Lint on non-trait implementations

Author: botahamec

clippy_lints/src/unnecessary_box_returns.rs

@@ -1,6 +1,6 @@
 use clippy_utils::{diagnostics::span_lint_and_sugg, ty::implements_trait};
 use rustc_errors::Applicability;
-use rustc_hir::{def_id::LocalDefId, FnDecl, FnRetTy, Item, ItemKind, TraitItem, TraitItemKind};
+use rustc_hir::{def_id::LocalDefId, FnDecl, FnRetTy, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind};
 use rustc_hir_analysis::hir_ty_to_ty;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_tool_lint, impl_lint_pass};
@@ -87,6 +87,19 @@ impl LateLintPass<'_> for UnnecessaryBoxReturns {
         self.check_fn_decl(cx, signature.decl, item.owner_id.def_id);
     }
 
+    fn check_impl_item(&mut self, cx: &LateContext<'_>, item: &rustc_hir::ImplItem<'_>) {
+        // Ignore implementations of traits, because the lint should be on the
+        // trait, not on the implmentation of it.
+        let Node::Item(parent) = cx.tcx.hir().get_parent(item.hir_id()) else { return };
+        let ItemKind::Impl(parent) = parent.kind else { return };
+        if parent.of_trait.is_some() {
+            return;
+        }
+
+        let ImplItemKind::Fn(signature, ..) = &item.kind else { return };
+        self.check_fn_decl(cx, signature.decl, item.owner_id.def_id);
+    }
+
     fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
         let ItemKind::Fn(signature, ..) = &item.kind else { return };
         self.check_fn_decl(cx, signature.decl, item.owner_id.def_id);

tests/ui/unnecessary_box_returns.rs

@@ -14,6 +14,12 @@ impl Bar for Foo {
     }
 }
 
+impl Foo {
+    fn baz(&self) -> Box<usize> {
+        Box::new(13)
+    }
+}
+
 // lint
 fn boxed_usize() -> Box<usize> {
     Box::new(5)

tests/ui/unnecessary_box_returns.stderr

@@ -7,16 +7,22 @@ LL |     fn baz(&self) -> Box<usize>;
    = note: `-D clippy::unnecessary-box-returns` implied by `-D warnings`
 
 error: boxed return of the sized type `usize`
-  --> $DIR/unnecessary_box_returns.rs:18:21
+  --> $DIR/unnecessary_box_returns.rs:18:22
+   |
+LL |     fn baz(&self) -> Box<usize> {
+   |                      ^^^^^^^^^^ help: try: `usize`
+
+error: boxed return of the sized type `usize`
+  --> $DIR/unnecessary_box_returns.rs:24:21
    |
 LL | fn boxed_usize() -> Box<usize> {
    |                     ^^^^^^^^^^ help: try: `usize`
 
 error: boxed return of the sized type `Foo`
-  --> $DIR/unnecessary_box_returns.rs:23:20
+  --> $DIR/unnecessary_box_returns.rs:29:20
    |
 LL | fn _boxed_foo() -> Box<Foo> {
    |                    ^^^^^^^^ help: try: `Foo`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors