fix(tvix/eval): inherit scope poisoning data in nested contexts

Scope poisoning must be inherited across lambda context boundaries,
e.g. if an outer scope has a poisoned `null`, any lambdas defined on
the same level must reference that poisoned identifier correctly.

Change-Id: I1aac64e1c048a6f3bacadb6d78ed295fa439e8b4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6410
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/scope.rs

@@ -152,6 +152,15 @@ impl Scope {
         }
     }
 
+    /// Inherit scope details from a parent scope (required for
+    /// correctly nesting scopes in lambdas and thunks when special
+    /// scope features like poisoning are present).
+    pub fn inherit(&self) -> Self {
+        let mut scope = Self::default();
+        scope.poisoned_tokens = self.poisoned_tokens.clone();
+        scope
+    }
+
     /// Check whether a given token is poisoned.
     pub fn is_poisoned(&self, name: &str) -> bool {
         self.poisoned_tokens.contains_key(name)