Add basic "nix build" command

Currently only builds by attribute from <nixpkgs> or the specified
file, e.g. "nix build hello".

src/nix/installables.cc

@@ -0,0 +1,75 @@
+#include "attr-path.hh"
+#include "common-opts.hh"
+#include "derivations.hh"
+#include "eval-inline.hh"
+#include "eval.hh"
+#include "get-drvs.hh"
+#include "installables.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+UserEnvElems MixInstallables::evalInstallables(ref<Store> store)
+{
+    UserEnvElems res;
+
+    for (auto & installable : installables) {
+
+        if (std::string(installable, 0, 1) == "/") {
+
+            if (isStorePath(installable)) {
+
+                if (isDerivation(installable)) {
+                    UserEnvElem elem;
+                    // FIXME: handle empty case, drop version
+                    elem.attrPath = {storePathToName(installable)};
+                    elem.isDrv = true;
+                    elem.drvPath = installable;
+                    res.push_back(elem);
+                }
+
+                else {
+                    UserEnvElem elem;
+                    // FIXME: handle empty case, drop version
+                    elem.attrPath = {storePathToName(installable)};
+                    elem.isDrv = false;
+                    elem.outPaths = {installable};
+                    res.push_back(elem);
+                }
+            }
+
+            else
+                throw UsageError(format("don't know what to do with ‘%1%’") % installable);
+        }
+
+        else {
+
+            EvalState state({}, store);
+
+            Expr * e = state.parseExprFromFile(resolveExprPath(lookupFileArg(state, file)));
+
+            Value vRoot;
+            state.eval(e, vRoot);
+
+            std::map<string, string> autoArgs_;
+            Bindings & autoArgs(*evalAutoArgs(state, autoArgs_));
+
+            Value & v(*findAlongAttrPath(state, installable, autoArgs, vRoot));
+            state.forceValue(v);
+
+            DrvInfos drvs;
+            getDerivations(state, v, "", autoArgs, drvs, false);
+
+            for (auto & i : drvs) {
+                UserEnvElem elem;
+                elem.isDrv = true;
+                elem.drvPath = i.queryDrvPath();
+                res.push_back(elem);
+            }
+        }
+    }
+
+    return res;
+}
+
+}