Correctly process flatten fields in enum variants

- Fix incorrect deserialization of variants that doesn't contain flatten field when other contains
- Fix a panic when deriving `Deserialize` for an enum with tuple and struct with flatten field

Fixes (2):
    regression::issue2565::simple_variant
    regression::issue1904 (compilation)

serde_derive/src/de.rs

@@ -281,11 +281,21 @@ fn deserialize_body(cont: &Container, params: &Parameters) -> Fragment {
     } else if let attr::Identifier::No = cont.attrs.identifier() {
         match &cont.data {
             Data::Enum(variants) => deserialize_enum(params, variants, &cont.attrs),
-            Data::Struct(Style::Struct, fields) => {
-                deserialize_struct(params, fields, &cont.attrs, StructForm::Struct)
-            }
+            Data::Struct(Style::Struct, fields) => deserialize_struct(
+                params,
+                fields,
+                &cont.attrs,
+                cont.attrs.has_flatten(),
+                StructForm::Struct,
+            ),
             Data::Struct(Style::Tuple, fields) | Data::Struct(Style::Newtype, fields) => {
-                deserialize_tuple(params, fields, &cont.attrs, TupleForm::Tuple)
+                deserialize_tuple(
+                    params,
+                    fields,
+                    &cont.attrs,
+                    cont.attrs.has_flatten(),
+                    TupleForm::Tuple,
+                )
             }
             Data::Struct(Style::Unit, _) => deserialize_unit_struct(params, &cont.attrs),
         }
@@ -459,9 +469,13 @@ fn deserialize_tuple(
     params: &Parameters,
     fields: &[Field],
     cattrs: &attr::Container,
+    has_flatten: bool,
     form: TupleForm,
 ) -> Fragment {
-    assert!(!cattrs.has_flatten());
+    assert!(
+        !has_flatten,
+        "tuples and tuple variants cannot have flatten fields"
+    );
 
     let field_count = fields
         .iter()
@@ -579,7 +593,10 @@ fn deserialize_tuple_in_place(
     fields: &[Field],
     cattrs: &attr::Container,
 ) -> Fragment {
-    assert!(!cattrs.has_flatten());
+    assert!(
+        !cattrs.has_flatten(),
+        "tuples and tuple variants cannot have flatten fields"
+    );
 
     let field_count = fields
         .iter()
@@ -910,6 +927,7 @@ fn deserialize_struct(
     params: &Parameters,
     fields: &[Field],
     cattrs: &attr::Container,
+    has_flatten: bool,
     form: StructForm,
 ) -> Fragment {
     let this_type = &params.this_type;
@@ -958,13 +976,13 @@ fn deserialize_struct(
             )
         })
         .collect();
-    let field_visitor = deserialize_field_identifier(&field_names_idents, cattrs);
+    let field_visitor = deserialize_field_identifier(&field_names_idents, cattrs, has_flatten);
 
     // untagged struct variants do not get a visit_seq method. The same applies to
     // structs that only have a map representation.
     let visit_seq = match form {
         StructForm::Untagged(..) => None,
-        _ if cattrs.has_flatten() => None,
+        _ if has_flatten => None,
         _ => {
             let mut_seq = if field_names_idents.is_empty() {
                 quote!(_)
@@ -987,10 +1005,16 @@ fn deserialize_struct(
             })
         }
     };
-    let visit_map = Stmts(deserialize_map(&type_path, params, fields, cattrs));
+    let visit_map = Stmts(deserialize_map(
+        &type_path,
+        params,
+        fields,
+        cattrs,
+        has_flatten,
+    ));
 
     let visitor_seed = match form {
-        StructForm::ExternallyTagged(..) if cattrs.has_flatten() => Some(quote! {
+        StructForm::ExternallyTagged(..) if has_flatten => Some(quote! {
             impl #de_impl_generics _serde::de::DeserializeSeed<#delife> for __Visitor #de_ty_generics #where_clause {
                 type Value = #this_type #ty_generics;
 
@@ -1005,7 +1029,7 @@ fn deserialize_struct(
         _ => None,
     };
 
-    let fields_stmt = if cattrs.has_flatten() {
+    let fields_stmt = if has_flatten {
         None
     } else {
         let field_names = field_names_idents
@@ -1025,7 +1049,7 @@ fn deserialize_struct(
         }
     };
     let dispatch = match form {
-        StructForm::Struct if cattrs.has_flatten() => quote! {
+        StructForm::Struct if has_flatten => quote! {
             _serde::Deserializer::deserialize_map(__deserializer, #visitor_expr)
         },
         StructForm::Struct => {
@@ -1034,7 +1058,7 @@ fn deserialize_struct(
                 _serde::Deserializer::deserialize_struct(__deserializer, #type_name, FIELDS, #visitor_expr)
             }
         }
-        StructForm::ExternallyTagged(_) if cattrs.has_flatten() => quote! {
+        StructForm::ExternallyTagged(_) if has_flatten => quote! {
             _serde::de::VariantAccess::newtype_variant_seed(__variant, #visitor_expr)
         },
         StructForm::ExternallyTagged(_) => quote! {
@@ -1116,7 +1140,7 @@ fn deserialize_struct_in_place(
         })
         .collect();
 
-    let field_visitor = deserialize_field_identifier(&field_names_idents, cattrs);
+    let field_visitor = deserialize_field_identifier(&field_names_idents, cattrs, false);
 
     let mut_seq = if field_names_idents.is_empty() {
         quote!(_)
@@ -1210,10 +1234,7 @@ fn deserialize_homogeneous_enum(
     }
 }
 
-fn prepare_enum_variant_enum(
-    variants: &[Variant],
-    cattrs: &attr::Container,
-) -> (TokenStream, Stmts) {
+fn prepare_enum_variant_enum(variants: &[Variant]) -> (TokenStream, Stmts) {
     let mut deserialized_variants = variants
         .iter()
         .enumerate()
@@ -1247,7 +1268,7 @@ fn prepare_enum_variant_enum(
 
     let variant_visitor = Stmts(deserialize_generated_identifier(
         &variant_names_idents,
-        cattrs,
+        false, // variant identifiers does not depend on the presence of flatten fields
         true,
         None,
         fallthrough,
@@ -1270,7 +1291,7 @@ fn deserialize_externally_tagged_enum(
     let expecting = format!("enum {}", params.type_name());
     let expecting = cattrs.expecting().unwrap_or(&expecting);
 
-    let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants, cattrs);
+    let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants);
 
     // Match arms to extract a variant from a string
     let variant_arms = variants
@@ -1355,7 +1376,7 @@ fn deserialize_internally_tagged_enum(
     cattrs: &attr::Container,
     tag: &str,
 ) -> Fragment {
-    let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants, cattrs);
+    let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants);
 
     // Match arms to extract a variant from a string
     let variant_arms = variants
@@ -1409,7 +1430,7 @@ fn deserialize_adjacently_tagged_enum(
         split_with_de_lifetime(params);
     let delife = params.borrowed.de_lifetime();
 
-    let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants, cattrs);
+    let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants);
 
     let variant_arms: &Vec<_> = &variants
         .iter()
@@ -1810,12 +1831,14 @@ fn deserialize_externally_tagged_variant(
             params,
             &variant.fields,
             cattrs,
+            variant.attrs.has_flatten(),
             TupleForm::ExternallyTagged(variant_ident),
         ),
         Style::Struct => deserialize_struct(
             params,
             &variant.fields,
             cattrs,
+            variant.attrs.has_flatten(),
             StructForm::ExternallyTagged(variant_ident),
         ),
     }
@@ -1859,6 +1882,7 @@ fn deserialize_internally_tagged_variant(
             params,
             &variant.fields,
             cattrs,
+            variant.attrs.has_flatten(),
             StructForm::InternallyTagged(variant_ident, deserializer),
         ),
         Style::Tuple => unreachable!("checked in serde_derive_internals"),
@@ -1909,12 +1933,14 @@ fn deserialize_untagged_variant(
             params,
             &variant.fields,
             cattrs,
+            variant.attrs.has_flatten(),
             TupleForm::Untagged(variant_ident, deserializer),
         ),
         Style::Struct => deserialize_struct(
             params,
             &variant.fields,
             cattrs,
+            variant.attrs.has_flatten(),
             StructForm::Untagged(variant_ident, deserializer),
         ),
     }
@@ -1985,7 +2011,7 @@ fn deserialize_untagged_newtype_variant(
 
 fn deserialize_generated_identifier(
     fields: &[(&str, Ident, &BTreeSet<String>)],
-    cattrs: &attr::Container,
+    has_flatten: bool,
     is_variant: bool,
     ignore_variant: Option<TokenStream>,
     fallthrough: Option<TokenStream>,
@@ -1999,11 +2025,11 @@ fn deserialize_generated_identifier(
         is_variant,
         fallthrough,
         None,
-        !is_variant && cattrs.has_flatten(),
+        !is_variant && has_flatten,
         None,
     ));
 
-    let lifetime = if !is_variant && cattrs.has_flatten() {
+    let lifetime = if !is_variant && has_flatten {
         Some(quote!(<'de>))
     } else {
         None
@@ -2043,8 +2069,9 @@ fn deserialize_generated_identifier(
 fn deserialize_field_identifier(
     fields: &[(&str, Ident, &BTreeSet<String>)],
     cattrs: &attr::Container,
+    has_flatten: bool,
 ) -> Stmts {
-    let (ignore_variant, fallthrough) = if cattrs.has_flatten() {
+    let (ignore_variant, fallthrough) = if has_flatten {
         let ignore_variant = quote!(__other(_serde::__private::de::Content<'de>),);
         let fallthrough = quote!(_serde::__private::Ok(__Field::__other(__value)));
         (Some(ignore_variant), Some(fallthrough))
@@ -2058,7 +2085,7 @@ fn deserialize_field_identifier(
 
     Stmts(deserialize_generated_identifier(
         fields,
-        cattrs,
+        has_flatten,
         false,
         ignore_variant,
         fallthrough,
@@ -2460,6 +2487,7 @@ fn deserialize_map(
     params: &Parameters,
     fields: &[Field],
     cattrs: &attr::Container,
+    has_flatten: bool,
 ) -> Fragment {
     // Create the field names for the fields.
     let fields_names: Vec<_> = fields
@@ -2480,9 +2508,6 @@ fn deserialize_map(
         });
 
     // Collect contents for flatten fields into a buffer
-    let has_flatten = fields
-        .iter()
-        .any(|field| field.attrs.flatten() && !field.attrs.skip_deserializing());
     let let_collect = if has_flatten {
         Some(quote! {
             let mut __collect = _serde::__private::Vec::<_serde::__private::Option<(
@@ -2681,7 +2706,10 @@ fn deserialize_map_in_place(
     fields: &[Field],
     cattrs: &attr::Container,
 ) -> Fragment {
-    assert!(!cattrs.has_flatten());
+    assert!(
+        !cattrs.has_flatten(),
+        "inplace deserialization of maps doesn't support flatten fields"
+    );
 
     // Create the field names for the fields.
     let fields_names: Vec<_> = fields