Swap the if let with if matches

Author: botahamec

clippy_lints/src/unused_box.rs

@@ -48,7 +48,7 @@ impl LateLintPass<'_> for UnusedBox {
     ) {
         // it's unclear what part of a closure you would span, so for now it's ignored
         // if this is changed, please also make sure not to call `hir_ty_to_ty` below
-        if let FnKind::Closure = fn_kind {
+        if matches!(fn_kind, FnKind::Closure) {
             return;
         }