warning: variable does not need to be mutable

[kernel test robot <lkp@intel.com>]

tree:   https://github.com/intel-lab-lkp/linux/commits/Manos-Pitsidianakis/rust-bindings-generate-virtio-bindings/20260513-065912
head:   dcbb217532e1f25cb932bf8b1019d1085373a6df
commit: dcbb217532e1f25cb932bf8b1019d1085373a6df samples/rust: Add sample virtio-rtc driver [WIP]
date:   4 days ago
config: x86_64-rhel-9.4-rust (https://download.01.org/0day-ci/archive/20260516/202605161734.3uEaaFFK-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
rustc: rustc 1.88.0 (6b00bc388 2025-06-23)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260516/202605161734.3uEaaFFK-lkp@intel.com/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
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202605161734.3uEaaFFK-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> warning: variable does not need to be mutable
   --> samples/rust/rust_virtio_rtc.rs:338:17
   |
   338 |             let mut is_exposed = false;
   |                 ----^^^^^^^^^^
   |                 |
   |                 help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default
--
>> warning: variants `Tai` and `Monotonic` are never constructed
   --> samples/rust/rust_virtio_rtc.rs:113:5
   |
   109 | enum ClockType {
   |      --------- variants in this enum
   ...
   113 |     Tai = 1,
   |     ^^^
   114 |     #[doc(alias = "VIRTIO_RTC_CLOCK_MONOTONIC")]
   115 |     Monotonic = 2,
   |     ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

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