fix(tvix/eval): thread Display & PartialEq through to thunk values

If a thunk is already evaluated, there are cases where due to the
memoisation implementation something might observe a value wrapped in
a thunk.

In these cases, the implementation of `Display` and `PartialEq` must
delegate to the underlying value.

Note that there are a handful of other cases like these which we need
to cover.

It is a little tricky to write integration tests for these directly,
especially as some of the open-upvalue optimisations coming down the
pipe will reduce the number of observable thunks.

One test that covers a part of this behaviour is currently
disabled (needs some more machinery), but it's being brought back in
the next commits.

Change-Id: Iaa8cd338c12236af844bbc99d8cec2205f0d0095
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6370
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -172,8 +172,11 @@ impl Display for Value {
                 write!(f, "{}", format!("{:.5}", num).trim_end_matches(['.', '0']))
             }
 
+            // Delegate thunk display to the type, as it must handle
+            // the case of already evaluated thunks.
+            Value::Thunk(t) => t.fmt(f),
+
             // internal types
-            Value::Thunk(_) => f.write_str("internal[thunk]"),
             Value::AttrPath(path) => write!(f, "internal[attrpath({})]", path.len()),
             Value::AttrNotFound => f.write_str("internal[not found]"),
             Value::Blueprint(_) => f.write_str("internal[blueprint]"),
@@ -203,6 +206,13 @@ impl PartialEq for Value {
             // Optimised attribute set comparison
             (Value::Attrs(a1), Value::Attrs(a2)) => Rc::ptr_eq(a1, a2) || { a1 == a2 },
 
+            // If either value is a thunk, the inner value must be
+            // compared instead. The compiler should ensure that
+            // thunks under comparison have been forced, otherwise it
+            // is a bug.
+            (Value::Thunk(lhs), rhs) => &*lhs.value() == rhs,
+            (lhs, Value::Thunk(rhs)) => lhs == &*rhs.value(),
+
             // Everything else is either incomparable (e.g. internal
             // types) or false.
             // TODO(tazjin): mirror Lambda equality behaviour

tvix/eval/src/value/thunk.rs

@@ -20,6 +20,7 @@
 
 use std::{
     cell::{Ref, RefCell, RefMut},
+    fmt::Display,
     rc::Rc,
 };
 
@@ -131,3 +132,12 @@ impl UpvalueCarrier for Thunk {
         })
     }
 }
+
+impl Display for Thunk {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &*self.0.borrow() {
+            ThunkRepr::Evaluated(v) => v.fmt(f),
+            _ => f.write_str("internal[thunk]"),
+        }
+    }
+}