Split build system into library and driver

1 files changed, 1817 insertions, 0 deletions

diff --git a/libbuild2/target.hxx b/libbuild2/target.hxx
new file mode 100644
index 0000000..cfbd9bc
--- /dev/null
+++ b/libbuild2/target.hxx
@@ -0,0 +1,1817 @@
+// file      : libbuild2/target.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_TARGET_HXX
+#define LIBBUILD2_TARGET_HXX
+
+#include <iterator>     // tags, etc.
+#include <type_traits>  // aligned_storage
+#include <unordered_map>
+
+#include <libbutl/multi-index.mxx> // map_iterator_adapter
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/scope.hxx>
+#include <libbuild2/action.hxx>
+#include <libbuild2/variable.hxx>
+#include <libbuild2/target-key.hxx>
+#include <libbuild2/target-type.hxx>
+#include <libbuild2/target-state.hxx>
+#include <libbuild2/prerequisite.hxx>
+
+#include <libbuild2/export.hxx>
+
+namespace build2
+{
+  class rule;
+  class scope;
+  class target;
+
+  // From <libbuild2/context.hxx>.
+  //
+  LIBBUILD2_SYMEXPORT extern size_t current_on;
+
+  // From <libbuild2/algorithm.hxx>.
+  //
+  const target& search (const target&, const prerequisite&);
+  const target* search_existing (const prerequisite&);
+
+  // Recipe.
+  //
+  // The returned target state is normally changed or unchanged. If there is
+  // an error, then the recipe should throw failed rather than returning (this
+  // is the only exception that a recipe can throw).
+  //
+  // The return value of the recipe is used to update the target state. If it
+  // is target_state::group then the target's state is the group's state.
+  //
+  // The recipe may also return postponed in which case the target state is
+  // assumed to be unchanged (normally this means a prerequisite was postponed
+  // and while the prerequisite will be re-examined via another dependency,
+  // this target is done).
+  //
+  // Note that max size for the "small capture optimization" in std::function
+  // ranges (in pointer sizes) from 0 (GCC prior to 5) to 2 (GCC 5) to 6 (VC
+  // 14.2). With the size ranging (in bytes for 64-bit target) from 32 (GCC)
+  // to 64 (VC).
+  //
+  using recipe_function = target_state (action, const target&);
+  using recipe = function<recipe_function>;
+
+  // Commonly-used recipes. The default recipe executes the action on
+  // all the prerequisites in a loop, skipping ignored. Specifically,
+  // for actions with the "first" execution mode, it calls
+  // execute_prerequisites() while for those with the "last" mode --
+  // reverse_execute_prerequisites(); see <libbuild2/operation.hxx>,
+  // <libbuild2/algorithm.hxx> for details. The group recipe call's the
+  // group's recipe.
+  //
+  LIBBUILD2_SYMEXPORT extern const recipe empty_recipe;
+  LIBBUILD2_SYMEXPORT extern const recipe noop_recipe;
+  LIBBUILD2_SYMEXPORT extern const recipe default_recipe;
+  LIBBUILD2_SYMEXPORT extern const recipe group_recipe;
+
+  // Defined in <libbuild2/algorithm.hxx>.
+  //
+  LIBBUILD2_SYMEXPORT target_state
+  noop_action (action, const target&);
+
+  // Defined in <libbuild2/algorithm.hxx>.
+  //
+  LIBBUILD2_SYMEXPORT target_state
+  group_action (action, const target&);
+
+  // A view of target group members.
+  //
+  struct group_view
+  {
+    const target* const* members; // NULL means not yet known.
+    size_t count;
+  };
+
+  // List of prerequisites resolved to targets. Unless additional storage is
+  // needed, it can be used as just vector<const target*> (which is what we
+  // used to have initially).
+  //
+  struct prerequisite_target
+  {
+    using target_type = build2::target;
+
+    prerequisite_target (const target_type* t, bool a = false, uintptr_t d = 0)
+        : target (t), adhoc (a), data (d) {}
+
+    prerequisite_target (const target_type* t, include_type a, uintptr_t d = 0)
+        : prerequisite_target (t, a == include_type::adhoc, d) {}
+
+    operator const target_type*&  ()       {return target;}
+    operator const target_type*   () const {return target;}
+    const target_type* operator-> () const {return target;}
+
+    const target_type* target;
+    bool               adhoc;  // True if include=adhoc.
+    uintptr_t          data;
+  };
+  using prerequisite_targets = vector<prerequisite_target>;
+
+  // A rule match is an element of hint_rule_map.
+  //
+  using rule_match = pair<const string, reference_wrapper<const rule>>;
+
+  // Target.
+  //
+  class LIBBUILD2_SYMEXPORT target
+  {
+    optional<string>* ext_; // Reference to value in target_key.
+
+  public:
+    // For targets that are in the src tree of a project we also keep the
+    // corresponding out directory. As a result we may end up with multiple
+    // targets for the same file if we are building multiple configurations of
+    // the same project at once. We do it this way because, in a sense, a
+    // target's out directory is its "configuration" (in terms of variables).
+    // As an example, consider installing the same README file (src) but for
+    // two different project configurations at once. Which installation
+    // directory should we use? The answer depends on which configuration you
+    // ask.
+    //
+    // Empty out directory indicates this target is in the out tree (including
+    // 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 () const; // Return NULL if not specified.
+    const string& ext (string);
+
+    const dir_path&
+    out_dir () const {return out.empty () ? dir : out;}
+
+    // A target that is not (yet) entered as part of a real dependency
+    // declaration (for example, that is entered as part of a target-specific
+    // variable assignment, dependency extraction, etc) is called implied.
+    //
+    // The implied flag should only be cleared during the load phase via the
+    // MT-safe target_set::insert().
+    //
+    bool implied;
+
+    // Target group to which this target belongs, if any. Note that we assume
+    // that the group and all its members are in the same scope (for example,
+    // in variable lookup). We also don't support nested groups (with an
+    // exception for ad hoc groups; see below).
+    //
+    // The semantics of the interaction between the group and its members and
+    // what it means to, say, update the group, is unspecified and is
+    // determined by the group's type. In particular, a group can be created
+    // out of member types that have no idea they are part of this group
+    // (e.g., cli.cxx{}).
+    //
+    // Normally, however, there are two kinds of groups: "all" and "choice".
+    // In a choice-group, normally one of the members is selected when the
+    // group is mentioned as a prerequisite with, perhaps, an exception for
+    // special rules, like aliases, where it makes more sense to treat such
+    // group prerequisites as a whole. In this case we say that the rule
+    // "semantically recognizes" the group and picks some of its members.
+    //
+    // Updating a choice-group as a whole can mean updating some subset of its
+    // members (e.g., lib{}). Or the group may not support this at all (e.g.,
+    // obj{}).
+    //
+    // In an all-group, when a group is updated, normally all its members are
+    // updates (and usually with a single command), though there could be some
+    // members that are omitted, depending on the configuration (e.g., an
+    // inline file not/being generated). When an all-group is mentioned as a
+    // prerequisite, the rule is usually interested in the individual members
+    // rather than the whole group. For example, a C++ compile rule would like
+    // to "see" the ?xx{} members when it gets a cli.cxx{} group.
+    //
+    // Which brings us to the group iteration mode. The target type contains a
+    // member called see_through that indicates whether the default iteration
+    // mode for the group should be "see through"; that is, whether we see the
+    // members or the group itself. For the iteration support itself, see the
+    // *_prerequisite_members() machinery below.
+    //
+    // In an all-group we usually want the state (and timestamp; see mtime())
+    // for members to come from the group. This is achieved with the special
+    // target_state::group state. You would normally also use the group_recipe
+    // for group members.
+    //
+    // Note that the group-member link-up can happen anywhere between the
+    // member creation and rule matching so reading the group before the
+    // member has been matched can be racy.
+    //
+    const target* group = nullptr;
+
+    // What has been described above is a "explicit" group. That is, there is
+    // a dedicated target type that explicitly serves as a group and there is
+    // an explicit mechanism for discovering the group's members.
+    //
+    // However, sometimes, we may want to create a group on the fly out of a
+    // normal target type. For example, we have the libs{} target type. But
+    // on Windows a shared library consist of (at least) two files: the import
+    // library and the DLL itself. So we somehow need to be able to capture
+    // that. One approach would be to imply the presence of the second file.
+    // However, that means that a lot of generic rules (e.g., clean, install,
+    // etc) will need to know about this special semantics on Windows. Also,
+    // there would be no convenient way to customize things like extensions,
+    // etc (for which we use target-specific variables). In other words, it
+    // would be much easier and more consistent to make these extra files
+    // proper targets.
+    //
+    // So to support this requirement we have "ad hoc" groups. The idea is
+    // that any target can be turned either by a user's declaration in a
+    // buildfile or by the rule that matches it into an ad hoc group by
+    // chaining several targets together.
+    //
+    // Ad hoc groups have a more restricted semantics compared to the normal
+    // groups. In particular:
+    //
+    // - The ad hoc group itself is in a sense its first/primary target.
+    //
+    // - Group member's recipes, if set, should be group_recipe. Normally, a
+    //   rule-managed member isn't matched by the rule since all that's
+    //   usually needed is to derive its path.
+    //
+    // - Unless declared, members are discovered lazily, they are only known
+    //   after the group's rule's apply() call.
+    //
+    // - Only declared members can be used as prerequisites but all can be
+    //   used as targets (e.g., to set variables, etc).
+    //
+    // - Members don't have prerequisites.
+    //
+    // - Ad hoc group cannot have sub-groups (of any kind) though an ad hoc
+    //   group can be a sub-group of an explicit group.
+    //
+    // - Member variable lookup skips the ad hoc group (since the group is the
+    //   first member, this is normally what we want).
+    //
+    // Note that ad hoc groups can be part of explicit groups. In a sense, we
+    // have a two-level grouping: an explicit group with its members each of
+    // which can be an ad hoc group. For example, lib{} contains libs{} which
+    // may have an import stub as its ad hoc member.
+    //
+    // Use add_adhoc_member(), find_adhoc_member() from algorithms to manage
+    // ad hoc members.
+    //
+    const_ptr<target> member = nullptr;
+
+    bool
+    adhoc_group () const
+    {
+      // An ad hoc group can be a member of a normal group.
+      //
+      return member != nullptr &&
+        (group == nullptr || group->member == nullptr);
+    }
+
+    bool
+    adhoc_member () const
+    {
+      return group != nullptr && group->member != nullptr;
+    }
+
+  public:
+    // Normally you should not call this function directly and rather use
+    // resolve_members() from algorithm.hxx.
+    //
+    virtual group_view
+    group_members (action) const;
+
+    // Note that the returned key "tracks" the target (except for the
+    // extension).
+    //
+    target_key
+    key () const;
+
+    // Scoping.
+    //
+   public:
+    // Most qualified scope that contains this target.
+    //
+    const scope&
+    base_scope () const;
+
+    // Root scope of a project that contains this target. Note that
+    // a target can be out of any (known) project root in which case
+    // this function asserts. If you need to detect this situation,
+    // then use base_scope().root_scope() expression instead.
+    //
+    const scope&
+    root_scope () const;
+
+    // Root scope of a strong amalgamation that contains this target.
+    // The same notes as to root_scope() apply.
+    //
+    const scope&
+    strong_scope () const {return *root_scope ().strong_scope ();}
+
+    // Root scope of the outermost amalgamation that contains this target.
+    // The same notes as to root_scope() apply.
+    //
+    const scope&
+    weak_scope () const {return *root_scope ().weak_scope ();}
+
+    bool
+    in (const scope& s) const
+    {
+      return out_dir ().sub (s.out_path ());
+    }
+
+    // Prerequisites.
+    //
+    // We use an atomic-empty semantics that allows one to "swap in" a set of
+    // prerequisites if none were specified. This is used to implement
+    // "synthesized" dependencies.
+    //
+  public:
+    using prerequisites_type = build2::prerequisites;
+
+    const prerequisites_type&
+    prerequisites () const;
+
+    // Swap-in a list of prerequisites. Return false if unsuccessful (i.e.,
+    // someone beat us to it). Note that it can be called on const target.
+    //
+    bool
+    prerequisites (prerequisites_type&&) const;
+
+    // Check if there are any prerequisites. Note that the group version may
+    // be racy (see target::group).
+    //
+    bool
+    has_prerequisites () const;
+
+    bool
+    has_group_prerequisites () const;
+
+  private:
+    friend class parser;
+
+    // Note that the state is also used to synchronize the prerequisites
+    // value so we use the release-acquire ordering.
+    //
+    // 0 - absent
+    // 1 - being set
+    // 2 - present
+    //
+    atomic<uint8_t> prerequisites_state_ {0};
+    prerequisites_type prerequisites_;
+
+    static const prerequisites_type empty_prerequisites_;
+
+    // Target-specific variables.
+    //
+    // See also rule-specific variables below.
+    //
+  public:
+    variable_map vars;
+
+    // Lookup, including in groups to which this target belongs and then in
+    // outer scopes (including target type/pattern-specific variables). If you
+    // only want to lookup in this target, do it on the variable map directly
+    // (and note that there will be no overrides).
+    //
+    lookup
+    operator[] (const variable& var) const
+    {
+      return find (var).first;
+    }
+
+    lookup
+    operator[] (const variable* var) const // For cached variables.
+    {
+      assert (var != nullptr);
+      return operator[] (*var);
+    }
+
+    lookup
+    operator[] (const string& name) const
+    {
+      const variable* var (var_pool.find (name));
+      return var != nullptr ? operator[] (*var) : lookup ();
+    }
+
+    // As above but also return the depth at which the value is found. The
+    // depth is calculated by adding 1 for each test performed. So a value
+    // that is from the target will have depth 1. That from the group -- 2.
+    // From the innermost scope's target type/patter-specific variables --
+    // 3. From the innermost scope's variables -- 4. And so on.  The idea is
+    // that given two lookups from the same target, we can say which one came
+    // earlier. If no value is found, then the depth is set to ~0.
+    //
+    pair<lookup, size_t>
+    find (const variable& var) const
+    {
+      auto p (find_original (var));
+      return var.overrides == nullptr
+        ? p
+        : base_scope ().find_override (var, move (p), true);
+    }
+
+    // If target_only is true, then only look in target and its target group
+    // without continuing in scopes.
+    //
+    pair<lookup, size_t>
+    find_original (const variable&, bool target_only = false) const;
+
+    // Return a value suitable for assignment. See scope for details.
+    //
+    value&
+    assign (const variable& var) {return vars.assign (var);}
+
+    value&
+    assign (const variable* var) {return vars.assign (var);} // For cached.
+
+    // Return a value suitable for appending. See scope for details.
+    //
+    value&
+    append (const variable&);
+
+    // Target operation state.
+    //
+  public:
+    // Atomic task count that is used during match and execution to track the
+    // target's "meta-state" as well as the number of its sub-tasks (e.g.,
+    // busy+1, busy+2, and so on, for instance, number of prerequisites
+    // being matched or executed).
+    //
+    // For each operation in a meta-operation batch (current_on) we have a
+    // "band" of counts, [touched, executed], that represent the target
+    // meta-state. Once the next operation is started, this band "moves" thus
+    // automatically resetting the target to "not yet touched" state for this
+    // operation.
+    //
+    // The target is said to be synchronized (in this thread) if we have
+    // either observed the task count to reach applied or executed or we have
+    // successfully changed it (via compare_exchange) to locked or busy. If
+    // the target is synchronized, then we can access and modify (second case)
+    // its state etc.
+    //
+    static const size_t offset_touched  = 1; // Target has been locked.
+    static const size_t offset_tried    = 2; // Rule match has been tried.
+    static const size_t offset_matched  = 3; // Rule has been matched.
+    static const size_t offset_applied  = 4; // Rule has been applied.
+    static const size_t offset_executed = 5; // Recipe has been executed.
+    static const size_t offset_busy     = 6; // Match/execute in progress.
+
+    static size_t count_base     () {return 5 * (current_on - 1);}
+
+    static size_t count_touched  () {return offset_touched  + count_base ();}
+    static size_t count_tried    () {return offset_tried    + count_base ();}
+    static size_t count_matched  () {return offset_matched  + count_base ();}
+    static size_t count_applied  () {return offset_applied  + count_base ();}
+    static size_t count_executed () {return offset_executed + count_base ();}
+    static size_t count_busy     () {return offset_busy     + count_base ();}
+
+    // Inner/outer operation state. See operation.hxx for details.
+    //
+    class LIBBUILD2_SYMEXPORT opstate
+    {
+    public:
+      mutable atomic_count task_count {0}; // Start offset_touched - 1.
+
+      // Number of direct targets that depend on this target in the current
+      // operation. It is incremented during match and then decremented during
+      // execution, before running the recipe. As a result, the recipe can
+      // detect the last chance (i.e., last dependent) to execute the command
+      // (see also the first/last execution modes in <operation.hxx>).
+      //
+      mutable atomic_count dependents {0};
+
+      // Matched rule (pointer to hint_rule_map element). Note that in case of
+      // a direct recipe assignment we may not have a rule (NULL).
+      //
+      const rule_match* rule;
+
+      // Applied recipe.
+      //
+      build2::recipe recipe;
+
+      // Target state for this operation. Note that it is undetermined until
+      // a rule is matched and recipe applied (see set_recipe()).
+      //
+      target_state state;
+
+      // Rule-specific variables.
+      //
+      // The rule (for this action) has to be matched before these variables
+      // can be accessed and only the rule being matched can modify them (so
+      // no iffy modifications of the group's variables by member's rules).
+      //
+      // They are also automatically cleared before another rule is matched,
+      // similar to the data pad. In other words, rule-specific variables are
+      // only valid for this match-execute phase.
+      //
+      variable_map vars;
+
+      // Lookup, continuing in the target-specific variables, etc. Note that
+      // the group's rule-specific variables are not included. If you only
+      // want to lookup in this target, do it on the variable map directly
+      // (and note that there will be no overrides).
+      //
+      lookup
+      operator[] (const variable& var) const
+      {
+        return find (var).first;
+      }
+
+      lookup
+      operator[] (const variable* var) const // For cached variables.
+      {
+        assert (var != nullptr);
+        return operator[] (*var);
+      }
+
+      lookup
+      operator[] (const string& name) const
+      {
+        const variable* var (var_pool.find (name));
+        return var != nullptr ? operator[] (*var) : lookup ();
+      }
+
+      // As above but also return the depth at which the value is found. The
+      // depth is calculated by adding 1 for each test performed. So a value
+      // that is from the rule will have depth 1. That from the target - 2,
+      // and so on, similar to target-specific variables.
+      //
+      pair<lookup, size_t>
+      find (const variable& var) const
+      {
+        auto p (find_original (var));
+        return var.overrides == nullptr
+          ? p
+          : target_->base_scope ().find_override (var, move (p), true, true);
+      }
+
+      // If target_only is true, then only look in target and its target group
+      // without continuing in scopes.
+      //
+      pair<lookup, size_t>
+      find_original (const variable&, bool target_only = false) const;
+
+      // Return a value suitable for assignment. See target for details.
+      //
+      value&
+      assign (const variable& var) {return vars.assign (var);}
+
+      value&
+      assign (const variable* var) {return vars.assign (var);} // For cached.
+
+    public:
+      opstate (): vars (false /* global */) {}
+
+    private:
+      friend class target_set;
+
+      const target* target_ = nullptr; // Back-pointer, set by target_set.
+    };
+
+    action_state<opstate> state;
+
+    opstate&       operator[] (action a)       {return state[a];}
+    const opstate& operator[] (action a) const {return state[a];}
+
+    // This function should only be called during match if we have observed
+    // (synchronization-wise) that this target has been matched (i.e., the
+    // rule has been applied) for this action.
+    //
+    target_state
+    matched_state (action, bool fail = true) const;
+
+    // See try_match().
+    //
+    pair<bool, target_state>
+    try_matched_state (action, bool fail = true) const;
+
+    // After the target has been matched and synchronized, check if the target
+    // is known to be unchanged. Used for optimizations during search & match.
+    //
+    bool
+    unchanged (action) const;
+
+    // This function should only be called during execution if we have
+    // observed (synchronization-wise) that this target has been executed.
+    //
+    target_state
+    executed_state (action, bool fail = true) const;
+
+  protected:
+    // Version that should be used during match after the target has been
+    // matched for this action.
+    //
+    // Indicate whether there is a rule match with the first half of the
+    // result (see try_match()).
+    //
+    pair<bool, target_state>
+    matched_state_impl (action) const;
+
+    // Return fail-untranslated (but group-translated) state assuming the
+    // target is executed and synchronized.
+    //
+    target_state
+    executed_state_impl (action) const;
+
+    // Return true if the state comes from the group. Target must be at least
+    // matched.
+    //
+    bool
+    group_state (action) const;
+
+  public:
+    // Targets to which prerequisites resolve for this action. Note that
+    // unlike prerequisite::target, these can be resolved to group members.
+    // NULL means the target should be skipped (or the rule may simply not add
+    // such a target to the list).
+    //
+    // Note also that it is possible the target can vary from action to
+    // action, just like recipes. We don't need to keep track of the action
+    // here since the targets will be updated if the recipe is updated,
+    // normally as part of rule::apply().
+    //
+    // Note that the recipe may modify this list.
+    //
+    mutable action_state<build2::prerequisite_targets> prerequisite_targets;
+
+    // Auxilary data storage.
+    //
+    // A rule that matches (i.e., returns true from its match() function) may
+    // use this pad to pass data between its match and apply functions as well
+    // as the recipe. After the recipe is executed, the data is destroyed by
+    // calling data_dtor (if not NULL). The rule should static assert that the
+    // size of the pad is sufficient for its needs.
+    //
+    // Note also that normally at least 2 extra pointers may be stored without
+    // a dynamic allocation in the returned recipe (small object optimization
+    // in std::function). So if you need to pass data only between apply() and
+    // the recipe, then this might be a more convenient way.
+    //
+    // Note also that a rule that delegates to another rule may not be able to
+    // use this mechanism fully since the delegated-to rule may also need the
+    // data pad.
+    //
+    // Currenly the data is not destroyed until the next match.
+    //
+    // Note that the recipe may modify the data. Currently reserved for the
+    // inner part of the action.
+    //
+    static constexpr size_t data_size = sizeof (string) * 16;
+    mutable std::aligned_storage<data_size>::type data_pad;
+
+    mutable void (*data_dtor) (void*)                = nullptr;
+
+    template <typename R,
+              typename T = typename std::remove_cv<
+                typename std::remove_reference<R>::type>::type>
+    typename std::enable_if<std::is_trivially_destructible<T>::value,T&>::type
+    data (R&& d) const
+    {
+      assert (sizeof (T) <= data_size && data_dtor == nullptr);
+      return *new (&data_pad) T (forward<R> (d));
+    }
+
+    template <typename R,
+              typename T = typename std::remove_cv<
+                typename std::remove_reference<R>::type>::type>
+    typename std::enable_if<!std::is_trivially_destructible<T>::value,T&>::type
+    data (R&& d) const
+    {
+      assert (sizeof (T) <= data_size && data_dtor == nullptr);
+      T& r (*new (&data_pad) T (forward<R> (d)));
+      data_dtor = [] (void* p) {static_cast<T*> (p)->~T ();};
+      return r;
+    }
+
+    template <typename T>
+    T&
+    data () const {return *reinterpret_cast<T*> (&data_pad);}
+
+    void
+    clear_data () const
+    {
+      if (data_dtor != nullptr)
+      {
+        data_dtor (&data_pad);
+        data_dtor = nullptr;
+      }
+    }
+
+    // Target type info and casting.
+    //
+  public:
+    const target*
+    is_a (const target_type& tt) const {
+      return type ().is_a (tt) ? this : nullptr;}
+
+    template <typename T>
+    T*
+    is_a () {return dynamic_cast<T*> (this);}
+
+    template <typename T>
+    const T*
+    is_a () const {return dynamic_cast<const T*> (this);}
+
+    // Unchecked cast.
+    //
+    template <typename T>
+    T&
+    as () {return static_cast<T&> (*this);}
+
+    template <typename T>
+    const T&
+    as () const {return static_cast<const T&> (*this);}
+
+    // Dynamic derivation to support define.
+    //
+    const target_type* derived_type = nullptr;
+
+    const target_type&
+    type () const
+    {
+      return derived_type != nullptr ? *derived_type : dynamic_type ();
+    }
+
+    virtual const target_type& dynamic_type () const = 0;
+    static const target_type static_type;
+
+  public:
+    // Split the name leaf into target name (in place) and extension
+    // (returned).
+    //
+    static optional<string>
+    split_name (string&, const location&);
+
+    // Combine the target name and extension into the name leaf.
+    //
+    // If the target type has the default extension, then "escape" the
+    // existing extension if any.
+    //
+    static void
+    combine_name (string&, const optional<string>&, bool default_extension);
+
+    // Targets should be created via the targets set below.
+    //
+  public:
+    target (dir_path d, dir_path o, string n)
+        : dir (move (d)), out (move (o)), name (move (n)),
+          vars (false /* global */) {}
+
+    target (target&&) = delete;
+    target& operator= (target&&) = delete;
+
+    target (const target&) = delete;
+    target& operator= (const target&) = delete;
+
+    virtual
+    ~target ();
+
+    friend class target_set;
+  };
+
+  // All targets are from the targets set below.
+  //
+  inline bool
+  operator== (const target& x, const target& y) {return &x == &y;}
+
+  inline bool
+  operator!= (const target& x, const target& y) {return !(x == y);}
+
+  ostream&
+  operator<< (ostream&, const target&);
+
+  // Sometimes it is handy to "mark" a pointer to a target (for example, in
+  // prerequisite_targets). We use the last 2 bits in a pointer for that (aka
+  // the "bit stealing" technique). Note that the pointer needs to be unmarked
+  // before it can be usable so care must be taken in the face of exceptions,
+  // etc.
+  //
+  void
+  mark (const target*&, uint8_t = 1);
+
+  uint8_t
+  marked (const target*); // Can be used as a predicate or to get the mark.
+
+  uint8_t
+  unmark (const target*&);
+
+  // A "range" that presents the prerequisites of a group and one of
+  // its members as one continuous sequence, or, in other words, as
+  // if they were in a single container. The group's prerequisites
+  // come first followed by the member's. If you need to see them
+  // in the other direction, iterate in reverse, for example:
+  //
+  // for (prerequisite& p: group_prerequisites (t))
+  //
+  // for (prerequisite& p: reverse_iterate (group_prerequisites (t))
+  //
+  // Note that in this case the individual elements of each list will
+  // also be traversed in reverse, but that's what you usually want,
+  // anyway.
+  //
+  // Note that you either should be iterating over a locked target (e.g., in
+  // rule's match() or apply()) or you should call resolve_group().
+  //
+  class group_prerequisites
+  {
+  public:
+    explicit
+    group_prerequisites (const target& t);
+
+    group_prerequisites (const target& t, const target* g);
+
+    using prerequisites_type = target::prerequisites_type;
+    using base_iterator      = prerequisites_type::const_iterator;
+
+    struct iterator
+    {
+      using value_type        = base_iterator::value_type;
+      using pointer           = base_iterator::pointer;
+      using reference         = base_iterator::reference;
+      using difference_type   = base_iterator::difference_type;
+      using iterator_category = std::bidirectional_iterator_tag;
+
+      iterator () {}
+      iterator (const target* t,
+                const target* g,
+                const prerequisites_type* c,
+                base_iterator i): t_ (t), g_ (g), c_ (c), i_ (i) {}
+
+      iterator&
+      operator++ ();
+
+      iterator
+      operator++ (int) {iterator r (*this); operator++ (); return r;}
+
+      iterator&
+      operator-- ();
+
+      iterator
+      operator-- (int) {iterator r (*this); operator-- (); return r;}
+
+      reference operator* () const {return *i_;}
+      pointer operator-> () const {return i_.operator -> ();}
+
+      friend bool
+      operator== (const iterator& x, const iterator& y)
+      {
+        return x.t_ == y.t_ && x.g_ == y.g_ && x.c_ == y.c_ && x.i_ == y.i_;
+      }
+
+      friend bool
+      operator!= (const iterator& x, const iterator& y) {return !(x == y);}
+
+    private:
+      const target*             t_ = nullptr;
+      const target*             g_ = nullptr;
+      const prerequisites_type* c_ = nullptr;
+      base_iterator             i_;
+    };
+
+    using reverse_iterator = std::reverse_iterator<iterator>;
+
+    iterator
+    begin () const;
+
+    iterator
+    end () const;
+
+    reverse_iterator
+    rbegin () const {return reverse_iterator (end ());}
+
+    reverse_iterator
+    rend () const {return reverse_iterator (begin ());}
+
+    size_t
+    size () const;
+
+  private:
+    const target& t_;
+    const target* g_;
+  };
+
+  // A member of a prerequisite. If 'member' is NULL, then this is the
+  // prerequisite itself. Otherwise, it is its member. In this case
+  // 'prerequisite' still refers to the prerequisite.
+  //
+  struct prerequisite_member
+  {
+    using scope_type = build2::scope;
+    using target_type = build2::target;
+    using prerequisite_type = build2::prerequisite;
+    using target_type_type = build2::target_type;
+
+    const prerequisite_type& prerequisite;
+    const target_type* member;
+
+    template <typename T>
+    bool
+    is_a () const
+    {
+      return member != nullptr
+        ? member->is_a<T> () != nullptr
+        : prerequisite.is_a<T> ();
+    }
+
+    bool
+    is_a (const target_type_type& tt) const
+    {
+      return member != nullptr
+        ? member->is_a (tt) != nullptr
+        : prerequisite.is_a (tt);
+    }
+
+    prerequisite_key
+    key () const;
+
+    const target_type_type&
+    type () const
+    {
+      return member != nullptr ? member->type () : prerequisite.type;
+    }
+
+    const string&
+    name () const
+    {
+      return member != nullptr ? member->name : prerequisite.name;
+    }
+
+    const dir_path&
+    dir () const
+    {
+      return member != nullptr ? member->dir : prerequisite.dir;
+    }
+
+    const optional<project_name>&
+    proj () const
+    {
+      // Member cannot be project-qualified.
+      //
+      return member != nullptr ? nullopt_project_name : prerequisite.proj;
+    }
+
+    const scope_type&
+    scope () const
+    {
+      return member != nullptr ? member->base_scope () : prerequisite.scope;
+    }
+
+    const target_type&
+    search (const target_type& t) const
+    {
+      return member != nullptr ? *member : build2::search (t, prerequisite);
+    }
+
+    const target_type*
+    search_existing () const
+    {
+      return member != nullptr
+        ? member
+        : build2::search_existing (prerequisite);
+    }
+
+    const target_type*
+    load (memory_order mo = memory_order_consume)
+    {
+      return member != nullptr ? member : prerequisite.target.load (mo);
+    }
+
+    // Return as a new prerequisite instance.
+    //
+    prerequisite_type
+    as_prerequisite () const;
+  };
+
+  // It is often stored as the target's auxiliary data so make sure there is
+  // no destructor overhead.
+  //
+  static_assert (std::is_trivially_destructible<prerequisite_member>::value,
+                 "prerequisite_member is not trivially destructible");
+
+  inline ostream&
+  operator<< (ostream& os, const prerequisite_member& pm)
+  {
+    return os << pm.key ();
+  }
+
+  inline include_type
+  include (action a, const target& t, const prerequisite_member& pm)
+  {
+    return include (a, t, pm.prerequisite, pm.member);
+  }
+
+  // A "range" that presents a sequence of prerequisites (e.g., from
+  // group_prerequisites()) as a sequence of prerequisite_member's. For each
+  // group prerequisite you will "see" either the prerequisite itself or all
+  // its members, depending on the default iteration mode of the target group
+  // type (ad hoc groups are never implicitly see through since one can only
+  // safely access members after a synchronous match). You can skip the
+  // rest of the group members with leave_group() and you can force iteration
+  // over the members with enter_group(). Usage:
+  //
+  // for (prerequisite_member pm: prerequisite_members (a, ...))
+  //
+  // Where ... can be:
+  //
+  //   t.prerequisites
+  //   reverse_iterate(t.prerequisites)
+  //   group_prerequisites (t)
+  //   reverse_iterate (group_prerequisites (t))
+  //
+  // But use shortcuts instead:
+  //
+  // prerequisite_members (a, t)
+  // reverse_prerequisite_members (a, t)
+  // group_prerequisite_members (a, t)
+  // reverse_group_prerequisite_members (a, t)
+  //
+  template <typename R>
+  class prerequisite_members_range;
+
+  // See-through group members iteration mode. Ad hoc members must always
+  // be entered explicitly.
+  //
+  enum class members_mode
+  {
+    always, // Iterate over members, assert if not resolvable.
+    maybe,  // Iterate over members if resolvable, group otherwise.
+    never   // Iterate over group (can still use enter_group()).
+  };
+
+  template <typename R>
+  inline prerequisite_members_range<R>
+  prerequisite_members (action a, const target& t,
+                        R&& r,
+                        members_mode m = members_mode::always)
+  {
+    return prerequisite_members_range<R> (a, t, forward<R> (r), m);
+  }
+
+  template <typename R>
+  class prerequisite_members_range
+  {
+  public:
+    prerequisite_members_range (action a, const target& t,
+                                R&& r,
+                                members_mode m)
+        : a_ (a), t_ (t), mode_ (m), r_ (forward<R> (r)), e_ (r_.end ()) {}
+
+    using base_iterator = decltype (declval<R> ().begin ());
+
+    struct iterator
+    {
+      using value_type        = prerequisite_member;
+      using pointer           = const value_type*;
+      using reference         = const value_type&;
+      using difference_type   = typename base_iterator::difference_type;
+      using iterator_category = std::forward_iterator_tag;
+
+      iterator (): r_ (nullptr) {}
+      iterator (const prerequisite_members_range* r, const base_iterator& i)
+          : r_ (r), i_ (i), g_ {nullptr, 0}, k_ (nullptr)
+      {
+        if (r_->mode_ != members_mode::never &&
+            i_ != r_->e_                     &&
+            i_->type.see_through)
+          switch_mode ();
+      }
+
+      iterator& operator++ ();
+      iterator operator++ (int) {iterator r (*this); operator++ (); return r;}
+
+      // Skip iterating over the rest of this group's members, if any. Note
+      // that the only valid operation after this call is to increment the
+      // iterator.
+      //
+      void
+      leave_group ();
+
+      // Iterate over this group's members. Return false if the member
+      // information is not available. Similar to leave_group(), you should
+      // increment the iterator after calling this function (provided it
+      // returned true).
+      //
+      bool
+      enter_group ();
+
+      // Return true if the next element is this group's members. Normally
+      // used to iterate over group members only, for example:
+      //
+      // for (...; ++i)
+      // {
+      //   if (i->prerequisite.type.see_through)
+      //   {
+      //     for (i.enter_group (); i.group (); )
+      //     {
+      //       ++i;
+      //       ...
+      //     }
+      //   }
+      // }
+      //
+      bool
+      group () const;
+
+      value_type operator* () const
+      {
+        const target* t (k_ != nullptr ? k_:
+                         g_.count != 0 ? g_.members[j_ - 1] : nullptr);
+
+        return value_type {*i_, t};
+      }
+
+      pointer operator-> () const
+      {
+        static_assert (
+          std::is_trivially_destructible<value_type>::value,
+          "prerequisite_member is not trivially destructible");
+
+        const target* t (k_ != nullptr ? k_:
+                         g_.count != 0 ? g_.members[j_ - 1] : nullptr);
+
+        return new (&m_) value_type {*i_, t};
+      }
+
+      friend bool
+      operator== (const iterator& x, const iterator& y)
+      {
+        return x.i_ == y.i_ &&
+          x.g_.count == y.g_.count &&
+          (x.g_.count == 0 || x.j_ == y.j_) &&
+          x.k_ == y.k_;
+      }
+
+      friend bool
+      operator!= (const iterator& x, const iterator& y) {return !(x == y);}
+
+      // What we have here is a state for three nested iteration modes (and
+      // no, I am not proud of it). The innermost mode is iteration over an ad
+      // hoc group (k_). Then we have iteration over a normal group (g_ and
+      // j_). Finally, at the outer level, we have the range itself (i_).
+      //
+      // Also, the enter/leave group support is full of ugly, special cases.
+      //
+    private:
+      void
+      switch_mode ();
+
+    private:
+      const prerequisite_members_range* r_;
+      base_iterator i_;
+      group_view g_;
+      size_t j_;        // 1-based index, to support enter_group().
+      const target* k_; // Current member of ad hoc group or NULL.
+      mutable typename std::aligned_storage<sizeof (value_type),
+                                            alignof (value_type)>::type m_;
+    };
+
+    iterator
+    begin () const {return iterator (this, r_.begin ());}
+
+    iterator
+    end () const {return iterator (this, e_);}
+
+  private:
+    action a_;
+    const target& t_;
+    members_mode mode_;
+    R r_;
+    base_iterator e_;
+  };
+
+  // prerequisite_members(t.prerequisites ())
+  //
+  auto
+  prerequisite_members (action a, const target& t,
+                        members_mode m = members_mode::always);
+
+  // prerequisite_members(reverse_iterate(t.prerequisites ()))
+  //
+  auto
+  reverse_prerequisite_members (action a, const target& t,
+                                members_mode m = members_mode::always);
+
+  // prerequisite_members(group_prerequisites (t))
+  //
+  inline auto
+  group_prerequisite_members (action a, target& t,
+                              members_mode m = members_mode::always)
+  {
+    return prerequisite_members (a, t, group_prerequisites (t), m);
+  }
+
+  inline auto
+  group_prerequisite_members (action a, const target& t,
+                              members_mode m = members_mode::always)
+  {
+    return prerequisite_members (a, t, group_prerequisites (t), m);
+  }
+
+  // prerequisite_members(reverse_iterate (group_prerequisites (t)))
+  //
+  inline auto
+  reverse_group_prerequisite_members (action a, target& t,
+                                      members_mode m = members_mode::always)
+  {
+    return prerequisite_members (
+      a, t, reverse_iterate (group_prerequisites (t)), m);
+  }
+
+  inline auto
+  reverse_group_prerequisite_members (action a, const target& t,
+                                      members_mode m = members_mode::always)
+  {
+    return prerequisite_members (
+      a, t, reverse_iterate (group_prerequisites (t)), m);
+  }
+
+  // A target with an unspecified extension is considered equal to the one
+  // with the specified one. And when we find a target with an unspecified
+  // extension via a key with the specified one, we update the extension,
+  // essentially modifying the map's key. To make this work we use a hash
+  // map. The key's hash ignores the extension, so the hash will stay stable
+  // across extension updates.
+  //
+  // Note also that once the extension is specified, it becomes immutable.
+  //
+  class LIBBUILD2_SYMEXPORT target_set
+  {
+  public:
+    using map_type = std::unordered_map<target_key, unique_ptr<target>>;
+
+    // Return existing target or NULL.
+    //
+    const target*
+    find (const target_key& k, tracer& trace) const;
+
+    const target*
+    find (const target_type& type,
+          const dir_path& dir,
+          const dir_path& out,
+          const string& name,
+          const optional<string>& ext,
+          tracer& trace) const
+    {
+      return find (target_key {&type, &dir, &out, &name, ext}, trace);
+    }
+
+    template <typename T>
+    const T*
+    find (const target_type& type,
+          const dir_path& dir,
+          const dir_path& out,
+          const string& name,
+          const optional<string>& ext,
+          tracer& trace) const
+    {
+      return static_cast<const T*> (find (type, dir, out, name, ext, trace));
+    }
+
+    // As above but ignore the extension.
+    //
+    const target*
+    find (const target_type& type,
+          const dir_path& dir,
+          const dir_path& out,
+          const string& name) const
+    {
+      slock l (mutex_);
+      auto i (map_.find (target_key {&type, &dir, &out, &name, nullopt}));
+      return i != map_.end () ? i->second.get () : nullptr;
+    }
+
+    template <typename T>
+    const T*
+    find (const dir_path& dir, const dir_path& out, const string& name) const
+    {
+      return static_cast<const T*> (find (T::static_type, dir, out, name));
+    }
+
+    // If the target was inserted, keep the map exclusive-locked and return
+    // the lock. In this case, the target is effectively still being created
+    // since nobody can see it until the lock is released.
+    //
+    pair<target&, ulock>
+    insert_locked (const target_type&,
+                   dir_path dir,
+                   dir_path out,
+                   string name,
+                   optional<string> ext,
+                   bool implied,
+                   tracer&);
+
+    pair<target&, bool>
+    insert (const target_type& tt,
+            dir_path dir,
+            dir_path out,
+            string name,
+            optional<string> ext,
+            bool implied,
+            tracer& t)
+    {
+      auto p (insert_locked (tt,
+                             move (dir),
+                             move (out),
+                             move (name),
+                             move (ext),
+                             implied,
+                             t));
+
+      return pair<target&, bool> (p.first, p.second.owns_lock ());
+    }
+
+    // Note that the following versions always enter implied targets.
+    //
+    template <typename T>
+    T&
+    insert (const target_type& tt,
+            dir_path dir,
+            dir_path out,
+            string name,
+            optional<string> ext,
+            tracer& t)
+    {
+      return insert (tt,
+                     move (dir),
+                     move (out),
+                     move (name),
+                     move (ext),
+                     true,
+                     t).first.template as<T> ();
+    }
+
+    template <typename T>
+    T&
+    insert (const dir_path& dir,
+            const dir_path& out,
+            const string& name,
+            const optional<string>& ext,
+            tracer& t)
+    {
+      return insert<T> (T::static_type, dir, out, name, ext, t);
+    }
+
+    template <typename T>
+    T&
+    insert (const dir_path& dir,
+            const dir_path& out,
+            const string& name,
+            tracer& t)
+    {
+      return insert<T> (dir, out, name, nullopt, t);
+    }
+
+    // Note: not MT-safe so can only be used during serial execution.
+    //
+  public:
+    using iterator = butl::map_iterator_adapter<map_type::const_iterator>;
+
+    iterator begin () const {return map_.begin ();}
+    iterator end ()   const {return map_.end ();}
+
+    void
+    clear () {map_.clear ();}
+
+  private:
+    friend class target; // Access to mutex.
+
+    mutable shared_mutex mutex_;
+    map_type map_;
+  };
+
+  LIBBUILD2_SYMEXPORT extern target_set targets;
+
+  // Modification time-based target.
+  //
+  class LIBBUILD2_SYMEXPORT mtime_target: public target
+  {
+  public:
+    using target::target;
+
+    // Modification time is an "atomic cash". That is, it can be set at any
+    // time (including on a const instance) and we assume everything will be
+    // ok regardless of the order in which racing updates happen because we do
+    // not modify the external state (which is the source of timestemps) while
+    // updating the internal.
+    //
+    // The modification time is reserved for the inner operation thus there is
+    // no action argument.
+    //
+    // The rule for groups that utilize target_state::group is as follows: if
+    // it has any members that are mtime_targets, then the group should be
+    // mtime_target and the members get the mtime from it. During match and
+    // execute the target should be synchronized.
+    //
+    // Note that this function can be called before the target is matched in
+    // which case the value always comes from the target itself. In other
+    // words, that group logic only kicks in once the target is matched.
+    //
+    timestamp
+    mtime () const;
+
+    // Note also that while we can cache the mtime, it may be ignored if the
+    // target state is set to group (see above).
+    //
+    void
+    mtime (timestamp) const;
+
+    // If the mtime is unknown, then load it from the filesystem also caching
+    // the result.
+    //
+    // Note: can only be called during executing and must not be used if the
+    // target state is group.
+    //
+    timestamp
+    load_mtime (const path&) const;
+
+    // Return true if this target is newer than the specified timestamp.
+    //
+    // Note: can only be called during execute on a synchronized target.
+    //
+    bool
+    newer (timestamp) const;
+
+  public:
+    static const target_type static_type;
+
+  protected:
+
+    // Complain if timestamp is not lock-free unless we were told non-lock-
+    // free is ok.
+    //
+#ifndef LIBBUILD2_ATOMIC_NON_LOCK_FREE
+    // C++17:
+    //
+    // static_assert (atomic<timestamp::rep>::is_always_lock_free,
+    //               "timestamp is not lock-free on this architecture");
+    //
+#if !defined(ATOMIC_LLONG_LOCK_FREE) || ATOMIC_LLONG_LOCK_FREE != 2
+#  error timestamp is not lock-free on this architecture
+#endif
+#endif
+
+    // Note that the value is not used to synchronize any other state so we
+    // use the release-consume ordering (i.e., we are only interested in the
+    // mtime value being synchronized).
+    //
+    // Store it as an underlying representation (normally int64_t) since
+    // timestamp is not usable with atomic (non-noexcept default ctor).
+    //
+    mutable atomic<timestamp::rep> mtime_ {timestamp_unknown_rep};
+  };
+
+  // Filesystem path-based target.
+  //
+  class LIBBUILD2_SYMEXPORT path_target: public mtime_target
+  {
+  public:
+    using mtime_target::mtime_target;
+
+    typedef build2::path path_type;
+
+    // Target path is an "atomic consistent cash". That is, it can be set at
+    // any time (including on a const instance) but any subsequent updates
+    // must set the same path. Or, in other words, once the path is set, it
+    // never changes.
+    //
+    // An empty path may signify special unknown/undetermined/unreal location
+    // (for example, a binless library or an installed import library -- we
+    // know the DLL is there, just not exactly where). In this case you would
+    // also normally set its mtime.
+    //
+    // We used to return a pointer to properly distinguish between not set and
+    // empty but that proved too tedious to work with. So now we return empty
+    // path both when not set (which will be empty_path so you can distinguish
+    // the two case if you really want to) and when set to empty. Note that
+    // this means there could be a race between path and mtime (unless you
+    // lock the target in some other way; see file_rule) so in this case it
+    // makes sense to set the timestamp first.
+    //
+    const path_type&
+    path () const;
+
+    const path_type&
+    path (path_type) const;
+
+    timestamp
+    load_mtime () const;
+
+    // Derive a path from target's dir, name, and, if set, ext. If ext is not
+    // set, try to derive it using the target type extension function and
+    // fallback to default_ext, if specified. In both cases also update the
+    // target's extension (this becomes important if later we need to reliably
+    // determine whether this file has an extension; think hxx{foo.bar.} and
+    // hxx{*}:extension is empty).
+    //
+    // If name_prefix is not NULL, add it before the name part and after the
+    // directory. Similarly, if name_suffix is not NULL, add it after the name
+    // part and before the extension.
+    //
+    // Finally, if the path was already assigned to this target, then this
+    // function verifies that the two are the same.
+    //
+    const path_type&
+    derive_path (const char* default_ext = nullptr,
+                 const char* name_prefix = nullptr,
+                 const char* name_suffix = nullptr);
+
+    // This version can be used to derive the path from another target's path
+    // by adding another extension.
+    //
+    const path_type&
+    derive_path (path_type base, const char* default_ext = nullptr);
+
+    // As above but only derives (and returns) the extension (empty means no
+    // extension used).
+    //
+    const string&
+    derive_extension (const char* default_ext = nullptr)
+    {
+      return *derive_extension (false, default_ext);
+    }
+
+    // As above but if search is true then look for the extension as if it was
+    // a prerequisite, not a target. In this case, if no extension can be
+    // derived, return NULL instead of failing (like search_existing_file()).
+    //
+    const string*
+    derive_extension (bool search, const char* default_ext = nullptr);
+
+    // Const versions of the above that can be used on unlocked targets. Note
+    // that here we don't allow providing any defaults since you probably
+    // should only use this version if everything comes from the target itself
+    // (and is therefore atomic).
+    //
+    const path_type&
+    derive_path () const
+    {
+      return const_cast<path_target*> (this)->derive_path (); // MT-aware.
+    }
+
+    const string&
+    derive_extension () const
+    {
+      return const_cast<path_target*> (this)->derive_extension (); // MT-aware.
+    }
+
+  public:
+    static const target_type static_type;
+
+  private:
+    // Note that the state is also used to synchronize the path value so
+    // we use the release-acquire ordering.
+    //
+    // 0 - absent
+    // 1 - being set
+    // 2 - present
+    //
+    mutable atomic<uint8_t> path_state_ {0};
+    mutable path_type path_;
+  };
+
+  // File target.
+  //
+  class LIBBUILD2_SYMEXPORT file: public path_target
+  {
+  public:
+    using path_target::path_target;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  // Alias target. It represents a list of targets (its prerequisites)
+  // as a single "name".
+  //
+  class LIBBUILD2_SYMEXPORT alias: public target
+  {
+  public:
+    using target::target;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  // Directory target. Note that this is not a filesystem directory
+  // but rather an alias target with the directory name. For actual
+  // filesystem directory (creation), see fsdir.
+  //
+  class LIBBUILD2_SYMEXPORT dir: public alias
+  {
+  public:
+    using alias::alias;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+
+  public:
+    template <typename K>
+    static const target*
+    search_implied (const scope&, const K&, tracer&);
+
+    // Return true if the implied buildfile is plausible for the specified
+    // subdirectory of a project with the specified root scope. That is, there
+    // is a buildfile in at least one of its subdirectories. Note that the
+    // directory must exist.
+    //
+    static bool
+    check_implied (const scope& root, const dir_path&);
+
+  private:
+    static prerequisites_type
+    collect_implied (const scope&);
+  };
+
+  // While a filesystem directory is mtime-based, the semantics is not very
+  // useful in our case. In particular, if another target depends on fsdir{},
+  // then all that's desired is the creation of the directory if it doesn't
+  // already exist. In particular, we don't want to update the target just
+  // because some unrelated entry was created in that directory.
+  //
+  class LIBBUILD2_SYMEXPORT fsdir: public target
+  {
+  public:
+    using target::target;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  // Executable file.
+  //
+  class LIBBUILD2_SYMEXPORT exe: public file
+  {
+  public:
+    using file::file;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  class LIBBUILD2_SYMEXPORT buildfile: public file
+  {
+  public:
+    using file::file;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  // Common documentation file targets.
+  //
+  class LIBBUILD2_SYMEXPORT doc: public file
+  {
+  public:
+    using file::file;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  // The problem with man pages is this: different platforms have
+  // different sets of sections. What seems to be the "sane" set
+  // is 1-9 (Linux and BSDs). SysV (e.g., Solaris) instead maps
+  // 8 to 1M (system administration). The section determines two
+  // things: the directory where the page is installed (e.g.,
+  // /usr/share/man/man1) as well as the extension of the file
+  // (e.g., test.1). Note also that there could be sub-sections,
+  // e.g., 1p (for POSIX). Such a page would still go into man1
+  // but will have the .1p extension (at least that's what happens
+  // on Linux). The challenge is to somehow handle this in a
+  // portable manner. So here is the plan:
+  //
+  // First of all, we have the man{} target type which can be used
+  // for a custom man page. That is, you can have any extension and
+  // install it anywhere you please:
+  //
+  // man{foo.X}: install = man/manX
+  //
+  // Then we have man1..9{} target types which model the "sane"
+  // section set and that would be automatically installed into
+  // correct locations on other platforms. In other words, the
+  // idea is that you should be able to have the foo.8 file,
+  // write man8{foo} and have it installed as man1m/foo.1m on
+  // some SysV host.
+  //
+  // Re-mapping the installation directory is easy: to help with
+  // that we have assigned install.man1..9 directory names. The
+  // messy part is to change the extension. It seems the only
+  // way to do that would be to have special logic for man pages
+  // in the generic install rule. @@ This is still a TODO.
+  //
+  // Note that handling subsections with man1..9{} is easy, we
+  // simply specify the extension explicitly, e.g., man{foo.1p}.
+  //
+  class LIBBUILD2_SYMEXPORT man: public doc
+  {
+  public:
+    using doc::doc;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  class LIBBUILD2_SYMEXPORT man1: public man
+  {
+  public:
+    using man::man;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  // We derive manifest from doc rather than file so that it get automatically
+  // installed into the same place where the rest of the documentation goes.
+  // If you think about it, it's kind of a documentation, similar to (but
+  // better than) the version file that many projects come with.
+  //
+  class LIBBUILD2_SYMEXPORT manifest: public doc
+  {
+  public:
+    using doc::doc;
+
+  public:
+    static const target_type static_type;
+    virtual const target_type& dynamic_type () const {return static_type;}
+  };
+
+  // Common implementation of the target factory, extension, and search
+  // functions.
+  //
+  template <typename T>
+  target*
+  target_factory (const target_type&, dir_path d, dir_path o, string n)
+  {
+    return new T (move (d), move (o), move (n));
+  }
+
+  // Return fixed target extension unless one was specified.
+  //
+  template <const char* ext>
+  const char*
+  target_extension_fix (const target_key&, const scope*);
+
+  template <const char* ext>
+  bool
+  target_pattern_fix (const target_type&, const scope&,
+                      string&, optional<string>&, const location&,
+                      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>
+  optional<string>
+  target_extension_var (const target_key&, const scope&, const char*, bool);
+
+  template <const char* var, const char* def>
+  bool
+  target_pattern_var (const target_type&, const scope&,
+                      string&, optional<string>&, const location&,
+                      bool);
+
+  // Target print functions.
+  //
+
+  // Target type uses the extension but it is fixed and there is no use
+  // printing it (e.g., man1{}).
+  //
+  LIBBUILD2_SYMEXPORT void
+  target_print_0_ext_verb (ostream&, const target_key&);
+
+  // Target type uses the extension and there is normally no default so it
+  // should be printed (e.g., file{}).
+  //
+  LIBBUILD2_SYMEXPORT void
+  target_print_1_ext_verb (ostream&, const target_key&);
+
+  // The default behavior, that is, look for an existing target in the
+  // prerequisite's directory scope.
+  //
+  LIBBUILD2_SYMEXPORT const target*
+  target_search (const target&, const prerequisite_key&);
+
+  // First look for an existing target as above. If not found, then look
+  // for an existing file in the target-type-specific list of paths.
+  //
+  LIBBUILD2_SYMEXPORT const target*
+  file_search (const target&, const prerequisite_key&);
+}
+
+#include <libbuild2/target.ixx>
+#include <libbuild2/target.txx>
+
+#endif // LIBBUILD2_TARGET_HXX