1 files changed, 121 insertions, 0 deletions
diff --git a/tvix/eval/src/spans.rs b/tvix/eval/src/spans.rs
new file mode 100644
index 000000000000..c0130a665428
--- /dev/null
+++ b/tvix/eval/src/spans.rs
@@ -0,0 +1,121 @@
+//! Utilities for dealing with span tracking in the compiler and in
+//! error reporting.
+
+use crate::opcode::CodeIdx;
+use crate::value::Lambda;
+use codemap::{File, Span};
+use rnix::ast;
+use rowan::ast::AstNode;
+use std::rc::Rc;
+
+/// Helper struct to carry information required for making a span, but
+/// without actually performing the (expensive) span lookup.
+///
+/// This is used for tracking spans across thunk boundaries, as they
+/// are frequently instantiated but spans are only used in error or
+/// warning cases.
+#[derive(Clone, Debug)]
+pub enum LightSpan {
+    /// The span has already been computed and can just be used right
+    /// away.
+    Actual { span: Span },
+
+    /// The span needs to be computed from the provided data, but only
+    /// when it is required.
+    Delayed { lambda: Rc<Lambda>, offset: CodeIdx },
+}
+
+impl LightSpan {
+    pub fn new_delayed(lambda: Rc<Lambda>, offset: CodeIdx) -> Self {
+        Self::Delayed { lambda, offset }
+    }
+
+    pub fn new_actual(span: Span) -> Self {
+        Self::Actual { span }
+    }
+
+    pub fn span(&self) -> Span {
+        match self {
+            LightSpan::Actual { span } => *span,
+            LightSpan::Delayed { lambda, offset } => lambda.chunk.get_span(*offset),
+        }
+    }
+}
+
+impl From<Span> for LightSpan {
+    fn from(span: Span) -> Self {
+        LightSpan::Actual { span }
+    }
+}
+
+/// Trait implemented by all types from which we can retrieve a span.
+pub trait ToSpan {
+    fn span_for(&self, file: &File) -> Span;
+}
+
+impl ToSpan for Span {
+    fn span_for(&self, _: &File) -> Span {
+        *self
+    }
+}
+
+impl ToSpan for rnix::TextRange {
+    fn span_for(&self, file: &File) -> Span {
+        file.span
+            .subspan(u32::from(self.start()) as u64, u32::from(self.end()) as u64)
+    }
+}
+
+impl ToSpan for rnix::SyntaxToken {
+    fn span_for(&self, file: &File) -> Span {
+        self.text_range().span_for(file)
+    }
+}
+
+impl ToSpan for rnix::SyntaxNode {
+    fn span_for(&self, file: &File) -> Span {
+        self.text_range().span_for(file)
+    }
+}
+
+/// Generates a `ToSpan` implementation for a type implementing
+/// `rowan::AstNode`. This is impossible to do as a blanket
+/// implementation because `rustc` forbids these implementations for
+/// traits from third-party crates due to a belief that semantic
+/// versioning truly could work (it doesn't).
+macro_rules! expr_to_span {
+    ( $type:path ) => {
+        impl ToSpan for $type {
+            fn span_for(&self, file: &File) -> Span {
+                self.syntax().span_for(file)
+            }
+        }
+    };
+}
+
+expr_to_span!(ast::Expr);
+expr_to_span!(ast::Apply);
+expr_to_span!(ast::Assert);
+expr_to_span!(ast::Attr);
+expr_to_span!(ast::AttrSet);
+expr_to_span!(ast::Attrpath);
+expr_to_span!(ast::AttrpathValue);
+expr_to_span!(ast::BinOp);
+expr_to_span!(ast::HasAttr);
+expr_to_span!(ast::Ident);
+expr_to_span!(ast::IdentParam);
+expr_to_span!(ast::IfElse);
+expr_to_span!(ast::Inherit);
+expr_to_span!(ast::Interpol);
+expr_to_span!(ast::Lambda);
+expr_to_span!(ast::LegacyLet);
+expr_to_span!(ast::LetIn);
+expr_to_span!(ast::List);
+expr_to_span!(ast::Literal);
+expr_to_span!(ast::PatBind);
+expr_to_span!(ast::Path);
+expr_to_span!(ast::Pattern);
+expr_to_span!(ast::Select);
+expr_to_span!(ast::Str);
+expr_to_span!(ast::UnaryOp);
+expr_to_span!(ast::With);