use clippy_utils::diagnostics::span_lint; use clippy_utils::ty::contains_ty; use rustc_hir::intravisit; use rustc_hir::{self, AssocItemKind, Body, FnDecl, HirId, HirIdSet, Impl, ItemKind, Node}; use rustc_infer::infer::TyCtxtInferExt; use rustc_lint::{LateContext, LateLintPass}; use rustc_middle::mir::FakeReadCause; use rustc_middle::ty::layout::LayoutOf; use rustc_middle::ty::{self, TraitRef, Ty}; use rustc_session::{declare_tool_lint, impl_lint_pass}; use rustc_span::source_map::Span; use rustc_span::symbol::kw; use rustc_target::spec::abi::Abi; use rustc_typeck::expr_use_visitor::{Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId}; #[derive(Copy, Clone)] pub struct BoxedLocal { pub too_large_for_stack: u64, } declare_clippy_lint! { /// ### What it does /// Checks for usage of `Box` where an unboxed `T` would /// work fine. /// /// ### Why is this bad? /// This is an unnecessary allocation, and bad for /// performance. It is only necessary to allocate if you wish to move the box /// into something. /// /// ### Example /// ```rust /// # fn foo(bar: usize) {} /// // Bad /// let x = Box::new(1); /// foo(*x); /// println!("{}", *x); /// /// // Good /// let x = 1; /// foo(x); /// println!("{}", x); /// ``` pub BOXED_LOCAL, perf, "using `Box` where unnecessary" } fn is_non_trait_box(ty: Ty<'_>) -> bool { ty.is_box() && !ty.boxed_ty().is_trait() } struct EscapeDelegate<'a, 'tcx> { cx: &'a LateContext<'tcx>, set: HirIdSet, trait_self_ty: Option>, too_large_for_stack: u64, } impl_lint_pass!(BoxedLocal => [BOXED_LOCAL]); impl<'tcx> LateLintPass<'tcx> for BoxedLocal { fn check_fn( &mut self, cx: &LateContext<'tcx>, fn_kind: intravisit::FnKind<'tcx>, _: &'tcx FnDecl<'_>, body: &'tcx Body<'_>, _: Span, hir_id: HirId, ) { if let Some(header) = fn_kind.header() { if header.abi != Abi::Rust { return; } } let parent_id = cx.tcx.hir().get_parent_item(hir_id); let parent_node = cx.tcx.hir().find(parent_id); let mut trait_self_ty = None; if let Some(Node::Item(item)) = parent_node { // If the method is an impl for a trait, don't warn. if let ItemKind::Impl(Impl { of_trait: Some(_), .. }) = item.kind { return; } // find `self` ty for this trait if relevant if let ItemKind::Trait(_, _, _, _, items) = item.kind { for trait_item in items { if trait_item.id.hir_id() == hir_id { // be sure we have `self` parameter in this function if let AssocItemKind::Fn { has_self: true } = trait_item.kind { trait_self_ty = Some(TraitRef::identity(cx.tcx, trait_item.id.def_id.to_def_id()).self_ty().skip_binder()); } } } } } let mut v = EscapeDelegate { cx, set: HirIdSet::default(), trait_self_ty, too_large_for_stack: self.too_large_for_stack, }; let fn_def_id = cx.tcx.hir().local_def_id(hir_id); cx.tcx.infer_ctxt().enter(|infcx| { ExprUseVisitor::new(&mut v, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body); }); for node in v.set { span_lint( cx, BOXED_LOCAL, cx.tcx.hir().span(node), "local variable doesn't need to be boxed here", ); } } } // TODO: Replace with Map::is_argument(..) when it's fixed fn is_argument(map: rustc_middle::hir::map::Map<'_>, id: HirId) -> bool { match map.find(id) { Some(Node::Binding(_)) => (), _ => return false, } matches!(map.find(map.get_parent_node(id)), Some(Node::Param(_))) } impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> { fn consume(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) { if cmt.place.projections.is_empty() { if let PlaceBase::Local(lid) = cmt.place.base { self.set.remove(&lid); let map = &self.cx.tcx.hir(); if let Some(Node::Binding(_)) = map.find(cmt.hir_id) { if self.set.contains(&lid) { // let y = x where x is known // remove x, insert y self.set.insert(cmt.hir_id); self.set.remove(&lid); } } } } } fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) { if cmt.place.projections.is_empty() { if let PlaceBase::Local(lid) = cmt.place.base { self.set.remove(&lid); } } } fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) { if cmt.place.projections.is_empty() { let map = &self.cx.tcx.hir(); if is_argument(*map, cmt.hir_id) { // Skip closure arguments let parent_id = map.get_parent_node(cmt.hir_id); if let Some(Node::Expr(..)) = map.find(map.get_parent_node(parent_id)) { return; } // skip if there is a `self` parameter binding to a type // that contains `Self` (i.e.: `self: Box`), see #4804 if let Some(trait_self_ty) = self.trait_self_ty { if map.name(cmt.hir_id) == kw::SelfLower && contains_ty(self.cx.tcx, cmt.place.ty(), trait_self_ty) { return; } } if is_non_trait_box(cmt.place.ty()) && !self.is_large_box(cmt.place.ty()) { self.set.insert(cmt.hir_id); } } } } fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {} } impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> { fn is_large_box(&self, ty: Ty<'tcx>) -> bool { // Large types need to be boxed to avoid stack overflows. if ty.is_box() { self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack } else { false } } }