nix build: Use Nix search path

That is, unless --file is specified, the Nix search path is
synthesized into an attribute set. Thus you can say

  $ nix build nixpkgs.hello

assuming $NIX_PATH contains an entry of the form "nixpkgs=...". This
is more verbose than

  $ nix build hello

but is less ambiguous.

2 files changed, 48 insertions, 6 deletions

diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 6257c7679af9..8341bbc5a3a4 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -9,6 +9,41 @@
 
 namespace nix {
 
+Value * MixInstallables::buildSourceExpr(EvalState & state)
+{
+    Value * vRoot = state.allocValue();
+
+    if (file != "") {
+        Expr * e = state.parseExprFromFile(resolveExprPath(lookupFileArg(state, file)));
+        state.eval(e, *vRoot);
+    }
+
+    else {
+
+        /* Construct the installation source from $NIX_PATH. */
+
+        auto searchPath = state.getSearchPath();
+
+        state.mkAttrs(*vRoot, searchPath.size());
+
+        std::unordered_set<std::string> seen;
+
+        for (auto & i : searchPath) {
+            if (i.first == "") continue;
+            if (seen.count(i.first)) continue;
+            seen.insert(i.first);
+            if (!pathExists(i.second)) continue;
+            mkApp(*state.allocAttr(*vRoot, state.symbols.create(i.first)),
+                state.getBuiltin("import"),
+                mkString(*state.allocValue(), i.second));
+        }
+
+        vRoot->attrs->sort();
+    }
+
+    return vRoot;
+}
+
 UserEnvElems MixInstallables::evalInstallables(ref<Store> store)
 {
     UserEnvElems res;
@@ -46,15 +81,12 @@ UserEnvElems MixInstallables::evalInstallables(ref<Store> store)
 
             EvalState state({}, store);
 
-            Expr * e = state.parseExprFromFile(resolveExprPath(lookupFileArg(state, file)));
-
-            Value vRoot;
-            state.eval(e, vRoot);
+            auto vRoot = buildSourceExpr(state);
 
             std::map<string, string> autoArgs_;
             Bindings & autoArgs(*evalAutoArgs(state, autoArgs_));
 
-            Value & v(*findAlongAttrPath(state, installable, autoArgs, vRoot));
+            Value & v(*findAlongAttrPath(state, installable, autoArgs, *vRoot));
             state.forceValue(v);
 
             DrvInfos drvs;
diff --git a/src/nix/installables.hh b/src/nix/installables.hh
index 5eb897d46148..a58f7dc59bb4 100644
--- a/src/nix/installables.hh
+++ b/src/nix/installables.hh
@@ -21,10 +21,13 @@ struct UserEnvElem
 
 typedef std::vector<UserEnvElem> UserEnvElems;
 
+struct Value;
+class EvalState;
+
 struct MixInstallables : virtual Args
 {
     Strings installables;
-    Path file = "<nixpkgs>";
+    Path file;
 
     MixInstallables()
     {
@@ -33,6 +36,13 @@ struct MixInstallables : virtual Args
     }
 
     UserEnvElems evalInstallables(ref<Store> store);
+
+    /* Return a value representing the Nix expression from which we
+       are installing. This is either the file specified by ‘--file’,
+       or an attribute set constructed from $NIX_PATH, e.g. ‘{ nixpkgs
+       = import ...; bla = import ...; }’. */
+    Value * buildSourceExpr(EvalState & state);
+
 };
 
 }