Rust LLVMIR causes errors after LTO on x86-32

#[repr(C)]
pub struct UrlpPattern(pub *mut c_void);

#[unsafe(no_mangle)]
pub unsafe extern "C" fn square(num: UrlpPattern) -> *mut c_void {
    num.0
}

generates

define noundef ptr @square(ptr noalias nocapture noundef readonly byval([4 x i8]) align 4 dereferenceable(4) %num) unnamed_addr {
start:
  %_0 = load ptr, ptr %num, align 4
  ret ptr %_0
}

but

struct wrapper {
    void *p;
};


void *square(wrapper p) {
    return p.p;
}

generates

define dso_local i8* @square(wrapper)(i8* readnone returned %p.0) local_unnamed_addr {
entry:
  ret i8* %p.0
}

It seems like this mostly works because it compiles to the same thing x86-32 code:

square(wrapper):                      # @square(wrapper)
        mov     eax, dword ptr [esp + 4]
        ret

However, if the rust code is inlined into C++ with LTO we end up with an extra dereference.

We ran into this in Firefox here: https://bugzilla.mozilla.org/show_bug.cgi?id=1973805

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)

Start by reproducing the Rust and C/C++ examples with rustc 1.88.0 and compare their LLVM IR, including an LTO build. Use the Firefox Bugzilla report as context, then verify that x86-32 LTO no longer introduces an extra dereference while preserving the expected ABI behavior.