hermit: Clarify isize -> usize conversion.

Author: briansmith

src/hermit.rs

@@ -9,18 +9,14 @@ extern "C" {
 pub fn getrandom_inner(mut dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     while !dest.is_empty() {
         let res = unsafe { sys_read_entropy(dest.as_mut_ptr().cast::<u8>(), dest.len(), 0) };
-        // Positive `isize`s can be safely casted to `usize`
-        if res > 0 && (res as usize) <= dest.len() {
-            dest = &mut dest[res as usize..];
-        } else {
-            let err = if res < 0 {
-                u32::try_from(res.unsigned_abs())
+        match usize::try_from(res) {
+            Ok(len) if len > 0 => dest = &mut dest[res as usize..],
+            _ => {
+                let err = u32::try_from(res.unsigned_abs())
                     .ok()
-                    .map_or(Error::UNEXPECTED, Error::from_os_error)
-            } else {
-                Error::UNEXPECTED
-            };
-            return Err(err);
+                    .map_or(Error::UNEXPECTED, Error::from_os_error);
+                return Err(err);
+            }
         }
     }
     Ok(())