docs(tvix/eval): StackIdx, LocalIdx UpvalueIdx r/5201

This adds a comment noting that StackIdx is an offset relative to
the base of the current CallFrame, whereas UpvalueIdx is an absolute
index into the upvalues array.

It also removes the confusing mention of StackIdx in the descriptive
comment for LocalIdx.  They index into totally different structures;
one exists at runtime and the other exists at compile time.

Change-Id: Ib932b1b0679734c15001e8c5c95a08293fa016b4
Signed-off-by: Adam Joseph <adam@westernsemico.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7017
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 1 insertions, 3 deletions

diff --git a/tvix/eval/src/compiler/scope.rs b/tvix/eval/src/compiler/scope.rs
index 83fba6eed7..327766bcb5 100644
--- a/tvix/eval/src/compiler/scope.rs
+++ b/tvix/eval/src/compiler/scope.rs
@@ -121,9 +121,7 @@ pub struct Upvalue {
     pub span: codemap::Span,
 }
 
-/// Represents the index of a local in the scope's local array, which
-/// is subtly different from its `StackIdx` (which excludes
-/// uninitialised values in between).
+/// The index of a local in the scope's local array at compile time.
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
 pub struct LocalIdx(usize);