fix(tvix/eval): use normal thunking behavior for default in formals r/6244

When comparing to C++ Nix, we notice that the thunking of default
expressions in function formals corresponds to their normal thunking,
e.g. literals are not thunked. This means that we can just invoke
compile() without much of a care and trust that it will sort it out
correctly.

If function formals blow up as a result of this, it likely indicates
that the expression is treated incorrectly by compile(), not
compile_param_pattern().

Change-Id: I64acbff2f251423eb72ce43e56a0603379305e1d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8704
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

2 files changed, 7 insertions, 9 deletions

diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 450799d683d1..43dab506c5ed 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -950,15 +950,8 @@ impl Compiler<'_> {
 
                 self.patch_jump(jump_to_default);
 
-                // 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)
-                    });
-                }
+                // Does not need to thunked since compile() already does so when necessary
+                self.compile(*idx, default_expr);
 
                 self.patch_jump(jump_over_default);
             } else {
diff --git a/tvix/eval/tests/nix_oracle.rs b/tvix/eval/tests/nix_oracle.rs
index ecaf53a1b21a..386d49c51ccd 100644
--- a/tvix/eval/tests/nix_oracle.rs
+++ b/tvix/eval/tests/nix_oracle.rs
@@ -154,4 +154,9 @@ compare_lazy_eval_tests! {
     thunked_lambda_in_list("[ (x: x) ]");
     thunked_function_application_in_list("[ (builtins.add 1 2) ]");
     thunked_legacy_let_in_list("[ (let { foo = 12; body = foo; }) ]");
+
+    unthunked_formals_fallback_literal("({ foo ? 12 }: [ foo ]) { }");
+    unthunked_formals_fallback_string_literal("({ foo ? \"wiggly\" }: [ foo ]) { }");
+    thunked_formals_fallback_application("({ foo ? builtins.add 1 2 }: [ foo ]) { }");
+    thunked_formals_fallback_name_resolution_literal("({ foo ? bar, bar ? 12 }: [ foo ]) { }");
 }