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
  23 /// necessary to allocate if you wish to move the box into something.
  24 ///
  25 /// **Known problems:** None
  26 ///
  27 /// **Example:**
  28 ///
  29 /// ```rust
  30 /// fn main() {
  31 ///     let x = Box::new(1);
  32 ///     foo(*x);
  33 ///     println!("{}", *x);
  34 /// }
  35 /// ```
  36 declare_lint! {
  37     pub BOXED_LOCAL, Warn, "using `Box<T>` where unnecessary"
  38 }
  39
  40 fn is_non_trait_box(ty: ty::Ty) -> bool {
  41     match ty.sty {
  42         ty::TyBox(inner) => !inner.is_trait(),
  43         _ => false,
  44     }
  45 }
  46
  47 struct EscapeDelegate<'a, 'tcx: 'a+'gcx, 'gcx: 'a> {
  48     tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
  49     set: NodeSet,
  50     infcx: &'a InferCtxt<'a, 'gcx, 'gcx>,
  51     target: TargetDataLayout,
  52     too_large_for_stack: u64,
  53 }
  54
  55 impl LintPass for Pass {
  56     fn get_lints(&self) -> LintArray {
  57         lint_array!(BOXED_LOCAL)
  58     }
  59 }
  60
  61 impl LateLintPass for Pass {
  62     fn check_fn(&mut self, cx: &LateContext, _: visit::FnKind, decl: &FnDecl, body: &Block, _: Span, id: NodeId) {
  63         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  64
  65         let infcx = cx.tcx.borrowck_fake_infer_ctxt(param_env);
  66
  67         // we store the infcx because it is expensive to recreate
  68         // the context each time.
  69         let mut v = EscapeDelegate {
  70             tcx: cx.tcx,
  71             set: NodeSet(),
  72             infcx: &infcx,
  73             target: TargetDataLayout::parse(cx.sess()),
  74             too_large_for_stack: self.too_large_for_stack,
  75         };
  76
  77         {
  78             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  79             vis.walk_fn(decl, body);
  80         }
  81
  82         for node in v.set {
  83             span_lint(cx,
  84                       BOXED_LOCAL,
  85                       cx.tcx.map.span(node),
  86                       "local variable doesn't need to be boxed here");
  87         }
  88     }
  89 }
  90
  91 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx, 'gcx> {
  92     fn consume(&mut self, _: NodeId, _: Span, cmt: cmt<'tcx>, mode: ConsumeMode) {
  93         if let Categorization::Local(lid) = cmt.cat {
  94             if self.set.contains(&lid) {
  95                 if let Move(DirectRefMove) = mode {
  96                     // moved out or in. clearly can't be localized
  97                     self.set.remove(&lid);
  98                 }
  99             }
 100         }
 101     }
 102     fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
 103     fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
 104         let map = &self.tcx.map;
 105         if map.is_argument(consume_pat.id) {
 106             // Skip closure arguments
 107             if let Some(NodeExpr(..)) = map.find(map.get_parent_node(consume_pat.id)) {
 108                 return;
 109             }
 110             if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 111                 self.set.insert(consume_pat.id);
 112             }
 113             return;
 114         }
 115         if let Categorization::Rvalue(..) = cmt.cat {
 116             if let Some(NodeStmt(st)) = map.find(map.get_parent_node(cmt.id)) {
 117                 if let StmtDecl(ref decl, _) = st.node {
 118                     if let DeclLocal(ref loc) = decl.node {
 119                         if let Some(ref ex) = loc.init {
 120                             if let ExprBox(..) = ex.node {
 121                                 if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 122                                     // let x = box (...)
 123                                     self.set.insert(consume_pat.id);
 124                                 }
 125                                 // TODO Box::new
 126                                 // TODO vec![]
 127                                 // TODO "foo".to_owned() and friends
 128                             }
 129                         }
 130                     }
 131                 }
 132             }
 133         }
 134         if let Categorization::Local(lid) = cmt.cat {
 135             if self.set.contains(&lid) {
 136                 // let y = x where x is known
 137                 // remove x, insert y
 138                 self.set.insert(consume_pat.id);
 139                 self.set.remove(&lid);
 140             }
 141         }
 142
 143     }
 144     fn borrow(&mut self, borrow_id: NodeId, _: Span, cmt: cmt<'tcx>, _: ty::Region, _: ty::BorrowKind,
 145               loan_cause: LoanCause) {
 146
 147         if let Categorization::Local(lid) = cmt.cat {
 148             if self.set.contains(&lid) {
 149                 if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 150                                                                         .tables
 151                                                                         .borrow()
 152                                                                         .adjustments
 153                                                                         .get(&borrow_id) {
 154                     if LoanCause::AutoRef == loan_cause {
 155                         // x.foo()
 156                         if adj.autoderefs == 0 {
 157                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 158                         }
 159                     } else {
 160                         span_bug!(cmt.span, "Unknown adjusted AutoRef");
 161                     }
 162                 } else if LoanCause::AddrOf == loan_cause {
 163                     // &x
 164                     if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 165                                                                             .tables
 166                                                                             .borrow()
 167                                                                             .adjustments
 168                                                                             .get(&self.tcx
 169                                                                                       .map
 170                                                                                       .get_parent_node(borrow_id)) {
 171                         if adj.autoderefs <= 1 {
 172                             // foo(&x) where no extra autoreffing is happening
 173                             self.set.remove(&lid);
 174                         }
 175                     }
 176
 177                 } else if LoanCause::MatchDiscriminant == loan_cause {
 178                     self.set.remove(&lid); // `match x` can move
 179                 }
 180                 // do nothing for matches, etc. These can't escape
 181             }
 182         }
 183     }
 184     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 185     fn mutate(&mut self, _: NodeId, _: Span, _: cmt<'tcx>, _: MutateMode) {}
 186 }
 187
 188 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> EscapeDelegate<'a, 'tcx, 'gcx> {
 189     fn is_large_box(&self, ty: ty::Ty<'gcx>) -> bool {
 190         // Large types need to be boxed to avoid stack
 191         // overflows.
 192         match ty.sty {
 193             ty::TyBox(inner) => {
 194                 if let Ok(layout) = inner.layout(self.infcx) {
 195                     let size = layout.size(&self.target);
 196                     size.bytes() > self.too_large_for_stack
 197                 } else {
 198                     false
 199                 }
 200             },
 201             _ => false,
 202         }
 203     }
 204 }