expand: Add error check if derive has wrong item

The DeriveVisitor previously dispatched calls to specific handlers (like
DeriveClone) without validating the input item type. This caused internal
compiler errors (ICEs) when users attempted to derive traits on invalid items
like functions or traits, as the handlers assumed the input was a struct,
enum, or union.

This patch adds a validation predicate `validate_item` to the DeriveVisitor
to reject invalid items gracefully with a diagnostic error before dispatching.

Fixes #3875

gcc/rust/ChangeLog:

	* expand/rust-derive.h (DeriveVisitor::validate_item): Add prototype.
	* expand/rust-derive.cc (DeriveVisitor::derive): Use visitor validation.
	(DeriveVisitor::validate_item): New function to check item kind.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3875.rs: New test.

Signed-off-by: Jayant Chauhan <0001jayant@gmail.com>

Author: Pasta-coder

gcc/rust/expand/rust-derive.cc

@@ -40,10 +40,7 @@ DeriveVisitor::derive (Item &item, const Attribute &attr,
 {
   auto loc = attr.get_locus ();
 
-  using Kind = AST::Item::Kind;
-  auto item_kind = item.get_item_kind ();
-  if (item_kind != Kind::Enum && item_kind != Kind::Struct
-      && item_kind != Kind::Union)
+  if (!DeriveVisitor::validate_item (item))
     {
       rust_error_at (loc,
 		     "derive may only be applied to structs, enums and unions");
@@ -159,5 +156,18 @@ DeriveVisitor::setup_impl_generics (
   return ImplGenerics{std::move (self_type_path), std::move (impl_generics)};
 }
 
+bool
+DeriveVisitor::validate_item (const Item &item)
+{
+  using Kind = AST::Item::Kind;
+  auto item_kind = item.get_item_kind ();
+
+  if (item_kind == Kind::Enum || item_kind == Kind::Struct
+      || item_kind == Kind::Union)
+    return true;
+
+  return false;
+}
+
 } // namespace AST
 } // namespace Rust

gcc/rust/expand/rust-derive.h

@@ -71,6 +71,7 @@ class DeriveVisitor : public AST::ASTVisitor
     = tl::nullopt) const;
 
 private:
+  static bool validate_item (const AST::Item &item);
   // the 4 "allowed" visitors, which a derive-visitor can specify and override
   virtual void visit_struct (StructStruct &struct_item) = 0;
   virtual void visit_tuple (TupleStruct &tuple_item) = 0;

gcc/testsuite/rust/compile/issue-3875.rs

@@ -0,0 +1,12 @@
+// { dg-options "-w" }
+
+#[derive(Clone)] // { dg-error "derive may only be applied to structs, enums and unions" }
+fn foo() {}
+
+#[derive(Debug)] // { dg-error "derive may only be applied to structs, enums and unions" }
+trait Bar {}
+
+#[derive(Copy)] // { dg-error "derive may only be applied to structs, enums and unions" }
+impl Bar for i32 {}
+
+fn main() {}