Point out serde(untagged) variants which are out of order

Previously if someone wrote an enum containing:

- `A` (untagged)
- `B` (tagged)
- `C` (tagged)
- `D` (untagged)
- `E` (tagged)
- `F` (untagged)

serde_derive would produce errors referring to B and E only, saying
you're supposed to put untagged variants at the end. The choice of B and
E for this error doesn't make a lot of sense because in order to resolve
the issue, the user must either:

- move A and D down

or:

- move B, C, and E up.

This commit changes the error to appear on A and D instead.

serde_derive/src/internals/ast.rs

@@ -140,8 +140,7 @@ fn enum_from_ast<'a>(
  variants: &'a Punctuated<syn::Variant, Token![,]>,
  container_default: &attr::Default,
 ) -> Vec<Variant<'a>> {
- let mut seen_untagged = false;
- variants
+ let variants: Vec<Variant> = variants
  .iter()
  .map(|variant| {
  let attrs = attr::Variant::from_ast(cx, variant);
@@ -154,12 +153,21 @@ fn enum_from_ast<'a>(
  fields,
  original: variant,
  }
- }).inspect(|variant| {
- if !variant.attrs.untagged() && seen_untagged {
+ })
+ .collect();
+
+ let index_of_last_tagged_variant = variants
+ .iter()
+ .rposition(|variant| !variant.attrs.untagged());
+ if let Some(index_of_last_tagged_variant) = index_of_last_tagged_variant {
+ for variant in &variants[..index_of_last_tagged_variant] {
+ if variant.attrs.untagged() {
  cx.error_spanned_by(&variant.ident, "all variants with the #[serde(untagged)] attribute must be placed at the end of the enum");
  }
- seen_untagged = variant.attrs.untagged();
- }).collect()
+ }
+ }
+
+ variants
 }
 
 fn struct_from_ast<'a>(

test_suite/tests/ui/enum-representation/partially_tagged_wrong_order.stderr

@@ -1,5 +1,5 @@
 error: all variants with the #[serde(untagged)] attribute must be placed at the end of the enum
- --> tests/ui/enum-representation/partially_tagged_wrong_order.rs:7:5
+ --> tests/ui/enum-representation/partially_tagged_wrong_order.rs:6:5
  |
-7 | B(String),
+6 | A(u8),
  | ^