clippy_lints/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::infer::InferCtxt;
   5 use rustc::lint::*;
   6 use rustc::middle::expr_use_visitor::*;
   7 use rustc::middle::mem_categorization::{cmt, Categorization};
   8 use rustc::ty::adjustment::AutoAdjustment;
   9 use rustc::ty;
  10 use rustc::ty::layout::TargetDataLayout;
  11 use rustc::util::nodemap::NodeSet;
  12 use syntax::ast::NodeId;
  13 use syntax::codemap::Span;
  14 use utils::span_lint;
  15
  16 pub struct Pass {
  17     pub too_large_for_stack: u64,
  18 }
  19
  20 /// **What it does:** Checks for usage of `Box<T>` where an unboxed `T` would
  21 /// work fine.
  22 ///
  23 /// **Why is this bad?** This is an unnecessary allocation, and bad for
  24 /// performance. It is only necessary to allocate if you wish to move the box
  25 /// into something.
  26 ///
  27 /// **Known problems:** None.
  28 ///
  29 /// **Example:**
  30 /// ```rust
  31 /// fn main() {
  32 ///     let x = Box::new(1);
  33 ///     foo(*x);
  34 ///     println!("{}", *x);
  35 /// }
  36 /// ```
  37 declare_lint! {
  38     pub BOXED_LOCAL,
  39     Warn,
  40     "using `Box<T>` where unnecessary"
  41 }
  42
  43 fn is_non_trait_box(ty: ty::Ty) -> bool {
  44     match ty.sty {
  45         ty::TyBox(inner) => !inner.is_trait(),
  46         _ => false,
  47     }
  48 }
  49
  50 struct EscapeDelegate<'a, 'tcx: 'a+'gcx, 'gcx: 'a> {
  51     tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
  52     set: NodeSet,
  53     infcx: &'a InferCtxt<'a, 'gcx, 'gcx>,
  54     target: TargetDataLayout,
  55     too_large_for_stack: u64,
  56 }
  57
  58 impl LintPass for Pass {
  59     fn get_lints(&self) -> LintArray {
  60         lint_array!(BOXED_LOCAL)
  61     }
  62 }
  63
  64 impl LateLintPass for Pass {
  65     fn check_fn(&mut self, cx: &LateContext, _: visit::FnKind, decl: &FnDecl, body: &Block, _: Span, id: NodeId) {
  66         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  67
  68         let infcx = cx.tcx.borrowck_fake_infer_ctxt(param_env);
  69
  70         // we store the infcx because it is expensive to recreate
  71         // the context each time.
  72         let mut v = EscapeDelegate {
  73             tcx: cx.tcx,
  74             set: NodeSet(),
  75             infcx: &infcx,
  76             target: TargetDataLayout::parse(cx.sess()),
  77             too_large_for_stack: self.too_large_for_stack,
  78         };
  79
  80         {
  81             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  82             vis.walk_fn(decl, body);
  83         }
  84
  85         for node in v.set {
  86             span_lint(cx,
  87                       BOXED_LOCAL,
  88                       cx.tcx.map.span(node),
  89                       "local variable doesn't need to be boxed here");
  90         }
  91     }
  92 }
  93
  94 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx, 'gcx> {
  95     fn consume(&mut self, _: NodeId, _: Span, cmt: cmt<'tcx>, mode: ConsumeMode) {
  96         if let Categorization::Local(lid) = cmt.cat {
  97             if self.set.contains(&lid) {
  98                 if let Move(DirectRefMove) = mode {
  99                     // moved out or in. clearly can't be localized
 100                     self.set.remove(&lid);
 101                 }
 102             }
 103         }
 104     }
 105     fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
 106     fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
 107         let map = &self.tcx.map;
 108         if map.is_argument(consume_pat.id) {
 109             // Skip closure arguments
 110             if let Some(NodeExpr(..)) = map.find(map.get_parent_node(consume_pat.id)) {
 111                 return;
 112             }
 113             if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 114                 self.set.insert(consume_pat.id);
 115             }
 116             return;
 117         }
 118         if let Categorization::Rvalue(..) = cmt.cat {
 119             if let Some(NodeStmt(st)) = map.find(map.get_parent_node(cmt.id)) {
 120                 if let StmtDecl(ref decl, _) = st.node {
 121                     if let DeclLocal(ref loc) = decl.node {
 122                         if let Some(ref ex) = loc.init {
 123                             if let ExprBox(..) = ex.node {
 124                                 if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 125                                     // let x = box (...)
 126                                     self.set.insert(consume_pat.id);
 127                                 }
 128                                 // TODO Box::new
 129                                 // TODO vec![]
 130                                 // TODO "foo".to_owned() and friends
 131                             }
 132                         }
 133                     }
 134                 }
 135             }
 136         }
 137         if let Categorization::Local(lid) = cmt.cat {
 138             if self.set.contains(&lid) {
 139                 // let y = x where x is known
 140                 // remove x, insert y
 141                 self.set.insert(consume_pat.id);
 142                 self.set.remove(&lid);
 143             }
 144         }
 145
 146     }
 147     fn borrow(&mut self, borrow_id: NodeId, _: Span, cmt: cmt<'tcx>, _: &ty::Region, _: ty::BorrowKind,
 148               loan_cause: LoanCause) {
 149
 150         if let Categorization::Local(lid) = cmt.cat {
 151             if self.set.contains(&lid) {
 152                 if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 153                                                                         .tables
 154                                                                         .borrow()
 155                                                                         .adjustments
 156                                                                         .get(&borrow_id) {
 157                     if LoanCause::AutoRef == loan_cause {
 158                         // x.foo()
 159                         if adj.autoderefs == 0 {
 160                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 161                         }
 162                     } else {
 163                         span_bug!(cmt.span, "Unknown adjusted AutoRef");
 164                     }
 165                 } else if LoanCause::AddrOf == loan_cause {
 166                     // &x
 167                     if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.tcx
 168                                                                             .tables
 169                                                                             .borrow()
 170                                                                             .adjustments
 171                                                                             .get(&self.tcx
 172                                                                                       .map
 173                                                                                       .get_parent_node(borrow_id)) {
 174                         if adj.autoderefs <= 1 {
 175                             // foo(&x) where no extra autoreffing is happening
 176                             self.set.remove(&lid);
 177                         }
 178                     }
 179
 180                 } else if LoanCause::MatchDiscriminant == loan_cause {
 181                     self.set.remove(&lid); // `match x` can move
 182                 }
 183                 // do nothing for matches, etc. These can't escape
 184             }
 185         }
 186     }
 187     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 188     fn mutate(&mut self, _: NodeId, _: Span, _: cmt<'tcx>, _: MutateMode) {}
 189 }
 190
 191 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> EscapeDelegate<'a, 'tcx, 'gcx> {
 192     fn is_large_box(&self, ty: ty::Ty<'gcx>) -> bool {
 193         // Large types need to be boxed to avoid stack
 194         // overflows.
 195         match ty.sty {
 196             ty::TyBox(inner) => {
 197                 if let Ok(layout) = inner.layout(self.infcx) {
 198                     let size = layout.size(&self.target);
 199                     size.bytes() > self.too_large_for_stack
 200                 } else {
 201                     false
 202                 }
 203             },
 204             _ => false,
 205         }
 206     }
 207 }