Auto merge of #9102 - botahamec:unused-box, r=xFrednet

Added the `[unnecessary_box_returns]` lint

fixes #5

I'm not confident in the name of this lint. Let me know if you can think of something better

---

changelog: New lint: ``[`unnecessary_box_returns`]``
[#9102](#9102)
<!-- changelog_checked -->

Author: bors

CHANGELOG.md

@@ -4975,6 +4975,7 @@ 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

book/src/lint_configuration.md

@@ -130,6 +130,7 @@ Suppress lints whenever the suggested change would cause breakage for other crat
 * [option_option](https://rust-lang.github.io/rust-clippy/master/index.html#option_option)
 * [linkedlist](https://rust-lang.github.io/rust-clippy/master/index.html#linkedlist)
 * [rc_mutex](https://rust-lang.github.io/rust-clippy/master/index.html#rc_mutex)
+* [unnecessary_box_returns](https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_box_returns)
 
 
 ### msrv

clippy_lints/src/declared_lints.rs

@@ -617,6 +617,7 @@ 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 +941,11 @@ 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(move |_| {
+        Box::new(unnecessary_box_returns::UnnecessaryBoxReturns::new(
+            avoid_breaking_exported_api,
+        ))
+    });
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/unnecessary_box_returns.rs

@@ -0,0 +1,120 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use rustc_errors::Applicability;
+use rustc_hir::{def_id::LocalDefId, FnDecl, FnRetTy, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_tool_lint, impl_lint_pass};
+use rustc_span::Symbol;
+
+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"
+}
+
+pub struct UnnecessaryBoxReturns {
+    avoid_breaking_exported_api: bool,
+}
+
+impl_lint_pass!(UnnecessaryBoxReturns => [UNNECESSARY_BOX_RETURNS]);
+
+impl UnnecessaryBoxReturns {
+    pub fn new(avoid_breaking_exported_api: bool) -> Self {
+        Self {
+            avoid_breaking_exported_api,
+        }
+    }
+
+    fn check_fn_item(&mut self, cx: &LateContext<'_>, decl: &FnDecl<'_>, def_id: LocalDefId, name: Symbol) {
+        // we don't want to tell someone to break an exported function if they ask us not to
+        if self.avoid_breaking_exported_api && cx.effective_visibilities.is_exported(def_id) {
+            return;
+        }
+
+        // functions which contain the word "box" are exempt from this lint
+        if name.as_str().contains("box") {
+            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");
+                },
+            );
+        }
+    }
+}
+
+impl LateLintPass<'_> for UnnecessaryBoxReturns {
+    fn check_trait_item(&mut self, cx: &LateContext<'_>, item: &TraitItem<'_>) {
+        let TraitItemKind::Fn(signature, _) = &item.kind else { return };
+        self.check_fn_item(cx, signature.decl, item.owner_id.def_id, item.ident.name);
+    }
+
+    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_item(cx, signature.decl, item.owner_id.def_id, item.ident.name);
+    }
+
+    fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
+        let ItemKind::Fn(signature, ..) = &item.kind else { return };
+        self.check_fn_item(cx, signature.decl, item.owner_id.def_id, item.ident.name);
+    }
+}

clippy_lints/src/utils/conf.rs

@@ -249,7 +249,7 @@ define_Conf! {
     /// arithmetic-side-effects-allowed-unary = ["SomeType", "AnotherType"]
     /// ```
     (arithmetic_side_effects_allowed_unary: rustc_data_structures::fx::FxHashSet<String> = <_>::default()),
-    /// Lint: ENUM_VARIANT_NAMES, LARGE_TYPES_PASSED_BY_VALUE, TRIVIALLY_COPY_PASS_BY_REF, UNNECESSARY_WRAPS, UNUSED_SELF, UPPER_CASE_ACRONYMS, WRONG_SELF_CONVENTION, BOX_COLLECTION, REDUNDANT_ALLOCATION, RC_BUFFER, VEC_BOX, OPTION_OPTION, LINKEDLIST, RC_MUTEX.
+    /// Lint: ENUM_VARIANT_NAMES, LARGE_TYPES_PASSED_BY_VALUE, TRIVIALLY_COPY_PASS_BY_REF, UNNECESSARY_WRAPS, UNUSED_SELF, UPPER_CASE_ACRONYMS, WRONG_SELF_CONVENTION, BOX_COLLECTION, REDUNDANT_ALLOCATION, RC_BUFFER, VEC_BOX, OPTION_OPTION, LINKEDLIST, RC_MUTEX, UNNECESSARY_BOX_RETURNS.
     ///
     /// Suppress lints whenever the suggested change would cause breakage for other crates.
     (avoid_breaking_exported_api: bool = true),

tests/ui/unnecessary_box_returns.rs

@@ -0,0 +1,60 @@
+#![warn(clippy::unnecessary_box_returns)]
+
+trait Bar {
+    // lint
+    fn baz(&self) -> Box<usize>;
+}
+
+pub struct Foo {}
+
+impl Bar for Foo {
+    // don't lint: this is a problem with the trait, not the implementation
+    fn baz(&self) -> Box<usize> {
+        Box::new(42)
+    }
+}
+
+impl Foo {
+    fn baz(&self) -> Box<usize> {
+        // lint
+        Box::new(13)
+    }
+}
+
+// lint
+fn bxed_usize() -> Box<usize> {
+    Box::new(5)
+}
+
+// lint
+fn _bxed_foo() -> Box<Foo> {
+    Box::new(Foo {})
+}
+
+// don't lint: this is exported
+pub fn bxed_foo() -> Box<Foo> {
+    Box::new(Foo {})
+}
+
+// don't lint: str is unsized
+fn bxed_str() -> Box<str> {
+    "Hello, world!".to_string().into_boxed_str()
+}
+
+// don't lint: function contains the word, "box"
+fn boxed_usize() -> Box<usize> {
+    Box::new(7)
+}
+
+// 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 +1,35 @@
+error: boxed return of the sized type `usize`
+  --> $DIR/unnecessary_box_returns.rs:5:22
+   |
+LL |     fn baz(&self) -> 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 `usize`
+  --> $DIR/unnecessary_box_returns.rs:18:22
+   |
+LL |     fn baz(&self) -> Box<usize> {
+   |                      ^^^^^^^^^^ help: try: `usize`
+   |
+   = help: changing this also requires a change to the return expressions in this function
+
+error: boxed return of the sized type `usize`
+  --> $DIR/unnecessary_box_returns.rs:25:20
+   |
+LL | fn bxed_usize() -> Box<usize> {
+   |                    ^^^^^^^^^^ help: try: `usize`
+   |
+   = help: changing this also requires a change to the return expressions in this function
+
+error: boxed return of the sized type `Foo`
+  --> $DIR/unnecessary_box_returns.rs:30:19
+   |
+LL | fn _bxed_foo() -> Box<Foo> {
+   |                   ^^^^^^^^ help: try: `Foo`
+   |
+   = help: changing this also requires a change to the return expressions in this function
+
+error: aborting due to 4 previous errors
+