Added some comments

Author: botahamec

clippy_lints/src/unused_box.rs

@@ -44,10 +44,15 @@ impl LateLintPass<'_> for UnusedBox {
         _: Span,
         _: HirId,
     ) {
+        // 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 {
             return;
         }
+
         let FnRetTy::Return(return_ty_hir) = &decl.output else { return };
+
+        // this is safe, since we're not in a body
         let return_ty = hir_ty_to_ty(cx.tcx, return_ty_hir);
 
         if !return_ty.is_box() {
@@ -57,6 +62,7 @@ impl LateLintPass<'_> for UnusedBox {
         let boxed_ty = return_ty.boxed_ty();
         let Some(sized_trait) = cx.tcx.lang_items().sized_trait() else { return };
 
+        // it's sometimes useful to return Box<T> if T is unsized, so don't lint those
         if implements_trait(cx, boxed_ty, sized_trait, &[]) {
             span_lint_and_sugg(
                 cx,