From 8e5fbf4d730b9fcf39eddf5539a206cf19d2cdce Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Fri, 4 Apr 2014 22:52:14 +0200
Subject: [PATCH] Show position info in attribute selection errors

---
 src/libexpr/eval.cc    | 14 +++++++-------
 src/libexpr/nixexpr.hh |  5 +++--
 src/libexpr/parser.y   |  8 ++++----
 3 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 44d75bd8c..0f7e8e385 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -736,7 +736,7 @@ unsigned long nrLookups = 0;
 void ExprSelect::eval(EvalState & state, Env & env, Value & v)
 {
     Value vTmp;
-    Pos * pos = 0;
+    Pos * pos2 = 0;
     Value * vAttrs = &vTmp;
 
     e->eval(state, env, vTmp);
@@ -756,21 +756,21 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     return;
                 }
             } else {
-                state.forceAttrs(*vAttrs);
+                state.forceAttrs(*vAttrs, pos);
                 if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
-                    throwEvalError("attribute `%1%' missing", showAttrPath(attrPath));
+                    throwEvalError("attribute `%1%' missing, at %2%", showAttrPath(attrPath), pos);
             }
             vAttrs = j->value;
-            pos = j->pos;
-            if (state.countCalls && pos) state.attrSelects[*pos]++;
+            pos2 = j->pos;
+            if (state.countCalls && pos2) state.attrSelects[*pos2]++;
         }
 
         state.forceValue(*vAttrs);
 
     } catch (Error & e) {
-        if (pos && pos->file != state.sDerivationNix)
+        if (pos2 && pos2->file != state.sDerivationNix)
             addErrorPrefix(e, "while evaluating the attribute `%1%' at %2%:\n",
-                showAttrPath(attrPath), *pos);
+                showAttrPath(attrPath), *pos2);
         throw;
     }
 
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index f07b85c83..813efbe21 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -149,10 +149,11 @@ struct ExprVar : Expr
 
 struct ExprSelect : Expr
 {
+    Pos pos;
     Expr * e, * def;
     AttrPath attrPath;
-    ExprSelect(Expr * e, const AttrPath & attrPath, Expr * def) : e(e), def(def), attrPath(attrPath) { };
-    ExprSelect(Expr * e, const Symbol & name) : e(e), def(0) { attrPath.push_back(AttrName(name)); };
+    ExprSelect(const Pos & pos, Expr * e, const AttrPath & attrPath, Expr * def) : pos(pos), e(e), def(def), attrPath(attrPath) { };
+    ExprSelect(const Pos & pos, Expr * e, const Symbol & name) : pos(pos), e(e), def(0) { attrPath.push_back(AttrName(name)); };
     COMMON_METHODS
 };
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 021f8b026..dbcffff99 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -361,9 +361,9 @@ expr_app
 
 expr_select
   : expr_simple '.' attrpath
-    { $$ = new ExprSelect($1, *$3, 0); }
+    { $$ = new ExprSelect(CUR_POS, $1, *$3, 0); }
   | expr_simple '.' attrpath OR_KW expr_select
-    { $$ = new ExprSelect($1, *$3, $5); }
+    { $$ = new ExprSelect(CUR_POS, $1, *$3, $5); }
   | /* Backwards compatibility: because Nixpkgs has a rarely used
        function named ‘or’, allow stuff like ‘map or [...]’. */
     expr_simple OR_KW
@@ -403,7 +403,7 @@ expr_simple
   /* Let expressions `let {..., body = ...}' are just desugared
      into `(rec {..., body = ...}).body'. */
   | LET '{' binds '}'
-    { $3->recursive = true; $$ = new ExprSelect($3, data->symbols.create("body")); }
+    { $3->recursive = true; $$ = new ExprSelect(noPos, $3, data->symbols.create("body")); }
   | REC '{' binds '}'
     { $3->recursive = true; $$ = $3; }
   | '{' binds '}'
@@ -458,7 +458,7 @@ binds
       foreach (AttrPath::iterator, i, *$6) {
           if ($$->attrs.find(i->symbol) != $$->attrs.end())
               dupAttr(i->symbol, makeCurPos(@6, data), $$->attrs[i->symbol].pos);
-          $$->attrs[i->symbol] = ExprAttrs::AttrDef(new ExprSelect($4, i->symbol), makeCurPos(@6, data));
+          $$->attrs[i->symbol] = ExprAttrs::AttrDef(new ExprSelect(CUR_POS, $4, i->symbol), makeCurPos(@6, data));
       }
     }
   | { $$ = new ExprAttrs; }