feat(tvix/eval): report all known spans on infinite recursion r/6026

This reports the span

1. of the code within a thunk,
2. of the place where the thunk was instantiated,
3. of the place where the thunk was first forced,
4. of the place where the thunk was forced again,

when yielding an infinite recursion error, which hopefully makes it
easier to debug them.

The spans are tracked in the ThunkRepr::Blackhole variant when putting
a thunk under evaluation.

Note that we currently have some loss of span precision in the VM loop
when switching between frame types, so spans 3/4 are currently a bit
wonky. Working on it.

Change-Id: Icbd2a9df903d00e8c2545b3fc46dcd2a9e3e3e55
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8270
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

1 files changed, 37 insertions, 12 deletions

diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 76a3da3ff52f..2fbb6496ceea 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -79,6 +79,8 @@ pub enum ErrorKind {
     /// Infinite recursion encountered while forcing thunks.
     InfiniteRecursion {
         first_force: Span,
+        suspended_at: Option<Span>,
+        content_span: Option<Span>,
     },
 
     ParseErrors(Vec<rnix::parser::ParseError>),
@@ -871,19 +873,42 @@ impl Error {
                 ]
             }
 
-            ErrorKind::InfiniteRecursion { first_force } => {
-                vec![
-                    SpanLabel {
-                        label: Some("first requested here".into()),
-                        span: *first_force,
+            ErrorKind::InfiniteRecursion {
+                first_force,
+                suspended_at,
+                content_span,
+            } => {
+                let mut spans = vec![];
+
+                if let Some(content_span) = content_span {
+                    spans.push(SpanLabel {
+                        label: Some("this lazily-evaluated code".into()),
+                        span: *content_span,
                         style: SpanStyle::Secondary,
-                    },
-                    SpanLabel {
-                        label: Some("requested again here".into()),
-                        span: self.span,
-                        style: SpanStyle::Primary,
-                    },
-                ]
+                    })
+                }
+
+                if let Some(suspended_at) = suspended_at {
+                    spans.push(SpanLabel {
+                        label: Some("which was instantiated here".into()),
+                        span: *suspended_at,
+                        style: SpanStyle::Secondary,
+                    })
+                }
+
+                spans.push(SpanLabel {
+                    label: Some("was first requested to be evaluated here".into()),
+                    span: *first_force,
+                    style: SpanStyle::Secondary,
+                });
+
+                spans.push(SpanLabel {
+                    label: Some("but then requested again here during its own evaluation".into()),
+                    span: self.span,
+                    style: SpanStyle::Primary,
+                });
+
+                spans
             }
 
             // All other errors pretty much have the same shape.