capi: fix 'unused return value' warnings

Somewhat recently, 'CString::from_raw' got a '#[must_use]' slapped
on it. Arguably, writing 'drop' around its return value is indeed much
clearer. So we do that here.

We also do that for 'Box::from_raw' even though it doesn't have a
'#[must_use]' on it. But the same principle applies.

PR #882

regex-capi/src/error.rs

@@ -54,7 +54,7 @@ ffi_fn! {
 
 ffi_fn! {
     fn rure_error_free(err: *mut Error) {
-        unsafe { Box::from_raw(err); }
+        unsafe { drop(Box::from_raw(err)); }
     }
 }
 

regex-capi/src/rure.rs

@@ -151,7 +151,7 @@ ffi_fn! {
 
 ffi_fn! {
     fn rure_free(re: *const Regex) {
-        unsafe { Box::from_raw(re as *mut Regex); }
+        unsafe { drop(Box::from_raw(re as *mut Regex)); }
     }
 }
 
@@ -257,10 +257,10 @@ ffi_fn! {
     fn rure_iter_capture_names_free(it: *mut IterCaptureNames) {
         unsafe {
             let it = &mut *it;
-            while let Some(ptr) = it.name_ptrs.pop(){
-                CString::from_raw(ptr);
+            while let Some(ptr) = it.name_ptrs.pop() {
+                drop(CString::from_raw(ptr));
             }
-            Box::from_raw(it);
+            drop(Box::from_raw(it));
         }
     }
 }
@@ -316,7 +316,7 @@ ffi_fn! {
 
 ffi_fn! {
     fn rure_iter_free(it: *mut Iter) {
-        unsafe { Box::from_raw(it); }
+        unsafe { drop(Box::from_raw(it)); }
     }
 }
 
@@ -407,7 +407,7 @@ ffi_fn! {
 
 ffi_fn! {
     fn rure_captures_free(captures: *const Captures) {
-        unsafe { Box::from_raw(captures as *mut Captures); }
+        unsafe { drop(Box::from_raw(captures as *mut Captures)); }
     }
 }
 
@@ -447,7 +447,7 @@ ffi_fn! {
 
 ffi_fn! {
     fn rure_options_free(options: *mut Options) {
-        unsafe { Box::from_raw(options); }
+        unsafe { drop(Box::from_raw(options)); }
     }
 }
 
@@ -527,7 +527,7 @@ ffi_fn! {
 
 ffi_fn! {
     fn rure_set_free(re: *const RegexSet) {
-        unsafe { Box::from_raw(re as *mut RegexSet); }
+        unsafe { drop(Box::from_raw(re as *mut RegexSet)); }
     }
 }
 
@@ -624,6 +624,6 @@ fn rure_escape(
 
 ffi_fn! {
     fn rure_cstring_free(s: *mut c_char) {
-        unsafe { CString::from_raw(s); }
+        unsafe { drop(CString::from_raw(s)); }
     }
 }