fix(tvix/eval): allow use of ? operator on non-set types

Nix allows this, but always returns false. Tvix needs to do the same.

Change-Id: Ic9eec90834a0d0969eea5316d5c25032d3691d94
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6209
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

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

@@ -0,0 +1 @@
+[ false false false false ]

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

@@ -0,0 +1,3 @@
+# Nix allows using the ? operator on non-set types, in which case it
+# should always return false.
+[ (123 ? key) ("foo" ? key) (null ? key) ([ "key" ] ? key) ]

tvix/eval/src/value/mod.rs

@@ -116,7 +116,7 @@ impl Display for Value {
             }
 
             // internal types
-            Value::AttrPath(_) => f.write_str("internal[attrpath]"),
+            Value::AttrPath(path) => write!(f, "internal[attrpath({})]", path.len()),
             Value::Blackhole => f.write_str("internal[blackhole]"),
             Value::NotFound => f.write_str("internal[not found]"),
         }

tvix/eval/src/vm.rs

@@ -192,9 +192,15 @@ impl VM {
 
                 OpCode::OpAttrsIsSet => {
                     let key = self.pop().to_string()?;
-                    let attrs = self.pop().to_attrs()?;
-                    let result = Value::Bool(attrs.select(key.as_str()).is_some());
-                    self.push(result);
+                    let result = match self.pop() {
+                        Value::Attrs(attrs) => attrs.select(key.as_str()).is_some(),
+
+                        // Nix allows use of `?` on non-set types, but
+                        // always returns false in those cases.
+                        _ => false,
+                    };
+
+                    self.push(Value::Bool(result));
                 }
 
                 OpCode::OpList(count) => {