* nix-env now maintains meta info (from the `meta' derivation

attribute) about installed packages in user environments.  Thus, an
  operation like `nix-env -q --description' shows useful information
  not only on available packages but also on installed packages.

* nix-env now passes the entire manifest as an argument to the Nix
  expression of the user environment builder (not just a list of
  paths), so that in particular the user environment builder has
  access to the meta attributes.
  
* New operation `--set-flag' in nix-env to change meta info of
  installed packages.  This will be useful to pass per-package
  policies to the user environment builder (e.g., how to resolve
  collision or whether to disable a package (NIX-80)) or upgrade
  policies in nix-env (e.g., that a package should be "masked", that
  is, left untouched by upgrade actions).  Example:

  $ nix-env --set-flag enabled false ghc-6.4

src/libexpr/nixexpr.cc

@@ -319,7 +319,7 @@ struct Canonicalise : TermFun
 };
 
 
-Expr canonicaliseExpr(Expr & e)
+Expr canonicaliseExpr(Expr e)
 {
     Canonicalise canonicalise;
     return bottomupRewrite(canonicalise, e);