Implemented the lint with a small test

Author: botahamec

clippy_lints/src/unused_box.rs

@@ -1,5 +1,10 @@
-use rustc_lint::LateLintPass;
+use clippy_utils::{diagnostics::span_lint_and_sugg, ty::implements_trait};
+use rustc_errors::Applicability;
+use rustc_hir::{intravisit::FnKind, Body, FnDecl, FnRetTy, HirId};
+use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::Span;
+use rustc_typeck::hir_ty_to_ty;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -29,4 +34,39 @@ declare_clippy_lint! {
 }
 declare_lint_pass!(UnusedBox => [UNUSED_BOX]);
 
-impl LateLintPass<'_> for UnusedBox {}
+impl LateLintPass<'_> for UnusedBox {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'_>,
+        fn_kind: FnKind<'_>,
+        decl: &FnDecl<'_>,
+        _: &Body<'_>,
+        _: Span,
+        _: HirId,
+    ) {
+        if let FnKind::Closure = fn_kind {
+            return;
+        }
+        let FnRetTy::Return(return_ty_hir) = &decl.output else { return };
+        let return_ty = hir_ty_to_ty(cx.tcx, return_ty_hir);
+
+        if !return_ty.is_box() {
+            return;
+        }
+
+        let boxed_ty = return_ty.boxed_ty();
+        let Some(sized_trait) = cx.tcx.lang_items().sized_trait() else { return };
+
+        if implements_trait(cx, boxed_ty, sized_trait, &[]) {
+            span_lint_and_sugg(
+                cx,
+                UNUSED_BOX,
+                return_ty_hir.span,
+                format!("function returns `Box<{0}>` when `{0}` implements `Sized`", boxed_ty).as_str(),
+                "change the return type to",
+                boxed_ty.to_string(),
+                Applicability::MaybeIncorrect,
+            );
+        }
+    }
+}

tests/ui/unused_box.rs

@@ -1,5 +1,9 @@
 #![warn(clippy::unused_box)]
 
+fn foo() -> Box<usize> {
+    Box::new(5)
+}
+
 fn main() {
     // test code goes here
 }