feat(tvix/eval): implement builtins.toJSON

Implements `Serialize` for `tvix_eval::Value`. Special care is taken
with serialisation of attribute sets, and forcing of thunks.

The tests should cover both cases well.

Change-Id: I9bb135bacf6f87bc6bd0bd88cef0a42308e6c335
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7803
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

tvix/eval/src/value/attrs.rs

@@ -9,7 +9,8 @@ use std::iter::FromIterator;
 
 use imbl::{ordmap, OrdMap};
 use serde::de::{Deserializer, Error, Visitor};
-use serde::Deserialize;
+use serde::ser::SerializeMap;
+use serde::{Deserialize, Serialize};
 
 use crate::errors::ErrorKind;
 use crate::vm::VM;
@@ -140,6 +141,24 @@ impl TotalDisplay for NixAttrs {
     }
 }
 
+impl Serialize for NixAttrs {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        match &self.0 {
+            AttrsRep::Empty => serializer.serialize_map(Some(0))?.end(),
+            AttrsRep::KV { name, value } => {
+                let mut map = serializer.serialize_map(Some(2))?;
+                map.serialize_entry("name", name)?;
+                map.serialize_entry("value", value)?;
+                map.end()
+            }
+            AttrsRep::Im(map) => map.serialize(serializer),
+        }
+    }
+}
+
 impl<'de> Deserialize<'de> for NixAttrs {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where

tvix/eval/src/value/list.rs

@@ -3,7 +3,7 @@ use std::ops::Index;
 
 use imbl::{vector, Vector};
 
-use serde::Deserialize;
+use serde::{Deserialize, Serialize};
 
 use crate::errors::ErrorKind;
 use crate::vm::VM;
@@ -13,7 +13,7 @@ use super::TotalDisplay;
 use super::Value;
 
 #[repr(transparent)]
-#[derive(Clone, Debug, Deserialize)]
+#[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct NixList(Vector<Value>);
 
 impl TotalDisplay for NixList {

tvix/eval/src/value/mod.rs

@@ -6,7 +6,7 @@ use std::path::PathBuf;
 use std::rc::Rc;
 use std::{cell::Ref, fmt::Display};
 
-use serde::Deserialize;
+use serde::{Deserialize, Serialize};
 
 #[cfg(feature = "arbitrary")]
 mod arbitrary;
@@ -33,7 +33,7 @@ pub use thunk::Thunk;
 use self::thunk::ThunkSet;
 
 #[warn(variant_size_differences)]
-#[derive(Clone, Debug, Deserialize)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 #[serde(untagged)]
 pub enum Value {
     Null,
@@ -49,12 +49,13 @@ pub enum Value {
 
     #[serde(skip)]
     Closure(Rc<Closure>), // must use Rc<Closure> here in order to get proper pointer equality
+
     #[serde(skip)]
     Builtin(Builtin),
 
     // Internal values that, while they technically exist at runtime,
     // are never returned to or created directly by users.
-    #[serde(skip)]
+    #[serde(skip_deserializing)]
     Thunk(Thunk),
 
     // See [`compiler::compile_select_or()`] for explanation

tvix/eval/src/value/string.rs

@@ -9,16 +9,17 @@ use std::path::Path;
 use std::{borrow::Cow, fmt::Display, str::Chars};
 
 use serde::de::{Deserializer, Visitor};
-use serde::Deserialize;
+use serde::{Deserialize, Serialize};
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize)]
+#[serde(untagged)]
 enum StringRepr {
     Smol(SmolStr),
     Heap(String),
 }
 
 #[repr(transparent)]
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize)]
 pub struct NixString(StringRepr);
 
 impl PartialEq for NixString {

tvix/eval/src/value/thunk.rs

@@ -24,6 +24,8 @@ use std::{
     rc::Rc,
 };
 
+use serde::Serialize;
+
 use crate::{
     chunk::Chunk,
     errors::{Error, ErrorKind},
@@ -329,6 +331,15 @@ impl TotalDisplay for Thunk {
     }
 }
 
+impl Serialize for Thunk {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        self.value().serialize(serializer)
+    }
+}
+
 /// A wrapper type for tracking which thunks have already been seen in a
 /// context. This is necessary for cycle detection.
 ///