[PATCH v2] rust: allow `clippy::unwrap_or_default` globally

[Alexandre Courbot <acourbot@nvidia.com>]

Starting with rustc 1.88, the `clippy::unwrap_or_default` lint triggers
on `rust/kernel/soc.rs` if `CONFIG_CC_OPTIMIZE_FOR_SIZE=y`:

    warning: use of `unwrap_or` to construct default value
      --> ../rust/kernel/soc.rs:66:10
      |
   66 |         .unwrap_or(core::ptr::null())
      |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `unwrap_or_default()`

This is a clippy bug [1]: the lint decides whether an expression is
equivalent to `Default::default()` by inspecting the optimized MIR of
`<*const T as Default>::default` exported by `core`, so its outcome
depends on the optimization level `core` was built with. Moreover, its
suggestion ignores our MSRV of 1.85 (`Default` for `*const T` is only
stable since Rust 1.88), so we could not apply it anyway.

Disable the lint globally rather than working around this single
occurrence; it can be re-enabled conditionally using `rustc-min-version`
once clippy is fixed.

Link: https://github.com/rust-lang/rust-clippy/issues/17379 [1]
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
Changes in v2:
- Disable the `clippy::unwrap_or_default` lint globally instead of
  allowing it on the site triggering the issue.
- Link to v1: https://patch.msgid.link/20260707-soc_unwrap_or-v1-1-1ca1757a259f@nvidia.com

To: Nathan Chancellor <nathan@kernel.org>
To: Nicolas Schier <nsc@kernel.org>
To: Miguel Ojeda <ojeda@kernel.org>
To: Boqun Feng <boqun@kernel.org>
To: Gary Guo <gary@garyguo.net>
To: Björn Roy Baron <bjorn3_gh@protonmail.com>
To: Benno Lossin <lossin@kernel.org>
To: Andreas Hindborg <a.hindborg@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
To: Trevor Gross <tmgross@umich.edu>
To: Danilo Krummrich <dakr@kernel.org>
To: Daniel Almeida <daniel.almeida@collabora.com>
To: Tamir Duberstein <tamird@kernel.org>
To: Alexandre Courbot <acourbot@nvidia.com>
To: Onur Özkan <work@onurozkan.dev>
Cc: linux-kbuild@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: rust-for-linux@vger.kernel.org
---
 Makefile | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index b9c5792c79e0..22be861c5e7d 100644
--- a/Makefile
+++ b/Makefile
@@ -863,8 +863,13 @@ export WARN_ON_UNUSED_TRACEPOINTS
 # include bitmasking and shift operations. 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.
+#
+# `-Aclippy::unwrap_or_default`: the lint is buggy [1] and ignores our
+# MSRV. It can trigger depending on the optimization level.
+# [1] https://github.com/rust-lang/rust-clippy/issues/17379
 rust_common_flags_per_version := \
-    $(if $(call rustc-min-version,108600),,-Aclippy::precedence)
+    $(if $(call rustc-min-version,108600),,-Aclippy::precedence) \
+    -Aclippy::unwrap_or_default
 
 rust_common_flags += $(rust_common_flags_per_version)
 KBUILD_HOSTRUSTFLAGS += $(rust_common_flags_per_version) $(HOSTRUSTFLAGS)

---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260707-soc_unwrap_or-ed37e674a759

Best regards,
--  
Alexandre Courbot <acourbot@nvidia.com>