Diagnostic infrastructure revamp

20 files changed, 650 insertions, 426 deletions

diff --git a/build/algorithm.cxx b/build/algorithm.cxx
index af0e3f3..616d21e 100644
--- a/build/algorithm.cxx
+++ b/build/algorithm.cxx
@@ -7,13 +7,13 @@
 #include <memory>   // unique_ptr
 #include <utility>  // move
 #include <cassert>
-#include <iostream>
 
 #include <build/path>
 #include <build/scope>
 #include <build/target>
 #include <build/prerequisite>
 #include <build/rule>
+#include <build/utility>
 #include <build/diagnostics>
 
 using namespace std;
@@ -23,7 +23,7 @@ namespace build
   target&
   search (prerequisite& p)
   {
-    tracer tr ("search");
+    tracer trace ("search");
 
     assert (p.target == nullptr);
 
@@ -40,11 +40,10 @@ namespace build
 
     // Find or insert.
     //
-    auto r (targets.insert (p.type, move (d), p.name, p.ext, tr));
+    auto r (targets.insert (p.type, move (d), p.name, p.ext, trace));
 
-    trace (4, [&]{
-        tr << (r.second ? "new" : "existing") << " target " << r.first
-           << " for prerequsite " << p;});
+    level4 ([&]{trace << (r.second ? "new" : "existing") << " target "
+                      << r.first << " for prerequsite " << p;});
 
     p.target = &r.first;
     return r.first;
@@ -81,10 +80,9 @@ namespace build
         {
           auto g (
             make_exception_guard (
-              [] (target& t, const string& n)
+              [](target& t, const string& n)
               {
-                cerr << "info: while matching rule " << n
-                     << " for target " << t << endl;
+                info << "while matching rule " << n << " for target " << t;
               },
               t, n));
 
@@ -97,6 +95,8 @@ namespace build
           //
           bool ambig (false);
 
+          diag_record dr;
+
           for (++i; i != rs.second; ++i)
           {
             const string& n1 (i->first);
@@ -106,10 +106,10 @@ namespace build
             {
               auto g (
                 make_exception_guard (
-                  [] (target& t, const string& n1)
+                  [](target& t, const string& n1)
                   {
-                    cerr << "info: while matching rule " << n1
-                         << " for target " << t << endl;
+                    info << "while matching rule " << n1 << " for target "
+                         << t;
                   },
                   t, n1));
 
@@ -120,23 +120,22 @@ namespace build
             {
               if (!ambig)
               {
-                cerr << "error: multiple rules matching target " << t << endl;
-                cerr << "info: rule " << n << " matches" << endl;
+                dr << fail << "multiple rules matching target " << t
+                   << info << "rule " << n << " matches";
                 ambig = true;
               }
 
-              cerr << "info: rule " << n1 << " also matches" << endl;
+              dr << info << "rule " << n1 << " also matches";
             }
           }
 
-          if (ambig)
+          if (!ambig)
           {
-            cerr << "info: use rule hint to disambiguate this match" << endl;
-            throw error ();
+            t.recipe (ru.select (t, m));
+            break;
           }
-
-          t.recipe (ru.select (t, m));
-          break;
+          else
+            dr << info << "use rule hint to disambiguate this match";
         }
       }
     }
diff --git a/build/b.cxx b/build/b.cxx
index 14b23fc..3d19131 100644
--- a/build/b.cxx
+++ b/build/b.cxx
@@ -13,7 +13,7 @@
 #include <vector>
 #include <cassert>
 #include <fstream>
-#include <iostream>
+#include <iostream>  //@@ TMP, for dump()
 #include <typeinfo>
 #include <system_error>
 
@@ -25,6 +25,7 @@
 #include <build/process>
 #include <build/diagnostics>
 #include <build/context>
+#include <build/utility>
 
 #include <build/lexer>
 #include <build/parser>
@@ -44,7 +45,7 @@ namespace build
     {
       if (!match (t))
       {
-        cerr << "error: no rule to update target " << t << endl;
+        error << "no rule to update target " << t;
         return false;
       }
     }
@@ -60,7 +61,7 @@ namespace build
 
       if (!match_recursive (*p.target))
       {
-        cerr << "info: required by " << t << endl;
+        info << "required by " << t;
         return false;
       }
     }
@@ -73,7 +74,10 @@ namespace build
   {
     assert (t.state () == target_state::unknown);
 
-    target_state ts;
+    auto g (
+      make_exception_guard (
+        [](target& t){info << "while building target " << t;},
+        t));
 
     for (prerequisite& p: t.prerequisites)
     {
@@ -81,26 +85,19 @@ namespace build
 
       if (pt.state () == target_state::unknown)
       {
-        pt.state ((ts = update (pt)));
+        target_state ts (update (pt));
 
         if (ts == target_state::failed)
           return ts;
       }
     }
 
+    // @@ Why do we indicate failure via code rather than throw? Now
+    //    there is no diagnostics via exception_guard above.
+
     const recipe& r (t.recipe ());
 
-    {
-      auto g (
-        make_exception_guard (
-          [] (target& t)
-          {
-            cerr << "info: while building target " << t << endl;
-          },
-          t));
-
-      ts = r (t);
-    }
+    target_state ts (r (t));
 
     assert (ts != target_state::unknown);
     t.state (ts);
@@ -142,142 +139,128 @@ using namespace build;
 int
 main (int argc, char* argv[])
 {
-  tracer tr ("main");
-
-  // Initialize time conversion data that is used by localtime_r().
-  //
-  tzset ();
+  try
+  {
+    tracer trace ("main");
 
-  // Trace verbosity.
-  //
-  verb = 5;
+    // Initialize time conversion data that is used by localtime_r().
+    //
+    tzset ();
 
-  // Register target types.
-  //
-  target_types.insert (file::static_type);
+    // Trace verbosity.
+    //
+    verb = 5;
 
-  target_types.insert (exe::static_type);
-  target_types.insert (obj::static_type);
+    // Register target types.
+    //
+    target_types.insert (file::static_type);
 
-  target_types.insert (cxx::h::static_type);
-  target_types.insert (cxx::c::static_type);
+    target_types.insert (exe::static_type);
+    target_types.insert (obj::static_type);
 
-  target_types.insert (cxx::cxx::static_type);
-  target_types.insert (cxx::hxx::static_type);
-  target_types.insert (cxx::ixx::static_type);
-  target_types.insert (cxx::txx::static_type);
+    target_types.insert (cxx::h::static_type);
+    target_types.insert (cxx::c::static_type);
 
-  // Figure out directories: work, home, and {src,out}_{root,base}.
-  //
-  work = path::current ();
+    target_types.insert (cxx::cxx::static_type);
+    target_types.insert (cxx::hxx::static_type);
+    target_types.insert (cxx::ixx::static_type);
+    target_types.insert (cxx::txx::static_type);
 
-  if (const char* h = getenv ("HOME"))
-    home = path (h);
-  else
-  {
-    struct passwd* pw (getpwuid (getuid ()));
+    // Figure out directories: work, home, and {src,out}_{root,base}.
+    //
+    work = path::current ();
 
-    if (pw == nullptr)
+    if (const char* h = getenv ("HOME"))
+      home = path (h);
+    else
     {
-      const char* msg (strerror (errno));
-      cerr << "error: unable to determine home directory: " << msg << endl;
-      return 1;
-    }
+      struct passwd* pw (getpwuid (getuid ()));
 
-    home = path (pw->pw_dir);
-  }
+      if (pw == nullptr)
+      {
+        const char* msg (strerror (errno));
+        fail << "unable to determine home directory: " << msg;
+      }
+
+      home = path (pw->pw_dir);
+    }
 
-  //@@ Must be normalized.
-  //
-  out_base = work;
-  src_base = out_base;
+    //@@ Must be normalized.
+    //
+    out_base = work;
+    src_base = out_base;
 
-  // The project's root directory is the one that contains the build/
-  // sub-directory which contains the pre.build file.
-  //
-  for (path d (src_base); !d.root () && d != home; d = d.directory ())
-  {
-    path f (d / path ("build/pre.build"));
-    if (path_mtime (f) != timestamp_nonexistent)
+    // The project's root directory is the one that contains the build/
+    // sub-directory which contains the pre.build file.
+    //
+    for (path d (src_base); !d.root () && d != home; d = d.directory ())
     {
-      src_root = d;
-      break;
+      path f (d / path ("build/pre.build"));
+      if (path_mtime (f) != timestamp_nonexistent)
+      {
+        src_root = d;
+        break;
+      }
     }
-  }
 
-  if (src_root.empty ())
-  {
-    src_root = src_base;
-    out_root = out_base;
-  }
-  else
-    out_root = out_base.directory (src_base.leaf (src_root));
+    if (src_root.empty ())
+    {
+      src_root = src_base;
+      out_root = out_base;
+    }
+    else
+      out_root = out_base.directory (src_base.leaf (src_root));
 
-  if (verb >= 4)
-  {
-    tr << "work dir: " << work.string ();
-    tr << "home dir: " << home.string ();
-    tr << "out_base: " << out_base.string ();
-    tr << "src_base: " << src_base.string ();
-    tr << "out_root: " << out_root.string ();
-    tr << "src_root: " << src_root.string ();
-  }
+    if (verb >= 4)
+    {
+      trace << "work dir: " << work.string ();
+      trace << "home dir: " << home.string ();
+      trace << "out_base: " << out_base.string ();
+      trace << "src_base: " << src_base.string ();
+      trace << "out_root: " << out_root.string ();
+      trace << "src_root: " << src_root.string ();
+    }
 
-  // Parse buildfile.
-  //
-  path bf ("buildfile");
+    // Parse buildfile.
+    //
+    path bf ("buildfile");
 
-  ifstream ifs (bf.string ().c_str ());
-  if (!ifs.is_open ())
-  {
-    cerr << "error: unable to open " << bf << " in read mode" << endl;
-    return 1;
-  }
+    ifstream ifs (bf.string ().c_str ());
+    if (!ifs.is_open ())
+      fail << "unable to open " << bf;
 
-  ifs.exceptions (ifstream::failbit | ifstream::badbit);
-  parser p (cerr);
+    ifs.exceptions (ifstream::failbit | ifstream::badbit);
+    parser p;
 
-  try
-  {
-    p.parse (ifs, bf, scopes[path::current ()]);
-  }
-  catch (const lexer_error&)
-  {
-    return 1; // Diagnostics has already been issued.
-  }
-  catch (const parser_error&)
-  {
-    return 1; // Diagnostics has already been issued.
-  }
-  catch (const std::ios_base::failure&)
-  {
-    cerr << "error: failed to read from " << bf << endl;
-    return 1;
-  }
+    try
+    {
+      p.parse (ifs, bf, scopes[path::current ()]);
+    }
+    catch (const std::ios_base::failure&)
+    {
+      fail << "failed to read from " << bf;
+    }
 
-  dump ();
+    dump ();
 
-  // Register rules.
-  //
-  cxx::link cxx_link;
-  rules[typeid (exe)].emplace ("cxx.gnu.link", cxx_link);
+    // Register rules.
+    //
+    cxx::link cxx_link;
+    rules[typeid (exe)].emplace ("cxx.gnu.link", cxx_link);
 
-  cxx::compile cxx_compile;
-  rules[typeid (obj)].emplace ("cxx.gnu.compile", cxx_compile);
+    cxx::compile cxx_compile;
+    rules[typeid (obj)].emplace ("cxx.gnu.compile", cxx_compile);
 
-  default_path_rule path_exists;
-  rules[typeid (path_target)].emplace ("", path_exists);
+    default_path_rule path_exists;
+    rules[typeid (path_target)].emplace ("", path_exists);
 
-  // Build.
-  //
-  if (default_target == nullptr)
-  {
-    cerr << "error: no default target" << endl;
-    return 1;
-  }
+    // Build.
+    //
+    if (default_target == nullptr)
+    {
+      fail << "no default target";
+    }
 
-  try
-  {
     target& d (*default_target);
 
     if (!match_recursive (d))
@@ -289,27 +272,26 @@ main (int argc, char* argv[])
     {
     case target_state::uptodate:
       {
-        cerr << "info: target " << d << " is up to date" << endl;
+        info << "target " << d << " is up to date";
         break;
       }
     case target_state::updated:
       break;
     case target_state::failed:
       {
-        cerr << "error: failed to update target " << d << endl;
-        return 1;
+        fail << "failed to update target " << d;
       }
     case target_state::unknown:
       assert (false);
     }
   }
-  catch (const error&)
+  catch (const failed&)
   {
     return 1; // Diagnostics has already been issued.
   }
   catch (const std::exception& e)
   {
-    cerr << "error: " << e.what () << endl;
+    error << e.what ();
     return 1;
   }
 }
diff --git a/build/buildfile b/build/buildfile
index 3575baa..819f2e4 100644
--- a/build/buildfile
+++ b/build/buildfile
@@ -1,4 +1,3 @@
-exe{b1}: cxx{b algorithm scope parser lexer trace target prerequisite rule \
+exe{b1}: cxx{b algorithm scope parser lexer target prerequisite rule \
   native context diagnostics cxx/target cxx/rule process timestamp path \
   utility}
-
diff --git a/build/cxx/rule.cxx b/build/cxx/rule.cxx
index c9dd7d6..8d2c0b2 100644
--- a/build/cxx/rule.cxx
+++ b/build/cxx/rule.cxx
@@ -9,7 +9,6 @@
 #include <cstddef>  // size_t
 #include <cstdlib>  // exit
 #include <utility>  // move()
-#include <iostream>
 
 #include <ext/stdio_filebuf.h>
 
@@ -31,7 +30,7 @@ namespace build
     void* compile::
     match (target& t, const string&) const
     {
-      tracer tr ("cxx::compile::match");
+      tracer trace ("cxx::compile::match");
 
       // @@ TODO:
       //
@@ -57,7 +56,7 @@ namespace build
           return &p;
       }
 
-      trace (3, [&]{tr << "no c++ source file for target " << t;});
+      level3 ([&]{trace << "no c++ source file for target " << t;});
       return nullptr;
     }
 
@@ -139,7 +138,7 @@ namespace build
     void compile::
     inject_prerequisites (obj& o, const cxx& s, scope& ds) const
     {
-      tracer tr ("cxx::compile::inject_prerequisites");
+      tracer trace ("cxx::compile::inject_prerequisites");
 
       // We are using absolute source file path in order to get
       // absolute paths in the result.
@@ -157,8 +156,7 @@ namespace build
       if (verb >= 2)
         print_process (args);
 
-      if (verb >= 5)
-        tr << "target: " << o;
+      level5 ([&]{trace << "target: " << o;});
 
       try
       {
@@ -173,10 +171,7 @@ namespace build
           getline (is, l);
 
           if (is.fail () && !is.eof ())
-          {
-            cerr << "error: io error while parsing g++ -M output" << endl;
-            throw error ();
-          }
+            fail << "io error while parsing g++ -M output";
 
           size_t pos (0);
 
@@ -199,8 +194,7 @@ namespace build
             path file (next (l, pos));
             file.normalize ();
 
-            if (verb >= 5)
-              tr << "prerequisite path: " << file.string ();
+            level5 ([&]{trace << "prerequisite path: " << file.string ();});
 
             // If there is no extension (e.g., standard C++ headers),
             // then assume it is a header. Otherwise, let the standard
@@ -223,7 +217,7 @@ namespace build
             //
             prerequisite& p (
               ds.prerequisites.insert (
-                hxx::static_type, move (d), move (n), e, ds, tr).first);
+                hxx::static_type, move (d), move (n), e, ds, trace).first);
 
             // Resolve to target so that we can assign its path.
             //
@@ -241,12 +235,11 @@ namespace build
         // We assume the child process issued some diagnostics.
         //
         if (!pr.wait ())
-          throw error ();
+          throw failed ();
       }
       catch (const process_error& e)
       {
-        cerr << "error: unable to execute '" << args[0] << "': " <<
-          e.what () << endl;
+        error << "unable to execute " << args[0] << ": " << e.what ();
 
         // In a multi-threaded program that fork()'ed but did not exec(),
         // it is unwise to try to do any kind of cleanup (like unwinding
@@ -255,7 +248,7 @@ namespace build
         if (e.child ())
           exit (1);
 
-        throw error ();
+        throw failed ();
       }
     }
 
@@ -319,7 +312,7 @@ namespace build
       if (verb >= 1)
         print_process (args);
       else
-        cerr << "c++ " << *s << endl;
+        text << "c++ " << *s;
 
       try
       {
@@ -338,8 +331,7 @@ namespace build
       }
       catch (const process_error& e)
       {
-        cerr << "error: unable to execute '" << args[0] << "': " <<
-          e.what () << endl;
+        error << "unable to execute " << args[0] << ": " << e.what ();
 
         // In a multi-threaded program that fork()'ed but did not exec(),
         // it is unwise to try to do any kind of cleanup (like unwinding
@@ -357,7 +349,7 @@ namespace build
     void* link::
     match (target& t, const string& hint) const
     {
-      tracer tr ("cxx::link::match");
+      tracer trace ("cxx::link::match");
 
       // @@ TODO:
       //
@@ -395,7 +387,7 @@ namespace build
         }
         else
         {
-          trace (3, [&]{tr << "unexpected prerequisite type " << p.type;});
+          level3 ([&]{trace << "unexpected prerequisite type " << p.type;});
           return nullptr;
         }
       }
@@ -405,7 +397,7 @@ namespace build
       //
       if (seen_c && !seen_cxx && hint < "cxx")
       {
-        trace (3, [&]{tr << "c prerequisite(s) without c++ or hint";});
+        level3 ([&]{trace << "c prerequisite(s) without c++ or hint";});
         return nullptr;
       }
 
@@ -415,7 +407,7 @@ namespace build
     recipe link::
     select (target& t, void*) const
     {
-      tracer tr ("cxx::link::select");
+      tracer trace ("cxx::link::select");
 
       // Derive executable file name from target name.
       //
@@ -449,10 +441,8 @@ namespace build
         {
           if (!cp.directory.sub (src_root))
           {
-            cerr << "error: out of project prerequisite " << cp << endl;
-            cerr << "info: specify corresponding obj{} target explicitly"
-                 << endl;
-            throw error ();
+            fail << "out of project prerequisite " << cp <<
+              info << "specify corresponding obj{} target explicitly";
           }
 
           d = out_root / cp.directory.leaf (src_root);
@@ -460,7 +450,12 @@ namespace build
 
         prerequisite& op (
           cp.scope.prerequisites.insert (
-            obj::static_type, move (d), cp.name, nullptr, cp.scope, tr).first);
+            obj::static_type,
+            move (d),
+            cp.name,
+            nullptr,
+            cp.scope,
+            trace).first);
 
         // Resolve this prerequisite to target.
         //
@@ -495,20 +490,18 @@ namespace build
             }
           }
 
-          cerr << "error: synthesized target for prerequisite " << cp
-               << " would be incompatible with existing target " << ot
-               << endl;
+          diag_record r;
+
+          r << fail << "synthesized target for prerequisite " << cp
+            << " would be incompatible with existing target " << ot;
 
           if (p.type.id == typeid (cxx))
-            cerr << "info: existing prerequsite " << p << " does not "
-                 << "match " << cp << endl;
+            r << info << "existing prerequsite " << p << " does not "
+              << "match " << cp;
           else
-            cerr << "info: unknown existing prerequsite " << p << endl;
-
-          cerr << "info: specify corresponding obj{} target explicitly"
-               << endl;
+            r << info << "unknown existing prerequsite " << p;
 
-          throw error ();
+          r << info << "specify corresponding obj{} target explicitly";
         }
 
         if (add)
@@ -583,7 +576,7 @@ namespace build
       if (verb >= 1)
         print_process (args);
       else
-        cerr << "ld " << e << endl;
+        text << "ld " << e;
 
       try
       {
@@ -602,8 +595,7 @@ namespace build
       }
       catch (const process_error& e)
       {
-        cerr << "error: unable to execute '" << args[0] << "': " <<
-          e.what () << endl;
+        error << "unable to execute " << args[0] << ": " << e.what ();
 
         // In a multi-threaded program that fork()'ed but did not exec(),
         // it is unwise to try to do any kind of cleanup (like unwinding
diff --git a/build/diagnostics b/build/diagnostics
index 98f481f..f85d118 100644
--- a/build/diagnostics
+++ b/build/diagnostics
@@ -5,20 +5,17 @@
 #ifndef BUILD_DIAGNOSTICS
 #define BUILD_DIAGNOSTICS
 
-#include <tuple>
 #include <vector>
+#include <cstdint>
 #include <utility>
+#include <cassert>
+#include <sstream>
+#include <ostream>
 #include <exception>
-
-#include <build/trace>
+#include <type_traits>
 
 namespace build
 {
-  // Throw this exception to terminate the build. The handler should
-  // assume that the diagnostics has already been issued.
-  //
-  class error: public std::exception {};
-
   // Print process commmand line.
   //
   void
@@ -30,39 +27,296 @@ namespace build
     print_process (args.data ());
   }
 
-  // Call a function if there is an exception.
+  // Throw this exception to terminate the build. The handler should
+  // assume that the diagnostics has already been issued.
+  //
+  class failed: public std::exception {};
+
+  // Trace verbosity level.
   //
-  template <typename F, typename T>
-  struct exception_guard;
+  // 1 - command lines to update explicit targets (e.g., .o)
+  // 2 - command lines to update implicit targets (e.g., .d)
+  // 3 - things didn't work out (e.g., rule did not match)
+  // 4 - additional information
+  // 5 - more additional information
+  //
+  extern std::uint8_t verb;
+
+  template <typename F> inline void level1 (const F& f) {if (verb >= 1) f ();}
+  template <typename F> inline void level2 (const F& f) {if (verb >= 2) f ();}
+  template <typename F> inline void level3 (const F& f) {if (verb >= 3) f ();}
+  template <typename F> inline void level4 (const F& f) {if (verb >= 4) f ();}
+  template <typename F> inline void level5 (const F& f) {if (verb >= 5) f ();}
+
+  // Diagnostic facility, base infrastructure (potentially reusable).
+  //
+  extern std::ostream* diag_stream;
+
+  template <typename> struct diag_prologue;
+  template <typename> struct diag_mark;
 
-  template <typename F, typename... A>
-  inline exception_guard<F, std::tuple<A&&...>>
-  make_exception_guard (F f, A&&... a)
+  struct diag_record;
+
+  typedef void (*diag_epilogue) (const diag_record&);
+
+  struct diag_record
   {
-    return exception_guard<F, std::tuple<A&&...>> (
-      std::move (f), std::forward_as_tuple (a...));
-  }
+    template <typename T>
+    friend const diag_record&
+    operator<< (const diag_record& r, const T& x)
+    {
+      r.os_ << x;
+      return r;
+    }
+
+    diag_record (): empty_ (true), epilogue_ (nullptr) {}
+
+    template <typename B>
+    explicit
+    diag_record (const diag_prologue<B>& p)
+        : empty_ (true), epilogue_ (nullptr) { *this << p;}
+
+    template <typename B>
+    explicit
+    diag_record (const diag_mark<B>& m)
+        : empty_ (true), epilogue_ (nullptr) { *this << m;}
+
+    ~diag_record () noexcept(false);
+
+    void
+    append (diag_epilogue e) const
+    {
+      if (e != nullptr)
+      {
+        assert (epilogue_ == nullptr); // No multiple epilogues support.
+        epilogue_ = e;
+      }
+
+      if (empty_)
+        empty_ = false;
+      else
+        os_ << "\n  ";
+    }
+
+    // Move constructible-only type.
+    //
+    /*
+    @@ libstdc++ doesn't yet have the ostringstream move support.
+
+    diag_record (diag_record&& r)
+        : os_ (std::move (r.os_))
+    {
+      empty_ = r.empty_;
+      r.empty_ = true;
+
+      epilogue_ = r.epilogue_;
+      r.epilogue_ = nullptr;
+    }
+    */
+
+    diag_record (diag_record&& r)
+    {
+      empty_ = r.empty_;
+      epilogue_ = r.epilogue_;
+
+      if (!empty_)
+      {
+        os_ << r.os_.str ();
+
+        r.empty_ = true;
+        r.epilogue_ = nullptr;
+      }
+    }
+
+    diag_record& operator= (diag_record&&) = delete;
+
+    diag_record (const diag_record&) = delete;
+    diag_record& operator= (const diag_record&) = delete;
+
+  private:
+    mutable bool empty_;
+    mutable std::ostringstream os_;
+    mutable diag_epilogue epilogue_;
+  };
+
+  template <typename B>
+  struct diag_prologue: B
+  {
+    diag_prologue (diag_epilogue e = nullptr): B (), epilogue_ (e) {}
+
+    template <typename... A>
+    diag_prologue (A&&... a)
+        : B (std::forward<A> (a)...), epilogue_ (nullptr) {}
+
+    template <typename... A>
+    diag_prologue (diag_epilogue e, A&&... a)
+        : B (std::forward<A> (a)...), epilogue_ (e) {}
+
+    template <typename T>
+    diag_record
+    operator<< (const T& x) const
+    {
+      diag_record r;
+      r.append (epilogue_);
+      B::operator() (r);
+      r << x;
+      return r;
+    }
+
+    friend const diag_record&
+    operator<< (const diag_record& r, const diag_prologue& p)
+    {
+      r.append (p.epilogue_);
+      p (r);
+      return r;
+    }
+
+  private:
+    diag_epilogue epilogue_;
+  };
 
-  template <typename F, typename... A>
-  struct exception_guard<F, std::tuple<A...>>
+  template <typename B>
+  struct diag_mark: B
   {
-    typedef std::tuple<A...> T;
+    diag_mark (): B () {}
 
-    exception_guard (F f, T a): f_ (std::move (f)), a_ (std::move (a)) {}
-    ~exception_guard ()
+    template <typename... A>
+    diag_mark (A&&... a): B (std::forward<A> (a)...) {}
+
+    template <typename T>
+    diag_record
+    operator<< (const T& x) const
     {
-      if (std::uncaught_exception ())
-        call (std::index_sequence_for<A...> ());
+      return B::operator() () << x;
     }
 
+    friend const diag_record&
+    operator<< (const diag_record& r, const diag_mark& m)
+    {
+      return r << m ();
+    }
+  };
+
+  // Diagnostic facility, project specifics.
+  //
+  struct simple_prologue_base
+  {
+    explicit
+    simple_prologue_base (const char* type, const char* name)
+        : type_ (type), name_ (name) {}
+
+    void
+    operator() (const diag_record& r) const;
+
   private:
-    template <std::size_t... I>
+    const char* type_;
+    const char* name_;
+  };
+  typedef diag_prologue<simple_prologue_base> simple_prologue;
+
+  struct location
+  {
+    location () {}
+    location (const char* f, std::uint64_t l, std::uint64_t c)
+        : file (f), line (l), column (c) {}
+
+    const char* file;
+    std::uint64_t line;
+    std::uint64_t column;
+  };
+
+  struct location_prologue_base
+  {
+    location_prologue_base (const char* type,
+                            const char* name,
+                            const location& l)
+        : type_ (type), name_ (name), loc_ (l) {}
+
     void
-    call (std::index_sequence<I...>) {f_ (std::get<I> (a_)...);}
+    operator() (const diag_record& r) const;
+
+  private:
+    const char* type_;
+    const char* name_;
+    const location& loc_;
+  };
+  typedef diag_prologue<location_prologue_base> location_prologue;
+
+  struct basic_mark_base
+  {
+    explicit
+    basic_mark_base (const char* type, const char* name = nullptr)
+        : type_ (type), name_ (name) {}
+
+    simple_prologue
+    operator() () const
+    {
+      return simple_prologue (type_, name_);
+    }
+
+    location_prologue
+    operator() (const location& l) const
+    {
+      return location_prologue (type_, name_, l);
+    }
+
+    template <typename L>
+    location_prologue
+    operator() (const L& l) const
+    {
+      return location_prologue (type_, name_, get_location (l));
+    }
 
-    F f_;
-    T a_;
+  private:
+    const char* type_;
+    const char* name_;
   };
+  typedef diag_mark<basic_mark_base> basic_mark;
+
+  extern const basic_mark error;
+  extern const basic_mark warn;
+  extern const basic_mark info;
+  extern const basic_mark text;
+
+  struct trace_mark_base: basic_mark_base
+  {
+    explicit
+    trace_mark_base (const char* name): basic_mark_base ("trace", name) {}
+  };
+  typedef diag_mark<trace_mark_base> trace_mark;
+
+  typedef trace_mark tracer;
+
+  template <typename E>
+  struct fail_mark_base
+  {
+    simple_prologue
+    operator() () const
+    {
+      return simple_prologue (&epilogue, "error", nullptr);
+    }
+
+    location_prologue
+    operator() (const location& l) const
+    {
+      return location_prologue (&epilogue, "error", nullptr, l);
+    }
+
+    template <typename L>
+    location_prologue
+    operator() (const L& l) const
+    {
+      return location_prologue (&epilogue, "error", nullptr, get_location (l));
+    }
+
+    static void
+    epilogue (const diag_record&) {throw E ();}
+  };
+
+  template <typename E>
+  using fail_mark = diag_mark<fail_mark_base<E>>;
+
+  extern const fail_mark<failed> fail;
 }
 
 #endif // BUILD_DIAGNOSTICS
diff --git a/build/diagnostics.cxx b/build/diagnostics.cxx
index f213707..6b524a5 100644
--- a/build/diagnostics.cxx
+++ b/build/diagnostics.cxx
@@ -6,6 +6,8 @@
 
 #include <iostream>
 
+#include <build/utility>
+
 using namespace std;
 
 namespace build
@@ -13,8 +15,68 @@ namespace build
   void
   print_process (const char* const* args)
   {
+    diag_record r (text);
+
     for (const char* const* p (args); *p != nullptr; p++)
-      cerr << (p != args ? " " : "") << *p;
-    cerr << endl;
+      r << (p != args ? " " : "") << *p;
+  }
+
+  // Trace verbosity level.
+  //
+  uint8_t verb;
+
+  // Diagnostic facility, base infrastructure.
+  //
+  ostream* diag_stream = &cerr;
+
+  diag_record::
+  ~diag_record () noexcept(false)
+  {
+    // Don't flush the record if this destructor was called as part of
+    // the stack unwinding. Right now this means we cannot use this
+    // mechanism in destructors, which is not a big deal, except for
+    // one place: exception_guard. So for now we are going to have
+    // this ugly special check which we will be able to get rid of
+    // once C++17 uncaught_exceptions() becomes available.
+    //
+    if (!empty_ && (!std::uncaught_exception () || exception_unwinding_dtor))
+    {
+      *diag_stream << os_.str () << std::endl;
+
+      if (epilogue_ != nullptr)
+        epilogue_ (*this); // Can throw.
+    }
+  }
+
+  // Diagnostic facility, project specifics.
+  //
+
+  void simple_prologue_base::
+  operator() (const diag_record& r) const
+  {
+    if (type_ != nullptr)
+      r << type_ << ": ";
+
+    if (name_ != nullptr)
+      r << name_ << ": ";
+  }
+
+  void location_prologue_base::
+  operator() (const diag_record& r) const
+  {
+    r << loc_.file << ':' << loc_.line << ':' << loc_.column << ": ";
+
+    if (type_ != nullptr)
+      r << type_ << ": ";
+
+    if (name_ != nullptr)
+      r << name_ << ": ";
   }
+
+  const basic_mark error ("error");
+  const basic_mark warn ("warning");
+  const basic_mark info ("info");
+  const basic_mark text (nullptr);
+
+  const fail_mark<failed> fail;
 }
diff --git a/build/lexer b/build/lexer
index cf67eec..1944727 100644
--- a/build/lexer
+++ b/build/lexer
@@ -11,18 +11,14 @@
 #include <exception>
 
 #include <build/token>
+#include <build/diagnostics>
 
 namespace build
 {
-  // The handler must assume the diagnostics has already been issued.
-  //
-  struct lexer_error: std::exception {};
-
   class lexer
   {
   public:
-    lexer (std::istream& is, const std::string& name, std::ostream& diag)
-        : is_ (is), name_ (name), diag_ (diag) {}
+    lexer (std::istream& is, const std::string& name): is_ (is), fail (name) {}
 
     token
     next ();
@@ -83,16 +79,23 @@ namespace build
     token
     name (xchar);
 
-    // Utilities.
+    // Diagnostics.
     //
   private:
-    std::ostream&
-    error (const xchar&);
+    struct fail_mark_base: build::fail_mark_base<failed>
+    {
+      fail_mark_base (const std::string& n): name_ (n) {}
+
+      location_prologue
+      operator() (const xchar&) const;
+
+      std::string name_;
+    };
+    typedef diag_mark<fail_mark_base> fail_mark;
 
   private:
     std::istream& is_;
-    std::string name_;
-    std::ostream& diag_;
+    fail_mark fail;
 
     std::uint64_t l_ {1};
     std::uint64_t c_ {1};
diff --git a/build/lexer.cxx b/build/lexer.cxx
index a1aa375..ea11680 100644
--- a/build/lexer.cxx
+++ b/build/lexer.cxx
@@ -4,8 +4,6 @@
 
 #include <build/lexer>
 
-#include <iostream>
-
 using namespace std;
 
 namespace build
@@ -56,8 +54,7 @@ namespace build
     if (!is_eos (c))
       return c;
 
-    error (c) << "unterminated escape sequence" << endl;
-    throw lexer_error ();
+    fail (c) << "unterminated escape sequence";
   }
 
   void lexer::
@@ -217,10 +214,10 @@ namespace build
     unget_ = true;
   }
 
-  ostream& lexer::
-  error (const xchar& c)
+  location_prologue lexer::fail_mark_base::
+  operator() (const xchar& c) const
   {
-    return diag_ << name_ << ':' << c.line () << ':' <<
-      c.column () << ": error: ";
+    return build::fail_mark_base<failed>::operator() (
+      location (name_.c_str (), c.line (), c.column ()));
   }
 }
diff --git a/build/parser b/build/parser
index 66c0357..7515908 100644
--- a/build/parser
+++ b/build/parser
@@ -9,10 +9,9 @@
 #include <vector>
 #include <iosfwd>
 #include <utility>   // std::move
-#include <exception>
-
 
 #include <build/path>
+#include <build/diagnostics>
 
 namespace build
 {
@@ -22,15 +21,11 @@ namespace build
   enum class token_type;
   class lexer;
 
-  // The handler must assume the diagnostics has already been issued.
-  //
-  struct parser_error: std::exception {};
-
   class parser
   {
   public:
-    parser (std::ostream& diag): diag_ (diag) {}
-
+    // Issues diagnostics and throws failed in case of an error.
+    //
     void
     parse (std::istream&, const path&, scope&);
 
@@ -70,14 +65,22 @@ namespace build
     token_type
     next (token&, token_type&);
 
-    std::ostream&
-    error (const token&);
+    // Diagnostics.
+    //
+  private:
+    struct fail_mark_base: build::fail_mark_base<failed>
+    {
+      location_prologue
+      operator() (const token&) const;
+
+      const path* path_;
+    };
+    typedef diag_mark<fail_mark_base> fail_mark;
 
   private:
-    std::ostream& diag_;
+    fail_mark fail;
 
     lexer* lexer_;
-    const path* path_;
     scope* scope_;
   };
 }
diff --git a/build/parser.cxx b/build/parser.cxx
index ccb41b4..696e2a3 100644
--- a/build/parser.cxx
+++ b/build/parser.cxx
@@ -5,7 +5,6 @@
 #include <build/parser>
 
 #include <memory>   // unique_ptr
-#include <iostream>
 
 #include <build/token>
 #include <build/lexer>
@@ -29,9 +28,9 @@ namespace build
   void parser::
   parse (istream& is, const path& p, scope& s)
   {
-    lexer l (is, p.string (), diag_);
+    lexer l (is, p.string ());
     lexer_ = &l;
-    path_ = &p;
+    fail.path_ = &p;
     scope_ = &s;
 
     token t (type::eos, 0, 0);
@@ -41,16 +40,13 @@ namespace build
     parse_clause (t, tt);
 
     if (tt != type::eos)
-    {
-      error (t) << "unexpected " << t << endl;
-      throw parser_error ();
-    }
+      fail (t) << "unexpected " << t;
   }
 
   void parser::
   parse_clause (token& t, token_type& tt)
   {
-    tracer tr ("parser::parse_clause");
+    tracer trace ("parser::parse_clause");
 
     while (tt != type::eos)
     {
@@ -90,8 +86,7 @@ namespace build
             {
               //@@ TODO name (or better yet, type) location
 
-              error (t) << "unknown prerequisite type '" << tt << "'" << endl;
-              throw parser_error ();
+              fail (t) << "unknown prerequisite type " << tt;
             }
 
             const target_type& ti (i->second);
@@ -133,7 +128,7 @@ namespace build
             //
             prerequisite& p (
               scope_->prerequisites.insert (
-                ti, move (d), move (n), e, *scope_, tr).first);
+                ti, move (d), move (n), e, *scope_, trace).first);
 
             ps.push_back (p);
           }
@@ -190,15 +185,16 @@ namespace build
             {
               //@@ TODO name (or better yet, type) location
 
-              error (t) << "unknown target type '" << tt << "'" << endl;
-              throw parser_error ();
+              fail (t) << "unknown target type " << tt;
             }
 
             const target_type& ti (i->second);
 
             // Find or insert.
             //
-            target& t (targets.insert (ti, move (d), move (n), e, tr).first);
+            target& t (
+              targets.insert (
+                ti, move (d), move (n), e, trace).first);
 
             t.prerequisites = ps; //@@ OPT: move if last target.
 
@@ -209,10 +205,7 @@ namespace build
           if (tt == type::newline)
             next (t, tt);
           else if (tt != type::eos)
-          {
-            error (t) << "expected newline instead of " << t << endl;
-            throw parser_error ();
-          }
+            fail (t) << "expected newline instead of " << t;
 
           continue;
         }
@@ -226,10 +219,7 @@ namespace build
             // Should be on its own line.
             //
             if (next (t, tt) != type::newline)
-            {
-              error (t) << "expected newline after '{'" << endl;
-              throw parser_error ();
-            }
+              fail (t) << "expected newline after {";
 
             // See if this is a directory or target scope. Different
             // things can appear inside depending on which one it is.
@@ -243,8 +233,7 @@ namespace build
                 {
                   // @@ TODO: point to name.
                   //
-                  error (t) << "multiple names in directory scope" << endl;
-                  throw parser_error ();
+                  fail (t) << "multiple names in directory scope";
                 }
 
                 dir = true;
@@ -276,20 +265,14 @@ namespace build
             }
 
             if (tt != type::rcbrace)
-            {
-              error (t) << "expected '}' instead of " << t << endl;
-              throw parser_error ();
-            }
+              fail (t) << "expected '}' instead of " << t;
 
             // Should be on its own line.
             //
             if (next (t, tt) == type::newline)
               next (t, tt);
             else if (tt != type::eos)
-            {
-              error (t) << "expected newline after '}'" << endl;
-              throw parser_error ();
-            }
+              fail (t) << "expected newline after }";
           }
 
           continue;
@@ -298,12 +281,10 @@ namespace build
         if (tt == type::eos)
           continue;
 
-        error (t) << "expected newline insetad of " << t << endl;
-        throw parser_error ();
+        fail (t) << "expected newline insetad of " << t;
       }
 
-      error (t) << "unexpected " << t << endl;
-      throw parser_error ();
+      fail (t) << "unexpected " << t;
     }
   }
 
@@ -320,10 +301,7 @@ namespace build
         parse_names (t, tt, ns, dp, tp);
 
         if (tt != type::rcbrace)
-        {
-          error (t) << "expected '}' instead of " << t << endl;
-          throw parser_error ();
-        }
+          fail (t) << "expected '}' instead of " << t;
 
         next (t, tt);
         continue;
@@ -343,10 +321,7 @@ namespace build
           string::size_type p (name.rfind ('/')), n (name.size () - 1);
 
           if (p != n && tp != nullptr)
-          {
-            error (t) << "nested type name '" << name << "'" << endl;
-            throw parser_error ();
-          }
+            fail (t) << "nested type name " << name;
 
           path d1;
           const path* dp1 (dp);
@@ -382,10 +357,7 @@ namespace build
           parse_names (t, tt, ns, dp1, tp1);
 
           if (tt != type::rcbrace)
-          {
-            error (t) << "expected '}' instead of " << t << endl;
-            throw parser_error ();
-          }
+            fail (t) << "expected '}' instead of " << t;
 
           next (t, tt);
           continue;
@@ -400,8 +372,7 @@ namespace build
       if (!first)
         break;
 
-      error (t) << "expected name instead of " << t << endl;
-      throw parser_error ();
+      fail (t) << "expected name instead of " << t;
     }
   }
 
@@ -413,11 +384,11 @@ namespace build
     return tt;
   }
 
-  ostream& parser::
-  error (const token& t)
+  location_prologue parser::fail_mark_base::
+  operator() (const token& t) const
   {
-    return diag_ << path_->string () << ':' << t.line () << ':' <<
-      t.column () << ": error: ";
+    return build::fail_mark_base<failed>::operator() (
+      location (path_->string ().c_str (), t.line (), t.column ()));
   }
 
   // Output the token type and value in a format suitable for diagnostics.
@@ -429,10 +400,10 @@ namespace build
     {
     case token_type::eos:     os << "<end-of-stream>"; break;
     case token_type::newline: os << "<newline>"; break;
-    case token_type::colon:   os << "':'"; break;
-    case token_type::lcbrace: os << "'{'"; break;
-    case token_type::rcbrace: os << "'}'"; break;
-    case token_type::name:    os << '\'' << t.name () << '\''; break;
+    case token_type::colon:   os << ":"; break;
+    case token_type::lcbrace: os << "{"; break;
+    case token_type::rcbrace: os << "}"; break;
+    case token_type::name:    os << t.name (); break;
     }
 
     return os;
diff --git a/build/prerequisite b/build/prerequisite
index a14621f..da90df4 100644
--- a/build/prerequisite
+++ b/build/prerequisite
@@ -13,13 +13,13 @@
 
 #include <build/path>
 #include <build/utility> // extension_pool
+#include <build/diagnostics>
 
 namespace build
 {
   class scope;
   class target;
   class target_type;
-  class tracer;
 
   class prerequisite
   {
diff --git a/build/prerequisite.cxx b/build/prerequisite.cxx
index f5461ef..8d86392 100644
--- a/build/prerequisite.cxx
+++ b/build/prerequisite.cxx
@@ -82,7 +82,7 @@ namespace build
           std::string name,
           const std::string* ext,
           scope& s,
-          tracer& tr) -> pair<prerequisite&, bool>
+          tracer& trace) -> pair<prerequisite&, bool>
   {
     //@@ OPT: would be nice to somehow first check if this prerequisite is
     //   already in the set before allocating a new instance.
@@ -96,8 +96,8 @@ namespace build
     //
     if (p.ext != ext)
     {
-      trace (4, [&]{
-          tracer::record r (tr);
+      level4 ([&]{
+          diag_record r (trace);
           r << "assuming prerequisite " << p << " is the same as the "
             << "one with ";
           if (ext == nullptr)
diff --git a/build/rule.cxx b/build/rule.cxx
index b73b053..d40eebf 100644
--- a/build/rule.cxx
+++ b/build/rule.cxx
@@ -5,7 +5,8 @@
 #include <build/rule>
 
 #include <utility>  // move()
-#include <iostream>
+
+#include <build/diagnostics>
 
 using namespace std;
 
@@ -74,9 +75,9 @@ namespace build
       {
         if (mt < mtp->mtime ())
         {
-          cerr << "error: no rule to update target " << t << endl
-               << "info: prerequisite " << pt << " is ahead of " << t <<
-            " by " << (mtp->mtime () - mt) << endl;
+          error << "no rule to update target " << t <<
+            info << "prerequisite " << pt << " is ahead of " << t
+                << " by " << (mtp->mtime () - mt);
 
           return target_state::failed;
         }
@@ -87,9 +88,9 @@ namespace build
         //
         if (pt.state () == target_state::updated)
         {
-          cerr << "error: no rule to update target " << t << endl
-               << "info: prerequisite " << pt << " is ahead of " << t <<
-            " because it was updated" << endl;
+          error << "no rule to update target " << t <<
+            info << "prerequisite " << pt << " is ahead of " << t
+                << " because it was updated";
 
           return target_state::failed;
         }
diff --git a/build/target.cxx b/build/target.cxx
index b20f53b..3852bc4 100644
--- a/build/target.cxx
+++ b/build/target.cxx
@@ -43,7 +43,7 @@ namespace build
           path dir,
           std::string name,
           const std::string* ext,
-          tracer& tr) -> pair<target&, bool>
+          tracer& trace) -> pair<target&, bool>
   {
     //@@ OPT: would be nice to somehow first check if this target is
     //   already in the set before allocating a new instance.
@@ -60,8 +60,8 @@ namespace build
     //
     if (t.ext != ext)
     {
-      trace (4, [&]{
-          tracer::record r (tr);
+      level4 ([&]{
+          diag_record r (trace);
           r << "assuming target " << t << " is the same as the one with ";
           if (ext == nullptr)
             r << "unspecified extension";
diff --git a/build/timestamp b/build/timestamp
index 2386125..04a0c29 100644
--- a/build/timestamp
+++ b/build/timestamp
@@ -38,10 +38,10 @@ namespace build
   const timestamp timestamp_nonexistent {duration {0}};
 
   std::ostream&
-  operator<< (std::ostream&, timestamp);
+  operator<< (std::ostream&, const timestamp&);
 
   std::ostream&
-  operator<< (std::ostream&, duration);
+  operator<< (std::ostream&, const duration&);
 
   // Returns timestamp_nonexistent if the entry at the specified path
   // does not exist. All other errors are reported by throwing
diff --git a/build/timestamp.cxx b/build/timestamp.cxx
index 0ad3f3f..0d450ea 100644
--- a/build/timestamp.cxx
+++ b/build/timestamp.cxx
@@ -58,7 +58,7 @@ namespace build
   }
 
   ostream&
-  operator<< (ostream& os, timestamp ts)
+  operator<< (ostream& os, const timestamp& ts)
   {
     // @@ replace with put_time()
     //
@@ -97,7 +97,7 @@ namespace build
   }
 
   ostream&
-  operator<< (ostream& os, duration d)
+  operator<< (ostream& os, const duration& d)
   {
     // @@ replace with put_time()
     //
diff --git a/build/trace b/build/trace
deleted file mode 100644
index 1764a69..0000000
--- a/build/trace
+++ /dev/null
@@ -1,80 +0,0 @@
-// file      : build/trace -*- C++ -*-
-// copyright : Copyright (c) 2014-2015 Code Synthesis Tools CC
-// license   : MIT; see accompanying LICENSE file
-
-#ifndef BUILD_TRACE
-#define BUILD_TRACE
-
-#include <cstdint>
-#include <iostream>
-
-namespace build
-{
-  // 1 - command lines to update explicit targets (e.g., .o)
-  // 2 - command lines to update implicit targets (e.g., .d)
-  // 3 - things didn't work out (e.g., rule did not match)
-  // 4 - additional information
-  // 5 - more additional information
-  //
-  extern std::uint8_t verb;
-
-  struct tracer
-  {
-    explicit
-    tracer (const char* name): name_ (name) {}
-
-    struct record
-    {
-      ~record () {if (!empty_) std::cerr << std::endl;}
-
-      template <typename T>
-      std::ostream&
-      operator<< (const T& x) const
-      {
-        return std::cerr << x;
-      }
-
-      explicit record (tracer& t): empty_ (false) {t.begin ();}
-      explicit record (bool e = true): empty_ (e) {}
-
-      // Movable-only type.
-      //
-      record (record&& r) {empty_ = r.empty_; r.empty_ = true;}
-      record& operator= (record&& r) {empty_ = r.empty_; r.empty_ = true;}
-
-      record (const record&) = delete;
-      record& operator= (const record&) = delete;
-
-    private:
-      mutable bool empty_;
-    };
-
-    template <typename T>
-    record
-    operator<< (const T& x) const
-    {
-      begin ();
-      std::cerr << x;
-      return record (false);
-    }
-
-    void
-    begin () const
-    {
-      std::cerr << "trace: " << name_ << ": ";
-    }
-
-  private:
-    const char* name_;
-  };
-
-  template <typename F>
-  inline void
-  trace (std::uint8_t level, const F& f)
-  {
-    if (verb >= level)
-      f ();
-  }
-}
-
-#endif // BUILD_TRACE
diff --git a/build/trace.cxx b/build/trace.cxx
deleted file mode 100644
index a7e6566..0000000
--- a/build/trace.cxx
+++ /dev/null
@@ -1,12 +0,0 @@
-// file      : build/trace.cxx -*- C++ -*-
-// copyright : Copyright (c) 2014-2015 Code Synthesis Tools CC
-// license   : MIT; see accompanying LICENSE file
-
-#include <build/trace>
-
-using namespace std;
-
-namespace build
-{
-  uint8_t verb;
-}
diff --git a/build/utility b/build/utility
index bcbf834..5523e8b 100644
--- a/build/utility
+++ b/build/utility
@@ -5,9 +5,12 @@
 #ifndef BUILD_UTILITY
 #define BUILD_UTILITY
 
+#include <tuple>
 #include <string>
-#include <unordered_set>
+#include <utility>
 #include <cstring> // strcmp
+#include <exception>
+#include <unordered_set>
 
 
 namespace build
@@ -26,6 +29,54 @@ namespace build
     bool operator() (const P& x, const P& y) const {return *x < *y;}
   };
 
+  // Call a function if there is an exception.
+  //
+
+  // Means we are in the body of a destructor that is being called
+  // as part of the exception stack unwindining. Used to compensate
+  // for the deficiencies of uncaught_exception() until C++17
+  // uncaught_exceptions() becomes available.
+  //
+  // @@ MT: will have to be TLS.
+  //
+  extern bool exception_unwinding_dtor;
+
+  template <typename F, typename T>
+  struct exception_guard;
+
+  template <typename F, typename... A>
+  inline exception_guard<F, std::tuple<A&&...>>
+  make_exception_guard (F f, A&&... a)
+  {
+    return exception_guard<F, std::tuple<A&&...>> (
+      std::move (f), std::forward_as_tuple (a...));
+  }
+
+  template <typename F, typename... A>
+  struct exception_guard<F, std::tuple<A...>>
+  {
+    typedef std::tuple<A...> T;
+
+    exception_guard (F f, T a): f_ (std::move (f)), a_ (std::move (a)) {}
+    ~exception_guard ()
+    {
+      if (std::uncaught_exception ())
+      {
+        exception_unwinding_dtor = true;
+        call (std::index_sequence_for<A...> ());
+        exception_unwinding_dtor = false;
+      }
+    }
+
+  private:
+    template <std::size_t... I>
+    void
+    call (std::index_sequence<I...>) {f_ (std::get<I> (a_)...);}
+
+    F f_;
+    T a_;
+  };
+
   // Pools (@@ perhaps move into a separate header).
   //
   struct string_pool: std::unordered_set<std::string>
diff --git a/build/utility.cxx b/build/utility.cxx
index f3fd51b..cd92740 100644
--- a/build/utility.cxx
+++ b/build/utility.cxx
@@ -8,5 +8,7 @@ using namespace std;
 
 namespace build
 {
+  bool exception_unwinding_dtor = false;
+
   string_pool extension_pool;
 }