fix(tvix/eval): thread thunk forcing errors through correctly r/4742

With this, if an error occurs while forcing a thunk (which is very
likely) it is threaded through to the top by wrapping it in the
ErrorKind::ThunkForce variant.

We could use this to generate "stacktrace-like" error output if we
wanted, or simply jump through and discard everything except the
innermost error.

Change-Id: I3c1c8708c2f73ae062815adf490ce935b1979da8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6409
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

2 files changed, 7 insertions, 3 deletions

diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 59142f2efe64..5f1a24b54b4b 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -48,6 +48,10 @@ pub enum ErrorKind {
     // These are user-generated errors through builtins.
     Throw(String),
     Abort(String),
+
+    // An error occured while forcing a thunk, and needs to be chained
+    // up.
+    ThunkForce(Box<Error>),
 }
 
 #[derive(Clone, Debug)]
diff --git a/tvix/eval/src/value/thunk.rs b/tvix/eval/src/value/thunk.rs
index c2552284fe20..4fd41689c70c 100644
--- a/tvix/eval/src/value/thunk.rs
+++ b/tvix/eval/src/value/thunk.rs
@@ -85,9 +85,9 @@ impl Thunk {
                         std::mem::replace(&mut *thunk_mut, ThunkRepr::Blackhole)
                     {
                         vm.call(lambda, upvalues, 0);
-                        // TODO: find a cheap way to actually retain
-                        // the original error span
-                        *thunk_mut = ThunkRepr::Evaluated(vm.run().map_err(|e| e.kind)?);
+                        *thunk_mut = ThunkRepr::Evaluated(
+                            vm.run().map_err(|e| ErrorKind::ThunkForce(Box::new(e)))?,
+                        );
                     }
                 }
             }