refactor(tvix/eval): Don't (ab)use PartialEq for Nix equality r/4908

Using rust's PartialEq trait to implement Nix equality semantics is
reasonably fraught with peril, both because the actual laws are
different than what nix expects, and (more importantly) because certain
things actually require extra context to compare for equality (for
example, thunks need to be forced). This converts the manual PartialEq
impl for Value (and all its descendants) to a *derived* PartialEq
impl (which requires a lot of extra PartialEq derives on miscellanious
other types within the codebase), and converts the previous
nix-semantics equality comparison into a new `nix_eq` method. This
returns an EvalResult, even though it can't currently return an error,
to allow it to fail when eg forcing thunks (which it will do soon).

Since the PartialEq impls for Value and NixAttrs are now quite boring,
this converts the generated proptests for those into handwritten ones
that cover `nix_eq` instead

Change-Id: If3da7171f88c22eda5b7a60030d8b00c3b76f672
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6650
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 67 insertions, 45 deletions

diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 594c1fd4735c..34dfbddfa167 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -24,7 +24,7 @@ pub use string::NixString;
 pub use thunk::Thunk;
 
 #[warn(variant_size_differences)]
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, PartialEq)]
 pub enum Value {
     Null,
     Bool(bool),
@@ -253,6 +253,40 @@ impl Value {
 
     gen_is!(is_number, Value::Integer(_) | Value::Float(_));
     gen_is!(is_bool, Value::Bool(_));
+
+    /// Compare `self` against `other` for equality using Nix equality semantics
+    pub fn nix_eq(&self, other: &Self) -> Result<bool, ErrorKind> {
+        match (self, other) {
+            // Trivial comparisons
+            (Value::Null, Value::Null) => Ok(true),
+            (Value::Bool(b1), Value::Bool(b2)) => Ok(b1 == b2),
+            (Value::List(l1), Value::List(l2)) => l1.nix_eq(l2),
+            (Value::String(s1), Value::String(s2)) => Ok(s1 == s2),
+            (Value::Path(p1), Value::Path(p2)) => Ok(p1 == p2),
+
+            // Numerical comparisons (they work between float & int)
+            (Value::Integer(i1), Value::Integer(i2)) => Ok(i1 == i2),
+            (Value::Integer(i), Value::Float(f)) => Ok(*i as f64 == *f),
+            (Value::Float(f1), Value::Float(f2)) => Ok(f1 == f2),
+            (Value::Float(f), Value::Integer(i)) => Ok(*i as f64 == *f),
+
+            // Optimised attribute set comparison
+            (Value::Attrs(a1), Value::Attrs(a2)) => Ok(Rc::ptr_eq(a1, a2) || a1.nix_eq(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), Value::Thunk(rhs)) => Ok(*lhs.value() == *rhs.value()),
+            (Value::Thunk(lhs), rhs) => Ok(&*lhs.value() == rhs),
+            (lhs, Value::Thunk(rhs)) => Ok(lhs == &*rhs.value()),
+
+            // Everything else is either incomparable (e.g. internal
+            // types) or false.
+            // TODO(tazjin): mirror Lambda equality behaviour
+            _ => Ok(false),
+        }
+    }
 }
 
 impl Display for Value {
@@ -291,41 +325,6 @@ impl Display for Value {
     }
 }
 
-impl PartialEq for Value {
-    fn eq(&self, other: &Self) -> bool {
-        match (self, other) {
-            // Trivial comparisons
-            (Value::Null, Value::Null) => true,
-            (Value::Bool(b1), Value::Bool(b2)) => b1 == b2,
-            (Value::List(l1), Value::List(l2)) => l1 == l2,
-            (Value::String(s1), Value::String(s2)) => s1 == s2,
-            (Value::Path(p1), Value::Path(p2)) => p1 == p2,
-
-            // Numerical comparisons (they work between float & int)
-            (Value::Integer(i1), Value::Integer(i2)) => i1 == i2,
-            (Value::Integer(i), Value::Float(f)) => *i as f64 == *f,
-            (Value::Float(f1), Value::Float(f2)) => f1 == f2,
-            (Value::Float(f), Value::Integer(i)) => *i as f64 == *f,
-
-            // 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), Value::Thunk(rhs)) => *lhs.value() == *rhs.value(),
-            (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
-            _ => false,
-        }
-    }
-}
-
 fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
     ErrorKind::TypeError {
         expected,
@@ -335,16 +334,39 @@ fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
 
 #[cfg(test)]
 mod tests {
-    use crate::properties::eq_laws;
-    use proptest::prelude::ProptestConfig;
-
     use super::*;
 
-    eq_laws!(
-        Value,
-        ProptestConfig {
-            cases: 20,
-            ..Default::default()
+    #[test]
+    fn test_name() {}
+
+    mod nix_eq {
+        use super::*;
+        use proptest::prelude::ProptestConfig;
+        use test_strategy::proptest;
+
+        #[proptest(ProptestConfig { cases: 5, ..Default::default() })]
+        fn reflexive(x: Value) {
+            assert!(x.nix_eq(&x).unwrap())
+        }
+
+        #[proptest(ProptestConfig { cases: 5, ..Default::default() })]
+        fn symmetric(x: Value, y: Value) {
+            assert_eq!(x.nix_eq(&y).unwrap(), y.nix_eq(&x).unwrap())
+        }
+
+        #[proptest(ProptestConfig { cases: 5, ..Default::default() })]
+        fn transitive(x: Value, y: Value, z: Value) {
+            if x.nix_eq(&y).unwrap() && y.nix_eq(&z).unwrap() {
+                assert!(x.nix_eq(&z).unwrap())
+            }
+        }
+
+        #[test]
+        fn list_int_float_fungibility() {
+            let v1 = Value::List(NixList::from(vec![Value::Integer(1)]));
+            let v2 = Value::List(NixList::from(vec![Value::Float(1.0)]));
+
+            assert!(v1.nix_eq(&v2).unwrap())
         }
-    );
+    }
 }