Add primop ‘scopedImport’

‘scopedImport’ works like ‘import’, except that it takes a set of
attributes to be added to the lexical scope of the expression,
essentially extending or overriding the builtin variables.  For
instance, the expression

  scopedImport { x = 1; } ./foo.nix

where foo.nix contains ‘x’, will evaluate to 1.

This has a few applications:

* It allows getting rid of function argument specifications in package
  expressions. For instance, a package expression like:

    { stdenv, fetchurl, libfoo }:

    stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }

  can now we written as just

    stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }

  and imported in all-packages.nix as:

    bar = scopedImport pkgs ./bar.nix;

  So whereas we once had dependencies listed in three places
  (buildInputs, the function, and the call site), they now only need
  to appear in one place.

* It allows overriding builtin functions. For instance, to trace all
  calls to ‘map’:

  let
    overrides = {
      map = f: xs: builtins.trace "map called!" (map f xs);

      # Ensure that our override gets propagated by calls to
      # import/scopedImport.
      import = fn: scopedImport overrides fn;

      scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;

      # Also update ‘builtins’.
      builtins = builtins // overrides;
    };
  in scopedImport overrides ./bla.nix

* Similarly, it allows extending the set of builtin functions. For
  instance, during Nixpkgs/NixOS evaluation, the Nixpkgs library
  functions could be added to the default scope.

There is a downside: calls to scopedImport are not memoized, unlike
import. So importing a file multiple times leads to multiple parsings
/ evaluations. It would be possible to construct the AST only once,
but that would require careful handling of variables/environments.

src/libexpr/primops.cc

@@ -93,6 +93,30 @@ static void prim_import(EvalState & state, const Pos & pos, Value * * args, Valu
 }
 
 
+static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    PathSet context;
+    state.forceAttrs(*args[0]);
+    Path path = resolveExprPath(state.coerceToPath(pos, *args[1], context));
+
+    Env * env = &state.allocEnv(args[0]->attrs->size());
+    env->up = &state.baseEnv;
+
+    StaticEnv staticEnv(false, &state.staticBaseEnv);
+
+    unsigned int displ = 0;
+    for (auto & attr : *args[0]->attrs) {
+        staticEnv.vars[attr.name] = displ;
+        env->values[displ++] = attr.value;
+    }
+
+    startNest(nest, lvlTalkative, format("evaluating file `%1%'") % path);
+    Expr * e = state.parseExprFromFile(path, staticEnv);
+
+    e->eval(state, *env, v);
+}
+
+
 /* Return a string representing the type of the expression. */
 static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
@@ -1247,6 +1271,7 @@ void EvalState::createBaseEnv()
 
     // Miscellaneous
     addPrimOp("import", 1, prim_import);
+    addPrimOp("scopedImport", 2, prim_scopedImport);
     addPrimOp("__typeOf", 1, prim_typeOf);
     addPrimOp("isNull", 1, prim_isNull);
     addPrimOp("__isFunction", 1, prim_isFunction);