5 files changed, 99 insertions, 34 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 92084b031c6c..4a50a3be4935 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -157,12 +157,14 @@ impl<'observer> Compiler<'observer> {
         let mut root_dir = match location {
             Some(dir) if cfg!(target_arch = "wasm32") || dir.is_absolute() => Ok(dir),
             _ => {
-                let current_dir = std::env::current_dir().map_err(|e| Error {
-                    kind: ErrorKind::RelativePathResolution(format!(
-                        "could not determine current directory: {}",
-                        e
-                    )),
-                    span: file.span,
+                let current_dir = std::env::current_dir().map_err(|e| {
+                    Error::new(
+                        ErrorKind::RelativePathResolution(format!(
+                            "could not determine current directory: {}",
+                            e
+                        )),
+                        file.span,
+                    )
                 })?;
                 if let Some(dir) = location {
                     Ok(current_dir.join(dir))
@@ -1220,7 +1222,7 @@ impl Compiler<'_> {
 
     fn emit_error<N: ToSpan>(&mut self, node: &N, kind: ErrorKind) {
         let span = self.span_for(node);
-        self.errors.push(Error { kind, span })
+        self.errors.push(Error::new(kind, span))
     }
 }
 
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index ed161f4155f5..ccc2d8976440 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -158,6 +158,12 @@ pub enum ErrorKind {
     /// not actually implemented yet, and without which eval can not
     /// proceed.
     NotImplemented(&'static str),
+
+    /// Internal variant which should disappear during error construction.
+    WithContext {
+        context: String,
+        underlying: Box<ErrorKind>,
+    },
 }
 
 impl error::Error for Error {
@@ -242,8 +248,29 @@ impl From<serde_json::Error> for ErrorKind {
 
 #[derive(Clone, Debug)]
 pub struct Error {
-    pub kind: ErrorKind,
-    pub span: Span,
+    kind: ErrorKind,
+    span: Span,
+    contexts: Vec<String>,
+}
+
+impl Error {
+    pub fn new(mut kind: ErrorKind, span: Span) -> Self {
+        let mut contexts = vec![];
+        while let ErrorKind::WithContext {
+            context,
+            underlying,
+        } = kind
+        {
+            kind = *underlying;
+            contexts.push(context);
+        }
+
+        Error {
+            kind,
+            span,
+            contexts,
+        }
+    }
 }
 
 impl Display for ErrorKind {
@@ -434,6 +461,10 @@ to a missing value in the attribute set(s) included via `with`."#,
             ErrorKind::NotImplemented(feature) => {
                 write!(f, "feature not yet implemented in Tvix: {}", feature)
             }
+
+            ErrorKind::WithContext { .. } => {
+                panic!("internal ErrorKind::WithContext variant leaked")
+            }
         }
     }
 }
@@ -721,7 +752,8 @@ impl Error {
             | ErrorKind::Xml(_)
             | ErrorKind::TvixError(_)
             | ErrorKind::TvixBug { .. }
-            | ErrorKind::NotImplemented(_) => return None,
+            | ErrorKind::NotImplemented(_)
+            | ErrorKind::WithContext { .. } => return None,
         };
 
         Some(label.into())
@@ -782,11 +814,15 @@ impl Error {
             //
             // The error code for thunk forces is E017.
             ErrorKind::ThunkForce(ref err) => err.code(),
+
+            ErrorKind::WithContext { .. } => {
+                panic!("internal ErrorKind::WithContext variant leaked")
+            }
         }
     }
 
     fn spans(&self, source: &SourceCode) -> Vec<SpanLabel> {
-        match &self.kind {
+        let mut spans = match &self.kind {
             ErrorKind::ImportParseError { errors, file, .. } => {
                 spans_for_parse_errors(file, errors)
             }
@@ -840,7 +876,17 @@ impl Error {
                     style: SpanStyle::Primary,
                 }]
             }
+        };
+
+        for ctx in &self.contexts {
+            spans.push(SpanLabel {
+                label: Some(format!("while {}", ctx)),
+                span: self.span,
+                style: SpanStyle::Secondary,
+            });
         }
+
+        spans
     }
 
     /// Create the primary diagnostic for a given error.
@@ -869,3 +915,33 @@ impl Error {
         }
     }
 }
+
+// Convenience methods to add context on other types.
+pub trait AddContext {
+    /// Add context to the error-carrying type.
+    fn context<S: Into<String>>(self, ctx: S) -> Self;
+}
+
+impl AddContext for ErrorKind {
+    fn context<S: Into<String>>(self, ctx: S) -> Self {
+        ErrorKind::WithContext {
+            context: ctx.into(),
+            underlying: Box::new(self),
+        }
+    }
+}
+
+impl<T> AddContext for Result<T, ErrorKind> {
+    fn context<S: Into<String>>(self, ctx: S) -> Self {
+        self.map_err(|kind| kind.context(ctx))
+    }
+}
+
+impl<T> AddContext for Result<T, Error> {
+    fn context<S: Into<String>>(self, ctx: S) -> Self {
+        self.map_err(|err| Error {
+            kind: err.kind.context(ctx),
+            ..err
+        })
+    }
+}
diff --git a/tvix/eval/src/lib.rs b/tvix/eval/src/lib.rs
index 484d3e72e310..a759e0c0ab2c 100644
--- a/tvix/eval/src/lib.rs
+++ b/tvix/eval/src/lib.rs
@@ -48,7 +48,7 @@ use crate::vm::run_lambda;
 
 // Re-export the public interface used by other crates.
 pub use crate::compiler::{compile, prepare_globals, CompilationOutput};
-pub use crate::errors::{Error, ErrorKind, EvalResult};
+pub use crate::errors::{AddContext, Error, ErrorKind, EvalResult};
 pub use crate::io::{DummyIO, EvalIO, FileType};
 pub use crate::pretty_ast::pretty_print_expr;
 pub use crate::source::SourceCode;
@@ -278,10 +278,10 @@ fn parse_compile_internal(
     let parse_errors = parsed.errors();
 
     if !parse_errors.is_empty() {
-        result.errors.push(Error {
-            kind: ErrorKind::ParseErrors(parse_errors.to_vec()),
-            span: file.span,
-        });
+        result.errors.push(Error::new(
+            ErrorKind::ParseErrors(parse_errors.to_vec()),
+            file.span,
+        ));
         return None;
     }
 
diff --git a/tvix/eval/src/value/thunk.rs b/tvix/eval/src/value/thunk.rs
index 23c60aa37815..2d48550dad97 100644
--- a/tvix/eval/src/value/thunk.rs
+++ b/tvix/eval/src/value/thunk.rs
@@ -197,10 +197,8 @@ impl Thunk {
                                     self_clone.0.replace(ThunkRepr::Evaluated(vm.pop()));
                                 assert!(matches!(should_be_blackhole, ThunkRepr::Blackhole));
                                 vm.push(Value::Thunk(self_clone));
-                                Self::force_trampoline(vm).map_err(|kind| Error {
-                                    kind,
-                                    span: light_span.span(),
-                                })
+                                Self::force_trampoline(vm)
+                                    .map_err(|kind| Error::new(kind, light_span.span()))
                             })),
                         });
                     }
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 3dece0641281..60006ea76f9c 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -134,12 +134,7 @@ macro_rules! fallible {
     ( $self:ident, $body:expr) => {
         match $body {
             Ok(result) => result,
-            Err(kind) => {
-                return Err(Error {
-                    kind,
-                    span: $self.current_span(),
-                })
-            }
+            Err(kind) => return Err(Error::new(kind, $self.current_span())),
         }
     };
 }
@@ -281,10 +276,7 @@ impl<'o> VM<'o> {
     /// Construct an error from the given ErrorKind and the source
     /// span of the current instruction.
     pub fn error(&self, kind: ErrorKind) -> Error {
-        Error {
-            kind,
-            span: self.current_span(),
-        }
+        Error::new(kind, self.current_span())
     }
 
     /// Push an already constructed warning.
@@ -1197,10 +1189,7 @@ pub fn run_lambda(
 
     value
         .deep_force(&mut vm, &mut Default::default())
-        .map_err(|kind| Error {
-            kind,
-            span: root_span,
-        })?;
+        .map_err(|kind| Error::new(kind, root_span))?;
 
     Ok(RuntimeResult {
         value,