fix(tvix/eval): Actually trace spans for thunks r/5097

Currently, the span on *all* thunk force errors is the span at which the
thunk is forced, which for recursive thunk forcing ends up just being
the same span over and over again. This changes the span on thunk force
errors to be the span at which point the thunk is *created*, which is a
bit more helpful (though the printing atm is a little... crowded). To
make this work, we have to thread through the span at which a thunk is
created into a field on the thunk itself.

Change-Id: I81474810a763046e2eb3a8f07acf7d8ec708824a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6932
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 2 insertions, 2 deletions

diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 924e8a9ebd..c805b70218 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -186,7 +186,7 @@ impl<'o> VM<'o> {
 
     /// Returns the source span of the instruction currently being
     /// executed.
-    fn current_span(&self) -> codemap::Span {
+    pub(crate) fn current_span(&self) -> codemap::Span {
         self.chunk().get_span(self.frame().ip - 1)
     }
 
@@ -637,7 +637,7 @@ impl<'o> VM<'o> {
                     };
 
                     let upvalue_count = blueprint.upvalue_count;
-                    let thunk = Thunk::new(blueprint);
+                    let thunk = Thunk::new(blueprint, self.current_span());
                     let upvalues = thunk.upvalues_mut();
 
                     self.push(Value::Thunk(thunk.clone()));