fix(tvix/eval): ensure all thunks are forced in nested selects

Previously only the first one was guaranteed to be forced, but we need
to do this for all of them.

Fixes b/190

Change-Id: I76b5667dbfb2f3fde3587e7b91d268cbf32aca00
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6645
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>

tvix/eval/src/compiler/attrs.rs

@@ -270,13 +270,15 @@ impl Compiler<'_> {
 
         // Push the set onto the stack
         self.compile(slot, set.clone());
-        self.emit_force(&set);
 
         // Compile each key fragment and emit access instructions.
         //
         // TODO: multi-select instruction to avoid re-pushing attrs on
         // nested selects.
         for fragment in path.attrs() {
+            // Force the current set value.
+            self.emit_force(&fragment);
+
             self.compile_attr(slot, fragment.clone());
             self.push_op(OpCode::OpAttrsSelect, &fragment);
         }
@@ -319,10 +321,10 @@ impl Compiler<'_> {
         default: ast::Expr,
     ) {
         self.compile(slot, set.clone());
-        self.emit_force(&set);
         let mut jumps = vec![];
 
         for fragment in path.attrs() {
+            self.emit_force(&fragment);
             self.compile_attr(slot, fragment.clone());
             self.push_op(OpCode::OpAttrsTrySelect, &fragment);
             jumps.push(self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &fragment));

tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.exp

@@ -0,0 +1 @@
+42

tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.nix

@@ -0,0 +1,5 @@
+({
+  x = {
+    y = 42;
+  };
+}).x.y