cleanup: Get null pointers through core rather than casting 0

src/gnrc/ipv6.rs

@@ -215,7 +215,7 @@ impl<M: Mode> Pktsnip<M> {
         // unsafe: C API, and requirement on a Pktsnip that typed snips follow that type's
         // conventions
         let ptr = unsafe { riot_sys::gnrc_ipv6_get_header(self.ptr) };
-        if ptr == 0 as _ {
+        if ptr.is_null() {
             None
         } else {
             // unsafe: Header is a transparent wrapper around the actual ipv6_hdr_t, and the

src/gnrc/mod.rs

@@ -47,7 +47,7 @@ impl Netif {
 
     #[doc(alias = "gnrc_netif_get_by_pid")]
     pub fn by_pid(pid: KernelPID) -> Option<Self> {
-        const NULL: *mut riot_sys::gnrc_netif_t = 0 as _;
+        const NULL: *mut riot_sys::gnrc_netif_t = core::ptr::null_mut();
         // Not using as_ref: We can't guarantee that even for the short period between we're making
         // it into a reference and casting it back to a pointer again, it is not used by anyone
         // else

src/gnrc_pktbuf.rs

@@ -201,7 +201,7 @@ impl<'a> Pktsnip<Shared> {
         // unsafe: The C functions justify the new type
         unsafe {
             let new = riot_sys::gnrc_pktbuf_start_write(self.to_ptr());
-            if new == 0 as _ {
+            if new.is_null() {
                 Err(NotEnoughSpace)
             } else {
                 Ok(Pktsnip::<Writable>::from_ptr(new))

src/saul/registration.rs

@@ -195,7 +195,7 @@ where
     ) -> Self {
         Registration {
             reg: riot_sys::saul_reg_t {
-                next: 0 as _,
+                next: core::ptr::null_mut(),
                 dev: device as *const _ as *mut _,
                 name: name.map(|n| n.as_ptr() as _).unwrap_or(core::ptr::null()),
                 driver: &driver.driver as *const _,

src/ztimer/periodic.rs

@@ -80,7 +80,7 @@ impl<H: Handler, const HZ: u32> Timer<H, HZ> {
                 clock.0,
                 timer.as_mut_ptr(),
                 Some(Self::callback),
-                0 as _,
+                core::ptr::null_mut(),
                 ticks.0,
             );
             timer.assume_init()