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),
   |     ^