Support anonymous struct embedding via -fms-extensions#9022
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Could you please add test(s) and address the clang-format error for a start? |
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A *tagged* struct/union used as an unnamed (anonymous) member -- e.g. the Linux kernel's `struct __filename_head;` embedded in struct filename -- was silently dropped by the C front-end in GCC/Clang mode: it contributed no bytes (so sizeof was wrong, breaking the kernel's static_assert(sizeof(struct filename) % 64 == 0)) and its members were not injected (inaccessible by name). GCC/Clang accept this only with -fms-extensions (the Plan 9 C extension); without it they ignore the member, matching the existing regression/ansi-c/anonymous_member* tests. So: - config: add ansi_c.allow_anonymous_struct_embedding (default false), named to be explicit that only this one MS/Plan 9 extension is implemented, not all of -fms-extensions; - gcc_mode: set it when -fms-extensions is passed (queried as "fms-extensions" -- goto_cc_cmdlinet stores long options without the leading '-'); - c_typecheck_type: in GCC/Clang mode accept an *untagged* struct/union anonymous member always (C11) and a *tagged* one only under -fms-extensions; a non-struct/union unnamed member, or a tagged one without the flag, keeps the previous "ignore" behaviour. With the flag the member now contributes its size and its fields are injected (reachable by name). The new behaviour is documented in the goto-cc section of the CPROVER manual, and exercised from both sides: goto-gcc/ms_extensions_anonymous_member checks exact layout and member access with the flag, and goto-gcc/ms_extensions_anonymous_member_ignored checks the member is ignored without it. The existing anonymous_member* tests confirm the default cbmc behaviour is preserved. Co-authored-by: Michael Tautschnig <tautschn@amazon.com> Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>
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When -fms-extensions is passed, tagged struct/union members without a declarator name are treated as anonymous embedding (Plan 9 C extension). This allows code using the GCC extension where 'struct tag;' inside another struct inlines the tagged struct's fields.
The implementation lets the tagged anonymous member fall through to the existing anonymous member handling (the $anon naming + set_anonymous pass), rather than skipping it with 'continue'.
Only this one behavior is implemented — no other MSVC or Plan 9 extensions. The internal config field is named
allow_anonymous_struct_embedding to be explicit about scope.