1 use rustc_hir::intravisit;
2 use rustc_hir::{self, AssocItemKind, Body, FnDecl, HirId, HirIdSet, Impl, ItemKind, Node};
3 use rustc_infer::infer::TyCtxtInferExt;
4 use rustc_lint::{LateContext, LateLintPass};
5 use rustc_middle::ty::{self, TraitRef, Ty};
6 use rustc_session::{declare_tool_lint, impl_lint_pass};
7 use rustc_span::source_map::Span;
8 use rustc_span::symbol::kw;
9 use rustc_target::abi::LayoutOf;
10 use rustc_target::spec::abi::Abi;
11 use rustc_typeck::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
13 use crate::utils::{contains_ty, span_lint};
15 #[derive(Copy, Clone)]
16 pub struct BoxedLocal {
17 pub too_large_for_stack: u64,
20 declare_clippy_lint! {
21 /// **What it does:** Checks for usage of `Box<T>` where an unboxed `T` would
24 /// **Why is this bad?** This is an unnecessary allocation, and bad for
25 /// performance. It is only necessary to allocate if you wish to move the box
28 /// **Known problems:** None.
32 /// # fn foo(bar: usize) {}
34 /// let x = Box::new(1);
36 /// println!("{}", *x);
41 /// println!("{}", x);
45 "using `Box<T>` where unnecessary"
48 fn is_non_trait_box(ty: Ty<'_>) -> bool {
49 ty.is_box() && !ty.boxed_ty().is_trait()
52 struct EscapeDelegate<'a, 'tcx> {
53 cx: &'a LateContext<'tcx>,
55 trait_self_ty: Option<Ty<'a>>,
56 too_large_for_stack: u64,
59 impl_lint_pass!(BoxedLocal => [BOXED_LOCAL]);
61 impl<'tcx> LateLintPass<'tcx> for BoxedLocal {
64 cx: &LateContext<'tcx>,
65 fn_kind: intravisit::FnKind<'tcx>,
71 if let Some(header) = fn_kind.header() {
72 if header.abi != Abi::Rust {
77 let parent_id = cx.tcx.hir().get_parent_item(hir_id);
78 let parent_node = cx.tcx.hir().find(parent_id);
80 let mut trait_self_ty = None;
81 if let Some(Node::Item(item)) = parent_node {
82 // If the method is an impl for a trait, don't warn.
83 if let ItemKind::Impl(Impl { of_trait: Some(_), .. }) = item.kind {
87 // find `self` ty for this trait if relevant
88 if let ItemKind::Trait(_, _, _, _, items) = item.kind {
89 for trait_item in items {
90 if trait_item.id.hir_id == hir_id {
91 // be sure we have `self` parameter in this function
92 if let AssocItemKind::Fn { has_self: true } = trait_item.kind {
94 Some(TraitRef::identity(cx.tcx, trait_item.id.hir_id.owner.to_def_id()).self_ty());
101 let mut v = EscapeDelegate {
103 set: HirIdSet::default(),
105 too_large_for_stack: self.too_large_for_stack,
108 let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
109 cx.tcx.infer_ctxt().enter(|infcx| {
110 ExprUseVisitor::new(&mut v, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body);
117 cx.tcx.hir().span(node),
118 "local variable doesn't need to be boxed here",
124 // TODO: Replace with Map::is_argument(..) when it's fixed
125 fn is_argument(map: rustc_middle::hir::map::Map<'_>, id: HirId) -> bool {
127 Some(Node::Binding(_)) => (),
131 matches!(map.find(map.get_parent_node(id)), Some(Node::Param(_)))
134 impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
135 fn consume(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId, mode: ConsumeMode) {
136 if cmt.place.projections.is_empty() {
137 if let PlaceBase::Local(lid) = cmt.place.base {
138 if let ConsumeMode::Move = mode {
139 // moved out or in. clearly can't be localized
140 self.set.remove(&lid);
142 let map = &self.cx.tcx.hir();
143 if let Some(Node::Binding(_)) = map.find(cmt.hir_id) {
144 if self.set.contains(&lid) {
145 // let y = x where x is known
146 // remove x, insert y
147 self.set.insert(cmt.hir_id);
148 self.set.remove(&lid);
155 fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {
156 if cmt.place.projections.is_empty() {
157 if let PlaceBase::Local(lid) = cmt.place.base {
158 self.set.remove(&lid);
163 fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) {
164 if cmt.place.projections.is_empty() {
165 let map = &self.cx.tcx.hir();
166 if is_argument(*map, cmt.hir_id) {
167 // Skip closure arguments
168 let parent_id = map.get_parent_node(cmt.hir_id);
169 if let Some(Node::Expr(..)) = map.find(map.get_parent_node(parent_id)) {
173 // skip if there is a `self` parameter binding to a type
174 // that contains `Self` (i.e.: `self: Box<Self>`), see #4804
175 if let Some(trait_self_ty) = self.trait_self_ty {
176 if map.name(cmt.hir_id) == kw::SelfLower && contains_ty(cmt.place.ty(), trait_self_ty) {
181 if is_non_trait_box(cmt.place.ty()) && !self.is_large_box(cmt.place.ty()) {
182 self.set.insert(cmt.hir_id);
189 impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> {
190 fn is_large_box(&self, ty: Ty<'tcx>) -> bool {
191 // Large types need to be boxed to avoid stack overflows.
193 self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack