rust: init: wrap type checking struct initializers in a closure

In the implementation of the init macros there is a `if false` statement
that type checks the initializer to ensure every field is initialized.
Since the next patch has a stack variable to store the struct, the
function might allocate too much memory on debug builds. Putting the
struct into a closure that is never executed ensures that even in debug
builds no stack overflow error is caused. In release builds this was not
a problem since the code was optimized away due to the `if false`.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-6-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init/macros.rs

@@ -1040,14 +1040,14 @@ macro_rules! __init_internal {
                     // once, this struct initializer will still be type-checked and complain with a
                     // very natural error message if a field is forgotten/mentioned more than once.
                     #[allow(unreachable_code, clippy::diverging_sub_expression)]
-                    if false {
+                    let _ = || {
                         $crate::__init_internal!(make_initializer:
                             @slot(slot),
                             @type_name($t),
                             @munch_fields($($fields)*,),
                             @acc(),
                         );
-                    }
+                    };
                 }
                 Ok(__InitOk)
             }
@@ -1168,8 +1168,8 @@ macro_rules! __init_internal {
         @acc($($acc:tt)*),
     ) => {
         // Endpoint, nothing more to munch, create the initializer.
-        // Since we are in the `if false` branch, this will never get executed. We abuse `slot` to
+        // Since we are in the closure that is never called, this will never get executed.
-        // get the correct type inference here:
+        // We abuse `slot` to get the correct type inference here:
         unsafe {
             ::core::ptr::write($slot, $t {
                 $($acc)*