Fix clippy warnings (#498)

src/compaction_filter.rs

@@ -100,7 +100,7 @@ pub unsafe extern "C" fn destructor_callback<F>(raw_cb: *mut c_void)
 where
     F: CompactionFilter,
 {
-    let _: Box<F> = Box::from_raw(raw_cb as *mut F);
+    Box::from_raw(raw_cb as *mut F);
 }
 
 pub unsafe extern "C" fn name_callback<F>(raw_cb: *mut c_void) -> *const c_char

src/compaction_filter_factory.rs

@@ -30,7 +30,7 @@ pub unsafe extern "C" fn destructor_callback<F>(raw_self: *mut c_void)
 where
     F: CompactionFilterFactory,
 {
-    let _: Box<F> = Box::from_raw(raw_self as *mut F);
+    Box::from_raw(raw_self as *mut F);
 }
 
 pub unsafe extern "C" fn name_callback<F>(raw_self: *mut c_void) -> *const c_char

src/comparator.rs

@@ -16,7 +16,6 @@
 use libc::{c_char, c_int, c_void, size_t};
 use std::cmp::Ordering;
 use std::ffi::CString;
-use std::mem;
 use std::slice;
 
 pub type CompareFn = fn(&[u8], &[u8]) -> Ordering;
@@ -27,7 +26,7 @@ pub struct ComparatorCallback {
 }
 
 pub unsafe extern "C" fn destructor_callback(raw_cb: *mut c_void) {
-    let _: Box<ComparatorCallback> = mem::transmute(raw_cb);
+    Box::from_raw(raw_cb as *mut ComparatorCallback);
 }
 
 pub unsafe extern "C" fn name_callback(raw_cb: *mut c_void) -> *const c_char {

src/merge_operator.rs

@@ -77,8 +77,7 @@ pub struct MergeOperatorCallback<F: MergeFn, PF: MergeFn> {
 }
 
 pub unsafe extern "C" fn destructor_callback<F: MergeFn, PF: MergeFn>(raw_cb: *mut c_void) {
-    let _: Box<MergeOperatorCallback<F, PF>> =
+    Box::from_raw(raw_cb as *mut MergeOperatorCallback<F, PF>);
-        Box::from_raw(raw_cb as *mut MergeOperatorCallback<F, PF>);
 }
 
 pub unsafe extern "C" fn delete_callback(
@@ -87,7 +86,7 @@ pub unsafe extern "C" fn delete_callback(
     value_length: size_t,
 ) {
     if !value.is_null() {
-        let _ = Box::from_raw(slice::from_raw_parts_mut(
+        Box::from_raw(slice::from_raw_parts_mut(
             value as *mut u8,
             value_length as usize,
         ));