Lint on trait declarations

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::{intravisit::FnKind, Body, FnDecl, FnRetTy, HirId};
+use rustc_hir::{intravisit::FnKind, Body, FnDecl, FnRetTy, HirId, TraitItem, TraitItemKind};
 use rustc_hir_analysis::hir_ty_to_ty;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -36,7 +36,40 @@ declare_clippy_lint! {
 }
 declare_lint_pass!(UnnecessaryBoxReturns => [UNNECESSARY_BOX_RETURNS]);
 
+fn check_fn_decl(cx: &LateContext<'_>, decl: &FnDecl<'_>) {
+    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() {
+        return;
+    }
+
+    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,
+            UNNECESSARY_BOX_RETURNS,
+            return_ty_hir.span,
+            format!("function returns `Box<{boxed_ty}>` when `{boxed_ty}` implements `Sized`").as_str(),
+            "change the return type to",
+            boxed_ty.to_string(),
+            // the return value and function callers also needs to be changed, so this can't be MachineApplicable
+            Applicability::Unspecified,
+        );
+    }
+}
+
 impl LateLintPass<'_> for UnnecessaryBoxReturns {
+    fn check_trait_item(&mut self, cx: &LateContext<'_>, item: &TraitItem<'_>) {
+        let TraitItemKind::Fn(signature, _) = &item.kind else { return };
+        check_fn_decl(cx, signature.decl);
+    }
+
     fn check_fn(
         &mut self,
         cx: &LateContext<'_>,
@@ -51,30 +84,6 @@ impl LateLintPass<'_> for UnnecessaryBoxReturns {
         // If this is changed, please also make sure not to call `hir_ty_to_ty` below.
         let FnKind::ItemFn(..) = fn_kind else { 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() {
-            return;
-        }
-
-        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,
-                UNNECESSARY_BOX_RETURNS,
-                return_ty_hir.span,
-                format!("function returns `Box<{boxed_ty}>` when `{boxed_ty}` implements `Sized`").as_str(),
-                "change the return type to",
-                boxed_ty.to_string(),
-                // the return value and function callers also needs to be changed, so this can't be MachineApplicable
-                Applicability::Unspecified,
-            );
-        }
+        check_fn_decl(cx, decl);
     }
 }

tests/ui/unnecessary_box_returns.rs

@@ -1,6 +1,7 @@
 #![warn(clippy::unnecessary_box_returns)]
 
 trait Bar {
+    // lint
     fn baz(&self) -> Box<usize>;
 }
 

tests/ui/unnecessary_box_returns.stderr

@@ -1,16 +1,22 @@
 error: function returns `Box<usize>` when `usize` implements `Sized`
-  --> $DIR/unnecessary_box_returns.rs:17:21
+  --> $DIR/unnecessary_box_returns.rs:5:22
    |
-LL | fn boxed_usize() -> Box<usize> {
-   |                     ^^^^^^^^^^ help: change the return type to: `usize`
+LL |     fn baz(&self) -> Box<usize>;
+   |                      ^^^^^^^^^^ help: change the return type to: `usize`
    |
    = note: `-D clippy::unnecessary-box-returns` implied by `-D warnings`
 
+error: function returns `Box<usize>` when `usize` implements `Sized`
+  --> $DIR/unnecessary_box_returns.rs:18:21
+   |
+LL | fn boxed_usize() -> Box<usize> {
+   |                     ^^^^^^^^^^ help: change the return type to: `usize`
+
 error: function returns `Box<Foo>` when `Foo` implements `Sized`
-  --> $DIR/unnecessary_box_returns.rs:22:19
+  --> $DIR/unnecessary_box_returns.rs:23:19
    |
 LL | fn boxed_foo() -> Box<Foo> {
    |                   ^^^^^^^^ help: change the return type to: `Foo`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors