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, 2 insertions, 2 deletions

diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 5ab095b22056..6128e3a601a0 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -744,8 +744,8 @@ impl<'o> VM<'o> {
                     let gen_span = frame.current_light_span();
                     self.push_call_frame(span, frame);
 
-                    self.enqueue_generator("coerce_to_string", gen_span, |co| {
-                        value.coerce_to_string(co, CoercionKind::Weak)
+                    self.enqueue_generator("coerce_to_string", gen_span.clone(), |co| {
+                        value.coerce_to_string(co, CoercionKind::Weak, gen_span)
                     });
 
                     return Ok(false);