1 files changed, 50 insertions, 0 deletions
diff --git a/tvix/eval/src/chunk.rs b/tvix/eval/src/chunk.rs
index 7526add169d6..9075db11b3c0 100644
--- a/tvix/eval/src/chunk.rs
+++ b/tvix/eval/src/chunk.rs
@@ -1,10 +1,31 @@
 use crate::opcode::{CodeIdx, ConstantIdx, OpCode};
 use crate::value::Value;
 
+/// Represents a source location from which one or more operations
+/// were compiled.
+///
+/// The span itself is an index into a [codemap::Codemap], and the
+/// structure tracks the number of operations that were yielded from
+/// the same span.
+///
+/// At error reporting time, it becomes possible to either just fetch
+/// the textual representation of that span from the codemap, or to
+/// even re-parse the AST using rnix to create more semantically
+/// interesting errors.
+#[derive(Clone, Debug)]
+struct SourceSpan {
+    /// Span into the [codemap::Codemap].
+    span: codemap::Span,
+
+    /// Number of instructions derived from this span.
+    count: usize,
+}
+
 #[derive(Clone, Debug, Default)]
 pub struct Chunk {
     pub code: Vec<OpCode>,
     pub constants: Vec<Value>,
+    spans: Vec<SourceSpan>,
 }
 
 impl Chunk {
@@ -23,4 +44,33 @@ impl Chunk {
     pub fn constant(&self, idx: ConstantIdx) -> &Value {
         &self.constants[idx.0]
     }
+
+    // Span tracking implementation
+
+    fn push_span(&mut self, span: codemap::Span) {
+        match self.spans.last_mut() {
+            // We do not need to insert the same span again, as this
+            // instruction was compiled from the same span as the last
+            // one.
+            Some(last) if last.span == span => last.count += 1,
+
+            // In all other cases, this is a new source span.
+            _ => self.spans.push(SourceSpan { span, count: 1 }),
+        }
+    }
+
+    /// Retrieve the [codemap::Span] from which the instruction at
+    /// `offset` was compiled.
+    pub fn get_span(&self, offset: CodeIdx) -> codemap::Span {
+        let mut pos = 0;
+
+        for span in &self.spans {
+            pos += span.count;
+            if pos > offset.0 {
+                return span.span;
+            }
+        }
+
+        panic!("compiler error: chunk missing span for offset {}", offset.0);
+    }
 }