expand: Add error check if derive has wrong item

[Pasta-coder]

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 to the DeriveVisitor to reject invalid items gracefully with a diagnostic error before dispatching.

Fixes #3875

gcc/rust/ChangeLog:

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

gcc/testsuite/ChangeLog:

* rust/compile/derive-on-invalid-item.rs: New test.

• GCC development requires copyright assignment or the Developer's Certificate of Origin sign-off, see https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
• Read contributing guidlines
• make check-rust passes locally
• Run clang-format
• Added any relevant test cases to gcc/testsuite/rust/

[dkm]

Hello, thanks for your contribution!

Small nitpick on the Changelog part...

* expand/rust-derive.h: Add validate_item prototype.

Does not target a specific "item". You could be more precise using:

* expand/rust-derive.h (DeriveVisitor::validate_item): New.

[Pasta-coder]

@dkm hello,

commit message regarding changelog changed .

thanks