[PATCH 3/3] rust: error: replace `WARN_ON_ONCE` comment with `debug_assert!`

[Miguel Ojeda <ojeda@kernel.org>]

`warn_on!` support landed recently, and we had a very old comment
about using it when supported to catch invalid inputs passed to
`Error::from_errno`.

However, the kernel policy is that reaching a `WARN_ON` by user
interactions is a CVE, e.g. [1].

Since `from_errno` and other functions that use it such as `to_result`
will be used everywhere, sooner or later a caller may pass an invalid
value due to a user interaction.

Thus, instead, use a debug assertion -- this assumes hitting one of them
is not going to be considered a CVE (which requires
`CONFIG_RUST_DEBUG_ASSERTIONS=y`).

We don't want to potentially panic when testing the examples, thus
convert those to a build-test.

Cc: Greg KH <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/all/2024092340-renovate-cornflake-4b5e@gregkh/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/error.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 1ebdb798fd5d..7b9892a46505 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -115,18 +115,20 @@ impl Error {
     /// The following calls are considered a bug:
     ///
     /// ```
+    /// # fn no_run() {
     /// assert_eq!(Error::from_errno(0), EINVAL);
     /// assert_eq!(Error::from_errno(-1000000), EINVAL);
+    /// # }
     /// ```
     pub fn from_errno(errno: crate::ffi::c_int) -> Error {
         if let Some(error) = Self::try_from_errno(errno) {
             error
         } else {
-            // TODO: Make it a `WARN_ONCE` once available.
             crate::pr_warn!(
                 "attempted to create `Error` with out of range `errno`: {}\n",
                 errno
             );
+            debug_assert!(false);
             code::EINVAL
         }
     }
-- 
2.51.0