src/escape.rs

   1 use rustc::hir::*;
   2 use rustc::hir::intravisit as visit;
   3 use rustc::hir::map::Node::{NodeExpr, NodeStmt};
   4 use rustc::lint::*;
   5 use rustc::middle::expr_use_visitor::*;
   6 use rustc::middle::mem_categorization::{cmt, Categorization};
   7 use rustc::ty::adjustment::AutoAdjustment;
   8 use rustc::ty;
   9 use rustc::util::nodemap::NodeSet;
  10 use syntax::ast::NodeId;
  11 use syntax::codemap::Span;
  12 use utils::span_lint;
  13
  14 pub struct EscapePass;
  15
  16 /// **What it does:** This lint checks for usage of `Box<T>` where an unboxed `T` would work fine.
  17 ///
  18 /// **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.
  19 ///
  20 /// **Known problems:** None
  21 ///
  22 /// **Example:**
  23 ///
  24 /// ```rust
  25 /// fn main() {
  26 ///     let x = Box::new(1);
  27 ///     foo(*x);
  28 ///     println!("{}", *x);
  29 /// }
  30 /// ```
  31 declare_lint! {
  32     pub BOXED_LOCAL, Warn, "using `Box<T>` where unnecessary"
  33 }
  34
  35 fn is_non_trait_box(ty: ty::Ty) -> bool {
  36     match ty.sty {
  37         ty::TyBox(ref inner) => !inner.is_trait(),
  38         _ => false,
  39     }
  40 }
  41
  42 struct EscapeDelegate<'a, 'tcx: 'a> {
  43     tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
  44     set: NodeSet,
  45 }
  46
  47 impl LintPass for EscapePass {
  48     fn get_lints(&self) -> LintArray {
  49         lint_array!(BOXED_LOCAL)
  50     }
  51 }
  52
  53 impl LateLintPass for EscapePass {
  54     fn check_fn(&mut self, cx: &LateContext, _: visit::FnKind, decl: &FnDecl, body: &Block, _: Span, id: NodeId) {
  55         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  56
  57         let infcx = cx.tcx.borrowck_fake_infer_ctxt(param_env);
  58         let mut v = EscapeDelegate {
  59             tcx: cx.tcx,
  60             set: NodeSet(),
  61         };
  62
  63         {
  64             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  65             vis.walk_fn(decl, body);
  66         }
  67
  68         for node in v.set {
  69             span_lint(cx,
  70                       BOXED_LOCAL,
  71                       cx.tcx.map.span(node),
  72                       "local variable doesn't need to be boxed here");
  73         }
  74     }
  75 }
  76
  77 impl<'a, 'tcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
  78     fn consume(&mut self, _: NodeId, _: Span, cmt: cmt<'tcx>, mode: ConsumeMode) {
  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         let map = &self.tcx.map;
  91         if map.is_argument(consume_pat.id) {
  92             // Skip closure arguments
  93             if let Some(NodeExpr(..)) = map.find(map.get_parent_node(consume_pat.id)) {
  94                 return;
  95             }
  96             if is_non_trait_box(cmt.ty) {
  97                 self.set.insert(consume_pat.id);
  98             }
  99             return;
 100         }
 101         if let Categorization::Rvalue(..) = cmt.cat {
 102             if let Some(NodeStmt(st)) = map.find(map.get_parent_node(cmt.id)) {
 103                 if let StmtDecl(ref decl, _) = st.node {
 104                     if let DeclLocal(ref loc) = decl.node {
 105                         if let Some(ref ex) = loc.init {
 106                             if let ExprBox(..) = ex.node {
 107                                 if is_non_trait_box(cmt.ty) {
 108                                     // let x = box (...)
 109                                     self.set.insert(consume_pat.id);
 110                                 }
 111                                 // TODO Box::new
 112                                 // TODO vec![]
 113                                 // TODO "foo".to_owned() and friends
 114                             }
 115                         }
 116                     }
 117                 }
 118             }
 119         }
 120         if let Categorization::Local(lid) = cmt.cat {
 121             if self.set.contains(&lid) {
 122                 // let y = x where x is known
 123                 // remove x, insert y
 124                 self.set.insert(consume_pat.id);
 125                 self.set.remove(&lid);
 126             }
 127         }
 128
 129     }
 130     fn borrow(&mut self, borrow_id: NodeId, _: Span, cmt: cmt<'tcx>, _: ty::Region, _: ty::BorrowKind,
 131               loan_cause: LoanCause) {
 132
 133         if let Categorization::Local(lid) = cmt.cat {
 134             if self.set.contains(&lid) {
 135                 if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 136                                                                         .tables
 137                                                                         .borrow()
 138                                                                         .adjustments
 139                                                                         .get(&borrow_id) {
 140                     if LoanCause::AutoRef == loan_cause {
 141                         // x.foo()
 142                         if adj.autoderefs == 0 {
 143                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 144                         }
 145                     } else {
 146                         span_bug!(cmt.span, "Unknown adjusted AutoRef");
 147                     }
 148                 } else if LoanCause::AddrOf == loan_cause {
 149                     // &x
 150                     if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 151                                                                             .tables
 152                                                                             .borrow()
 153                                                                             .adjustments
 154                                                                             .get(&self.tcx
 155                                                                                       .map
 156                                                                                       .get_parent_node(borrow_id)) {
 157                         if adj.autoderefs <= 1 {
 158                             // foo(&x) where no extra autoreffing is happening
 159                             self.set.remove(&lid);
 160                         }
 161                     }
 162
 163                 } else if LoanCause::MatchDiscriminant == loan_cause {
 164                     self.set.remove(&lid); // `match x` can move
 165                 }
 166                 // do nothing for matches, etc. These can't escape
 167             }
 168         }
 169     }
 170     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 171     fn mutate(&mut self, _: NodeId, _: Span, _: cmt<'tcx>, _: MutateMode) {}
 172 }