feat(tvix/eval): contextful JSON operations r/7899

`toJSON` transform a Nix structure into a JSON string.

For each context in that Nix structure, the JSON string must possess it.

Thus, it is necessary to take the union of all contexts and attach it to
the final structure.

Unfortunately, the return type of `into_json` is a serde's JSON object,
not a string. Therefore, it is not possible to reuse `NixString`
machinery.

Context tests are reinforced as Nix does not test those behaviors.

Fixes b/393.

Change-Id: I5afdbc4e18dd70469192c1aa657d1049ba330149
Signed-off-by: Ryan Lahfa <tvl@lahfa.xyz>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11266
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

2 files changed, 56 insertions, 19 deletions

diff --git a/tvix/eval/src/value/json.rs b/tvix/eval/src/value/json.rs
index 7ab1655d9055..c48e9c1f4e85 100644
--- a/tvix/eval/src/value/json.rs
+++ b/tvix/eval/src/value/json.rs
@@ -6,6 +6,7 @@
 use super::{CoercionKind, Value};
 use crate::errors::{CatchableErrorKind, ErrorKind};
 use crate::generators::{self, GenCo};
+use crate::NixContext;
 
 use bstr::ByteSlice;
 use serde_json::value::to_value;
@@ -13,22 +14,32 @@ use serde_json::Value as Json; // name clash with *our* `Value`
 use serde_json::{Map, Number};
 
 impl Value {
-    pub async fn into_json(
+    /// Transforms the structure into a JSON
+    /// and accumulate all encountered context in the second's element
+    /// of the return type.
+    pub async fn into_contextful_json(
         self,
         co: &GenCo,
-    ) -> Result<Result<Json, CatchableErrorKind>, ErrorKind> {
+    ) -> Result<Result<(Json, NixContext), CatchableErrorKind>, ErrorKind> {
         let self_forced = generators::request_force(co, self).await;
+        let mut context = NixContext::new();
 
         let value = match self_forced {
             Value::Null => Json::Null,
             Value::Bool(b) => Json::Bool(b),
             Value::Integer(i) => Json::Number(Number::from(i)),
             Value::Float(f) => to_value(f)?,
-            Value::String(s) => Json::String(s.to_str()?.to_owned()),
+            Value::String(s) => {
+                context.mimic(&s);
+
+                Json::String(s.to_str()?.to_owned())
+            }
 
             Value::Path(p) => {
                 let imported = generators::request_path_import(co, *p).await;
-                Json::String(imported.to_string_lossy().to_string())
+                let path = imported.to_string_lossy().to_string();
+                context = context.append(crate::NixContextElement::Plain(path.clone()));
+                Json::String(path)
             }
 
             Value::List(l) => {
@@ -36,7 +47,10 @@ impl Value {
 
                 for val in l.into_iter() {
                     match generators::request_to_json(co, val).await {
-                        Ok(v) => out.push(v),
+                        Ok((v, mut ctx)) => {
+                            context = context.join(&mut ctx);
+                            out.push(v)
+                        }
                         Err(cek) => return Ok(Err(cek)),
                     }
                 }
@@ -62,7 +76,14 @@ impl Value {
                         .await?
                     {
                         Value::Catchable(cek) => return Ok(Err(*cek)),
-                        Value::String(s) => return Ok(Ok(Json::String(s.to_str()?.to_owned()))),
+                        Value::String(s) => {
+                            // We need a fresh context here because `__toString` will discard
+                            // everything.
+                            let mut fresh = NixContext::new();
+                            fresh.mimic(&s);
+
+                            return Ok(Ok((Json::String(s.to_str()?.to_owned()), fresh)));
+                        }
                         _ => panic!("Value::coerce_to_string_() returned a non-string!"),
                     }
                 }
@@ -79,7 +100,10 @@ impl Value {
                     out.insert(
                         name.to_str()?.to_owned(),
                         match generators::request_to_json(co, value).await {
-                            Ok(v) => v,
+                            Ok((v, mut ctx)) => {
+                                context = context.join(&mut ctx);
+                                v
+                            }
                             Err(cek) => return Ok(Err(cek)),
                         },
                     );
@@ -97,21 +121,34 @@ impl Value {
             | val @ Value::Blueprint(_)
             | val @ Value::DeferredUpvalue(_)
             | val @ Value::UnresolvedPath(_)
-            | val @ Value::Json(_)
+            | val @ Value::Json(..)
             | val @ Value::FinaliseRequest(_) => {
                 return Err(ErrorKind::NotSerialisableToJson(val.type_of()))
             }
         };
 
-        Ok(Ok(value))
+        Ok(Ok((value, context)))
     }
 
     /// Generator version of the above, which wraps responses in
-    /// Value::Json.
-    pub(crate) async fn into_json_generator(self, co: GenCo) -> Result<Value, ErrorKind> {
-        match self.into_json(&co).await? {
+    /// [`Value::Json`].
+    pub(crate) async fn into_contextful_json_generator(
+        self,
+        co: GenCo,
+    ) -> Result<Value, ErrorKind> {
+        match self.into_contextful_json(&co).await? {
             Err(cek) => Ok(Value::from(cek)),
-            Ok(json) => Ok(Value::Json(Box::new(json))),
+            Ok((json, ctx)) => Ok(Value::Json(Box::new((json, ctx)))),
         }
     }
+
+    /// Transforms the structure into a JSON
+    /// All the accumulated context is ignored, use [`into_contextful_json`]
+    /// to obtain the resulting context of the JSON object.
+    pub async fn into_json(
+        self,
+        co: &GenCo,
+    ) -> Result<Result<Json, CatchableErrorKind>, ErrorKind> {
+        Ok(self.into_contextful_json(co).await?.map(|(json, _)| json))
+    }
 }
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index e8e27e5968de..c171c9a04eb8 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -78,7 +78,7 @@ pub enum Value {
     #[serde(skip)]
     UnresolvedPath(Box<PathBuf>),
     #[serde(skip)]
-    Json(Box<serde_json::Value>),
+    Json(Box<(serde_json::Value, NixContext)>),
 
     #[serde(skip)]
     FinaliseRequest(bool),
@@ -294,7 +294,7 @@ impl Value {
                 | Value::Blueprint(_)
                 | Value::DeferredUpvalue(_)
                 | Value::UnresolvedPath(_)
-                | Value::Json(_)
+                | Value::Json(..)
                 | Value::FinaliseRequest(_) => panic!(
                     "Tvix bug: internal value left on stack: {}",
                     value.type_of()
@@ -444,7 +444,7 @@ impl Value {
                 | (Value::Blueprint(_), _)
                 | (Value::DeferredUpvalue(_), _)
                 | (Value::UnresolvedPath(_), _)
-                | (Value::Json(_), _)
+                | (Value::Json(..), _)
                 | (Value::FinaliseRequest(_), _) => {
                     panic!("tvix bug: .coerce_to_string() called on internal value")
                 }
@@ -681,7 +681,7 @@ impl Value {
             Value::Blueprint(_) => "internal[blueprint]",
             Value::DeferredUpvalue(_) => "internal[deferred_upvalue]",
             Value::UnresolvedPath(_) => "internal[unresolved_path]",
-            Value::Json(_) => "internal[json]",
+            Value::Json(..) => "internal[json]",
             Value::FinaliseRequest(_) => "internal[finaliser_sentinel]",
             Value::Catchable(_) => "internal[catchable]",
         }
@@ -877,7 +877,7 @@ impl Value {
             | Value::Blueprint(_)
             | Value::DeferredUpvalue(_)
             | Value::UnresolvedPath(_)
-            | Value::Json(_)
+            | Value::Json(..)
             | Value::FinaliseRequest(_) => "an internal Tvix evaluator value".into(),
         }
     }
@@ -991,7 +991,7 @@ impl TotalDisplay for Value {
             Value::Blueprint(_) => f.write_str("internal[blueprint]"),
             Value::DeferredUpvalue(_) => f.write_str("internal[deferred_upvalue]"),
             Value::UnresolvedPath(_) => f.write_str("internal[unresolved_path]"),
-            Value::Json(_) => f.write_str("internal[json]"),
+            Value::Json(..) => f.write_str("internal[json]"),
             Value::FinaliseRequest(_) => f.write_str("internal[finaliser_sentinel]"),
 
             // Delegate thunk display to the type, as it must handle