feat(tvix/eval): implement generator-based Nix equality logic r/5963

Adds a `Value::neo_nix_eq` method (the `neo_` prefix will be dropped
when we flip over to the generator implementation of the VM) which
implements Nix equality semantics using async, generator-based
comparisons.

Instead of tracking the "kind" of equality that is being compared (see
the pointer-equality doc) through a pair of booleans, I've introduced
an enum that explicitly lists the possible comparisons.

Change-Id: I3354cc1470eeccb3000a5ae24f2418db1a7a2edc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8241
Tested-by: BuildkiteCI
Reviewed-by: Adam Joseph <adam@westernsemico.com>

1 files changed, 1 insertions, 5 deletions

diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
index 3b822b0863..2a6a8fa730 100644
--- a/tvix/eval/src/vm/generators.rs
+++ b/tvix/eval/src/vm/generators.rs
@@ -14,17 +14,13 @@ use smol_str::SmolStr;
 use std::fmt::Display;
 use std::future::Future;
 
-use crate::value::SharedThunkSet;
+use crate::value::{PointerEquality, SharedThunkSet};
 use crate::warnings::WarningKind;
 use crate::FileType;
 use crate::NixString;
 
 use super::*;
 
-/// Dummy type, before the actual implementation is in place.
-#[derive(Debug)]
-pub struct PointerEquality {}
-
 // -- Implementation of generic generator logic.
 
 /// States that a generator can be in while being driven by the VM.