rust: kernel: move build_error hidden function to prevent mistakes

Users were using the hidden exported `kernel::build_error` function
instead of the intended `kernel::build_error!` macro, e.g. see the
previous commit.

To force to use the macro, move it into the `build_assert` module,
thus making it a compilation error and avoiding a collision in the same
"namespace". Using the function now would require typing the module name
(which is hidden), not just a single character.

Now attempting to use the function will trigger this error with the
right suggestion by the compiler:

      error[E0423]: expected function, found macro `kernel::build_error`
      --> samples/rust/rust_minimal.rs:29:9
         |
      29 |         kernel::build_error();
         |         ^^^^^^^^^^^^^^^^^^^ not a function
         |
      help: use `!` to invoke the macro
         |
      29 |         kernel::build_error!();
         |                            +

An alternative would be using an alias, but it would be more complex
and moving it into the module seems right since it belongs there and
reduces the amount of code at the crate root.

Keep the `#[doc(hidden)]` inside `build_assert` in case the module is
not hidden in the future.

Reviewed-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/kernel/build_assert.rs

@@ -2,6 +2,9 @@
 
 //! Build-time assert.
 
+#[doc(hidden)]
+pub use build_error::build_error;
+
 /// Fails the build if the code path calling `build_error!` can possibly be executed.
 ///
 /// If the macro is executed in const context, `build_error!` will panic.
@@ -23,10 +26,10 @@
 #[macro_export]
 macro_rules! build_error {
     () => {{
-        $crate::build_error("")
+        $crate::build_assert::build_error("")
     }};
     ($msg:expr) => {{
-        $crate::build_error($msg)
+        $crate::build_assert::build_error($msg)
     }};
 }
 
@@ -73,12 +76,12 @@ macro_rules! build_error {
 macro_rules! build_assert {
     ($cond:expr $(,)?) => {{
         if !$cond {
-            $crate::build_error(concat!("assertion failed: ", stringify!($cond)));
+            $crate::build_assert::build_error(concat!("assertion failed: ", stringify!($cond)));
         }
     }};
     ($cond:expr, $msg:expr) => {{
         if !$cond {
-            $crate::build_error($msg);
+            $crate::build_assert::build_error($msg);
         }
     }};
 }

rust/kernel/lib.rs

@@ -32,7 +32,8 @@ pub use ffi;
 pub mod alloc;
 #[cfg(CONFIG_BLOCK)]
 pub mod block;
-mod build_assert;
+#[doc(hidden)]
+pub mod build_assert;
 pub mod cred;
 pub mod device;
 pub mod error;
@@ -74,9 +75,6 @@ pub use bindings;
 pub use macros;
 pub use uapi;
 
-#[doc(hidden)]
-pub use build_error::build_error;
-
 /// Prefix to appear before log messages printed from within the `kernel` crate.
 const __LOG_PREFIX: &[u8] = b"rust_kernel\0";