clippy_lints/src/escape.rs

   1 use rustc::hir::*;
   2 use rustc::hir::intravisit as visit;
   3 use rustc::hir::map::Node::{NodeExpr, NodeStmt};
   4 use rustc::infer::InferCtxt;
   5 use rustc::lint::*;
   6 use rustc::middle::expr_use_visitor::*;
   7 use rustc::middle::mem_categorization::{cmt, Categorization};
   8 use rustc::ty::adjustment::AutoAdjustment;
   9 use rustc::ty;
  10 use rustc::ty::layout::TargetDataLayout;
  11 use rustc::util::nodemap::NodeSet;
  12 use syntax::ast::NodeId;
  13 use syntax::codemap::Span;
  14 use utils::span_lint;
  15
  16 pub struct Pass {
  17     pub too_large_for_stack: u64,
  18 }
  19
  20 /// **What it does:** This lint checks for usage of `Box<T>` where an unboxed `T` would work fine.
  21 ///
  22 /// **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.
  23 ///
  24 /// **Known problems:** None
  25 ///
  26 /// **Example:**
  27 ///
  28 /// ```rust
  29 /// fn main() {
  30 ///     let x = Box::new(1);
  31 ///     foo(*x);
  32 ///     println!("{}", *x);
  33 /// }
  34 /// ```
  35 declare_lint! {
  36     pub BOXED_LOCAL, Warn, "using `Box<T>` where unnecessary"
  37 }
  38
  39 fn is_non_trait_box(ty: ty::Ty) -> bool {
  40     match ty.sty {
  41         ty::TyBox(ref inner) => !inner.is_trait(),
  42         _ => false,
  43     }
  44 }
  45
  46 struct EscapeDelegate<'a, 'tcx: 'a+'gcx, 'gcx: 'a> {
  47     tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
  48     set: NodeSet,
  49     infcx: &'a InferCtxt<'a, 'gcx, 'gcx>,
  50     target: TargetDataLayout,
  51     too_large_for_stack: u64,
  52 }
  53
  54 impl LintPass for Pass {
  55     fn get_lints(&self) -> LintArray {
  56         lint_array!(BOXED_LOCAL)
  57     }
  58 }
  59
  60 impl LateLintPass for Pass {
  61     fn check_fn(&mut self, cx: &LateContext, _: visit::FnKind, decl: &FnDecl, body: &Block, _: Span, id: NodeId) {
  62         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  63
  64         let infcx = cx.tcx.borrowck_fake_infer_ctxt(param_env);
  65
  66         // we store the infcx because it is expensive to recreate
  67         // the context each time.
  68         let mut v = EscapeDelegate {
  69             tcx: cx.tcx,
  70             set: NodeSet(),
  71             infcx: &infcx,
  72             target: TargetDataLayout::parse(cx.sess()),
  73             too_large_for_stack: self.too_large_for_stack,
  74         };
  75
  76         {
  77             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  78             vis.walk_fn(decl, body);
  79         }
  80
  81         for node in v.set {
  82             span_lint(cx,
  83                       BOXED_LOCAL,
  84                       cx.tcx.map.span(node),
  85                       "local variable doesn't need to be boxed here");
  86         }
  87     }
  88 }
  89
  90 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx, 'gcx> {
  91     fn consume(&mut self, _: NodeId, _: Span, cmt: cmt<'tcx>, mode: ConsumeMode) {
  92         if let Categorization::Local(lid) = cmt.cat {
  93             if self.set.contains(&lid) {
  94                 if let Move(DirectRefMove) = mode {
  95                     // moved out or in. clearly can't be localized
  96                     self.set.remove(&lid);
  97                 }
  98             }
  99         }
 100     }
 101     fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
 102     fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
 103         let map = &self.tcx.map;
 104         if map.is_argument(consume_pat.id) {
 105             // Skip closure arguments
 106             if let Some(NodeExpr(..)) = map.find(map.get_parent_node(consume_pat.id)) {
 107                 return;
 108             }
 109             if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 110                 self.set.insert(consume_pat.id);
 111             }
 112             return;
 113         }
 114         if let Categorization::Rvalue(..) = cmt.cat {
 115             if let Some(NodeStmt(st)) = map.find(map.get_parent_node(cmt.id)) {
 116                 if let StmtDecl(ref decl, _) = st.node {
 117                     if let DeclLocal(ref loc) = decl.node {
 118                         if let Some(ref ex) = loc.init {
 119                             if let ExprBox(..) = ex.node {
 120                                 if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 121                                     // let x = box (...)
 122                                     self.set.insert(consume_pat.id);
 123                                 }
 124                                 // TODO Box::new
 125                                 // TODO vec![]
 126                                 // TODO "foo".to_owned() and friends
 127                             }
 128                         }
 129                     }
 130                 }
 131             }
 132         }
 133         if let Categorization::Local(lid) = cmt.cat {
 134             if self.set.contains(&lid) {
 135                 // let y = x where x is known
 136                 // remove x, insert y
 137                 self.set.insert(consume_pat.id);
 138                 self.set.remove(&lid);
 139             }
 140         }
 141
 142     }
 143     fn borrow(&mut self, borrow_id: NodeId, _: Span, cmt: cmt<'tcx>, _: ty::Region, _: ty::BorrowKind,
 144               loan_cause: LoanCause) {
 145
 146         if let Categorization::Local(lid) = cmt.cat {
 147             if self.set.contains(&lid) {
 148                 if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 149                                                                         .tables
 150                                                                         .borrow()
 151                                                                         .adjustments
 152                                                                         .get(&borrow_id) {
 153                     if LoanCause::AutoRef == loan_cause {
 154                         // x.foo()
 155                         if adj.autoderefs == 0 {
 156                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 157                         }
 158                     } else {
 159                         span_bug!(cmt.span, "Unknown adjusted AutoRef");
 160                     }
 161                 } else if LoanCause::AddrOf == loan_cause {
 162                     // &x
 163                     if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 164                                                                             .tables
 165                                                                             .borrow()
 166                                                                             .adjustments
 167                                                                             .get(&self.tcx
 168                                                                                       .map
 169                                                                                       .get_parent_node(borrow_id)) {
 170                         if adj.autoderefs <= 1 {
 171                             // foo(&x) where no extra autoreffing is happening
 172                             self.set.remove(&lid);
 173                         }
 174                     }
 175
 176                 } else if LoanCause::MatchDiscriminant == loan_cause {
 177                     self.set.remove(&lid); // `match x` can move
 178                 }
 179                 // do nothing for matches, etc. These can't escape
 180             }
 181         }
 182     }
 183     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 184     fn mutate(&mut self, _: NodeId, _: Span, _: cmt<'tcx>, _: MutateMode) {}
 185 }
 186
 187 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> EscapeDelegate<'a, 'tcx, 'gcx> {
 188     fn is_large_box(&self, ty: ty::Ty<'gcx>) -> bool {
 189         // Large types need to be boxed to avoid stack
 190         // overflows.
 191         match ty.sty {
 192             ty::TyBox(ref inner) => {
 193                 if let Ok(layout) = inner.layout(self.infcx) {
 194                     let size = layout.size(&self.target);
 195                     size.bytes() > self.too_large_for_stack
 196                 } else {
 197                     false
 198                 }
 199             },
 200             _ => false,
 201         }
 202     }
 203 }