Record AstPtr of the first Expr for merged attrsets

Otherwise, it causes a panic of missing node when reporting diagnostics
on the merged attrset.

Fixes #114

crates/ide/src/def/liveness.rs

@@ -234,7 +234,14 @@ mod tests {
         let (db, f) = TestDB::from_fixture(fixture).unwrap();
         assert_eq!(f.files().len(), 1);
         let file = f.files()[0];
-        assert_eq!(db.source_map(file).diagnostics(), Vec::new(), "Lower error");
+
+        let source_map = db.source_map(file);
+        let lower_diags = source_map.diagnostics();
+        assert!(
+            !lower_diags.iter().any(|diag| diag.severity().is_fatal()),
+            "Lower error: {lower_diags:?}",
+        );
+
         let expect = f.markers().iter().map(|p| p.pos).collect::<Vec<_>>();
         let mut got = db
             .liveness_check(file)
@@ -369,4 +376,15 @@ mod tests {
         check("x: _y: x");
         check("let __findFile = 42; in <nixpkgs>");
     }
+
+    // Issue #114
+    #[test]
+    fn merged_rec_attrset() {
+        check("{ test = $0rec { }; test.bar = 42; }");
+        check("{ test = rec { foo = bar; }; test.bar = 42; }");
+        check("{ test = rec { foo = 42; }; test.bar = foo; }");
+        check("{ test = rec { }; test.foo = bar; test.bar = 42; }");
+
+        check("{ test = $0rec { }; test = rec { };}");
+    }
 }

crates/ide/src/def/lower.rs

@@ -509,10 +509,15 @@ impl MergingSet {
                 ctx.source_map.name_map_rev[ent.name].push(attr_ptr.clone());
 
                 if let Some(prev_set) = &mut ent.set {
-                    // Erase the source information when merging occurs.
-                    // Since the previous definition is not exhaustive now.
-                    // `{ a = { b = 1; }; a.c = 2; }`
-                    prev_set.ptr = None;
+                    // Here we keep the `ptr` of the first definition attrset,
+                    // which is the determinant `rec` one if it happens to be.
+                    // This may be used to report diagnostics in liveness check.
+                    // Covered by test `liveness::tests::merge_rec_attrset`.
+                    // Eg. `{ a = rec { b = 1; }; a.c = 2; }`
+                    //            ^^^ Unused rec.
+                    //
+                    // This does result that the node pointed by `ptr` is non-exhaustive.
+                    // Hope this does not break anything.
 
                     if prev_set.name_kind.is_definition() {
                         ctx.diagnostic(Diagnostic::new(

crates/ide/src/diagnostic.rs

@@ -41,6 +41,12 @@ pub enum Severity {
     IncompleteSyntax,
 }
 
+impl Severity {
+    pub fn is_fatal(self) -> bool {
+        self != Self::Warning
+    }
+}
+
 impl Diagnostic {
     pub fn new(range: TextRange, kind: DiagnosticKind) -> Self {
         Self {