util_libc: Avoid as conversion in ssize_t-to-usize conversions.

Author: briansmith

src/util_libc.rs

@@ -55,20 +55,22 @@ pub fn sys_fill_exact(
 ) -> Result<(), Error> {
     while !buf.is_empty() {
         let res = sys_fill(buf);
-        match res {
-            res if res > 0 => buf = buf.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?,
-            -1 => {
-                let err = last_os_error();
-                // We should try again if the call was interrupted.
-                if err.raw_os_error() != Some(libc::EINTR) {
-                    return Err(err);
-                }
+        if let Ok(res) = usize::try_from(res) {
+            if res > 0 {
+                buf = buf.get_mut(res..).ok_or(Error::UNEXPECTED)?;
+                continue;
+            }
+        } else if res == -1 {
+            let err = last_os_error();
+            // We should try again if the call was interrupted.
+            if err.raw_os_error() != Some(libc::EINTR) {
+                return Err(err);
             }
-            // Negative return codes not equal to -1 should be impossible.
-            // EOF (ret = 0) should be impossible, as the data we are reading
-            // should be an infinite stream of random bytes.
-            _ => return Err(Error::UNEXPECTED),
         }
+        // Negative return codes not equal to -1 should be impossible.
+        // EOF (ret = 0) should be impossible, as the data we are reading
+        // should be an infinite stream of random bytes.
+        return Err(Error::UNEXPECTED);
     }
     Ok(())
 }