feat(tvix/eval): Allow passing in an env to evaluation r/8345

Allow passing in a top-level env, a map from name to value, to
evaluation. The intent is to support bound identifiers in the REPL just
like upstream nix does.

Getting this working involves mucking around a bit with internals - most
notably, locals now only optionally have a Span (since locals don't have
an easy span we can use) - and getting that working requires propagating
some minor hacks to places where we currently *need* a span (and which
would require too much changing now to make spans optional; my guess is
that that would essentially end up making spans optional throughout the
codebase).

Also, some extra care has to be taken to close out the scope in the case
that we do pass in an env, to avoid breaking our assumptions about the
size of the stack when we return from the toplevel

Change-Id: Ie475b2d3dfc72ccbf298d2a3ea28c63ac877d653
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11953
Tested-by: BuildkiteCI
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>

1 files changed, 27 insertions, 0 deletions

diff --git a/tvix/eval/src/spans.rs b/tvix/eval/src/spans.rs
index 9998e438b2..df2b9a7255 100644
--- a/tvix/eval/src/spans.rs
+++ b/tvix/eval/src/spans.rs
@@ -35,6 +35,33 @@ impl ToSpan for rnix::SyntaxNode {
     }
 }
 
+/// A placeholder [`ToSpan`] implementation covering the entire source file.
+#[derive(Debug, Clone, Copy)]
+pub struct EntireFile;
+
+impl ToSpan for EntireFile {
+    fn span_for(&self, file: &File) -> Span {
+        file.span
+    }
+}
+
+/// A placeholder [`ToSpan`] implementation which falls back to the entire file if its wrapped value
+/// is [`None`]
+#[derive(Debug, Clone, Copy)]
+pub struct OrEntireFile<T>(pub Option<T>);
+
+impl<T> ToSpan for OrEntireFile<T>
+where
+    T: ToSpan,
+{
+    fn span_for(&self, file: &File) -> Span {
+        match &self.0 {
+            Some(t) => t.span_for(file),
+            None => EntireFile.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