fix(tvix/eval): implement cppnix JSON-serialisation semantics r/5979

This drops the usage of serde::Serialize, as the trait can not be used
to implement the correct semantics (function colouring!).

Instead, a manual JSON serialisation function is written which
correctly handles toString, outPath and other similar weirdnesses.

Unexpectedly, the eval-okay-tojson test from the C++ Nix test suite
now passes, too.

This fixes an issue where serialising data structures containing
derivations to JSON would fail.

Change-Id: I5c39e3d8356ee93a07eda481410f88610f6dd9f8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8209
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

1 files changed, 9 insertions, 0 deletions

diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index ec697fa84288..2f00ecab8543 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -134,6 +134,9 @@ pub enum ErrorKind {
     /// Errors converting JSON to a value
     FromJsonError(String),
 
+    /// Nix value that can not be serialised to JSON.
+    NotSerialisableToJson(&'static str),
+
     /// Errors converting TOML to a value
     FromTomlError(String),
 
@@ -442,6 +445,10 @@ to a missing value in the attribute set(s) included via `with`."#,
                 write!(f, "Error converting JSON to a Nix value: {msg}")
             }
 
+            ErrorKind::NotSerialisableToJson(_type) => {
+                write!(f, "a {} cannot be converted to JSON", _type)
+            }
+
             ErrorKind::FromTomlError(msg) => {
                 write!(f, "Error converting TOML to a Nix value: {msg}")
             }
@@ -761,6 +768,7 @@ impl Error {
             | ErrorKind::ImportCompilerError { .. }
             | ErrorKind::IO { .. }
             | ErrorKind::FromJsonError(_)
+            | ErrorKind::NotSerialisableToJson(_)
             | ErrorKind::FromTomlError(_)
             | ErrorKind::Xml(_)
             | ErrorKind::TvixError(_)
@@ -809,6 +817,7 @@ impl Error {
             ErrorKind::DivisionByZero => "E033",
             ErrorKind::Xml(_) => "E034",
             ErrorKind::FromTomlError(_) => "E035",
+            ErrorKind::NotSerialisableToJson(_) => "E036",
 
             // Special error code for errors from other Tvix
             // components. We may want to introduce a code namespacing