fix(tvix/eval): or should handle non-attrset values, too

If a nested attrpath encounters a non-set value, the sentinel value
denoting a lack of next values should be emitted. This mirrors the
behaviour of Nix.

Change-Id: Ia80443d5a11243cc6d98dcab1249a3f5fdf77e27
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6210
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.exp

@@ -0,0 +1 @@
+"works fine"

tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.nix

@@ -0,0 +1,2 @@
+# `or` operator should keep working if it encounters a non-set type.
+{ a.b = 42; }.a.b.c or "works fine"

tvix/eval/src/vm.rs

@@ -182,12 +182,16 @@ impl VM {
 
                 OpCode::OpAttrOrNotFound => {
                     let key = self.pop().to_string()?;
-                    let attrs = self.pop().to_attrs()?;
+                    let value = match self.pop() {
+                        Value::Attrs(attrs) => match attrs.select(key.as_str()) {
+                            Some(value) => value.clone(),
+                            None => Value::NotFound,
+                        },
 
-                    match attrs.select(key.as_str()) {
+                        _ => Value::NotFound,
-                        Some(value) => self.push(value.clone()),
+                    };
-                        None => self.push(Value::NotFound),
+
-                    }
+                    self.push(value);
                 }
 
                 OpCode::OpAttrsIsSet => {