move (d), move (o), move (n), e);
+      return new testscript (move (d), move (o), move (n), move (e));
     }
 
-    static const string*
+    static optional<string>
     testscript_target_extension (const target_key& tk, scope&, bool)
     {
       // If the name is special 'testscript', then there is no extension,
       // otherwise it is .test.
       //
-      return &extension_pool.find (*tk.name == "testscript" ? "" : "test");
+      return *tk.name == "testscript" ? string () : "test";
     }
 
     const target_type testscript::static_type
diff --git a/build2/utility b/build2/utility
index 464c1fe..b162a9d 100644
--- a/build2/utility
+++ b/build2/utility
@@ -437,17 +437,6 @@ namespace build2
   unsigned int
   to_version (const string&);
 
-  // Pools (@@ perhaps move into a separate header).
-  //
-  struct string_pool: std::unordered_set<std::string>
-  {
-    const std::string&
-    find (const char* s) {return *emplace (s).first;}
-
-    const std::string&
-    find (const std::string& s) {return *emplace (s).first;}
-  };
-
   // Initialize build2 global state (verbosity, home/work directories, etc).
   // Should be called early in main() once.
   //
diff --git a/unit-tests/test/script/parser/driver.cxx b/unit-tests/test/script/parser/driver.cxx
index e167505..271fd2c 100644
--- a/unit-tests/test/script/parser/driver.cxx
+++ b/unit-tests/test/script/parser/driver.cxx
@@ -9,7 +9,7 @@
 #include <build2/utility>
 
 #include <build2/target>
-#include <build2/context>
+#include <build2/context>   // reset()
 #include <build2/scheduler>
 
 #include <build2/test/target>
@@ -186,16 +186,15 @@ namespace build2
             targets.insert<file> (work,
                                   dir_path (),
                                   "driver",
-                                  &extension_pool.find (""),
+                                  string (),
                                   trace));
 
           testscript& st (
-            targets.insert<testscript> (
-              work,
-              dir_path (),
-              name.leaf ().base ().string (),
-              &extension_pool.find (name.leaf ().extension ()),
-              trace));
+            targets.insert<testscript> (work,
+                                        dir_path (),
+                                        name.leaf ().base ().string (),
+                                        name.leaf ().extension (),
+                                        trace));
 
           tt.path (path ("driver"));
           st.path (name);