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Diffstat (limited to 'tvix')
-rw-r--r--tvix/eval/src/compiler/mod.rs3
-rw-r--r--tvix/eval/src/errors.rs129
-rw-r--r--tvix/eval/src/vm/generators.rs22
-rw-r--r--tvix/eval/src/vm/mod.rs217
4 files changed, 242 insertions, 129 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 39b706cb487a..9cbbf62dbd0a 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -1339,7 +1339,8 @@ fn compile_src_builtin(
             file.clone(),
             weak.upgrade().unwrap(),
             &mut crate::observer::NoOpObserver {},
-        )?;
+        )
+        .map_err(|e| ErrorKind::NativeError(Box::new(e)))?;
 
         if !result.errors.is_empty() {
             return Err(ErrorKind::ImportCompilerError {
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 2f00ecab8543..de17f8d332ed 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -81,9 +81,13 @@ pub enum ErrorKind {
 
     ParseErrors(Vec<rnix::parser::ParseError>),
 
-    /// An error occured while forcing a thunk, and needs to be
-    /// chained up.
-    ThunkForce(Box<Error>),
+    /// An error occured while executing some native code (e.g. a
+    /// builtin), and needs to be chained up.
+    NativeError(Box<Error>),
+
+    /// An error occured while executing Tvix bytecode, but needs to
+    /// be chained up.
+    BytecodeError(Box<Error>),
 
     /// Given type can't be coerced to a string in the respective context
     NotCoercibleToString {
@@ -175,7 +179,7 @@ pub enum ErrorKind {
 impl error::Error for Error {
     fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match &self.kind {
-            ErrorKind::ThunkForce(err) => err.source(),
+            ErrorKind::NativeError(err) | ErrorKind::BytecodeError(err) => err.source(),
             ErrorKind::ParseErrors(err) => err.first().map(|e| e as &dyn error::Error),
             ErrorKind::ParseIntError(err) => Some(err),
             ErrorKind::ImportParseError { errors, .. } => {
@@ -216,15 +220,6 @@ impl From<XmlError> for ErrorKind {
     }
 }
 
-/// Implementation used if errors occur while forcing thunks (which
-/// can potentially be threaded through a few contexts, i.e. nested
-/// thunks).
-impl From<Error> for ErrorKind {
-    fn from(e: Error) -> Self {
-        Self::ThunkForce(Box::new(e))
-    }
-}
-
 impl From<io::Error> for ErrorKind {
     fn from(e: io::Error) -> Self {
         ErrorKind::IO {
@@ -239,7 +234,7 @@ impl ErrorKind {
     pub fn is_catchable(&self) -> bool {
         match self {
             Self::Throw(_) | Self::AssertionFailed | Self::NixPathResolution(_) => true,
-            Self::ThunkForce(err) => err.kind.is_catchable(),
+            Self::NativeError(err) | Self::BytecodeError(err) => err.kind.is_catchable(),
             _ => false,
         }
     }
@@ -361,10 +356,8 @@ to a missing value in the attribute set(s) included via `with`."#,
             // Errors themselves ignored here & handled in Self::spans instead
             ErrorKind::ParseErrors(_) => write!(f, "failed to parse Nix code:"),
 
-            // TODO(tazjin): trace through the whole chain of thunk
-            // forcing errors with secondary spans, instead of just
-            // delegating to the inner error
-            ErrorKind::ThunkForce(err) => write!(f, "{err}"),
+            ErrorKind::NativeError(_) => write!(f, "while evaluating this native code"),
+            ErrorKind::BytecodeError(_) => write!(f, "while evaluating this Nix code"),
 
             ErrorKind::NotCoercibleToString { kind, from } => {
                 let kindly = match kind {
@@ -757,7 +750,8 @@ impl Error {
             | ErrorKind::NotCallable(_)
             | ErrorKind::InfiniteRecursion
             | ErrorKind::ParseErrors(_)
-            | ErrorKind::ThunkForce(_)
+            | ErrorKind::NativeError(_)
+            | ErrorKind::BytecodeError(_)
             | ErrorKind::NotCoercibleToString { .. }
             | ErrorKind::NotAnAbsolutePath(_)
             | ErrorKind::ParseIntError(_)
@@ -831,12 +825,9 @@ impl Error {
             // Placeholder error while Tvix is under construction.
             ErrorKind::NotImplemented(_) => "E999",
 
-            // TODO: thunk force errors should yield a chained
-            // diagnostic, but until then we just forward the error
-            // code from the inner error.
-            //
-            // The error code for thunk forces is E017.
-            ErrorKind::ThunkForce(ref err) => err.code(),
+            // Chained errors should yield the code of the innermost
+            // error.
+            ErrorKind::NativeError(ref err) | ErrorKind::BytecodeError(ref err) => err.code(),
 
             ErrorKind::WithContext { .. } => {
                 panic!("internal ErrorKind::WithContext variant leaked")
@@ -855,27 +846,6 @@ impl Error {
                 spans_for_parse_errors(&file, errors)
             }
 
-            // Unwrap thunk errors to the innermost one
-            // TODO: limit the number of intermediates!
-            ErrorKind::ThunkForce(err) => {
-                let mut labels = err.spans(source);
-
-                // Only add this thunk to the "cause chain" if it span isn't
-                // exactly identical to the next-higher level, which is very
-                // common for the last thunk in a chain.
-                if let Some(label) = labels.last() {
-                    if label.span != self.span {
-                        labels.push(SpanLabel {
-                            label: Some("while evaluating this".into()),
-                            span: self.span,
-                            style: SpanStyle::Secondary,
-                        });
-                    }
-                }
-
-                labels
-            }
-
             ErrorKind::UnexpectedArgument { formals_span, .. } => {
                 vec![
                     SpanLabel {
@@ -926,19 +896,82 @@ impl Error {
     /// any) of an error.
     fn diagnostics(&self, source: &SourceCode) -> Vec<Diagnostic> {
         match &self.kind {
-            ErrorKind::ThunkForce(err) => err.diagnostics(source),
-
             ErrorKind::ImportCompilerError { errors, .. } => {
                 let mut out = vec![self.diagnostic(source)];
                 out.extend(errors.iter().map(|e| e.diagnostic(source)));
                 out
             }
 
+            // When encountering either of these error kinds, we are dealing
+            // with the top of an error chain.
+            //
+            // An error chain creates a list of diagnostics which provide trace
+            // information.
+            //
+            // We don't know how deep this chain is, so we avoid recursing in
+            // this function while unrolling the chain.
+            ErrorKind::NativeError(next) | ErrorKind::BytecodeError(next) => {
+                // Accumulated diagnostics to return.
+                let mut diagnostics: Vec<Diagnostic> = vec![];
+
+                // The next (inner) error to add to the diagnostics, after this
+                // one.
+                let mut next = *next.clone();
+
+                // Diagnostic message for *this* error.
+                let mut this_message = self.to_string();
+
+                // Primary span for *this* error.
+                let mut this_span = self.span;
+
+                // Diagnostic spans for *this* error.
+                let mut this_spans = self.spans(source);
+
+                loop {
+                    if is_new_span(
+                        this_span,
+                        diagnostics.last().and_then(|last| last.spans.last()),
+                    ) {
+                        diagnostics.push(Diagnostic {
+                            level: Level::Note,
+                            message: this_message,
+                            spans: this_spans,
+                            code: None, // only the top-level error has one
+                        });
+                    }
+
+                    this_message = next.to_string();
+                    this_span = next.span;
+                    this_spans = next.spans(source);
+
+                    match next.kind {
+                        ErrorKind::NativeError(inner) | ErrorKind::BytecodeError(inner) => {
+                            next = *inner;
+                            continue;
+                        }
+                        _ => {
+                            diagnostics.extend(next.diagnostics(source));
+                            break;
+                        }
+                    }
+                }
+
+                diagnostics
+            }
+
             _ => vec![self.diagnostic(source)],
         }
     }
 }
 
+// Check if this error is in a different span from its immediate ancestor.
+fn is_new_span(this_span: Span, parent: Option<&SpanLabel>) -> bool {
+    match parent {
+        None => true,
+        Some(parent) => parent.span != this_span,
+    }
+}
+
 // Convenience methods to add context on other types.
 pub trait AddContext {
     /// Add context to the error-carrying type.
diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
index b05ba3d7c4df..853ab063465f 100644
--- a/tvix/eval/src/vm/generators.rs
+++ b/tvix/eval/src/vm/generators.rs
@@ -400,19 +400,15 @@ impl<'o> VM<'o> {
                         }
 
                         VMRequest::PathImport(path) => {
-                            let imported = self
-                                .io_handle
-                                .import_path(&path)
-                                .map_err(|kind| Error::new(kind, span.span()))?;
+                            let imported =
+                                self.io_handle.import_path(&path).with_span(&span, self)?;
 
                             message = VMResponse::Path(imported);
                         }
 
                         VMRequest::ReadToString(path) => {
-                            let content = self
-                                .io_handle
-                                .read_to_string(path)
-                                .map_err(|kind| Error::new(kind, span.span()))?;
+                            let content =
+                                self.io_handle.read_to_string(path).with_span(&span, self)?;
 
                             message = VMResponse::Value(Value::String(content.into()))
                         }
@@ -422,17 +418,13 @@ impl<'o> VM<'o> {
                                 .io_handle
                                 .path_exists(path)
                                 .map(Value::Bool)
-                                .map_err(|kind| Error::new(kind, span.span()))?;
+                                .with_span(&span, self)?;
 
                             message = VMResponse::Value(exists);
                         }
 
                         VMRequest::ReadDir(path) => {
-                            let dir = self
-                                .io_handle
-                                .read_dir(path)
-                                .map_err(|kind| Error::new(kind, span.span()))?;
-
+                            let dir = self.io_handle.read_dir(path).with_span(&span, self)?;
                             message = VMResponse::Directory(dir);
                         }
 
@@ -466,7 +458,7 @@ impl<'o> VM<'o> {
                 // Generator has completed, and its result value should
                 // be left on the stack.
                 genawaiter::GeneratorState::Complete(result) => {
-                    let value = result.map_err(|kind| Error::new(kind, span.span()))?;
+                    let value = result.with_span(&span, self)?;
                     self.stack.push(value);
                     return Ok(true);
                 }
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index fbecab959054..60039eab3342 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -12,6 +12,7 @@
 pub mod generators;
 mod macros;
 
+use codemap::Span;
 use serde_json::json;
 use std::{cmp::Ordering, collections::HashMap, ops::DerefMut, path::PathBuf, rc::Rc};
 
@@ -39,15 +40,75 @@ use generators::{call_functor, Generator, GeneratorState};
 
 use self::generators::{VMRequest, VMResponse};
 
+/// Internal helper trait for taking a span from a variety of types, to make use
+/// of `WithSpan` (defined below) more ergonomic at call sites.
+trait GetSpan {
+    fn get_span(self) -> Span;
+}
+
+impl<'o> GetSpan for &VM<'o> {
+    fn get_span(self) -> Span {
+        self.reasonable_span.span()
+    }
+}
+
+impl GetSpan for &CallFrame {
+    fn get_span(self) -> Span {
+        self.current_span()
+    }
+}
+
+impl GetSpan for &LightSpan {
+    fn get_span(self) -> Span {
+        self.span()
+    }
+}
+
+impl GetSpan for Span {
+    fn get_span(self) -> Span {
+        self
+    }
+}
+
 /// Internal helper trait for ergonomically converting from a `Result<T,
-/// ErrorKind>` to a `Result<T, Error>` using the current span of a call frame.
-trait WithSpan<T> {
-    fn with_span(self, frame: &CallFrame) -> Result<T, Error>;
+/// ErrorKind>` to a `Result<T, Error>` using the current span of a call frame,
+/// and chaining the VM's frame stack around it for printing a cause chain.
+trait WithSpan<T, S: GetSpan> {
+    fn with_span(self, top_span: S, vm: &VM) -> Result<T, Error>;
 }
 
-impl<T> WithSpan<T> for Result<T, ErrorKind> {
-    fn with_span(self, frame: &CallFrame) -> Result<T, Error> {
-        self.map_err(|kind| frame.error(kind))
+impl<T, S: GetSpan> WithSpan<T, S> for Result<T, ErrorKind> {
+    fn with_span(self, top_span: S, vm: &VM) -> Result<T, Error> {
+        match self {
+            Ok(something) => Ok(something),
+            Err(kind) => {
+                let mut error = Error::new(kind, top_span.get_span());
+
+                // Short-circuit the wrapping if we're dealing with tryEval, in
+                // which case the error is hidden and does not need to be
+                // exhaustive.
+                if !vm.try_eval_frames.is_empty() && error.kind.is_catchable() {
+                    return Err(error);
+                }
+
+                // Wrap the top-level error in chaining errors for each element
+                // of the frame stack.
+                for frame in vm.frames.iter().rev() {
+                    match frame {
+                        Frame::CallFrame { span, .. } => {
+                            error =
+                                Error::new(ErrorKind::BytecodeError(Box::new(error)), span.span());
+                        }
+                        Frame::Generator { span, .. } => {
+                            error =
+                                Error::new(ErrorKind::NativeError(Box::new(error)), span.span());
+                        }
+                    }
+                }
+
+                Err(error)
+            }
+        }
     }
 }
 
@@ -86,10 +147,16 @@ impl CallFrame {
         op
     }
 
-    /// Construct an error from the given ErrorKind and the source span of the
-    /// current instruction.
-    pub fn error(&self, kind: ErrorKind) -> Error {
-        Error::new(kind, self.chunk().get_span(self.ip - 1))
+    /// Construct an error result from the given ErrorKind and the source span
+    /// of the current instruction.
+    pub fn error<T>(&self, vm: &VM, kind: ErrorKind) -> Result<T, Error> {
+        Err(kind).with_span(self, vm)
+    }
+
+    /// Returns the current span. This is potentially expensive and should only
+    /// be used when actually constructing an error or warning.
+    pub fn current_span(&self) -> Span {
+        self.chunk().get_span(self.ip - 1)
     }
 
     /// Returns the information needed to calculate the current span,
@@ -475,23 +542,26 @@ impl<'o> VM<'o> {
                 }
 
                 OpCode::OpAttrsSelect => {
-                    let key = self.stack_pop().to_str().with_span(&frame)?;
-                    let attrs = self.stack_pop().to_attrs().with_span(&frame)?;
+                    let key = self.stack_pop().to_str().with_span(&frame, self)?;
+                    let attrs = self.stack_pop().to_attrs().with_span(&frame, self)?;
 
                     match attrs.select(key.as_str()) {
                         Some(value) => self.stack.push(value.clone()),
 
                         None => {
-                            return Err(frame.error(ErrorKind::AttributeNotFound {
-                                name: key.as_str().to_string(),
-                            }))
+                            return frame.error(
+                                self,
+                                ErrorKind::AttributeNotFound {
+                                    name: key.as_str().to_string(),
+                                },
+                            );
                         }
                     }
                 }
 
                 OpCode::OpJumpIfFalse(JumpOffset(offset)) => {
                     debug_assert!(offset != 0);
-                    if !self.stack_peek(0).as_bool().with_span(&frame)? {
+                    if !self.stack_peek(0).as_bool().with_span(&frame, self)? {
                         frame.ip += offset;
                     }
                 }
@@ -501,7 +571,7 @@ impl<'o> VM<'o> {
                 }
 
                 OpCode::OpAttrsTrySelect => {
-                    let key = self.stack_pop().to_str().with_span(&frame)?;
+                    let key = self.stack_pop().to_str().with_span(&frame, self)?;
                     let value = match self.stack_pop() {
                         Value::Attrs(attrs) => match attrs.select(key.as_str()) {
                             Some(value) => value.clone(),
@@ -550,24 +620,27 @@ impl<'o> VM<'o> {
                 OpCode::OpAssertBool => {
                     let val = self.stack_peek(0);
                     if !val.is_bool() {
-                        return Err(frame.error(ErrorKind::TypeError {
-                            expected: "bool",
-                            actual: val.type_of(),
-                        }));
+                        return frame.error(
+                            self,
+                            ErrorKind::TypeError {
+                                expected: "bool",
+                                actual: val.type_of(),
+                            },
+                        );
                     }
                 }
 
                 OpCode::OpAttrs(Count(count)) => self.run_attrset(&frame, count)?,
 
                 OpCode::OpAttrsUpdate => {
-                    let rhs = self.stack_pop().to_attrs().with_span(&frame)?;
-                    let lhs = self.stack_pop().to_attrs().with_span(&frame)?;
+                    let rhs = self.stack_pop().to_attrs().with_span(&frame, self)?;
+                    let lhs = self.stack_pop().to_attrs().with_span(&frame, self)?;
 
                     self.stack.push(Value::attrs(lhs.update(*rhs)))
                 }
 
                 OpCode::OpInvert => {
-                    let v = self.stack_pop().as_bool().with_span(&frame)?;
+                    let v = self.stack_pop().as_bool().with_span(&frame, self)?;
                     self.stack.push(Value::Bool(!v));
                 }
 
@@ -580,13 +653,13 @@ impl<'o> VM<'o> {
 
                 OpCode::OpJumpIfTrue(JumpOffset(offset)) => {
                     debug_assert!(offset != 0);
-                    if self.stack_peek(0).as_bool().with_span(&frame)? {
+                    if self.stack_peek(0).as_bool().with_span(&frame, self)? {
                         frame.ip += offset;
                     }
                 }
 
                 OpCode::OpHasAttr => {
-                    let key = self.stack_pop().to_str().with_span(&frame)?;
+                    let key = self.stack_pop().to_str().with_span(&frame, self)?;
                     let result = match self.stack_pop() {
                         Value::Attrs(attrs) => attrs.contains(key.as_str()),
 
@@ -599,13 +672,21 @@ impl<'o> VM<'o> {
                 }
 
                 OpCode::OpConcat => {
-                    let rhs = self.stack_pop().to_list().with_span(&frame)?.into_inner();
-                    let lhs = self.stack_pop().to_list().with_span(&frame)?.into_inner();
+                    let rhs = self
+                        .stack_pop()
+                        .to_list()
+                        .with_span(&frame, self)?
+                        .into_inner();
+                    let lhs = self
+                        .stack_pop()
+                        .to_list()
+                        .with_span(&frame, self)?
+                        .into_inner();
                     self.stack.push(Value::List(NixList::from(lhs + rhs)))
                 }
 
                 OpCode::OpResolveWith => {
-                    let ident = self.stack_pop().to_str().with_span(&frame)?;
+                    let ident = self.stack_pop().to_str().with_span(&frame, self)?;
 
                     // Re-enqueue this frame.
                     let op_span = frame.current_light_span();
@@ -664,13 +745,16 @@ impl<'o> VM<'o> {
                         "OpValidateClosedFormals called within the frame of a lambda without formals",
                     );
 
-                    let args = self.stack_peek(0).to_attrs().with_span(&frame)?;
+                    let args = self.stack_peek(0).to_attrs().with_span(&frame, self)?;
                     for arg in args.keys() {
                         if !formals.contains(arg) {
-                            return Err(frame.error(ErrorKind::UnexpectedArgument {
-                                arg: arg.clone(),
-                                formals_span: formals.span,
-                            }));
+                            return frame.error(
+                                self,
+                                ErrorKind::UnexpectedArgument {
+                                    arg: arg.clone(),
+                                    formals_span: formals.span,
+                                },
+                            );
                         }
                     }
                 }
@@ -692,14 +776,14 @@ impl<'o> VM<'o> {
                 OpCode::OpSub => {
                     let b = self.stack_pop();
                     let a = self.stack_pop();
-                    let result = arithmetic_op!(&a, &b, -).with_span(&frame)?;
+                    let result = arithmetic_op!(&a, &b, -).with_span(&frame, self)?;
                     self.stack.push(result);
                 }
 
                 OpCode::OpMul => {
                     let b = self.stack_pop();
                     let a = self.stack_pop();
-                    let result = arithmetic_op!(&a, &b, *).with_span(&frame)?;
+                    let result = arithmetic_op!(&a, &b, *).with_span(&frame, self)?;
                     self.stack.push(result);
                 }
 
@@ -707,15 +791,15 @@ impl<'o> VM<'o> {
                     let b = self.stack_pop();
 
                     match b {
-                        Value::Integer(0) => return Err(frame.error(ErrorKind::DivisionByZero)),
+                        Value::Integer(0) => return frame.error(self, ErrorKind::DivisionByZero),
                         Value::Float(b) if b == 0.0_f64 => {
-                            return Err(frame.error(ErrorKind::DivisionByZero))
+                            return frame.error(self, ErrorKind::DivisionByZero)
                         }
                         _ => {}
                     };
 
                     let a = self.stack_pop();
-                    let result = arithmetic_op!(&a, &b, /).with_span(&frame)?;
+                    let result = arithmetic_op!(&a, &b, /).with_span(&frame, self)?;
                     self.stack.push(result);
                 }
 
@@ -723,10 +807,13 @@ impl<'o> VM<'o> {
                     Value::Integer(i) => self.stack.push(Value::Integer(-i)),
                     Value::Float(f) => self.stack.push(Value::Float(-f)),
                     v => {
-                        return Err(frame.error(ErrorKind::TypeError {
-                            expected: "number (either int or float)",
-                            actual: v.type_of(),
-                        }));
+                        return frame.error(
+                            self,
+                            ErrorKind::TypeError {
+                                expected: "number (either int or float)",
+                                actual: v.type_of(),
+                            },
+                        );
                     }
                 },
 
@@ -740,7 +827,7 @@ impl<'o> VM<'o> {
                         let resolved = self
                             .nix_search_path
                             .resolve(&*self.io_handle, *path)
-                            .with_span(&frame)?;
+                            .with_span(&frame, self)?;
                         self.stack.push(resolved.into());
                     }
 
@@ -751,9 +838,12 @@ impl<'o> VM<'o> {
                     Value::UnresolvedPath(path) => {
                         match dirs::home_dir() {
                             None => {
-                                return Err(frame.error(ErrorKind::RelativePathResolution(
-                                    "failed to determine home directory".into(),
-                                )));
+                                return frame.error(
+                                    self,
+                                    ErrorKind::RelativePathResolution(
+                                        "failed to determine home directory".into(),
+                                    ),
+                                );
                             }
                             Some(mut buf) => {
                                 buf.push(*path);
@@ -774,7 +864,7 @@ impl<'o> VM<'o> {
                 }
 
                 OpCode::OpAssertFail => {
-                    return Err(frame.error(ErrorKind::AssertionFailed));
+                    frame.error(self, ErrorKind::AssertionFailed)?;
                 }
 
                 // Data-carrying operands should never be executed,
@@ -802,7 +892,7 @@ impl<'o> VM<'o> {
 
     fn run_attrset(&mut self, frame: &CallFrame, count: usize) -> EvalResult<()> {
         let attrs = NixAttrs::construct(count, self.stack.split_off(self.stack.len() - count * 2))
-            .with_span(frame)?;
+            .with_span(frame, self)?;
 
         self.stack.push(Value::attrs(attrs));
         Ok(())
@@ -842,7 +932,7 @@ impl<'o> VM<'o> {
         let mut out = String::new();
 
         for _ in 0..count {
-            out.push_str(self.stack_pop().to_str().with_span(frame)?.as_str());
+            out.push_str(self.stack_pop().to_str().with_span(frame, self)?.as_str());
         }
 
         self.stack.push(Value::String(out.into()));
@@ -855,12 +945,6 @@ impl<'o> VM<'o> {
         self.reasonable_span.clone()
     }
 
-    /// Construct an error from the given ErrorKind and the source
-    /// span of the current instruction.
-    pub fn error(&self, kind: ErrorKind) -> Error {
-        Error::new(kind, self.reasonable_span.span())
-    }
-
     /// Apply an argument from the stack to a builtin, and attempt to call it.
     ///
     /// All calls are tail-calls in Tvix, as every function application is a
@@ -932,7 +1016,7 @@ impl<'o> VM<'o> {
                 self.enqueue_generator("__functor call", gen_span, |co| call_functor(co, val));
                 Ok(())
             }
-            v => Err(self.error(ErrorKind::NotCallable(v.type_of()))),
+            v => Err(ErrorKind::NotCallable(v.type_of())).with_span(&span, self),
         }
     }
 
@@ -951,14 +1035,17 @@ impl<'o> VM<'o> {
                     let val = match self.stack.get(idx) {
                         Some(val) => val.clone(),
                         None => {
-                            return Err(frame.error(ErrorKind::TvixBug {
-                                msg: "upvalue to be captured was missing on stack",
-                                metadata: Some(Rc::new(json!({
-                                    "ip": format!("{:#x}", frame.ip.0 - 1),
-                                    "stack_idx(relative)": stack_idx,
-                                    "stack_idx(absolute)": idx,
-                                }))),
-                            }))
+                            return frame.error(
+                                self,
+                                ErrorKind::TvixBug {
+                                    msg: "upvalue to be captured was missing on stack",
+                                    metadata: Some(Rc::new(json!({
+                                        "ip": format!("{:#x}", frame.ip.0 - 1),
+                                        "stack_idx(relative)": stack_idx,
+                                        "stack_idx(absolute)": idx,
+                                    }))),
+                                },
+                            );
                         }
                     };