refactor(tvix/eval): add NixAttrs::contains function

This avoids copying around the value more than needed.

Change-Id: I35949d16dad7fb8f76e0f641eaccf48322144777
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6263
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/attrs.rs

@@ -66,6 +66,14 @@ impl AttrsRep {
             AttrsRep::Map(map) => map.get(&key.into()),
         }
     }
+
+    fn contains(&self, key: &str) -> bool {
+        match self {
+            AttrsRep::Empty => false,
+            AttrsRep::KV { .. } => key == "name" || key == "value",
+            AttrsRep::Map(map) => map.contains_key(&key.into()),
+        }
+    }
 }
 
 #[repr(transparent)]
@@ -202,6 +210,10 @@ impl NixAttrs {
         self.0.select(key)
     }
 
+    pub fn contains(&self, key: &str) -> bool {
+        self.0.contains(key)
+    }
+
     /// Implement construction logic of an attribute set, to encapsulate
     /// logic about attribute set optimisations inside of this module.
     pub fn construct(count: usize, mut stack_slice: Vec<Value>) -> EvalResult<Self> {

tvix/eval/src/vm.rs

@@ -242,7 +242,7 @@ impl VM {
                 OpCode::OpAttrsIsSet => {
                     let key = self.pop().to_string()?;
                     let result = match self.pop() {
-                        Value::Attrs(attrs) => attrs.select(key.as_str()).is_some(),
+                        Value::Attrs(attrs) => attrs.contains(key.as_str()),
 
                         // Nix allows use of `?` on non-set types, but
                         // always returns false in those cases.