src/escape.rs

   1 use rustc::lint::*;
   2 use rustc_front::hir::*;
   3 use rustc_front::intravisit as visit;
   4 use rustc::front::map::Node;
   5 use rustc::middle::ty;
   6 use rustc::middle::ty::adjustment::AutoAdjustment;
   7 use rustc::middle::expr_use_visitor::*;
   8 use rustc::middle::infer;
   9 use rustc::middle::mem_categorization::{cmt, Categorization};
  10 use rustc::util::nodemap::NodeSet;
  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
  20 ///
  21 /// It is only necessary to allocate if you wish to move the box into something.
  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!(pub BOXED_LOCAL, Warn, "using Box<T> where unnecessary");
  33
  34 fn is_box(ty: ty::Ty) -> bool {
  35     match ty.sty {
  36         ty::TyBox(..) => true,
  37         _ => false
  38     }
  39 }
  40
  41 struct EscapeDelegate<'a, 'tcx: 'a> {
  42     cx: &'a LateContext<'a, 'tcx>,
  43     set: NodeSet,
  44 }
  45
  46 impl LintPass for EscapePass {
  47     fn get_lints(&self) -> LintArray {
  48         lint_array!(BOXED_LOCAL)
  49     }
  50 }
  51
  52 impl LateLintPass for EscapePass {
  53     fn check_fn(&mut self,
  54                 cx: &LateContext,
  55                 _: visit::FnKind,
  56                 decl: &FnDecl,
  57                 body: &Block,
  58                 _: Span,
  59                 id: NodeId) {
  60         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  61         let infcx = infer::new_infer_ctxt(cx.tcx, &cx.tcx.tables, Some(param_env), false);
  62         let mut v = EscapeDelegate {
  63             cx: cx,
  64             set: NodeSet(),
  65         };
  66         {
  67             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  68             vis.walk_fn(decl, body);
  69         }
  70         for node in v.set {
  71             span_lint(cx,
  72                       BOXED_LOCAL,
  73                       cx.tcx.map.span(node),
  74                       "local variable doesn't need to be boxed here");
  75         }
  76     }
  77 }
  78
  79 impl<'a, 'tcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
  80     fn consume(&mut self,
  81                _: NodeId,
  82                _: Span,
  83                cmt: cmt<'tcx>,
  84                mode: ConsumeMode) {
  85
  86         if let Categorization::Local(lid) = cmt.cat {
  87             if self.set.contains(&lid) {
  88                 if let Move(DirectRefMove) = mode {
  89                     // moved out or in. clearly can't be localized
  90                     self.set.remove(&lid);
  91                 }
  92             }
  93         }
  94     }
  95     fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
  96     fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
  97         if self.cx.tcx.map.is_argument(consume_pat.id) {
  98             if is_box(cmt.ty) {
  99                 self.set.insert(consume_pat.id);
 100             }
 101             return;
 102         }
 103         if let Categorization::Rvalue(..) = cmt.cat {
 104             if let Some(Node::NodeStmt(st)) = self.cx
 105                                                   .tcx
 106                                                   .map
 107                                                   .find(self.cx.tcx.map.get_parent_node(cmt.id)) {
 108                 if let StmtDecl(ref decl, _) = st.node {
 109                     if let DeclLocal(ref loc) = decl.node {
 110                         if let Some(ref ex) = loc.init {
 111                             if let ExprBox(..) = ex.node {
 112                                 if is_box(cmt.ty) {
 113                                     // let x = box (...)
 114                                     self.set.insert(consume_pat.id);
 115                                 }
 116                                 // TODO Box::new
 117                                 // TODO vec![]
 118                                 // TODO "foo".to_owned() and friends
 119                             }
 120                         }
 121                     }
 122                 }
 123             }
 124         }
 125         if let Categorization::Local(lid) = cmt.cat {
 126             if self.set.contains(&lid) {
 127                 // let y = x where x is known
 128                 // remove x, insert y
 129                 self.set.insert(consume_pat.id);
 130                 self.set.remove(&lid);
 131             }
 132         }
 133
 134     }
 135     fn borrow(&mut self,
 136               borrow_id: NodeId,
 137               _: Span,
 138               cmt: cmt<'tcx>,
 139               _: ty::Region,
 140               _: ty::BorrowKind,
 141               loan_cause: LoanCause) {
 142
 143         if let Categorization::Local(lid) = cmt.cat {
 144             if self.set.contains(&lid) {
 145                 if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.cx
 146                                                                         .tcx
 147                                                                         .tables
 148                                                                         .borrow()
 149                                                                         .adjustments
 150                                                                         .get(&borrow_id) {
 151                     if LoanCause::AutoRef == loan_cause {
 152                         // x.foo()
 153                         if adj.autoderefs <= 0 {
 154                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 155                         }
 156                     } else {
 157                         self.cx.sess().span_bug(cmt.span, "Unknown adjusted AutoRef");
 158                     }
 159                 } else if LoanCause::AddrOf == loan_cause {
 160                     // &x
 161                     if let Some(&AutoAdjustment::AdjustDerefRef(adj)) =
 162                            self.cx.tcx.tables.borrow().adjustments
 163                                .get(&self.cx.tcx.map.get_parent_node(borrow_id)) {
 164                         if adj.autoderefs <= 1 {
 165                             // foo(&x) where no extra autoreffing is happening
 166                             self.set.remove(&lid);
 167                         }
 168                     }
 169
 170                 } else if LoanCause::MatchDiscriminant == loan_cause {
 171                     self.set.remove(&lid); // `match x` can move
 172                 }
 173                 // do nothing for matches, etc. These can't escape
 174             }
 175         }
 176     }
 177     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 178     fn mutate(&mut self,
 179               _: NodeId,
 180               _: Span,
 181               _: cmt<'tcx>,
 182               _: MutateMode) {
 183     }
 184 }