fix(tvix/nix-compat-derive): Better errors for default r/8892

This adds a span to default handling so that any error message
gives a more precise location as a hint for why the error occurred
instead of just pointing to the type.

Change-Id: I7bf6cf38e0284f9726d670ea50a94a6b1edd8a94
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12728
Reviewed-by: Vladimir Kryachko <v.kryachko@gmail.com>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: Brian Olsen <me@griff.name>

1 files changed, 6 insertions, 2 deletions

diff --git a/tvix/nix-compat-derive/src/de.rs b/tvix/nix-compat-derive/src/de.rs
index 8dac6964f612..2214254e2b32 100644
--- a/tvix/nix-compat-derive/src/de.rs
+++ b/tvix/nix-compat-derive/src/de.rs
@@ -111,7 +111,9 @@ fn nix_deserialize_struct(style: Style, fields: &[Field<'_>]) -> TokenStream {
         };
         if let Some(version) = f.attrs.version.as_ref() {
             let default = match &f.attrs.default {
-                Default::Default => quote_spanned!(ty.span()=>::std::default::Default::default),
+                Default::Default(span) => {
+                    quote_spanned!(span.span()=>::std::default::Default::default)
+                }
                 Default::Path(path) => path.to_token_stream(),
                 _ => panic!("No default for versioned field"),
             };
@@ -172,7 +174,9 @@ fn nix_deserialize_variant(variant: &Variant<'_>) -> TokenStream {
         };
         if let Some(version) = f.attrs.version.as_ref() {
             let default = match &f.attrs.default {
-                Default::Default => quote_spanned!(ty.span()=>::std::default::Default::default),
+                Default::Default(span) => {
+                    quote_spanned!(span.span()=>::std::default::Default::default)
+                }
                 Default::Path(path) => path.to_token_stream(),
                 _ => panic!("No default for versioned field"),
             };