* Ad hoc per-package logging. When Nix performs a derivation, it now

writes stdout/stderr of the builder to ${prefix}/var/log/nix/x,
  where x is the file name of the derivation expression, e.g.,

    /nix/var/log/nix/54256391624be04fcb426048ae3ea0a4-d-pan-0.14.2.nix

  Note that consecutive builds of the same expression overwrite,
  rather than append to, existing log files.

src/libnix/normalise.cc

@@ -164,7 +164,8 @@ Path normaliseNixExpr(const Path & _nePath, PathSet pending)
 
         /* Run the builder. */
         msg(lvlChatty, format("building..."));
-        runProgram(ne.derivation.builder, ne.derivation.args, env);
+        runProgram(ne.derivation.builder, ne.derivation.args, env,
+            nixLogDir + "/" + baseNameOf(nePath));
         msg(lvlChatty, format("build completed"));
         
     } else