parser: Allow mixed nested and top-level attrpaths

Fixes #2077.

src/libexpr/parser.y

@@ -83,6 +83,8 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
     AttrPath::iterator i;
     // All attrpaths have at least one attr
     assert(!attrPath.empty());
+    // Checking attrPath validity.
+    // ===========================
     for (i = attrPath.begin(); i + 1 < attrPath.end(); i++) { 
         if (i->symbol.set()) {
             ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
@@ -104,11 +106,29 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
             attrs = nested;
         }
     }
+    // Expr insertion.
+    // ==========================
     if (i->symbol.set()) { 
         ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
         if (j != attrs->attrs.end()) {
-            dupAttr(attrPath, pos, j->second.pos);
+            // This attr path is already defined. However, if both
+            // e and the expr pointed by the attr path are two attribute sets,
+            // we want to merge them.
+            // Otherwise, throw an error.
+            ExprAttrs* ae = dynamic_cast<ExprAttrs *>(e);
+            ExprAttrs* jAttrs = dynamic_cast<ExprAttrs *>(j->second.e);
+            if (jAttrs && ae) {
+                for (auto ad: ae->attrs) {
+                    ExprAttrs::AttrDefs::iterator j2 = jAttrs->attrs.find(ad.first);
+                    if (j2 != jAttrs->attrs.end()) // Attr already defined in iAttrs, error.
+                        dupAttr(ad.first, j2->second.pos, ad.second.pos);
+                    jAttrs->attrs[ad.first] = ad.second;
+                }
+            } else {
+                dupAttr(attrPath, pos, j->second.pos);
+            }
         } else {
+            // This attr path is not defined. Let's create it.
             attrs->attrs[i->symbol] = ExprAttrs::AttrDef(e, pos);
             e->setName(i->symbol);
         }

tests/lang/parse-fail-mixed-nested-attrs1.nix

@@ -1,4 +1,4 @@
 { 
-  x = { y = 3; z = 3; }; 
   x.z = 3; 
+  x = { y = 3; z = 3; }; 
 }

tests/lang/parse-fail-mixed-nested-attrs2.nix

@@ -0,0 +1,4 @@
+{ 
+  x.y.y = 3; 
+  x = { y.y= 3; z = 3; }; 
+}

tests/lang/parse-okay-nested-attrs.nix

@@ -1,4 +0,0 @@
-{ 
-  x = { y = 3; z = 3; }; 
-  x.q = 3; 
-}