fix(tvix/eval): add branch for directly comparing two thunks

Pointed out by sterni in cl/6370

Change-Id: I324d8049a2702ced8f30ad43a64d63ae79dd0eab
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6569
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -169,6 +169,7 @@ impl PartialEq for Value {
             // compared instead. The compiler should ensure that
             // thunks under comparison have been forced, otherwise it
             // is a bug.
+            (Value::Thunk(lhs), Value::Thunk(rhs)) => &*lhs.value() == &*rhs.value(),
             (Value::Thunk(lhs), rhs) => &*lhs.value() == rhs,
             (lhs, Value::Thunk(rhs)) => lhs == &*rhs.value(),