fix(tvix/eval): correctly thunk deferred formals access r/5970

Formals can be initialised with deferred default values (see the test
cases), in which case they need an extra thunk to have something that
can be finalised appropriately when the setup is done.

Fixes: b/255
Change-Id: I380e3770be68eaa83ace96d450c7cead32dacc9f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8196
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

3 files changed, 18 insertions, 1 deletions

diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 5460514ba1ed..39b706cb487a 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -937,7 +937,17 @@ impl Compiler<'_> {
                 let jump_over_default = self.push_op(OpCode::OpJump(JumpOffset(0)), &default_expr);
 
                 self.patch_jump(jump_to_default);
-                self.compile(idx, default_expr);
+
+                // Thunk the default expression, but only if it is something
+                // other than an identifier.
+                if let ast::Expr::Ident(_) = &default_expr {
+                    self.compile(idx, default_expr);
+                } else {
+                    self.thunk(idx, &self.span_for(&default_expr), move |c, s| {
+                        c.compile(s, default_expr)
+                    });
+                }
+
                 self.patch_jump(jump_over_default);
             } else {
                 self.push_op(OpCode::OpAttrsSelect, &entry.ident().unwrap());
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp
new file mode 100644
index 000000000000..5993db7ccc5a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp
@@ -0,0 +1 @@
+[ false -2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix
new file mode 100644
index 000000000000..1fbb3e853af4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix
@@ -0,0 +1,6 @@
+# Application of unary operators on deferred formals arguments (via
+# defaulting), see also b/255.
+[
+  (({ b ? !a, a }: b) { a = true; })
+  (({ b ? -a, a }: b) { a = 2; })
+]