1 use clippy_utils::diagnostics::span_lint;
2 use clippy_utils::ty::contains_ty;
3 use rustc_hir::intravisit;
4 use rustc_hir::{self, AssocItemKind, Body, FnDecl, HirId, HirIdSet, Impl, ItemKind, Node};
5 use rustc_infer::infer::TyCtxtInferExt;
6 use rustc_lint::{LateContext, LateLintPass};
7 use rustc_middle::mir::FakeReadCause;
8 use rustc_middle::ty::layout::LayoutOf;
9 use rustc_middle::ty::{self, TraitRef, Ty};
10 use rustc_session::{declare_tool_lint, impl_lint_pass};
11 use rustc_span::source_map::Span;
12 use rustc_span::symbol::kw;
13 use rustc_target::spec::abi::Abi;
14 use rustc_typeck::expr_use_visitor::{Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
16 #[derive(Copy, Clone)]
17 pub struct BoxedLocal {
18 pub too_large_for_stack: u64,
21 declare_clippy_lint! {
23 /// Checks for usage of `Box<T>` where an unboxed `T` would
26 /// ### Why is this bad?
27 /// This is an unnecessary allocation, and bad for
28 /// performance. It is only necessary to allocate if you wish to move the box
33 /// # fn foo(bar: usize) {}
35 /// let x = Box::new(1);
37 /// println!("{}", *x);
42 /// println!("{}", x);
44 #[clippy::version = "pre 1.29.0"]
47 "using `Box<T>` where unnecessary"
50 fn is_non_trait_box(ty: Ty<'_>) -> bool {
51 ty.is_box() && !ty.boxed_ty().is_trait()
54 struct EscapeDelegate<'a, 'tcx> {
55 cx: &'a LateContext<'tcx>,
57 trait_self_ty: Option<Ty<'tcx>>,
58 too_large_for_stack: u64,
61 impl_lint_pass!(BoxedLocal => [BOXED_LOCAL]);
63 impl<'tcx> LateLintPass<'tcx> for BoxedLocal {
66 cx: &LateContext<'tcx>,
67 fn_kind: intravisit::FnKind<'tcx>,
73 if let Some(header) = fn_kind.header() {
74 if header.abi != Abi::Rust {
79 let parent_id = cx.tcx.hir().get_parent_item(hir_id);
80 let parent_node = cx.tcx.hir().find(parent_id);
82 let mut trait_self_ty = None;
83 if let Some(Node::Item(item)) = parent_node {
84 // If the method is an impl for a trait, don't warn.
85 if let ItemKind::Impl(Impl { of_trait: Some(_), .. }) = item.kind {
89 // find `self` ty for this trait if relevant
90 if let ItemKind::Trait(_, _, _, _, items) = item.kind {
91 for trait_item in items {
92 if trait_item.id.hir_id() == hir_id {
93 // be sure we have `self` parameter in this function
94 if trait_item.kind == (AssocItemKind::Fn { has_self: true }) {
96 TraitRef::identity(cx.tcx, trait_item.id.def_id.to_def_id())
106 let mut v = EscapeDelegate {
108 set: HirIdSet::default(),
110 too_large_for_stack: self.too_large_for_stack,
113 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
114 cx.tcx.infer_ctxt().enter(|infcx| {
115 ExprUseVisitor::new(&mut v, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body);
122 cx.tcx.hir().span(node),
123 "local variable doesn't need to be boxed here",
129 // TODO: Replace with Map::is_argument(..) when it's fixed
130 fn is_argument(map: rustc_middle::hir::map::Map<'_>, id: HirId) -> bool {
132 Some(Node::Binding(_)) => (),
136 matches!(map.find(map.get_parent_node(id)), Some(Node::Param(_)))
139 impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
140 fn consume(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) {
141 if cmt.place.projections.is_empty() {
142 if let PlaceBase::Local(lid) = cmt.place.base {
143 self.set.remove(&lid);
144 let map = &self.cx.tcx.hir();
145 if let Some(Node::Binding(_)) = map.find(cmt.hir_id) {
146 if self.set.contains(&lid) {
147 // let y = x where x is known
148 // remove x, insert y
149 self.set.insert(cmt.hir_id);
150 self.set.remove(&lid);
157 fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {
158 if cmt.place.projections.is_empty() {
159 if let PlaceBase::Local(lid) = cmt.place.base {
160 self.set.remove(&lid);
165 fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) {
166 if cmt.place.projections.is_empty() {
167 let map = &self.cx.tcx.hir();
168 if is_argument(*map, cmt.hir_id) {
169 // Skip closure arguments
170 let parent_id = map.get_parent_node(cmt.hir_id);
171 if let Some(Node::Expr(..)) = map.find(map.get_parent_node(parent_id)) {
175 // skip if there is a `self` parameter binding to a type
176 // that contains `Self` (i.e.: `self: Box<Self>`), see #4804
177 if let Some(trait_self_ty) = self.trait_self_ty {
178 if map.name(cmt.hir_id) == kw::SelfLower && contains_ty(self.cx.tcx, cmt.place.ty(), trait_self_ty)
184 if is_non_trait_box(cmt.place.ty()) && !self.is_large_box(cmt.place.ty()) {
185 self.set.insert(cmt.hir_id);
191 fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}
194 impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> {
195 fn is_large_box(&self, ty: Ty<'tcx>) -> bool {
196 // Large types need to be boxed to avoid stack overflows.
198 self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack