* Allow the output/expression id to be forced to a certain

value; this potentially dangerous feature enables better 
  sharing for those paths for which the content is known in 
  advance (e.g., because a content hash is given).
* Fast builds: if we can expand all output paths of a derive 
  expression, we don't have to build.

src/fstate.cc

@@ -31,9 +31,10 @@ ATerm termFromId(const FSId & id)
 }
 
 
-FSId writeTerm(ATerm t, const string & suffix)
+FSId writeTerm(ATerm t, const string & suffix, FSId id)
 {
-    FSId id = hashTerm(t);
+    /* By default, the id of a term is its hash. */
+    if (id == FSId()) id = hashTerm(t);
 
     string path = canonPath(nixStore + "/" + 
         (string) id + suffix + ".nix");