expand: Mark some dead code in derive expansion as unreachable

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -407,13 +407,7 @@ impl<'a> TraitDef<'a> {
                             _ => false,
                         })
                     }
-                    _ => {
+                    _ => unreachable!(),
-                        // Non-ADT derive is an error, but it should have been
-                        // set earlier; see
-                        // librustc_expand/expand.rs:MacroExpander::fully_expand_fragment()
-                        // librustc_expand/base.rs:Annotatable::derive_allowed()
-                        return;
-                    }
                 };
                 let container_id = cx.current_expansion.id.expn_data().parent;
                 let always_copy = has_no_type_params && cx.resolver.has_derive_copy(container_id);
@@ -475,12 +469,7 @@ impl<'a> TraitDef<'a> {
                 );
                 push(Annotatable::Item(P(ast::Item { attrs, ..(*newitem).clone() })))
             }
-            _ => {
+            _ => unreachable!(),
-                // Non-Item derive is an error, but it should have been
-                // set earlier; see
-                // librustc_expand/expand.rs:MacroExpander::fully_expand_fragment()
-                // librustc_expand/base.rs:Annotatable::derive_allowed()
-            }
         }
     }
 

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -98,13 +98,7 @@ fn inject_impl_of_structural_trait(
 ) {
     let item = match *item {
         Annotatable::Item(ref item) => item,
-        _ => {
+        _ => unreachable!(),
-            // Non-Item derive is an error, but it should have been
-            // set earlier; see
-            // librustc_expand/expand.rs:MacroExpander::fully_expand_fragment()
-            // librustc_expand/base.rs:Annotatable::derive_allowed()
-            return;
-        }
     };
 
     let generics = match item.kind {

compiler/rustc_expand/src/expand.rs

@@ -767,9 +767,6 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             InvocationKind::Derive { path, item } => match ext {
                 SyntaxExtensionKind::Derive(expander)
                 | SyntaxExtensionKind::LegacyDerive(expander) => {
-                    if !item.derive_allowed() {
-                        return ExpandResult::Ready(fragment_kind.dummy(span));
-                    }
                     if let SyntaxExtensionKind::Derive(..) = ext {
                         self.gate_proc_macro_input(&item);
                     }

compiler/rustc_expand/src/proc_macro.rs

@@ -75,38 +75,9 @@ impl MultiItemModifier for ProcMacroDerive {
         item: Annotatable,
     ) -> ExpandResult<Vec<Annotatable>, Annotatable> {
         let item = match item {
-            Annotatable::Arm(..)
+            Annotatable::Item(item) => token::NtItem(item),
-            | Annotatable::Field(..)
+            _ => unreachable!(),
-            | Annotatable::FieldPat(..)
-            | Annotatable::GenericParam(..)
-            | Annotatable::Param(..)
-            | Annotatable::StructField(..)
-            | Annotatable::Variant(..) => panic!("unexpected annotatable"),
-            Annotatable::Item(item) => item,
-            Annotatable::ImplItem(_)
-            | Annotatable::TraitItem(_)
-            | Annotatable::ForeignItem(_)
-            | Annotatable::Stmt(_)
-            | Annotatable::Expr(_) => {
-                ecx.span_err(
-                    span,
-                    "proc-macro derives may only be applied to a struct, enum, or union",
-                );
-                return ExpandResult::Ready(Vec::new());
-            }
         };
-        match item.kind {
-            ItemKind::Struct(..) | ItemKind::Enum(..) | ItemKind::Union(..) => {}
-            _ => {
-                ecx.span_err(
-                    span,
-                    "proc-macro derives may only be applied to a struct, enum, or union",
-                );
-                return ExpandResult::Ready(Vec::new());
-            }
-        }
-
-        let item = token::NtItem(item);
         let input = if item.pretty_printing_compatibility_hack() {
             TokenTree::token(token::Interpolated(Lrc::new(item)), DUMMY_SP).into()
         } else {