* The recent change in nixpkgs of calling `stdenv.mkDerivation'

instead of `derivation' triggered a huge slowdown in the Nix
  expression evaluator.  Total execution time of `nix-env -qa' went up
  by a factor of 60 or so.

  This scalability problem was caused by expressions such as

    (x: y: ... x ...) a b

  where `a' is a large term (say, the one in
  `all-packages-generic.nix').  Then the first beta-reduction would
  produce

    (y: ... a ...) b

  by substituting `a' for `x'.  The second beta-reduction would then
  substitute `b' for `y' into the body `... a ...', which is a large
  term due to `a', and thus causes a large traversal to be performed
  by substitute() in the second reduction.  This is however entirely
  redundant, since `a' cannot contain free variables (since we never
  substitute below a weak head normal form).

  The solution is to wrap substituted terms into a `Closed'
  constructor, i.e.,

    subst(subs, Var(x)) = Closed(e) iff subs[x] = e

  have substitution not descent into closed terms,

    subst(subs, Closed(x)) = Closed(x)

  and otherwise ignore them for evaluation,

    eval(Closed(x)) = eval(x).

* Fix a typo that caused incorrect substitutions to be performed in
  simple lambdas, e.g., `(x: x: x) a' would reduce to `(x: a)'.

src/libexpr/eval.cc

@@ -201,6 +201,11 @@ Expr evalExpr2(EvalState & state, Expr e)
          cons == "List"))
         return e;
 
+    /* The `Closed' constructor is just a way to prevent substitutions
+       into expressions not containing free variables. */
+    if (atMatch(m, e) >> "Closed" >> e1)
+        return evalExpr(state, e1);
+
     /* Any encountered variables must be undeclared or primops. */
     if (atMatch(m, e) >> "Var" >> name) {
         PrimOp0 primOp = (PrimOp0) lookupPrimOp(state.primOps0, name);