Move in build system module to separate library

8 files changed, 937 insertions, 0 deletions

diff --git a/libbuild2/in/buildfile b/libbuild2/in/buildfile
new file mode 100644
index 0000000..ae07e27
--- /dev/null
+++ b/libbuild2/in/buildfile
@@ -0,0 +1,62 @@
+# file      : libbuild2/in/buildfile
+# copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+# license   : MIT; see accompanying LICENSE file
+
+import int_libs = libbutl%lib{butl}
+
+include ../
+int_libs += ../lib{build2}
+
+./: lib{build2-in}: libul{build2-in}: {hxx ixx txx cxx}{** -**.test...} \
+  $int_libs
+
+# Unit tests.
+#
+exe{*.test}:
+{
+  test = true
+  install = false
+}
+
+for t: cxx{**.test...}
+{
+  d = $directory($t)
+  n = $name($t)...
+
+  ./: $d/exe{$n}: $t $d/{hxx ixx txx}{+$n} $d/testscript{+$n}
+  $d/exe{$n}: libul{build2-in}: bin.whole = false
+}
+
+# Build options.
+#
+obja{*}: cxx.poptions += -DLIBBUILD2_IN_STATIC_BUILD
+objs{*}: cxx.poptions += -DLIBBUILD2_IN_SHARED_BUILD
+
+# Export options.
+#
+lib{build2-in}:
+{
+  cxx.export.poptions = "-I$out_root" "-I$src_root"
+  cxx.export.libs = $int_libs
+}
+
+liba{build2-in}: cxx.export.poptions += -DLIBBUILD2_IN_STATIC
+libs{build2-in}: cxx.export.poptions += -DLIBBUILD2_IN_SHARED
+
+# For pre-releases use the complete version to make sure they cannot be used
+# in place of another pre-release or the final version. See the version module
+# for details on the version.* variable values.
+#
+if $version.pre_release
+  lib{build2-in}: bin.lib.version = @"-$version.project_id"
+else
+  lib{build2-in}: bin.lib.version = @"-$version.major.$version.minor"
+
+# Install into the libbuild2/in/ subdirectory of, say, /usr/include/
+# recreating subdirectories.
+#
+{hxx ixx txx}{*}:
+{
+  install         = include/libbuild2/in/
+  install.subdirs = true
+}
diff --git a/libbuild2/in/export.hxx b/libbuild2/in/export.hxx
new file mode 100644
index 0000000..47909e7
--- /dev/null
+++ b/libbuild2/in/export.hxx
@@ -0,0 +1,34 @@
+#pragma once
+
+// Normally we don't export class templates (but do complete specializations),
+// inline functions, and classes with only inline member functions. Exporting
+// classes that inherit from non-exported/imported bases (e.g., std::string)
+// will end up badly. The only known workarounds are to not inherit or to not
+// export. Also, MinGW GCC doesn't like seeing non-exported functions being
+// used before their inline definition. The workaround is to reorder code. In
+// the end it's all trial and error.
+
+#if defined(LIBBUILD2_IN_STATIC)         // Using static.
+#  define LIBBUILD2_IN_SYMEXPORT
+#elif defined(LIBBUILD2_IN_STATIC_BUILD) // Building static.
+#  define LIBBUILD2_IN_SYMEXPORT
+#elif defined(LIBBUILD2_IN_SHARED)       // Using shared.
+#  ifdef _WIN32
+#    define LIBBUILD2_IN_SYMEXPORT __declspec(dllimport)
+#  else
+#    define LIBBUILD2_IN_SYMEXPORT
+#  endif
+#elif defined(LIBBUILD2_IN_SHARED_BUILD) // Building shared.
+#  ifdef _WIN32
+#    define LIBBUILD2_IN_SYMEXPORT __declspec(dllexport)
+#  else
+#    define LIBBUILD2_IN_SYMEXPORT
+#  endif
+#else
+// If none of the above macros are defined, then we assume we are being used
+// by some third-party build system that cannot/doesn't signal the library
+// type. Note that this fallback works for both static and shared but in case
+// of shared will be sub-optimal compared to having dllimport.
+//
+#  define LIBBUILD2_IN_SYMEXPORT         // Using static or shared.
+#endif
diff --git a/libbuild2/in/init.cxx b/libbuild2/in/init.cxx
new file mode 100644
index 0000000..6ef996b
--- /dev/null
+++ b/libbuild2/in/init.cxx
@@ -0,0 +1,126 @@
+// file      : libbuild2/in/init.cxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#include <libbuild2/in/init.hxx>
+
+#include <libbuild2/scope.hxx>
+#include <libbuild2/context.hxx>
+#include <libbuild2/variable.hxx>
+#include <libbuild2/diagnostics.hxx>
+
+#include <libbuild2/in/rule.hxx>
+#include <libbuild2/in/target.hxx>
+
+using namespace std;
+
+namespace build2
+{
+  namespace in
+  {
+    static const rule rule_ ("in", "in");
+
+    bool
+    base_init (scope& rs,
+               scope&,
+               const location&,
+               unique_ptr<module_base>&,
+               bool first,
+               bool,
+               const variable_map&)
+    {
+      tracer trace ("in::base_init");
+      l5 ([&]{trace << "for " << rs;});
+
+      assert (first);
+
+      // Enter variables.
+      //
+      {
+        auto& vp (var_pool.rw (rs));
+
+        // Alternative variable substitution symbol with '$' being the
+        // default.
+        //
+        vp.insert<string> ("in.symbol");
+
+        // Substitution mode. Valid values are 'strict' (default) and 'lax'.
+        // In the strict mode every substitution symbol is expected to start a
+        // substitution with the double symbol (e.g., $$) serving as an escape
+        // sequence.
+        //
+        // In the lax mode a pair of substitution symbols is only treated as a
+        // substitution if what's between them looks like a build2 variable
+        // name (i.e., doesn't contain spaces, etc). Everything else,
+        // including unterminated substitution symbols, is copied as is. Note
+        // also that in this mode the double symbol is not treated as an
+        // escape sequence.
+        //
+        // The lax mode is mostly useful when trying to reuse existing .in
+        // files, for example, from autoconf. Note, however, that the lax mode
+        // is still stricter than the autoconf's semantics which also leaves
+        // unknown substitutions as is.
+        //
+        vp.insert<string> ("in.substitution");
+      }
+
+      // Register target types.
+      //
+      rs.target_types.insert<in> ();
+
+      return true;
+    }
+
+    bool
+    init (scope& rs,
+          scope& bs,
+          const location& loc,
+          unique_ptr<module_base>&,
+          bool,
+          bool,
+          const variable_map&)
+    {
+      tracer trace ("in::init");
+      l5 ([&]{trace << "for " << bs;});
+
+      // Load in.base.
+      //
+      if (!cast_false<bool> (rs["in.base.loaded"]))
+        load_module (rs, rs, "in.base", loc);
+
+      // Register rules.
+      //
+      {
+        auto& r (bs.rules);
+
+        // There are rules that are "derived" from this generic in rule in
+        // order to provide extended preprocessing functionality (see the
+        // version module for an example). To make sure they are tried first
+        // we register for path_target, not file, but in rule::match() we only
+        // match if the target is a file. A bit of a hack.
+        //
+        r.insert<path_target> (perform_update_id,   "in", rule_);
+        r.insert<path_target> (perform_clean_id,    "in", rule_);
+        r.insert<path_target> (configure_update_id, "in", rule_);
+      }
+
+      return true;
+    }
+
+    static const module_functions mod_functions[] =
+    {
+      // NOTE: don't forget to also update the documentation in init.hxx if
+      //       changing anything here.
+
+      {"in.base", nullptr, base_init},
+      {"in",      nullptr, init},
+      {nullptr,   nullptr, nullptr}
+    };
+
+    const module_functions*
+    build2_in_load ()
+    {
+      return mod_functions;
+    }
+  }
+}
diff --git a/libbuild2/in/init.hxx b/libbuild2/in/init.hxx
new file mode 100644
index 0000000..b82c77e
--- /dev/null
+++ b/libbuild2/in/init.hxx
@@ -0,0 +1,30 @@
+// file      : libbuild2/in/init.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_IN_INIT_HXX
+#define LIBBUILD2_IN_INIT_HXX
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/module.hxx>
+
+#include <libbuild2/in/export.hxx>
+
+namespace build2
+{
+  namespace in
+  {
+    // Module `in` does not require bootstrapping.
+    //
+    // Submodules:
+    //
+    // `in.base`  -- only variables and target types.
+    //
+    extern "C" LIBBUILD2_IN_SYMEXPORT const module_functions*
+    build2_in_load ();
+  }
+}
+
+#endif // LIBBUILD2_IN_INIT_HXX
diff --git a/libbuild2/in/rule.cxx b/libbuild2/in/rule.cxx
new file mode 100644
index 0000000..bdc9b24
--- /dev/null
+++ b/libbuild2/in/rule.cxx
@@ -0,0 +1,485 @@
+// file      : libbuild2/in/rule.cxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#include <libbuild2/in/rule.hxx>
+
+#include <cstdlib> // strtoull()
+
+#include <libbuild2/depdb.hxx>
+#include <libbuild2/scope.hxx>
+#include <libbuild2/target.hxx>
+#include <libbuild2/function.hxx>
+#include <libbuild2/algorithm.hxx>
+#include <libbuild2/filesystem.hxx>
+#include <libbuild2/diagnostics.hxx>
+
+#include <libbuild2/in/target.hxx>
+
+using namespace std;
+using namespace butl;
+
+namespace build2
+{
+  namespace in
+  {
+    bool rule::
+    match (action a, target& xt, const string&) const
+    {
+      tracer trace ("in::rule::match");
+
+      if (!xt.is_a<file> ()) // See module init() for details.
+        return false;
+
+      file& t (static_cast<file&> (xt));
+
+      bool fi (false); // Found in.
+      for (prerequisite_member p: group_prerequisite_members (a, t))
+      {
+        if (include (a, t, p) != include_type::normal) // Excluded/ad hoc.
+          continue;
+
+        fi = fi || p.is_a<in> ();
+      }
+
+      // Note that while normally we print these at verbosity level 4, this
+      // one gets quite noisy since we try this rule for any file target.
+      //
+      if (!fi)
+        l5 ([&]{trace << "no in file prerequisite for target " << t;});
+
+      return fi;
+    }
+
+    recipe rule::
+    apply (action a, target& xt) const
+    {
+      file& t (static_cast<file&> (xt));
+
+      // Derive the file name.
+      //
+      t.derive_path ();
+
+      // Inject dependency on the output directory.
+      //
+      inject_fsdir (a, t);
+
+      // Match prerequisite members.
+      //
+      match_prerequisite_members (a,
+                                  t,
+                                  [this] (action a,
+                                          const target& t,
+                                          const prerequisite_member& p,
+                                          include_type i)
+                                  {
+                                    return search (a, t, p, i);
+                                  });
+
+      switch (a)
+      {
+      case perform_update_id: return [this] (action a, const target& t)
+        {
+          return perform_update (a, t);
+        };
+      case perform_clean_id: return &perform_clean_depdb; // Standard clean.
+      default:               return noop_recipe;          // Configure update.
+      }
+    }
+
+    target_state rule::
+    perform_update (action a, const target& xt) const
+    {
+      tracer trace ("in::rule::perform_update");
+
+      const file& t (xt.as<const file&> ());
+      const path& tp (t.path ());
+
+      // Substitution symbol.
+      //
+      char sym (symbol_);
+      if (const string* s = cast_null<string> (t["in.symbol"]))
+      {
+        if (s->size () == 1)
+          sym = s->front ();
+        else
+          fail << "invalid substitution symbol '" << *s << "'";
+      }
+
+      // Substitution mode.
+      //
+      bool strict (strict_);
+      if (const string* s = cast_null<string> (t["in.substitution"]))
+      {
+        if (*s == "lax")
+          strict = false;
+        else if (*s != "strict")
+          fail << "invalid substitution mode '" << *s << "'";
+      }
+
+      // Determine if anything needs to be updated.
+      //
+      timestamp mt (t.load_mtime ());
+      auto pr (execute_prerequisites<in> (a, t, mt));
+
+      bool update (!pr.first);
+      target_state ts (update ? target_state::changed : *pr.first);
+
+      const in& i (pr.second);
+      const path& ip (i.path ());
+
+      // We use depdb to track changes to the .in file name, symbol/mode, and
+      // variable values that have been substituted.
+      //
+      depdb dd (tp + ".d");
+
+      // First should come the rule name/version.
+      //
+      if (dd.expect (rule_id_ + " 1") != nullptr)
+        l4 ([&]{trace << "rule mismatch forcing update of " << t;});
+
+      // Then the substitution symbol.
+      //
+      if (dd.expect (string (1, sym)) != nullptr)
+        l4 ([&]{trace << "substitution symbol mismatch forcing update of"
+                      << t;});
+
+      // Then the substitution mode.
+      //
+      if (dd.expect (strict ? "strict" : "lax") != nullptr)
+        l4 ([&]{trace << "substitution mode mismatch forcing update of"
+                      << t;});
+
+      // Then the .in file.
+      //
+      if (dd.expect (i.path ()) != nullptr)
+        l4 ([&]{trace << "in file mismatch forcing update of " << t;});
+
+      // Update if any mismatch or depdb is newer that the output.
+      //
+      if (dd.writing () || dd.mtime > mt)
+        update = true;
+
+      // Substituted variable values.
+      //
+      // The plan is to save each substituted variable name and the hash of
+      // its value one entry per line. Plus the line location of its expansion
+      // for diagnostics.
+      //
+      // If update is true (i.e., the .in file has changes), then we simply
+      // overwrite the whole list.
+      //
+      // If update is false, then we need to read each name/hash, query and
+      // hash its current value, and compare. If hashes differ, then we need
+      // to start overwriting from this variable (the prefix of variables
+      // couldn't have changed since the .in file hasn't changed).
+      //
+      // Note that if the .in file substitutes the same variable multiple
+      // times, then we will end up with multiple entries for such a variable.
+      // For now we assume this is ok since this is probably not very common
+      // and it makes the overall logic simpler.
+      //
+      // Note also that because updating the depdb essentially requires
+      // performing the substitutions, this rule ignored the dry-run mode.
+      //
+      size_t dd_skip (0); // Number of "good" variable lines.
+
+      if (update)
+      {
+        // If we are still reading, mark the next line for overwriting.
+        //
+        if (dd.reading ())
+        {
+          dd.read ();  // Read the first variable line, if any.
+          dd.write (); // Mark it for overwriting.
+        }
+      }
+      else
+      {
+        while (dd.more ())
+        {
+          if (string* s = dd.read ())
+          {
+            // The line format is:
+            //
+            // <ln> <name> <hash>
+            //
+            // Note that <name> can contain spaces (see the constraint check
+            // expressions in the version module).
+            //
+            char* e (nullptr);
+            uint64_t ln (strtoull (s->c_str (), &e, 10));
+
+            size_t p1 (*e == ' ' ? e - s->c_str () : string::npos);
+            size_t p2 (s->rfind (' '));
+
+            if (p1 != string::npos && p2 != string::npos && p2 - p1 > 1)
+            {
+              string n (*s, p1 + 1, p2 - p1 - 1);
+
+              // Note that we have to call substitute(), not lookup() since it
+              // can be overriden with custom substitution semantics.
+              //
+              optional<string> v (
+                substitute (location (&ip, ln), a, t, n, strict));
+
+              assert (v); // Rule semantics change without version increment?
+
+              if (s->compare (p2 + 1,
+                              string::npos,
+                              sha256 (*v).string ()) == 0)
+              {
+                dd_skip++;
+                continue;
+              }
+              else
+                l4 ([&]{trace << n << " variable value mismatch forcing "
+                              << "update of " << t;});
+              // Fall through.
+            }
+
+            dd.write (); // Mark this line for overwriting.
+
+            // Fall through.
+          }
+
+          break;
+        }
+      }
+
+      if (dd.writing ()) // Recheck.
+        update = true;
+
+      // If nothing changed, then we are done.
+      //
+      if (!update)
+      {
+        dd.close ();
+        return ts;
+      }
+
+      if (verb >= 2)
+        text << program_ << ' ' << ip << " >" << tp;
+      else if (verb)
+        text << program_ << ' ' << ip;
+
+      // Read and process the file, one line at a time, while updating depdb.
+      //
+      const char* what;
+      const path* whom;
+      try
+      {
+        what = "open"; whom = &ip;
+        ifdstream ifs (ip, fdopen_mode::in, ifdstream::badbit);
+
+        // See fdopen() for details (umask, etc).
+        //
+        permissions prm (permissions::ru | permissions::wu |
+                         permissions::rg | permissions::wg |
+                         permissions::ro | permissions::wo);
+
+        if (t.is_a<exe> ())
+          prm |= permissions::xu | permissions::xg | permissions::xo;
+
+        // Remove the existing file to make sure permissions take effect. If
+        // this fails then presumable writing to it will fail as well and we
+        // will complain there.
+        //
+        try_rmfile (tp, true /* ignore_error */);
+
+        what = "open"; whom = &tp;
+        ofdstream ofs (fdopen (tp,
+                               fdopen_mode::out | fdopen_mode::create,
+                               prm));
+        auto_rmfile arm (tp);
+
+        string s; // Reuse the buffer.
+        for (size_t ln (1);; ++ln)
+        {
+          what = "read"; whom = &ip;
+          if (!getline (ifs, s))
+            break; // Could not read anything, not even newline.
+
+          // Not tracking column for now (see also depdb above).
+          //
+          const location l (&ip, ln);
+
+          // Scan the line looking for substiutions in the $<name>$ form. In
+          // the strict mode treat $$ as an escape sequence.
+          //
+          for (size_t b (0), n, d; b != (n = s.size ()); b += d)
+          {
+            d = 1;
+
+            if (s[b] != sym)
+              continue;
+
+            // Note that in the lax mode these should still be substitutions:
+            //
+            // @project@@
+            // @@project@
+
+            // Find the other end.
+            //
+            size_t e (b + 1);
+            for (; e != (n = s.size ()); ++e)
+            {
+              if (s[e] == sym)
+              {
+                if (strict && e + 1 != n && s[e + 1] == sym) // Escape.
+                  s.erase (e, 1); // Keep one, erase the other.
+                else
+                  break;
+              }
+            }
+
+            if (e == n)
+            {
+              if (strict)
+                fail (l) << "unterminated '" << sym << "'" << endf;
+
+              break;
+            }
+
+            if (e - b == 1) // Escape (or just double symbol in the lax mode).
+            {
+              if (strict)
+                s.erase (b, 1); // Keep one, erase the other.
+
+              continue;
+            }
+
+            // We have a (potential, in the lax mode) substition with b
+            // pointing to the opening symbol and e -- to the closing.
+            //
+            string name (s, b + 1, e - b -1);
+            if (optional<string> val = substitute (l, a, t, name, strict))
+            {
+              // Save in depdb.
+              //
+              if (dd_skip == 0)
+              {
+                // The line format is:
+                //
+                // <ln> <name> <hash>
+                //
+                string s (to_string (ln));
+                s += ' ';
+                s += name;
+                s += ' ';
+                s += sha256 (*val).string ();
+                dd.write (s);
+              }
+              else
+                --dd_skip;
+
+              // Patch the result in and adjust the delta.
+              //
+              s.replace (b, e - b + 1, *val);
+              d = val->size ();
+            }
+            else
+              d = e - b + 1; // Ignore this substitution.
+          }
+
+          what = "write"; whom = &tp;
+          if (ln != 1)
+            ofs << '\n'; // See below.
+          ofs << s;
+        }
+
+        // Close depdb before closing the output file so its mtime is not
+        // newer than of the output.
+        //
+        dd.close ();
+
+        what = "close"; whom = &tp;
+        ofs << '\n'; // Last write to make sure our mtime is older than dd.
+        ofs.close ();
+        arm.cancel ();
+
+        what = "close"; whom = &ip;
+        ifs.close ();
+      }
+      catch (const io_error& e)
+      {
+        fail << "unable to " << what << ' ' << *whom << ": " << e;
+      }
+
+      dd.check_mtime (tp);
+
+      t.mtime (system_clock::now ());
+      return target_state::changed;
+    }
+
+    prerequisite_target rule::
+    search (action,
+            const target& t,
+            const prerequisite_member& p,
+            include_type i) const
+    {
+      return prerequisite_target (&build2::search (t, p), i);
+    }
+
+    string rule::
+    lookup (const location& l, action, const target& t, const string& n) const
+    {
+      if (auto x = t[n])
+      {
+        value v (*x);
+
+        // For typed values call string() for conversion.
+        //
+        try
+        {
+          return convert<string> (
+            v.type == nullptr
+            ? move (v)
+            : functions.call (&t.base_scope (),
+                              "string",
+                              vector_view<value> (&v, 1),
+                              l));
+        }
+        catch (const invalid_argument& e)
+        {
+          fail (l) << e <<
+            info << "while substituting '" << n << "'" << endf;
+        }
+      }
+      else
+        fail (l) << "undefined variable '" << n << "'" << endf;
+    }
+
+    optional<string> rule::
+    substitute (const location& l,
+                action a,
+                const target& t,
+                const string& n,
+                bool strict) const
+    {
+      // In the lax mode scan the fragment to make sure it is a variable name
+      // (that is, it can be expanded in a buildfile as just $<name>; see
+      // lexer's variable mode for details).
+      //
+      if (!strict)
+      {
+        for (size_t i (0), e (n.size ()); i != e; )
+        {
+          bool f (i == 0); // First.
+          char c (n[i++]);
+          bool l (i == e); // Last.
+
+          if (c == '_' || (f ? alpha (c) : alnum (c)))
+            continue;
+
+          if (c == '.' && !l)
+            continue;
+
+          return nullopt; // Ignore this substitution.
+        }
+      }
+
+      return lookup (l, a, t, n);
+    }
+  }
+}
diff --git a/libbuild2/in/rule.hxx b/libbuild2/in/rule.hxx
new file mode 100644
index 0000000..0daf6c0
--- /dev/null
+++ b/libbuild2/in/rule.hxx
@@ -0,0 +1,90 @@
+// file      : libbuild2/in/rule.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_IN_RULE_HXX
+#define LIBBUILD2_IN_RULE_HXX
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/rule.hxx>
+
+#include <libbuild2/in/export.hxx>
+
+namespace build2
+{
+  namespace in
+  {
+    // Preprocess an .in file.
+    //
+    // Note that a derived rule can use the target data pad to cache data
+    // (e.g., in match()) to be used in substitute/lookup() calls.
+    //
+    // Note also that currently this rule ignores the dry-run mode (see
+    // perform_update() for the rationale).
+    //
+    class LIBBUILD2_IN_SYMEXPORT rule: public build2::rule
+    {
+    public:
+      // The rule id is used to form the rule name/version entry in depdb. The
+      // program argument is the pseudo-program name to use in the command
+      // line diagnostics.
+      //
+      rule (string rule_id,
+            string program,
+            char symbol = '$',
+            bool strict = true)
+          : rule_id_ (move (rule_id)),
+            program_ (move (program)),
+            symbol_ (symbol),
+            strict_ (strict) {}
+
+      virtual bool
+      match (action, target&, const string&) const override;
+
+      virtual recipe
+      apply (action, target&) const override;
+
+      virtual target_state
+      perform_update (action, const target&) const;
+
+      // Customization hooks.
+      //
+
+      // Perform prerequisite search.
+      //
+      virtual prerequisite_target
+      search (action,
+              const target&,
+              const prerequisite_member&,
+              include_type) const;
+
+      // Perform variable lookup.
+      //
+      virtual string
+      lookup (const location&,
+              action,
+              const target&,
+              const string& name) const;
+
+      // Perform variable substitution. Return nullopt if it should be
+      // ignored.
+      //
+      virtual optional<string>
+      substitute (const location&,
+                  action,
+                  const target&,
+                  const string& name,
+                  bool strict) const;
+
+    protected:
+      const string rule_id_;
+      const string program_;
+      char symbol_;
+      bool strict_;
+    };
+  }
+}
+
+#endif // LIBBUILD2_IN_RULE_HXX
diff --git a/libbuild2/in/target.cxx b/libbuild2/in/target.cxx
new file mode 100644
index 0000000..490ab10
--- /dev/null
+++ b/libbuild2/in/target.cxx
@@ -0,0 +1,62 @@
+// file      : libbuild2/in/target.cxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#include <libbuild2/in/target.hxx>
+
+using namespace std;
+
+namespace build2
+{
+  namespace in
+  {
+    static const target*
+    in_search (const target& xt, const prerequisite_key& cpk)
+    {
+      // If we have no extension then derive it from our target. Then delegate
+      // to file_search().
+      //
+      prerequisite_key pk (cpk);
+      optional<string>& e (pk.tk.ext);
+
+      if (!e)
+      {
+        if (const file* t = xt.is_a<file> ())
+        {
+          const string& te (t->derive_extension ());
+          e = te + (te.empty () ? "" : ".") + "in";
+        }
+        else
+          fail << "prerequisite " << pk << " for a non-file target " << xt;
+      }
+
+      return file_search (xt, pk);
+    }
+
+    static bool
+    in_pattern (const target_type&,
+                const scope&,
+                string&,
+                optional<string>&,
+                const location& l,
+                bool)
+    {
+      fail (l) << "pattern in in{} prerequisite" << endf;
+    }
+
+    extern const char in_ext_def[] = ""; // No extension by default.
+
+    const target_type in::static_type
+    {
+      "in",
+      &file::static_type,
+      &target_factory<in>,
+      &target_extension_fix<in_ext_def>,
+      nullptr, /* default_extension */   // Taken care of by search.
+      &in_pattern,
+      &target_print_1_ext_verb,          // Same as file.
+      &in_search,
+      false
+    };
+  }
+}
diff --git a/libbuild2/in/target.hxx b/libbuild2/in/target.hxx
new file mode 100644
index 0000000..5e3438d
--- /dev/null
+++ b/libbuild2/in/target.hxx
@@ -0,0 +1,48 @@
+// file      : libbuild2/in/target.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_IN_TARGET_HXX
+#define LIBBUILD2_IN_TARGET_HXX
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/target.hxx>
+
+#include <libbuild2/in/export.hxx>
+
+namespace build2
+{
+  namespace in
+  {
+    // This is the venerable .in ("input") file that needs some kind of
+    // preprocessing.
+    //
+    // One interesting aspect of this target type is that the prerequisite
+    // search is target-dependent. Consider:
+    //
+    // hxx{version}: in{version.hxx} // version.hxx.in -> version.hxx
+    //
+    // Having to specify the header extension explicitly is inelegant. Instead
+    // what we really want to write is this:
+    //
+    // hxx{version}: in{version}
+    //
+    // But how do we know that in{version} means version.hxx.in? That's where
+    // the target-dependent search comes in: we take into account the target
+    // we are a prerequisite of.
+    //
+    class LIBBUILD2_IN_SYMEXPORT in: public file
+    {
+    public:
+      using file::file;
+
+    public:
+      static const target_type static_type;
+      virtual const target_type& dynamic_type () const {return static_type;}
+    };
+  }
+}
+
+#endif // LIBBUILD2_IN_TARGET_HXX