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