feat(tvix/eval): nonrecursive coerce_to_string() r/7176

After this commit, the only non-builtins uses of generators are:

  - coerce_to_string() uses generators::request_enter_lambda()
  - Thunk::force() uses generators::request_enter_lambda()

That's it!  Once those two are taken care of, GenCo can become an
implementation detail of `builtins::BuiltinGen`.  No more crazy
nonlocal flow control within the interpreter: if you've got a GenCo
floating around in your code it's because you're writing a builtin,
which isn't part of the core interpreter.  The interpreter won't
need GenCos to talk to itself anymore.

Technically generators::request_path_import() is also used by
coerce_to_string(), but that's just because the io_handle happens to
be part of the VM.  There's no recursion-depth issue there, so the
call doesn't need to go through the generator mechanism
(request_path_import() doesn't call back to the interpreter!)

Change-Id: I83ce5774d49b88fdafdd61160975b4937a435bb0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10256
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: Adam Joseph <adam@westernsemico.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/tvix/eval/src/value/json.rs b/tvix/eval/src/value/json.rs
index bff04487fa50..f7ecf121de5e 100644
--- a/tvix/eval/src/value/json.rs
+++ b/tvix/eval/src/value/json.rs
@@ -44,8 +44,16 @@ impl Value {
                 // Attribute sets with a callable `__toString` attribute
                 // serialise to the string-coerced version of the result of
                 // calling that.
-                if let Some(s) = attrs.try_to_string(co, CoercionKind::Weak).await {
-                    return Ok(Ok(Json::String(s.as_str().to_string())));
+                if attrs.select("__toString").is_some() {
+                    let span = generators::request_span(co).await;
+                    match Value::Attrs(attrs)
+                        .coerce_to_string_(co, CoercionKind::Weak, span)
+                        .await?
+                    {
+                        Value::Catchable(cek) => return Ok(Err(cek)),
+                        Value::String(s) => return Ok(Ok(Json::String(s.as_str().to_owned()))),
+                        _ => panic!("Value::coerce_to_string_() returned a non-string!"),
+                    }
                 }
 
                 // Attribute sets with an `outPath` attribute