netbsd: Clarify type conversions are lossless.

src/netbsd.rs

@@ -4,11 +4,14 @@ use core::{ffi::c_void, mem::MaybeUninit, ptr};
 
 fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
     static MIB: [libc::c_int; 2] = [libc::CTL_KERN, libc::KERN_ARND];
-    let mut len = buf.len();
-    let ret = unsafe {
+    #[allow(clippy::cast_possible_truncation)]
+    let mib_len = MIB.len() as libc::c_uint;
+
+    let mut len = core::cmp::min(buf.len(), libc::ssize_t::MAX.unsignd_abs());
+    let ret: libc::c_int = unsafe {
         libc::sysctl(
             MIB.as_ptr(),
-            MIB.len() as libc::c_uint,
+            mib_len,
             buf.as_mut_ptr().cast::<c_void>(),
             &mut len,
             ptr::null(),
@@ -18,7 +21,13 @@ fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
     if ret == -1 {
         -1
     } else {
-        len as libc::ssize_t
+        // c_int to ssize_t conversion is lossless.
+        const _: () =
+            assert!(core::mem::size_of::<libc::c_int>() == core::mem::size_of::<libc::ssize_t>());
+        // We clamped the request to `ssize_t::MAX` bytes so this lossless.
+        #[allow(clippy::cast_possible_truncation)]
+        let len = len as libc::ssize_t;
+        len
     }
 }