Rollup merge of #134272 - RalfJung:destabilize-rustc_encodable_decodable, r=oli-obk

Remove rustc_encodable_decodable feature

This has been shown in future-compat reports since Rust 1.79 (#116016), released June 2024. Let's see if crater still finds any issues.

Part of #134301.

Cc ``@rust-lang/libs-api``

Author: matthiaskrgr

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -56,7 +56,7 @@ fn show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) ->
 
     let (ident, vdata, fields) = match substr.fields {
         Struct(vdata, fields) => (substr.type_ident, *vdata, fields),
-        EnumMatching(_, v, fields) => (v.ident, &v.data, fields),
+        EnumMatching(v, fields) => (v.ident, &v.data, fields),
         AllFieldlessEnum(enum_def) => return show_fieldless_enum(cx, span, enum_def, substr),
         EnumDiscr(..) | StaticStruct(..) | StaticEnum(..) => {
             cx.dcx().span_bug(span, "nonsensical .fields in `#[derive(Debug)]`")

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -42,7 +42,7 @@ pub(crate) fn expand_deriving_default(
                     StaticStruct(_, fields) => {
                         default_struct_substructure(cx, trait_span, substr, fields)
                     }
-                    StaticEnum(enum_def, _) => {
+                    StaticEnum(enum_def) => {
                         default_enum_substructure(cx, trait_span, enum_def, item.span())
                     }
                     _ => cx.dcx().span_bug(trait_span, "method in `derive(Default)`"),