fix(tvix/eval): manually count entries in recursive scopes

The previous version had a bug where we assumed that the number of
entries in an attribute set AST node would be equivalent to the number
of entries in the runtime attribute set, but due to inherit nodes
containing a variable number of entries, this did not work out.

Fixes b/199

Change-Id: I6f7f7729f3512b297cf29a2e046302ca28477854
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6749
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/attrs.rs

@@ -241,8 +241,8 @@ impl Compiler<'_> {
         self.scope_mut().begin_scope();
 
         if node.rec_token().is_some() {
-            self.compile_recursive_scope(slot, true, &node);
+            let count = self.compile_recursive_scope(slot, true, &node);
-            self.push_op(OpCode::OpAttrs(Count(node.entries().count())), &node);
+            self.push_op(OpCode::OpAttrs(Count(count)), &node);
         } else {
             let mut count = self.compile_inherit_attrs(slot, node.inherits());
 

tvix/eval/src/compiler/mod.rs

@@ -552,10 +552,11 @@ impl Compiler<'_> {
         self.patch_jump(else_idx); // patch jump *over* else body
     }
 
-    fn compile_recursive_scope<N>(&mut self, slot: LocalIdx, rec_attrs: bool, node: &N)
+    fn compile_recursive_scope<N>(&mut self, slot: LocalIdx, rec_attrs: bool, node: &N) -> usize
     where
         N: ToSpan + ast::HasEntry,
     {
+        let mut count = 0;
         self.scope_mut().begin_scope();
 
         // First pass to find all plain inherits (if they are not useless).
@@ -588,6 +589,8 @@ impl Compiler<'_> {
                             }
                         };
 
+                        count += 1;
+
                         // If the identifier resolves statically in a
                         // `let`, it has precedence over dynamic
                         // bindings, and the inherit is useless.
@@ -625,6 +628,7 @@ impl Compiler<'_> {
                             }
                         };
 
+                        count += 1;
                         inherit_froms.push((from.expr().unwrap(), name, self.span_for(&attr)));
                     }
                 }
@@ -632,7 +636,7 @@ impl Compiler<'_> {
         }
 
         // Data structures to track the bindings observed in the
-        // second path, and forward the information needed to compile
+        // second pass, and forward the information needed to compile
         // their value.
         enum BindingKind {
             InheritFrom {
@@ -689,6 +693,8 @@ impl Compiler<'_> {
 
         // Declare all regular bindings
         for entry in node.attrpath_values() {
+            count += 1;
+
             let mut path = match self.normalise_ident_path(entry.attrpath().unwrap().attrs()) {
                 Ok(p) => p,
                 Err(err) => {
@@ -773,6 +779,8 @@ impl Compiler<'_> {
                 self.push_op(OpCode::OpFinalise(stack_idx), node);
             }
         }
+
+        count
     }
 
     /// Compile a standard `let ...; in ...` expression.

tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.exp

@@ -0,0 +1 @@
+{ }

tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.nix

@@ -0,0 +1 @@
+rec { inherit; }