warning: unstable feature specified for `-Ctarget-feature`: `backchain`

[kernel test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   0e35b9b6ec0ffcc5e23cbdec09f5c622ad532b53
commit: 3f70ebe638581e73c8df6b3fa7eed9b45d90b083 s390: Enable Rust support
date:   4 weeks ago
config: s390-randconfig-001-20260707 (https://download.01.org/0day-ci/archive/20260707/202607071750.4f5d6hPj-lkp@xxxxxxxxx/config)
compiler: clang version 21.1.8 (https://github.com/llvm/llvm-project 2078da43e25a4623cab2d0d60decddf709aaea28)
rustc: rustc 1.96.0 (ac68faa20 2026-05-25)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260707/202607071750.4f5d6hPj-lkp@xxxxxxxxx/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Fixes: 3f70ebe63858 ("s390: Enable Rust support")
| Reported-by: kernel test robot <lkp@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-kbuild-all/202607071750.4f5d6hPj-lkp@xxxxxxxxx/

All warnings (new ones prefixed by >>):

>> warning: unstable feature specified for `-Ctarget-feature`: `backchain`
   |
   = note: this feature is not stably supported; its behavior can change in the future

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki