src/escape.rs

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