Added the new lint with some docs and tests

CHANGELOG.md

@@ -4974,6 +4974,7 @@ Released 2018-09-13
 [`unit_hash`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_hash
 [`unit_return_expecting_ord`]: https://rust-lang.github.io/rust-clippy/master/index.html#unit_return_expecting_ord
 [`unknown_clippy_lints`]: https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints
+[`unnecessary_box_returns`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_box_returns
 [`unnecessary_cast`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
 [`unnecessary_filter_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_filter_map
 [`unnecessary_find_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_find_map

clippy_lints/src/declared_lints.rs

@@ -616,6 +616,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::unit_types::UNIT_CMP_INFO,
     crate::unnamed_address::FN_ADDRESS_COMPARISONS_INFO,
     crate::unnamed_address::VTABLE_ADDRESS_COMPARISONS_INFO,
+    crate::unnecessary_box_returns::UNNECESSARY_BOX_RETURNS_INFO,
     crate::unnecessary_owned_empty_strings::UNNECESSARY_OWNED_EMPTY_STRINGS_INFO,
     crate::unnecessary_self_imports::UNNECESSARY_SELF_IMPORTS_INFO,
     crate::unnecessary_struct_initialization::UNNECESSARY_STRUCT_INITIALIZATION_INFO,

clippy_lints/src/lib.rs

@@ -300,6 +300,7 @@ mod uninit_vec;
 mod unit_return_expecting_ord;
 mod unit_types;
 mod unnamed_address;
+mod unnecessary_box_returns;
 mod unnecessary_owned_empty_strings;
 mod unnecessary_self_imports;
 mod unnecessary_struct_initialization;
@@ -940,6 +941,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(allow_attributes::AllowAttribute));
     store.register_late_pass(move |_| Box::new(manual_main_separator_str::ManualMainSeparatorStr::new(msrv())));
     store.register_late_pass(|_| Box::new(unnecessary_struct_initialization::UnnecessaryStruct));
+    store.register_late_pass(|_| Box::new(unnecessary_box_returns::UnnecessaryBoxReturns));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/unnecessary_box_returns.rs

@@ -0,0 +1,88 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use rustc_errors::Applicability;
+use rustc_hir::{def_id::LocalDefId, intravisit::FnKind, Body, FnDecl, FnRetTy};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::Span;
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for a return type containing a `Box<T>` where `T` implements `Sized`
+    ///
+    /// ### Why is this bad?
+    ///
+    /// It's better to just return `T` in these cases. The caller may not need
+    /// the value to be boxed, and it's expensive to free the memory once the
+    /// `Box<T>` been dropped.
+    ///
+    /// ### Example
+    /// ```rust
+    /// fn foo() -> Box<String> {
+    ///     Box::new(String::from("Hello, world!"))
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// fn foo() -> String {
+    ///     String::from("Hello, world!")
+    /// }
+    /// ```
+    #[clippy::version = "1.70.0"]
+    pub UNNECESSARY_BOX_RETURNS,
+    pedantic,
+    "Needlessly returning a Box"
+}
+declare_lint_pass!(UnnecessaryBoxReturns => [UNNECESSARY_BOX_RETURNS]);
+
+impl LateLintPass<'_> for UnnecessaryBoxReturns {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'_>,
+        fn_kind: FnKind<'_>,
+        decl: &FnDecl<'_>,
+        _: &Body<'_>,
+        _: Span,
+        def_id: LocalDefId,
+    ) {
+        // 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 matches!(fn_kind, FnKind::Closure) {
+            return;
+        }
+
+        let FnRetTy::Return(return_ty_hir) = &decl.output else { return };
+
+        let return_ty = cx
+            .tcx
+            .erase_late_bound_regions(cx.tcx.fn_sig(def_id).skip_binder())
+            .output();
+
+        if !return_ty.is_box() {
+            return;
+        }
+
+        let boxed_ty = return_ty.boxed_ty();
+
+        // it's sometimes useful to return Box<T> if T is unsized, so don't lint those
+        if boxed_ty.is_sized(cx.tcx, cx.param_env) {
+            span_lint_and_then(
+                cx,
+                UNNECESSARY_BOX_RETURNS,
+                return_ty_hir.span,
+                format!("boxed return of the sized type `{boxed_ty}`").as_str(),
+                |diagnostic| {
+                    diagnostic.span_suggestion(
+                        return_ty_hir.span,
+                        "try",
+                        boxed_ty.to_string(),
+                        // the return value and function callers also needs to
+                        // be changed, so this can't be MachineApplicable
+                        Applicability::Unspecified,
+                    );
+                    diagnostic.help("changing this also requires a change to the return expressions in this function");
+                },
+            );
+        }
+    }
+}

tests/ui/unnecessary_box_returns.rs

@@ -0,0 +1,31 @@
+#![warn(clippy::unnecessary_box_returns)]
+
+struct Foo {}
+
+// lint
+fn boxed_usize() -> Box<usize> {
+    Box::new(5)
+}
+
+// lint
+fn boxed_foo() -> Box<Foo> {
+    Box::new(Foo {})
+}
+
+// don't lint: str is unsized
+fn boxed_str() -> Box<str> {
+    "Hello, world!".to_string().into_boxed_str()
+}
+
+// don't lint: this has an unspecified return type
+fn default() {}
+
+// don't lint: this doesn't return a Box
+fn string() -> String {
+    String::from("Hello, world")
+}
+
+fn main() {
+    // don't lint: this is a closure
+    let a = || -> Box<usize> { Box::new(5) };
+}

tests/ui/unnecessary_box_returns.stderr

@@ -0,0 +1,19 @@
+error: boxed return of the sized type `usize`
+  --> $DIR/unnecessary_box_returns.rs:6:21
+   |
+LL | fn boxed_usize() -> Box<usize> {
+   |                     ^^^^^^^^^^ help: try: `usize`
+   |
+   = help: changing this also requires a change to the return expressions in this function
+   = note: `-D clippy::unnecessary-box-returns` implied by `-D warnings`
+
+error: boxed return of the sized type `Foo`
+  --> $DIR/unnecessary_box_returns.rs:11:19
+   |
+LL | fn boxed_foo() -> Box<Foo> {
+   |                   ^^^^^^^^ help: try: `Foo`
+   |
+   = help: changing this also requires a change to the return expressions in this function
+
+error: aborting due to 2 previous errors
+