Emit a warning when gold is used as the linker

[Noratrieb]

ld.gold is officially deprecated and doesn't seem entirely work properly for Rust: #139425

Once rustc starts emitting SHF_GNU_RETAIN sections for #[used] (#140872), it will get even worse, as gold miscompiles SHF_GNU_RETAIN (it doesn't just ignore it, it breaks the program in other ways).

It may be possible for rustc to detect usage of gold an emit an appropriate warning, telling the user to switch to a different linker like BFD or LLD instead.

@bjorn3 suggested looking at .comment after linking in #139425 (comment). From local testing, it looks like gold only adds this if you pass --enable-linker-version, which cc doesn't seem to pass by default.

But it looks like gold always emits a note, .note.gnu.gold-version/NT_GNU_GOLD_VERSION, which seems like a reliable way to detect gold.

[Noratrieb]

Oh, looks like -fuse-ld=gold is currently completely broken with the LLD default unless you disable it with -Zlinker-features=-lld:

error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "-m64" "/tmp/rustcFAdxzv/symbols.o" "<2 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,libcfg_if-*,liblibc-*,liballoc-*,librustc_std_workspace_core-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/tmp/rustcFAdxzv/raw-dylibs" "-B<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/bin/gcc-ld" "-fuse-ld=lld" "-Wl,-znostart-stop-gc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "a" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs" "-fuse-ld=gold"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: /nix/store/v63bxfiacw082c7ijshf60alvvrpfxsq-binutils-2.44/bin/ld.gold: nostart-stop-gc: unknown -z option
          /nix/store/v63bxfiacw082c7ijshf60alvvrpfxsq-binutils-2.44/bin/ld.gold: use the --help option for usage information
          collect2: error: ld returned 1 exit status

that's really funny.

[lqd]

People really want link args to remain opaque to rustc, and it’s becoming quite hard to handle these combinations of flags, driver versions, etc without understanding the link args or knowing which linker is going to be used. We can’t even really say "let’s not default to lld if there are any link args". We may need to retry linking without it if it causes issues instead. Very disheartening.