rust: kbuild: disambiguate `zerocopy` for `rusttest`

From: Antoni Boucher <[email protected]>

Starting with Rust 1.76.0, zerocopy was added as an (indirect) compiler dependency [1]. In turn, this meant that the rustc-dev component started including a precompiled zerocopy crate in the sysroot.

This makes rusttest fail because the compiler finds several candidates:

    error[E0464]: multiple candidates for `rmeta` dependency `zerocopy` found
      --> rust/kernel/prelude.rs:64:9
       |
    64 | pub use zerocopy::{
       |         ^^^^^^^^
       |
       = note: candidate #1: .../lib/rustlib/x86_64-unknown-linux-gnu/lib/libzerocopy-dfef4cb07ca752aa.rmeta
       = note: candidate #2: ./rust/test/libzerocopy.rlib

We cannot use --sysroot=/dev/null for these, thus point to the dependency explicitly.

[ Investigated when it started happening, reworded to add that and to
follow our usual style and sent on behalf of Antoni, who found this during his work to support Rust for Linux with the GCC backend, i.e. with rustc_codegen_gcc. - Miguel ]

---
 rust/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 627ed79dc6f5..fbe0accc51a3 100644
@@ -350,7 +350,7 @@ rusttestlib-pin_init: $(src)/pin-init/src/lib.rs rusttestlib-macros \
 rusttestlib-kernel: private rustc_target_flags = --extern ffi \
     --extern build_error --extern macros --extern pin_init \
     --extern bindings --extern uapi \
-    --extern zerocopy --extern zerocopy_derive
+    --extern zerocopy=$(objtree)/$(obj)/test/libzerocopy.rlib --extern zerocopy_derive
 rusttestlib-kernel: $(src)/kernel/lib.rs rusttestlib-bindings rusttestlib-uapi \
     rusttestlib-build_error rusttestlib-pin_init $(obj)/$(libmacros_name) \
     $(obj)/bindings.o rusttestlib-zerocopy rusttestlib-zerocopy_derive FORCE

base-commit: fc02acf6ac0ccde0c805c2daa9148683cdd01ba8

2.55.0

Applied to rust-next -- thanks!

I changed the Assisted-by tag with the details that Antoni gave me.

Cheers,
Miguel