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);
46 "using `Box<T>` where unnecessary"
49 fn is_non_trait_box(ty: Ty<'_>) -> bool {
50 ty.is_box() && !ty.boxed_ty().is_trait()
53 struct EscapeDelegate<'a, 'tcx> {
54 cx: &'a LateContext<'tcx>,
56 trait_self_ty: Option<Ty<'tcx>>,
57 too_large_for_stack: u64,
60 impl_lint_pass!(BoxedLocal => [BOXED_LOCAL]);
62 impl<'tcx> LateLintPass<'tcx> for BoxedLocal {
65 cx: &LateContext<'tcx>,
66 fn_kind: intravisit::FnKind<'tcx>,
72 if let Some(header) = fn_kind.header() {
73 if header.abi != Abi::Rust {
78 let parent_id = cx.tcx.hir().get_parent_item(hir_id);
79 let parent_node = cx.tcx.hir().find(parent_id);
81 let mut trait_self_ty = None;
82 if let Some(Node::Item(item)) = parent_node {
83 // If the method is an impl for a trait, don't warn.
84 if let ItemKind::Impl(Impl { of_trait: Some(_), .. }) = item.kind {
88 // find `self` ty for this trait if relevant
89 if let ItemKind::Trait(_, _, _, _, items) = item.kind {
90 for trait_item in items {
91 if trait_item.id.hir_id() == hir_id {
92 // be sure we have `self` parameter in this function
93 if let AssocItemKind::Fn { has_self: true } = trait_item.kind {
95 TraitRef::identity(cx.tcx, trait_item.id.def_id.to_def_id())
105 let mut v = EscapeDelegate {
107 set: HirIdSet::default(),
109 too_large_for_stack: self.too_large_for_stack,
112 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
113 cx.tcx.infer_ctxt().enter(|infcx| {
114 ExprUseVisitor::new(&mut v, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body);
121 cx.tcx.hir().span(node),
122 "local variable doesn't need to be boxed here",
128 // TODO: Replace with Map::is_argument(..) when it's fixed
129 fn is_argument(map: rustc_middle::hir::map::Map<'_>, id: HirId) -> bool {
131 Some(Node::Binding(_)) => (),
135 matches!(map.find(map.get_parent_node(id)), Some(Node::Param(_)))
138 impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
139 fn consume(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) {
140 if cmt.place.projections.is_empty() {
141 if let PlaceBase::Local(lid) = cmt.place.base {
142 self.set.remove(&lid);
143 let map = &self.cx.tcx.hir();
144 if let Some(Node::Binding(_)) = map.find(cmt.hir_id) {
145 if self.set.contains(&lid) {
146 // let y = x where x is known
147 // remove x, insert y
148 self.set.insert(cmt.hir_id);
149 self.set.remove(&lid);
156 fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {
157 if cmt.place.projections.is_empty() {
158 if let PlaceBase::Local(lid) = cmt.place.base {
159 self.set.remove(&lid);
164 fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) {
165 if cmt.place.projections.is_empty() {
166 let map = &self.cx.tcx.hir();
167 if is_argument(*map, cmt.hir_id) {
168 // Skip closure arguments
169 let parent_id = map.get_parent_node(cmt.hir_id);
170 if let Some(Node::Expr(..)) = map.find(map.get_parent_node(parent_id)) {
174 // skip if there is a `self` parameter binding to a type
175 // that contains `Self` (i.e.: `self: Box<Self>`), see #4804
176 if let Some(trait_self_ty) = self.trait_self_ty {
177 if map.name(cmt.hir_id) == kw::SelfLower && contains_ty(self.cx.tcx, cmt.place.ty(), trait_self_ty)
183 if is_non_trait_box(cmt.place.ty()) && !self.is_large_box(cmt.place.ty()) {
184 self.set.insert(cmt.hir_id);
190 fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}
193 impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> {
194 fn is_large_box(&self, ty: Ty<'tcx>) -> bool {
195 // Large types need to be boxed to avoid stack overflows.
197 self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack