refactor(tvix/eval): Make `strict` an EvalMode enum r/8796

Refactor the `strict` boolean passed into evaluation at the top-level to
be a (two-variant, so far) EvalMode enum of Lazy and Strict.

This is more explicit than a boolean, and if we ever add more EvalModes
it's a simple extension of the enum.

Change-Id: I3de50e74ec971011664f6cd0999d08b792118410
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12186
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: aspen <root@gws.fyi>

1 files changed, 16 insertions, 3 deletions

diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 49e9fc5864be..0630ed4174e8 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -1378,6 +1378,18 @@ async fn final_deep_force(co: GenCo) -> Result<Value, ErrorKind> {
     Ok(generators::request_deep_force(&co, value).await)
 }
 
+/// Specification for how to handle top-level values returned by evaluation
+#[derive(Debug, Clone, Copy, Default)]
+pub enum EvalMode {
+    /// The default. Values are returned from evaluations as-is, without any extra forcing or
+    /// special handling.
+    #[default]
+    Lazy,
+
+    /// Strictly and deeply evaluate top-level values returned by evaluation.
+    Strict,
+}
+
 pub fn run_lambda<IO>(
     nix_search_path: NixSearchPath,
     io_handle: IO,
@@ -1385,7 +1397,7 @@ pub fn run_lambda<IO>(
     source: SourceCode,
     globals: Rc<GlobalsMap>,
     lambda: Rc<Lambda>,
-    strict: bool,
+    mode: EvalMode,
 ) -> EvalResult<RuntimeResult>
 where
     IO: AsRef<dyn EvalIO> + 'static,
@@ -1409,8 +1421,9 @@ where
 
     // When evaluating strictly, synthesise a frame that will instruct
     // the VM to deep-force the final value before returning it.
-    if strict {
-        vm.enqueue_generator("final_deep_force", root_span, final_deep_force);
+    match mode {
+        EvalMode::Lazy => {}
+        EvalMode::Strict => vm.enqueue_generator("final_deep_force", root_span, final_deep_force),
     }
 
     vm.frames.push(Frame::CallFrame {