[PATCH] rust: io: always inline methods leading to build_assert

[Alexandre Courbot <acourbot@nvidia.com>]

`build_assert` relies on the compiler to optimize out its error path,
lest build fails with the dreaded error:

    ERROR: modpost: "rust_build_error" [drivers/gpu/nova-core/nova_core.ko] undefined!

It has been observed that very trivial code performing I/O accesses
(sometimes even using an immediate value) would seemingly randomly fail
with this error whenever `CLIPPY=1` was set. Removing the CLIPPY option
makes the error go away, but that's obviously not a great workaround.
Clippy appears to influence the way the compiler optimizes things,
making it on occasion generate a method where we would need it to inline
in order to satisfy a `build_assert`.

Fix this by instructing the compiler to always inline the methods
leading to `build_assert`. This stronger directive is effective even
when `CLIPPY=1` is specified, which gets rid of this error.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
This is the same fix as for another build error triggered by the use of
`build_assert` [1], which signals that all callers of this macro should
all be tagged with `#[inline(always)]`, as inlining is a requirement for
`build_assert` to perform properly anyway.

[1] https://lore.kernel.org/all/DEEUYUOAEZU3.1J1HM2YQ10EX1@nvidia.com/
---
 rust/kernel/io.rs | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index 98e8b84e68d1..f161ec8056ce 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -142,7 +142,8 @@ macro_rules! define_read {
         /// Bound checks are performed on compile time, hence if the offset is not known at compile
         /// time, the build will fail.
         $(#[$attr])*
-        #[inline]
+        // Always inline so the error path of `io_addr_assert` is optimized out.
+        #[inline(always)]
         pub fn $name(&self, offset: usize) -> $type_name {
             let addr = self.io_addr_assert::<$type_name>(offset);
 
@@ -171,7 +172,8 @@ macro_rules! define_write {
         /// Bound checks are performed on compile time, hence if the offset is not known at compile
         /// time, the build will fail.
         $(#[$attr])*
-        #[inline]
+        // Always inline so the error path of `io_addr_assert` is optimized out.
+        #[inline(always)]
         pub fn $name(&self, value: $type_name, offset: usize) {
             let addr = self.io_addr_assert::<$type_name>(offset);
 
@@ -239,7 +241,8 @@ fn io_addr<U>(&self, offset: usize) -> Result<usize> {
         self.addr().checked_add(offset).ok_or(EINVAL)
     }
 
-    #[inline]
+    // Always inline so the error path of `build_assert!` is optimized out.
+    #[inline(always)]
     fn io_addr_assert<U>(&self, offset: usize) -> usize {
         build_assert!(Self::offset_valid::<U>(offset, SIZE));
 

---
base-commit: ea34511aaf755349999a1067b2984a541bee1492
change-id: 20251127-io-build-assert-3579a5bfb81c

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