4 files changed, 37 insertions, 6 deletions
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 7fb38c0fd79c..c2db006c1414 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -410,6 +410,19 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 }
 
 
+void EvalState::mkPos(Value & v, Pos * pos)
+{
+    if (pos) {
+        mkAttrs(v, 3);
+        mkString(*allocAttr(v, sFile), pos->file);
+        mkInt(*allocAttr(v, sLine), pos->line);
+        mkInt(*allocAttr(v, sColumn), pos->column);
+        v.attrs->sort();
+    } else
+        mkNull(v);
+}
+
+
 /* Create a thunk for the delayed computation of the given expression
    in the given environment.  But if the expression is a variable,
    then look it up right away.  This significantly reduces the number
@@ -1044,11 +1057,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
 
 void ExprPos::eval(EvalState & state, Env & env, Value & v)
 {
-    state.mkAttrs(v, 3);
-    mkString(*state.allocAttr(v, state.sFile), pos.file);
-    mkInt(*state.allocAttr(v, state.sLine), pos.line);
-    mkInt(*state.allocAttr(v, state.sColumn), pos.column);
-    v.attrs->sort();
+    state.mkPos(v, &pos);
 }
 
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index bcc029f5b9b9..af408cd0bd76 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -243,6 +243,7 @@ public:
     void mkList(Value & v, unsigned int length);
     void mkAttrs(Value & v, unsigned int expected);
     void mkThunk_(Value & v, Expr * expr);
+    void mkPos(Value & v, Pos * pos);
 
     void concatLists(Value & v, unsigned int nrLists, Value * * lists);
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 180a24dffc91..5f2a58454627 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -776,6 +776,19 @@ void prim_getAttr(EvalState & state, Value * * args, Value & v)
 }
 
 
+/* Return position information of the specified attribute. */
+void prim_unsafeGetAttrPos(EvalState & state, Value * * args, Value & v)
+{
+    string attr = state.forceStringNoCtx(*args[0]);
+    state.forceAttrs(*args[1]);
+    Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
+    if (i == args[1]->attrs->end())
+        mkNull(v);
+    else
+        state.mkPos(v, i->pos);
+}
+
+
 /* Dynamic version of the `?' operator. */
 static void prim_hasAttr(EvalState & state, Value * * args, Value & v)
 {
@@ -1201,7 +1214,7 @@ void EvalState::createBaseEnv()
     mkBool(v, false);
     addConstant("false", v);
 
-    v.type = tNull;
+    mkNull(v);
     addConstant("null", v);
 
     mkInt(v, time(0));
@@ -1252,6 +1265,7 @@ void EvalState::createBaseEnv()
     // Sets
     addPrimOp("__attrNames", 1, prim_attrNames);
     addPrimOp("__getAttr", 2, prim_getAttr);
+    addPrimOp("__unsafeGetAttrPos", 2, prim_unsafeGetAttrPos);
     addPrimOp("__hasAttr", 2, prim_hasAttr);
     addPrimOp("__isAttrs", 1, prim_isAttrs);
     addPrimOp("removeAttrs", 2, prim_removeAttrs);
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index ee5c7397b3a9..982582793419 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -116,6 +116,13 @@ static inline void mkBool(Value & v, bool b)
 }
 
 
+static inline void mkNull(Value & v)
+{
+    v.type = tNull;
+    v.app.left = v.app.right = 00; // scrub
+}
+
+
 static inline void mkApp(Value & v, Value & left, Value & right)
 {
     v.type = tApp;