Add support for dynamic dependencies as byproduct of script body

Specifically, the `depdb dyndep` builtin now has the --byproduct option (which
must come first). In this mode only the --file input is supported. For example:

obje{hello.o}: cxx{hello}
{{
  o = $path($>)
  t = $(o).t

  depdb dyndep --byproduct --what=header --default-type=h --file $t

  diag c++ ($<[0])

  $cxx.path $cxx.poptions $cc.poptions $cc.coptions $cxx.coptions $cxx.mode -o $o -MD -MF $t -c $path($<[0])
}}

Naturally, this mode does not support dynamic auto-generated prerequisites.
If present, such prerequisites must be specified statically in the buildfile.

Note also that the --default-prereq-type option has been rename to
--default-type.

7 files changed, 368 insertions, 159 deletions

diff --git a/libbuild2/build/script/builtin-options.cxx b/libbuild2/build/script/builtin-options.cxx
index cf99b12..f66fe47 100644
--- a/libbuild2/build/script/builtin-options.cxx
+++ b/libbuild2/build/script/builtin-options.cxx
@@ -397,11 +397,11 @@ namespace build2
   {
     namespace script
     {
-      // depdb_dep_options
+      // depdb_dyndep_options
       //
 
-      depdb_dep_options::
-      depdb_dep_options ()
+      depdb_dyndep_options::
+      depdb_dyndep_options ()
       : file_ (),
         file_specified_ (false),
         format_ (),
@@ -410,12 +410,14 @@ namespace build2
         what_specified_ (false),
         include_path_ (),
         include_path_specified_ (false),
-        default_prereq_type_ (),
-        default_prereq_type_specified_ (false)
+        default_type_ (),
+        default_type_specified_ (false),
+        cwd_ (),
+        cwd_specified_ (false)
       {
       }
 
-      bool depdb_dep_options::
+      bool depdb_dyndep_options::
       parse (int& argc,
              char** argv,
              bool erase,
@@ -427,7 +429,7 @@ namespace build2
         return r;
       }
 
-      bool depdb_dep_options::
+      bool depdb_dyndep_options::
       parse (int start,
              int& argc,
              char** argv,
@@ -440,7 +442,7 @@ namespace build2
         return r;
       }
 
-      bool depdb_dep_options::
+      bool depdb_dyndep_options::
       parse (int& argc,
              char** argv,
              int& end,
@@ -454,7 +456,7 @@ namespace build2
         return r;
       }
 
-      bool depdb_dep_options::
+      bool depdb_dyndep_options::
       parse (int start,
              int& argc,
              char** argv,
@@ -469,7 +471,7 @@ namespace build2
         return r;
       }
 
-      bool depdb_dep_options::
+      bool depdb_dyndep_options::
       parse (::build2::build::script::cli::scanner& s,
              ::build2::build::script::cli::unknown_mode opt,
              ::build2::build::script::cli::unknown_mode arg)
@@ -479,44 +481,47 @@ namespace build2
       }
 
       typedef
-      std::map<std::string, void (*) (depdb_dep_options&, ::build2::build::script::cli::scanner&)>
-      _cli_depdb_dep_options_map;
+      std::map<std::string, void (*) (depdb_dyndep_options&, ::build2::build::script::cli::scanner&)>
+      _cli_depdb_dyndep_options_map;
 
-      static _cli_depdb_dep_options_map _cli_depdb_dep_options_map_;
+      static _cli_depdb_dyndep_options_map _cli_depdb_dyndep_options_map_;
 
-      struct _cli_depdb_dep_options_map_init
+      struct _cli_depdb_dyndep_options_map_init
       {
-        _cli_depdb_dep_options_map_init ()
-        {
-          _cli_depdb_dep_options_map_["--file"] =
-          &::build2::build::script::cli::thunk< depdb_dep_options, path, &depdb_dep_options::file_,
-            &depdb_dep_options::file_specified_ >;
-          _cli_depdb_dep_options_map_["--format"] =
-          &::build2::build::script::cli::thunk< depdb_dep_options, string, &depdb_dep_options::format_,
-            &depdb_dep_options::format_specified_ >;
-          _cli_depdb_dep_options_map_["--what"] =
-          &::build2::build::script::cli::thunk< depdb_dep_options, string, &depdb_dep_options::what_,
-            &depdb_dep_options::what_specified_ >;
-          _cli_depdb_dep_options_map_["--include-path"] =
-          &::build2::build::script::cli::thunk< depdb_dep_options, dir_paths, &depdb_dep_options::include_path_,
-            &depdb_dep_options::include_path_specified_ >;
-          _cli_depdb_dep_options_map_["-I"] =
-          &::build2::build::script::cli::thunk< depdb_dep_options, dir_paths, &depdb_dep_options::include_path_,
-            &depdb_dep_options::include_path_specified_ >;
-          _cli_depdb_dep_options_map_["--default-prereq-type"] =
-          &::build2::build::script::cli::thunk< depdb_dep_options, string, &depdb_dep_options::default_prereq_type_,
-            &depdb_dep_options::default_prereq_type_specified_ >;
+        _cli_depdb_dyndep_options_map_init ()
+        {
+          _cli_depdb_dyndep_options_map_["--file"] =
+          &::build2::build::script::cli::thunk< depdb_dyndep_options, path, &depdb_dyndep_options::file_,
+            &depdb_dyndep_options::file_specified_ >;
+          _cli_depdb_dyndep_options_map_["--format"] =
+          &::build2::build::script::cli::thunk< depdb_dyndep_options, string, &depdb_dyndep_options::format_,
+            &depdb_dyndep_options::format_specified_ >;
+          _cli_depdb_dyndep_options_map_["--what"] =
+          &::build2::build::script::cli::thunk< depdb_dyndep_options, string, &depdb_dyndep_options::what_,
+            &depdb_dyndep_options::what_specified_ >;
+          _cli_depdb_dyndep_options_map_["--include-path"] =
+          &::build2::build::script::cli::thunk< depdb_dyndep_options, dir_paths, &depdb_dyndep_options::include_path_,
+            &depdb_dyndep_options::include_path_specified_ >;
+          _cli_depdb_dyndep_options_map_["-I"] =
+          &::build2::build::script::cli::thunk< depdb_dyndep_options, dir_paths, &depdb_dyndep_options::include_path_,
+            &depdb_dyndep_options::include_path_specified_ >;
+          _cli_depdb_dyndep_options_map_["--default-type"] =
+          &::build2::build::script::cli::thunk< depdb_dyndep_options, string, &depdb_dyndep_options::default_type_,
+            &depdb_dyndep_options::default_type_specified_ >;
+          _cli_depdb_dyndep_options_map_["--cwd"] =
+          &::build2::build::script::cli::thunk< depdb_dyndep_options, dir_path, &depdb_dyndep_options::cwd_,
+            &depdb_dyndep_options::cwd_specified_ >;
         }
       };
 
-      static _cli_depdb_dep_options_map_init _cli_depdb_dep_options_map_init_;
+      static _cli_depdb_dyndep_options_map_init _cli_depdb_dyndep_options_map_init_;
 
-      bool depdb_dep_options::
+      bool depdb_dyndep_options::
       _parse (const char* o, ::build2::build::script::cli::scanner& s)
       {
-        _cli_depdb_dep_options_map::const_iterator i (_cli_depdb_dep_options_map_.find (o));
+        _cli_depdb_dyndep_options_map::const_iterator i (_cli_depdb_dyndep_options_map_.find (o));
 
-        if (i != _cli_depdb_dep_options_map_.end ())
+        if (i != _cli_depdb_dyndep_options_map_.end ())
         {
           (*(i->second)) (*this, s);
           return true;
@@ -525,7 +530,7 @@ namespace build2
         return false;
       }
 
-      bool depdb_dep_options::
+      bool depdb_dyndep_options::
       _parse (::build2::build::script::cli::scanner& s,
               ::build2::build::script::cli::unknown_mode opt_mode,
               ::build2::build::script::cli::unknown_mode arg_mode)
diff --git a/libbuild2/build/script/builtin-options.hxx b/libbuild2/build/script/builtin-options.hxx
index 85d67b9..15119f4 100644
--- a/libbuild2/build/script/builtin-options.hxx
+++ b/libbuild2/build/script/builtin-options.hxx
@@ -297,10 +297,10 @@ namespace build2
   {
     namespace script
     {
-      class depdb_dep_options
+      class depdb_dyndep_options
       {
         public:
-        depdb_dep_options ();
+        depdb_dyndep_options ();
 
         // Return true if anything has been parsed.
         //
@@ -404,19 +404,34 @@ namespace build2
         include_path_specified (bool);
 
         const string&
-        default_prereq_type () const;
+        default_type () const;
 
         string&
-        default_prereq_type ();
+        default_type ();
 
         void
-        default_prereq_type (const string&);
+        default_type (const string&);
 
         bool
-        default_prereq_type_specified () const;
+        default_type_specified () const;
 
         void
-        default_prereq_type_specified (bool);
+        default_type_specified (bool);
+
+        const dir_path&
+        cwd () const;
+
+        dir_path&
+        cwd ();
+
+        void
+        cwd (const dir_path&);
+
+        bool
+        cwd_specified () const;
+
+        void
+        cwd_specified (bool);
 
         // Implementation details.
         //
@@ -439,8 +454,10 @@ namespace build2
         bool what_specified_;
         dir_paths include_path_;
         bool include_path_specified_;
-        string default_prereq_type_;
-        bool default_prereq_type_specified_;
+        string default_type_;
+        bool default_type_specified_;
+        dir_path cwd_;
+        bool cwd_specified_;
       };
     }
   }
diff --git a/libbuild2/build/script/builtin-options.ixx b/libbuild2/build/script/builtin-options.ixx
index 06575c8..c6266d0 100644
--- a/libbuild2/build/script/builtin-options.ixx
+++ b/libbuild2/build/script/builtin-options.ixx
@@ -176,157 +176,187 @@ namespace build2
   {
     namespace script
     {
-      // depdb_dep_options
+      // depdb_dyndep_options
       //
 
-      inline const path& depdb_dep_options::
+      inline const path& depdb_dyndep_options::
       file () const
       {
         return this->file_;
       }
 
-      inline path& depdb_dep_options::
+      inline path& depdb_dyndep_options::
       file ()
       {
         return this->file_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       file (const path& x)
       {
         this->file_ = x;
       }
 
-      inline bool depdb_dep_options::
+      inline bool depdb_dyndep_options::
       file_specified () const
       {
         return this->file_specified_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       file_specified (bool x)
       {
         this->file_specified_ = x;
       }
 
-      inline const string& depdb_dep_options::
+      inline const string& depdb_dyndep_options::
       format () const
       {
         return this->format_;
       }
 
-      inline string& depdb_dep_options::
+      inline string& depdb_dyndep_options::
       format ()
       {
         return this->format_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       format (const string& x)
       {
         this->format_ = x;
       }
 
-      inline bool depdb_dep_options::
+      inline bool depdb_dyndep_options::
       format_specified () const
       {
         return this->format_specified_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       format_specified (bool x)
       {
         this->format_specified_ = x;
       }
 
-      inline const string& depdb_dep_options::
+      inline const string& depdb_dyndep_options::
       what () const
       {
         return this->what_;
       }
 
-      inline string& depdb_dep_options::
+      inline string& depdb_dyndep_options::
       what ()
       {
         return this->what_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       what (const string& x)
       {
         this->what_ = x;
       }
 
-      inline bool depdb_dep_options::
+      inline bool depdb_dyndep_options::
       what_specified () const
       {
         return this->what_specified_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       what_specified (bool x)
       {
         this->what_specified_ = x;
       }
 
-      inline const dir_paths& depdb_dep_options::
+      inline const dir_paths& depdb_dyndep_options::
       include_path () const
       {
         return this->include_path_;
       }
 
-      inline dir_paths& depdb_dep_options::
+      inline dir_paths& depdb_dyndep_options::
       include_path ()
       {
         return this->include_path_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       include_path (const dir_paths& x)
       {
         this->include_path_ = x;
       }
 
-      inline bool depdb_dep_options::
+      inline bool depdb_dyndep_options::
       include_path_specified () const
       {
         return this->include_path_specified_;
       }
 
-      inline void depdb_dep_options::
+      inline void depdb_dyndep_options::
       include_path_specified (bool x)
       {
         this->include_path_specified_ = x;
       }
 
-      inline const string& depdb_dep_options::
-      default_prereq_type () const
+      inline const string& depdb_dyndep_options::
+      default_type () const
       {
-        return this->default_prereq_type_;
+        return this->default_type_;
       }
 
-      inline string& depdb_dep_options::
-      default_prereq_type ()
+      inline string& depdb_dyndep_options::
+      default_type ()
       {
-        return this->default_prereq_type_;
+        return this->default_type_;
       }
 
-      inline void depdb_dep_options::
-      default_prereq_type (const string& x)
+      inline void depdb_dyndep_options::
+      default_type (const string& x)
       {
-        this->default_prereq_type_ = x;
+        this->default_type_ = x;
       }
 
-      inline bool depdb_dep_options::
-      default_prereq_type_specified () const
+      inline bool depdb_dyndep_options::
+      default_type_specified () const
       {
-        return this->default_prereq_type_specified_;
+        return this->default_type_specified_;
       }
 
-      inline void depdb_dep_options::
-      default_prereq_type_specified (bool x)
+      inline void depdb_dyndep_options::
+      default_type_specified (bool x)
       {
-        this->default_prereq_type_specified_ = x;
+        this->default_type_specified_ = x;
+      }
+
+      inline const dir_path& depdb_dyndep_options::
+      cwd () const
+      {
+        return this->cwd_;
+      }
+
+      inline dir_path& depdb_dyndep_options::
+      cwd ()
+      {
+        return this->cwd_;
+      }
+
+      inline void depdb_dyndep_options::
+      cwd (const dir_path& x)
+      {
+        this->cwd_ = x;
+      }
+
+      inline bool depdb_dyndep_options::
+      cwd_specified () const
+      {
+        return this->cwd_specified_;
+      }
+
+      inline void depdb_dyndep_options::
+      cwd_specified (bool x)
+      {
+        this->cwd_specified_ = x;
       }
     }
   }
diff --git a/libbuild2/build/script/builtin.cli b/libbuild2/build/script/builtin.cli
index 3ed3659..fafb330 100644
--- a/libbuild2/build/script/builtin.cli
+++ b/libbuild2/build/script/builtin.cli
@@ -15,17 +15,33 @@ namespace build2
     {
       // Pseudo-builtin options.
       //
-      class depdb_dep_options
+      class depdb_dyndep_options
       {
         // Note that --byproduct, if any, must be the first option and is
         // handled ad hoc, kind of as a sub-command.
         //
-        path --file;                   // Read from file rather than stdin.
-        string --format;               // Dependency format: make (default).
-        string --what;                 // Dependency kind, e.g., "header".
-        dir_paths --include-path|-I;   // Search paths for generated files.
-        string --default-prereq-type;  // Default prerequisite type to use
-                                       // if none could be derived from ext.
+        // Note that in the future we may extend --cwd support to the non-
+        // byproduct mode where it will also have the `env --cwd` semantics
+        // (thus the matching name). Note that it will also be incompatible
+        // with support for generated files (and thus -I) at least in the make
+        // format where we use relative paths for non-existent files.
+        //
+        // Note on naming: whenever we (may) have two options, one for target
+        // and the other for prerequisite, we omit "prerequisite" as that's
+        // what we extract by default and most commonly. For example:
+        //
+        // --what          --what-target
+        // --default-type  --default-target-type
+        //
+        path --file;                  // Read from file rather than stdin.
+        string --format;              // Dependency format: make (default).
+        string --what;                // Dependency kind, e.g., "header".
+        dir_paths --include-path|-I;  // Search paths for generated files.
+        string --default-type;        // Default prerequisite type to use
+                                      // if none could be derived from ext.
+        dir_path --cwd;               // Builtin's working directory used
+                                      // to complete relative paths (only
+                                      // in --byproduct mode).
       };
     }
   }
diff --git a/libbuild2/build/script/parser.cxx b/libbuild2/build/script/parser.cxx
index 67dbf69..15545cf 100644
--- a/libbuild2/build/script/parser.cxx
+++ b/libbuild2/build/script/parser.cxx
@@ -135,7 +135,10 @@ namespace build2
         //
         s.depdb_clear = depdb_clear_.has_value ();
         if (depdb_dyndep_)
+        {
           s.depdb_dyndep = depdb_dyndep_->second;
+          s.depdb_dyndep_byproduct = depdb_dyndep_byproduct_;
+        }
         s.depdb_preamble = move (depdb_preamble_);
 
         return s;
@@ -548,7 +551,7 @@ namespace build2
             }
             else
             {
-              // Verify depdb-dyndep is last.
+              // Verify depdb-dyndep is last and detect the byproduct flavor.
               //
               if (v == "dyndep")
               {
@@ -563,10 +566,8 @@ namespace build2
                   fail (l) << "multiple 'depdb dyndep' calls" <<
                     info (depdb_dyndep_->first) << "previous call is here";
 
-#if 0
                 if (peek () == type::word && peeked ().value == "--byproduct")
-                  ;
-#endif
+                  depdb_dyndep_byproduct_ = true;
               }
               else
               {
@@ -933,7 +934,8 @@ namespace build2
                            depdb& dd,
                            bool* update,
                            bool* deferred_failure,
-                           optional<timestamp> mt)
+                           optional<timestamp> mt,
+                           dyndep_byproduct* byp)
       {
         tracer trace ("exec_depdb_preamble");
 
@@ -961,8 +963,9 @@ namespace build2
           bool* update;
           bool* deferred_failure;
           optional<timestamp> mt;
+          dyndep_byproduct* byp;
 
-        } data {trace, a, bs, t, e, s, dd, update, deferred_failure, mt};
+        } data {trace, a, bs, t, e, s, dd, update, deferred_failure, mt, byp};
 
         auto exec_cmd = [this, &data] (token& t,
                                        build2::script::token_type& tt,
@@ -986,7 +989,8 @@ namespace build2
 
             if (cmd == "dyndep")
             {
-              // Note: cast is safe since this is always executed in apply().
+              // Note: the cast is safe since the part where the target is
+              // modified is always executed in apply().
               //
               exec_depdb_dyndep (t, tt,
                                  li, ll,
@@ -994,7 +998,8 @@ namespace build2
                                  data.dd,
                                  *data.update,
                                  *data.deferred_failure,
-                                 *data.mt);
+                                 *data.mt,
+                                 data.byp);
             }
             else
             {
@@ -1207,7 +1212,8 @@ namespace build2
                          depdb& dd,
                          bool& update,
                          bool& deferred_failure,
-                         timestamp mt)
+                         timestamp mt,
+                         dyndep_byproduct* byprod_result)
       {
         tracer trace ("exec_depdb_dyndep");
 
@@ -1215,13 +1221,22 @@ namespace build2
 
         // Similar approach to parse_env_builtin().
         //
-        depdb_dep_options ops;
+        depdb_dyndep_options ops;
         bool prog (false);
+        bool byprod (false);
         {
           auto& t (lt);
           auto& tt (ltt);
 
-          next (t, tt); // Skip 'dep' command.
+          next (t, tt); // Skip the 'dyndep' command.
+
+          if (tt == type::word && t.value == "--byproduct")
+          {
+            byprod = true;
+            next (t, tt);
+          }
+
+          assert (byprod == (byprod_result != nullptr));
 
           // Note that an option name and value can belong to different name
           // chunks. That's why we parse the arguments in the chunking mode
@@ -1273,6 +1288,10 @@ namespace build2
 
           if (prog)
           {
+            if (byprod)
+              fail (t) << "depdb dyndep: --byproduct cannot be used with "
+                       << "program";
+
             next (t, tt); // Skip '--'.
 
             if (tt == type::newline || tt == type::eos)
@@ -1311,12 +1330,10 @@ namespace build2
                 continue;
               }
 
-#if 0
               // Handle --byproduct in the wrong place.
               //
               if (strcmp (a, "--byproduct") == 0)
                 fail (ll) << "depdb dyndep: --byproduct must be first option";
-#endif
 
               // Handle unknown option.
               //
@@ -1334,6 +1351,64 @@ namespace build2
           }
         }
 
+        // --what
+        //
+        const char* what (ops.what_specified ()
+                          ? ops.what ().c_str ()
+                          : "file");
+
+        // --format
+        //
+        dyndep_format format (dyndep_format::make);
+
+        if (ops.format_specified ())
+        {
+          const string& f (ops.format ());
+
+          if (f != "make")
+            fail (ll) << "depdb dyndep: invalid --format option value '"
+                      << f << "'";
+        }
+
+        // --cwd
+        //
+        optional<dir_path> cwd;
+
+        if (ops.cwd_specified ())
+        {
+          if (!byprod)
+            fail (ll) << "depdb dyndep: --cwd only valid in --byproduct mode";
+
+          cwd = move (ops.cwd ());
+
+          if (cwd->relative ())
+            fail (ll) << "depdb dyndep: relative path specified with --cwd";
+        }
+
+        // --file
+        //
+        // Note that if --file is specified without a program, then we assume
+        // it is one of the static prerequisites.
+        //
+        optional<path> file;
+
+        if (ops.file_specified ())
+        {
+          file = move (ops.file ());
+
+          if (file->relative ())
+          {
+            if (!cwd)
+              fail (ll) << "depdb dyndep: relative path specified with --file";
+
+            *file = *cwd / *file;
+          }
+        }
+        else if (!prog)
+          fail (ll) << "depdb dyndep: program or --file expected";
+
+        // --default-type
+        //
         // Get the default prerequisite type falling back to file{} if not
         // specified.
         //
@@ -1344,17 +1419,34 @@ namespace build2
         // system headers to h{} targets analogous to the c module's rule.
         //
         const target_type* def_pt;
-        if (ops.default_prereq_type_specified ())
+        if (ops.default_type_specified ())
         {
-          const string& t (ops.default_prereq_type ());
+          const string& t (ops.default_type ());
 
           def_pt = bs.find_target_type (t);
           if (def_pt == nullptr)
-            fail (ll) << "unknown target type '" << t << "'";
+            fail (ll) << "unknown target type '" << t << "' specific with "
+                      << "--default-type";
         }
         else
           def_pt = &file::static_type;
 
+        if (byprod)
+        {
+          if (!ops.include_path ().empty ())
+            fail (ll) << "depdb dyndep: -I specified with --byproduct";
+
+          *byprod_result = dyndep_byproduct {
+            ll,
+            format,
+            move (cwd),
+            move (*file),
+            ops.what_specified () ? move (ops.what ()) : string (what),
+            def_pt};
+
+          return;
+        }
+
         // This code is based on the prior work in the cc module (specifically
         // extract_headers()) where you can often find more detailed rationale
         // for some of the steps performed.
@@ -1369,6 +1461,8 @@ namespace build2
         function<dyndep::map_extension_func> map_ext (
           [] (const scope& bs, const string& n, const string& e)
           {
+            // NOTE: another version in adhoc_buildscript_rule::apply().
+
             // @@ TODO: allow specifying base target types.
             //
             // Feels like the only reason one would want to specify base types
@@ -1403,7 +1497,7 @@ namespace build2
         {
           tracer& trace;
           const location& ll;
-          const depdb_dep_options& ops;
+          const depdb_dyndep_options& ops;
           optional<prefix_map> map;
         } pfx_data {trace, ll, ops, nullopt};
 
@@ -1440,11 +1534,6 @@ namespace build2
           };
         }
 
-        optional<path> file;
-        enum class format {make} fmt (format::make);
-        command_expr cmd;
-        srcout_map so_map;
-
         // Parse the remainder of the command line as a program (which can be
         // a pipe). If file is absent, then we save the command's stdout to a
         // pipe. Otherwise, assume the command writes to file and add it to
@@ -1454,31 +1543,13 @@ namespace build2
         // could do other broken tools). However, the user can always merge
         // stderr to stdout (2>&1).
         //
+        command_expr cmd;
+        srcout_map so_map;
+
         auto init_run = [this, &ctx,
                          &lt, &ltt, &ll,
-                         &ops, prog, &file, &cmd, &so_map] ()
+                         prog, &file, &ops, &cmd, &so_map] ()
         {
-          // --format
-          //
-          if (ops.format_specified ())
-          {
-            const string& f (ops.format ());
-
-            if (f != "make")
-              fail (ll) << "depdb dyndep: invalid --format option value '"
-                        << f << "'";
-          }
-
-          // --file
-          //
-          if (ops.file_specified ())
-          {
-            file = move (ops.file ());
-
-            if (file->relative ())
-              fail (ll) << "depdb dyndep: relative path specified with --file";
-          }
-
           // Populate the srcout map with the -I$out_base -I$src_base pairs.
           //
           {
@@ -1515,25 +1586,20 @@ namespace build2
                 {build2::script::cleanup_type::always, *file},
                 true /* implicit */);
           }
-          else
-          {
-            // Assume file is one of the prerequisites.
-            //
-            if (!file)
-              fail (ll) << "depdb dyndep: program or --file expected";
-          }
         };
 
         // Enter as a target, update, and add to the list of prerequisite
         // targets a file.
         //
-        const char* what (ops.what_specified ()
-                          ? ops.what ().c_str ()
-                          : "file");
-
+        // Note that these targets don't end up in $< (which is the right
+        // thing) because that variable has already been initialized (in the
+        // environment ctor).
+        //
         size_t skip_count (0);
+        auto& pts (t.prerequisite_targets[a]);
+
         auto add = [this, &trace, what,
-                    a, &bs, &t,
+                    a, &bs, &t, &pts, pts_n = pts.size (),
                     &map_ext, def_pt, &pfx_map, &so_map,
                     &dd, &skip_count] (path fp,
                                        bool cache,
@@ -1567,16 +1633,38 @@ namespace build2
           if (const build2::file* ft = dyndep::enter_file (
                 trace, what,
                 a, bs, t,
-                move (fp), cache, false /* normalize */,
+                fp, cache, cache /* normalized */,
                 map_ext, *def_pt, pfx_map, so_map).first)
           {
+            // Skip if this is one of the static prerequisites.
+            //
+            for (size_t i (0); i != pts_n; ++i)
+            {
+              const prerequisite_target& p (pts[i]);
+
+              if (const target* pt =
+                  (p.target != nullptr ? p.target :
+                   p.data   != 0       ? reinterpret_cast<target*> (p.data) :
+                   nullptr))
+              {
+                if (pt == ft)
+                {
+                  // Note that we have to increment the skip count since we
+                  // skip before performing this test.
+                  //
+                  skip_count++;
+                  return false;
+                }
+              }
+            }
+
             if (optional<bool> u = dyndep::inject_file (
                   trace, what,
                   a, t,
                   *ft, mt, false /* fail */))
             {
               if (!cache)
-                dd.expect (ft->path ());
+                dd.expect (ft->path ()); // @@ Use fp (or verify match)?
 
               skip_count++;
               return *u;
@@ -1721,14 +1809,16 @@ namespace build2
                                  : path_name ("<stdin>"));
 
             location il (in, 1);
+            size_t skip (skip_count);
 
             // The way we parse things is format-specific.
             //
-            size_t skip (skip_count);
-
-            switch (fmt)
+            // Note: similar code in
+            // adhoc_buildscript_rule::perform_update_file_dyndep_byproduct().
+            //
+            switch (format)
             {
-            case format::make:
+            case dyndep_format::make:
               {
                 using make_state = make_parser;
                 using make_type = make_parser::type;
diff --git a/libbuild2/build/script/parser.hxx b/libbuild2/build/script/parser.hxx
index da15509..bbdb052 100644
--- a/libbuild2/build/script/parser.hxx
+++ b/libbuild2/build/script/parser.hxx
@@ -127,6 +127,46 @@ namespace build2
             dd, &update, &deferred_failure, mt);
         }
 
+        // This version doesn't actually execute the depdb-dyndep builtin (but
+        // may execute some variable assignments) instead returning all the
+        // information (extracted from options) necessary to implement the
+        // depdb-dyndep --byproduct logic (which fits better into the rule
+        // implementation).
+        //
+        enum class dyndep_format {make};
+
+        struct dyndep_byproduct
+        {
+          location_value     location;
+          dyndep_format      format;
+          optional<dir_path> cwd;
+          path               file;
+          string             what;
+          const target_type* default_type;
+        };
+
+        dyndep_byproduct
+        execute_depdb_preamble_dyndep_byproduct (
+          action a, const scope& base, const file& t,
+          environment& e, const script& s, runner& r,
+          depdb& dd)
+        {
+          // This is getting really ugly (we also don't really need to pass
+          // depdb here). One day we will find a better way...
+          //
+          bool update, deferred_failure; // Dymmy.
+          timestamp mt;                  // Dummy.
+
+          dyndep_byproduct v;
+          exec_depdb_preamble (
+            a, base, t,
+            e, s, r,
+            s.depdb_preamble.begin () + *s.depdb_dyndep,
+            s.depdb_preamble.end (),
+            dd, &update, &deferred_failure, mt, &v);
+          return v;
+        }
+
         // Parse a special builtin line into names, performing the variable
         // and pattern expansions. If omit_builtin is true, then omit the
         // builtin name from the result.
@@ -166,7 +206,8 @@ namespace build2
                              depdb&,
                              bool* update = nullptr,
                              bool* deferred_failure = nullptr,
-                             optional<timestamp> mt = nullopt);
+                             optional<timestamp> mt = nullopt,
+                             dyndep_byproduct* = nullptr);
 
         void
         exec_depdb_dyndep (token&, build2::script::token_type&,
@@ -175,7 +216,8 @@ namespace build2
                            depdb&,
                            bool& update,
                            bool& deferred_failure,
-                           timestamp);
+                           timestamp,
+                           dyndep_byproduct*);
 
         // Helpers.
         //
@@ -274,13 +316,21 @@ namespace build2
         // depdb env <var-names> - Track the environment variables change as a
         //                         hash.
         //
-        // depdb dyndep ...      - Extract dynamic dependency information.
-        //                         Can only be the last depdb builtin call
-        //                         in the preamble.
+        // depdb dyndep ...      - Extract dynamic dependency information. Can
+        //                         only be the last depdb builtin call in the
+        //                         preamble. Note that such dependencies don't
+        //                         end up in $<. We also don't cause clean of
+        //                         such dependencies (since there may be no .d
+        //                         file) -- they should also be listed as
+        //                         static prerequisites of some other target
+        //                         (e.g., lib{} for headers) or a custom clean
+        //                         recipe should be provided.
+        //
         //
         optional<location> depdb_clear_;    // depdb-clear location.
         optional<pair<location, size_t>>
                            depdb_dyndep_;   // depdb-dyndep location/position.
+        bool               depdb_dyndep_byproduct_ = false; // --byproduct
         lines              depdb_preamble_; // Note: excluding depdb-clear.
 
         // If present, the first impure function called in the body of the
diff --git a/libbuild2/build/script/script.hxx b/libbuild2/build/script/script.hxx
index 9d7567c..4e88785 100644
--- a/libbuild2/build/script/script.hxx
+++ b/libbuild2/build/script/script.hxx
@@ -75,7 +75,8 @@ namespace build2
         // script parser for details).
         //
         bool             depdb_clear;
-        optional<size_t> depdb_dyndep;   // Position of first depdb-dyndep.
+        optional<size_t> depdb_dyndep;                   // Pos of first dyndep.
+        bool             depdb_dyndep_byproduct = false; // dyndep --byproduct
         lines_type       depdb_preamble;
         bool             depdb_preamble_temp_dir = false; // True if refs $~.