Because if calls to every one of these helpers are real calls, I would
expect performance to be pretty poor. There's probably a reason these
are macros or inlinable functions.

I would almost go so far and suggest that CONFIG_RUST be modified as
follows:

--- a/init/Kconfig
+++ b/init/Kconfig

@@ -2028,6 +2028,7 @@ config RUST
        depends on HAS_RUST
        depends on !COMPILE_TEST
        depends on !MODVERSIONS
+       depends on LTO || EXPERT
        default n
        help
          Enables Rust support in the kernel.

[ I'm sure there are configs that don't yet work with LTO, but could be
  useful to enable for debugging or testing purposes, and therefore would
  make it conditional on CONFIG_EXPERT as well. ]

[...]

quoted

+unsigned long rust_helper_copy_from_user(void *to, const void __user *from, unsigned long n)
+{
+     return copy_from_user(to, from, n);
+}
+

[...]

From some local tests, it looks like simply attaching
__attribute__((always_inline)) will do what one would expect when
compiling with Clang LTO (I checked -flto=thin).

If you confirm this also works across C and Rust TUs when enabling LTO,
I would then suggested adding __attribute__((always_inline)) to all
these helpers.

Thanks,
-- Marco