refactor(tvix/eval): introduce UpvalueCarrier trait r/4677

This trait abstracts over the commonalities of upvalue handling
between closures and thunks.

It allows the VM to simplify the code used for setting up upvalues,
without duplicating between the two different types.

Note that this does not yet refactor the VM code to optimally make use
of this.

Change-Id: If8de5181f26ae1fa00d554f1ae6ea473ee4b6070
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6347
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

1 files changed, 39 insertions, 0 deletions

diff --git a/tvix/eval/src/upvalues.rs b/tvix/eval/src/upvalues.rs
new file mode 100644
index 0000000000..9287e7abdf
--- /dev/null
+++ b/tvix/eval/src/upvalues.rs
@@ -0,0 +1,39 @@
+//! This module encapsulates some logic for upvalue handling, which is
+//! relevant to both thunks (delayed computations for lazy-evaluation)
+//! as well as closures (lambdas that capture variables from the
+//! surrounding scope).
+
+use std::cell::{Ref, RefMut};
+
+use crate::{opcode::UpvalueIdx, Value};
+
+/// `UpvalueCarrier` is implemented by all types that carry upvalues.
+pub trait UpvalueCarrier {
+    fn upvalue_count(&self) -> usize;
+
+    /// Read-only accessor for the stored upvalues.
+    fn upvalues(&self) -> Ref<'_, [Value]>;
+
+    /// Mutable accessor for stored upvalues.
+    fn upvalues_mut(&self) -> RefMut<'_, Vec<Value>>;
+
+    /// Read an upvalue at the given index.
+    fn upvalue(&self, idx: UpvalueIdx) -> Ref<'_, Value> {
+        Ref::map(self.upvalues(), |v| &v[idx.0])
+    }
+
+    /// Push an upvalue at the end of the upvalue list.
+    fn push_upvalue(&self, value: Value) {
+        self.upvalues_mut().push(value);
+    }
+
+    /// Resolve deferred upvalues from the provided stack slice,
+    /// mutating them in the internal upvalue slots.
+    fn resolve_deferred_upvalues(&self, stack: &[Value]) {
+        for upvalue in self.upvalues_mut().iter_mut() {
+            if let Value::DeferredUpvalue(idx) = upvalue {
+                *upvalue = stack[idx.0].clone();
+            }
+        }
+    }
+}