rust: kbuild: allow clippy::precedence for Rust < 1.86.0

The Clippy `precedence` lint was extended in Rust 1.85.0 to include
bitmasking and shift operations [1]. However, because it generated
many hits, in Rust 1.86.0 it was split into a new `precedence_bits`
lint which is not enabled by default [2].

In other words, only Rust 1.85 has a different behavior. For instance,
it reports:

    warning: operator precedence can trip the unwary
      --> drivers/gpu/nova-core/fb/hal/ga100.rs:16:5
       |
    16 | /     u64::from(regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR::read(bar).adr_39_08()) << FLUSH_SYSMEM_ADDR_SHIFT
    17 | |         | u64::from(regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR_HI::read(bar).adr_63_40())
    18 | |             << FLUSH_SYSMEM_ADDR_SHIFT_HI
       | |_________________________________________^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#precedence
       = note: `-W clippy::precedence` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::precedence)]`
    help: consider parenthesizing your expression
       |
    16 ~     (u64::from(regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR::read(bar).adr_39_08()) << FLUSH_SYSMEM_ADDR_SHIFT) | (u64::from(regs::NV_PFB_NISO_FLUSH_SYSMEM_ADDR_HI::read(bar).adr_63_40())
    17 +             << FLUSH_SYSMEM_ADDR_SHIFT_HI)
       |

    warning: operator precedence can trip the unwary
       --> drivers/gpu/nova-core/vbios.rs:511:17
        |
    511 | /                 u32::from(data[29]) << 24
    512 | |                     | u32::from(data[28]) << 16
    513 | |                     | u32::from(data[27]) << 8
        | |______________________________________________^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#precedence
    help: consider parenthesizing your expression
        |
    511 ~                 u32::from(data[29]) << 24
    512 +                     | u32::from(data[28]) << 16 | (u32::from(data[27]) << 8)
        |

    warning: operator precedence can trip the unwary
       --> drivers/gpu/nova-core/vbios.rs:511:17
        |
    511 | /                 u32::from(data[29]) << 24
    512 | |                     | u32::from(data[28]) << 16
        | |_______________________________________________^ help: consider parenthesizing your expression: `(u32::from(data[29]) << 24) | (u32::from(data[28]) << 16)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#precedence

While so far we try our best to keep all versions Clippy-clean, the
minimum (which is now Rust 1.85.0 after the bump) and the latest stable
are the most important ones; and this may be considered "false positives"
with respect to the behavior in other versions.

Thus allow this lint for this version using the per-version flags
mechanism introduced in the previous commit.

Link: rust-lang/rust-clippy#14097 [1]
Link: rust-lang/rust-clippy#14115 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20260401114540.30108-34-ojeda@kernel.org
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: ojeda

Makefile

@@ -836,7 +836,8 @@ export WARN_ON_UNUSED_TRACEPOINTS
 
 # Per-version Rust flags. These are like `rust_common_flags`, but may
 # depend on the Rust compiler version (e.g. using `rustc-min-version`).
-rust_common_flags_per_version :=
+rust_common_flags_per_version := \
+    $(if $(call rustc-min-version,108600),,-Aclippy::precedence)
 
 rust_common_flags += $(rust_common_flags_per_version)
 KBUILD_HOSTRUSTFLAGS += $(rust_common_flags_per_version)