src/escape.rs

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