src/tools/clippy/clippy_lints/src/escape.rs

   1 use clippy_utils::diagnostics::span_lint_hir;
   2 use rustc_hir::{self, intravisit, AssocItemKind, Body, FnDecl, HirId, HirIdSet, Impl, ItemKind, Node, Pat, PatKind};
   3 use rustc_hir_typeck::expr_use_visitor::{Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
   4 use rustc_infer::infer::TyCtxtInferExt;
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_middle::mir::FakeReadCause;
   7 use rustc_middle::ty::layout::LayoutOf;
   8 use rustc_middle::ty::{self, TraitRef, Ty};
   9 use rustc_session::{declare_tool_lint, impl_lint_pass};
  10 use rustc_span::def_id::LocalDefId;
  11 use rustc_span::Span;
  12 use rustc_span::symbol::kw;
  13 use rustc_target::spec::abi::Abi;
  14
  15 #[derive(Copy, Clone)]
  16 pub struct BoxedLocal {
  17     pub too_large_for_stack: u64,
  18 }
  19
  20 declare_clippy_lint! {
  21     /// ### What it does
  22     /// Checks for usage of `Box<T>` where an unboxed `T` would
  23     /// work fine.
  24     ///
  25     /// ### Why is this bad?
  26     /// This is an unnecessary allocation, and bad for
  27     /// performance. It is only necessary to allocate if you wish to move the box
  28     /// into something.
  29     ///
  30     /// ### Example
  31     /// ```no_run
  32     /// fn foo(x: Box<u32>) {}
  33     /// ```
  34     ///
  35     /// Use instead:
  36     /// ```no_run
  37     /// fn foo(x: u32) {}
  38     /// ```
  39     #[clippy::version = "pre 1.29.0"]
  40     pub BOXED_LOCAL,
  41     perf,
  42     "using `Box<T>` where unnecessary"
  43 }
  44
  45 fn is_non_trait_box(ty: Ty<'_>) -> bool {
  46     ty.is_box() && !ty.boxed_ty().is_trait()
  47 }
  48
  49 struct EscapeDelegate<'a, 'tcx> {
  50     cx: &'a LateContext<'tcx>,
  51     set: HirIdSet,
  52     trait_self_ty: Option<Ty<'tcx>>,
  53     too_large_for_stack: u64,
  54 }
  55
  56 impl_lint_pass!(BoxedLocal => [BOXED_LOCAL]);
  57
  58 impl<'tcx> LateLintPass<'tcx> for BoxedLocal {
  59     fn check_fn(
  60         &mut self,
  61         cx: &LateContext<'tcx>,
  62         fn_kind: intravisit::FnKind<'tcx>,
  63         _: &'tcx FnDecl<'_>,
  64         body: &'tcx Body<'_>,
  65         _: Span,
  66         fn_def_id: LocalDefId,
  67     ) {
  68         if let Some(header) = fn_kind.header() {
  69             if header.abi != Abi::Rust {
  70                 return;
  71             }
  72         }
  73
  74         let parent_id = cx
  75             .tcx
  76             .hir()
  77             .get_parent_item(cx.tcx.hir().local_def_id_to_hir_id(fn_def_id))
  78             .def_id;
  79         let parent_node = cx.tcx.hir().find_by_def_id(parent_id);
  80
  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 {
  85                 return;
  86             }
  87
  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.owner_id.def_id == fn_def_id {
  92                         // be sure we have `self` parameter in this function
  93                         if trait_item.kind == (AssocItemKind::Fn { has_self: true }) {
  94                             trait_self_ty =
  95                                 Some(TraitRef::identity(cx.tcx, trait_item.id.owner_id.to_def_id()).self_ty());
  96                         }
  97                     }
  98                 }
  99             }
 100         }
 101
 102         let mut v = EscapeDelegate {
 103             cx,
 104             set: HirIdSet::default(),
 105             trait_self_ty,
 106             too_large_for_stack: self.too_large_for_stack,
 107         };
 108
 109         let infcx = cx.tcx.infer_ctxt().build();
 110         ExprUseVisitor::new(&mut v, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body);
 111
 112         for node in v.set {
 113             span_lint_hir(
 114                 cx,
 115                 BOXED_LOCAL,
 116                 node,
 117                 cx.tcx.hir().span(node),
 118                 "local variable doesn't need to be boxed here",
 119             );
 120         }
 121     }
 122 }
 123
 124 // TODO: Replace with Map::is_argument(..) when it's fixed
 125 fn is_argument(map: rustc_middle::hir::map::Map<'_>, id: HirId) -> bool {
 126     match map.find(id) {
 127         Some(Node::Pat(Pat {
 128             kind: PatKind::Binding(..),
 129             ..
 130         })) => (),
 131         _ => return false,
 132     }
 133
 134     matches!(map.find_parent(id), Some(Node::Param(_)))
 135 }
 136
 137 impl<'a, 'tcx> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
 138     fn consume(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) {
 139         if cmt.place.projections.is_empty() {
 140             if let PlaceBase::Local(lid) = cmt.place.base {
 141                 self.set.remove(&lid);
 142             }
 143         }
 144     }
 145
 146     fn borrow(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {
 147         if cmt.place.projections.is_empty() {
 148             if let PlaceBase::Local(lid) = cmt.place.base {
 149                 self.set.remove(&lid);
 150             }
 151         }
 152     }
 153
 154     fn mutate(&mut self, cmt: &PlaceWithHirId<'tcx>, _: HirId) {
 155         if cmt.place.projections.is_empty() {
 156             let map = &self.cx.tcx.hir();
 157             if is_argument(*map, cmt.hir_id) {
 158                 // Skip closure arguments
 159                 let parent_id = map.parent_id(cmt.hir_id);
 160                 if let Some(Node::Expr(..)) = map.find_parent(parent_id) {
 161                     return;
 162                 }
 163
 164                 // skip if there is a `self` parameter binding to a type
 165                 // that contains `Self` (i.e.: `self: Box<Self>`), see #4804
 166                 if let Some(trait_self_ty) = self.trait_self_ty {
 167                     if map.name(cmt.hir_id) == kw::SelfLower && cmt.place.ty().contains(trait_self_ty) {
 168                         return;
 169                     }
 170                 }
 171
 172                 if is_non_trait_box(cmt.place.ty()) && !self.is_large_box(cmt.place.ty()) {
 173                     self.set.insert(cmt.hir_id);
 174                 }
 175             }
 176         }
 177     }
 178
 179     fn fake_read(&mut self, _: &rustc_hir_typeck::expr_use_visitor::PlaceWithHirId<'tcx>, _: FakeReadCause, _: HirId) {}
 180 }
 181
 182 impl<'a, 'tcx> EscapeDelegate<'a, 'tcx> {
 183     fn is_large_box(&self, ty: Ty<'tcx>) -> bool {
 184         // Large types need to be boxed to avoid stack overflows.
 185         if ty.is_box() {
 186             self.cx.layout_of(ty.boxed_ty()).map_or(0, |l| l.size.bytes()) > self.too_large_for_stack
 187         } else {
 188             false
 189         }
 190     }
 191 }