[PATCH] rust: pin_init: internal: use `loop {}` to produce never value

[Gary Guo]

From: Gary Guo <gary@xxxxxxxxxxx>

In the `init!`/`pin_init!` macros, we rely on a trick that assigns never
(`!`) values to all mentioned fields in never-executed code to let the
compiler check that all fields have been initialized.

Currently we use `::core::panic!()` to produce this value, but before Rust
1.91.0, it creates outlined `panic_cold_explicit` functions which do not
get removed by the optimizer, thus leaving dead code behind in the binary.
This has been fixed by [1], which lands in Rust 1.91.0+, higher than the
kernel minimum version 1.85.0.

This causes ~200 dead `panic_cold_explicit` instances being included in the
binary, with ~90 of them from nova-core's usage of pin-init.

Work around the issue by using `loop {}` which creates the never value
without macro expansion or function call at all. All instances of
`panic_cold_explicit` outside libcore are removed by this change in my
kernel build.

Link: https://github.com/rust-lang/rust/pull/145304 [1]
Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
---
 rust/pin-init/internal/src/init.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rust/pin-init/internal/src/init.rs b/rust/pin-init/internal/src/init.rs
index fbc8286263b2..b1cb2e53ee6f 100644
--- a/rust/pin-init/internal/src/init.rs
+++ b/rust/pin-init/internal/src/init.rs
@@ -338,7 +338,7 @@ fn make_field_check(
             ::core::ptr::write(slot, #path {
                 #(
                     #(#field_attrs)*
-                    #field_name: ::core::panic!(),
+                    #field_name: loop {},
                 )*
                 #zeroing_trailer
             })

-- 
2.51.2