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 declare_lint!(pub BOXED_LOCAL, Warn, "using Box<T> where unnecessary");
  32
  33 struct EscapeDelegate<'a, 'tcx: 'a> {
  34     cx: &'a LateContext<'a, 'tcx>,
  35     set: NodeSet,
  36 }
  37
  38 impl LintPass for EscapePass {
  39     fn get_lints(&self) -> LintArray {
  40         lint_array!(BOXED_LOCAL)
  41     }
  42 }
  43
  44 impl LateLintPass for EscapePass {
  45     fn check_fn(&mut self,
  46                 cx: &LateContext,
  47                 _: visit::FnKind,
  48                 decl: &FnDecl,
  49                 body: &Block,
  50                 _: Span,
  51                 id: NodeId) {
  52         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  53         let infcx = infer::new_infer_ctxt(cx.tcx, &cx.tcx.tables, Some(param_env), false);
  54         let mut v = EscapeDelegate {
  55             cx: cx,
  56             set: NodeSet(),
  57         };
  58         {
  59             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  60             vis.walk_fn(decl, body);
  61         }
  62         for node in v.set {
  63             span_lint(cx,
  64                       BOXED_LOCAL,
  65                       cx.tcx.map.span(node),
  66                       "local variable doesn't need to be boxed here");
  67         }
  68     }
  69 }
  70
  71 impl<'a, 'tcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
  72     fn consume(&mut self,
  73                _: NodeId,
  74                _: Span,
  75                cmt: cmt<'tcx>,
  76                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         if let Categorization::Rvalue(..) = cmt.cat {
  90             if let Some(Node::NodeStmt(st)) = self.cx
  91                                                   .tcx
  92                                                   .map
  93                                                   .find(self.cx.tcx.map.get_parent_node(cmt.id)) {
  94                 if let StmtDecl(ref decl, _) = st.node {
  95                     if let DeclLocal(ref loc) = decl.node {
  96                         if let Some(ref ex) = loc.init {
  97                             if let ExprBox(..) = ex.node {
  98                                 if let ty::TyBox(..) = cmt.ty.sty {
  99                                     // let x = box (...)
 100                                     self.set.insert(consume_pat.id);
 101                                 }
 102                                 // TODO Box::new
 103                                 // TODO vec![]
 104                                 // TODO "foo".to_owned() and friends
 105                             }
 106                         }
 107                     }
 108                 }
 109             }
 110         }
 111         if let Categorization::Local(lid) = cmt.cat {
 112             if self.set.contains(&lid) {
 113                 // let y = x where x is known
 114                 // remove x, insert y
 115                 self.set.insert(consume_pat.id);
 116                 self.set.remove(&lid);
 117             }
 118         }
 119
 120     }
 121     fn borrow(&mut self,
 122               borrow_id: NodeId,
 123               _: Span,
 124               cmt: cmt<'tcx>,
 125               _: ty::Region,
 126               _: ty::BorrowKind,
 127               loan_cause: LoanCause) {
 128
 129         if let Categorization::Local(lid) = cmt.cat {
 130             if self.set.contains(&lid) {
 131                 if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.cx
 132                                                                         .tcx
 133                                                                         .tables
 134                                                                         .borrow()
 135                                                                         .adjustments
 136                                                                         .get(&borrow_id) {
 137                     if LoanCause::AutoRef == loan_cause {
 138                         // x.foo()
 139                         if adj.autoderefs <= 0 {
 140                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 141                         }
 142                     } else {
 143                         self.cx.sess().span_bug(cmt.span, "Unknown adjusted AutoRef");
 144                     }
 145                 } else if LoanCause::AddrOf == loan_cause {
 146                     // &x
 147                     if let Some(&AutoAdjustment::AdjustDerefRef(adj)) =
 148                            self.cx.tcx.tables.borrow().adjustments
 149                                .get(&self.cx.tcx.map.get_parent_node(borrow_id)) {
 150                         if adj.autoderefs <= 1 {
 151                             // foo(&x) where no extra autoreffing is happening
 152                             self.set.remove(&lid);
 153                         }
 154                     }
 155
 156                 } else if LoanCause::MatchDiscriminant == loan_cause {
 157                     self.set.remove(&lid); // `match x` can move
 158                 }
 159                 // do nothing for matches, etc. These can't escape
 160             }
 161         }
 162     }
 163     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 164     fn mutate(&mut self,
 165               _: NodeId,
 166               _: Span,
 167               _: cmt<'tcx>,
 168               _: MutateMode) {
 169     }
 170 }