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