[PATCH] rust: allow `unknown_lints` in generated bindings for Rust < 1.88

[Miguel Ojeda <ojeda@kernel.org>]

Starting with bindgen 0.73.2 [1], `#[allow(unnecessary_transmutes)]`
are used, even when `--rust-target 1.85` is passed.

However, the lint was introduced in Rust 1.88.0. Thus building with
older Rust versions warns like:

    error: unknown lint: `unnecessary_transmutes`
         --> rust/uapi/uapi_generated.rs:26294:13
          |
    26294 |     #[allow(unnecessary_transmutes)]
          |             ^^^^^^^^^^^^^^^^^^^^^^
          |
          = note: `-D unknown-lints` implied by `-D warnings`
          = help: to override `-D warnings` add `#[allow(unknown_lints)]`

Thus allow `unknown_lints` in the generated bindings -- only when building
with older Rust versions.

I have asked upstream if this is intentional [1], i.e. if we are supposed
to always allow unknown lints in case `bindgen` uses such attributes,
or whether it is an oversight.

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Cc: Emilio Cobos Álvarez <emilio@crisal.io>
Link: https://github.com/rust-lang/rust-bindgen/pull/3455 [1]
Assisted-by: LLM
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/bindings/lib.rs | 1 +
 rust/uapi/lib.rs     | 1 +
 2 files changed, 2 insertions(+)

diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
index ad24c920b919..9e2c9a8e7e6c 100644
--- a/rust/bindings/lib.rs
+++ b/rust/bindings/lib.rs
@@ -27,6 +27,7 @@
 #[allow(clippy::ptr_as_ptr)]
 #[allow(clippy::ref_as_ptr)]
 #[allow(clippy::undocumented_unsafe_blocks)]
+#[cfg_attr(not(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES), allow(unknown_lints))]
 #[cfg_attr(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES, allow(unnecessary_transmutes))]
 #[cfg_attr(
     CONFIG_RUSTC_HAS_SUSPICIOUS_RUNTIME_SYMBOL_DEFINITIONS,
diff --git a/rust/uapi/lib.rs b/rust/uapi/lib.rs
index 2df0340e63d1..c1e27192ecce 100644
--- a/rust/uapi/lib.rs
+++ b/rust/uapi/lib.rs
@@ -24,6 +24,7 @@
     unreachable_pub,
     unsafe_op_in_unsafe_fn
 )]
+#![cfg_attr(not(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES), allow(unknown_lints))]
 #![cfg_attr(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES, allow(unnecessary_transmutes))]
 #![cfg_attr(
     CONFIG_RUSTC_HAS_SUSPICIOUS_RUNTIME_SYMBOL_DEFINITIONS,

base-commit: 2ac74c6db40adaa29c50cbb281ae9a6f63de18e1
-- 
2.55.0