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;
   9 use rustc::ty::layout::TargetDataLayout;
  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 Pass {
  16     pub too_large_for_stack: u64,
  17 }
  18
  19 /// **What it does:** Checks for usage of `Box<T>` where an unboxed `T` would
  20 /// work fine.
  21 ///
  22 /// **Why is this bad?** This is an unnecessary allocation, and bad for
  23 /// performance. It is only necessary to allocate if you wish to move the box
  24 /// into something.
  25 ///
  26 /// **Known problems:** None.
  27 ///
  28 /// **Example:**
  29 /// ```rust
  30 /// fn main() {
  31 ///     let x = Box::new(1);
  32 ///     foo(*x);
  33 ///     println!("{}", *x);
  34 /// }
  35 /// ```
  36 declare_lint! {
  37     pub BOXED_LOCAL,
  38     Warn,
  39     "using `Box<T>` where unnecessary"
  40 }
  41
  42 fn is_non_trait_box(ty: ty::Ty) -> bool {
  43     match ty.sty {
  44         ty::TyBox(inner) => !inner.is_trait(),
  45         _ => false,
  46     }
  47 }
  48
  49 struct EscapeDelegate<'a, 'tcx: 'a+'gcx, 'gcx: 'a> {
  50     tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
  51     set: NodeSet,
  52     infcx: &'a InferCtxt<'a, 'gcx, 'gcx>,
  53     target: TargetDataLayout,
  54     too_large_for_stack: u64,
  55 }
  56
  57 impl LintPass for Pass {
  58     fn get_lints(&self) -> LintArray {
  59         lint_array!(BOXED_LOCAL)
  60     }
  61 }
  62
  63 impl LateLintPass for Pass {
  64     fn check_fn(&mut self, cx: &LateContext, _: visit::FnKind, decl: &FnDecl, body: &Expr, _: Span, id: NodeId) {
  65         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  66
  67         let infcx = cx.tcx.borrowck_fake_infer_ctxt(param_env);
  68
  69         // we store the infcx because it is expensive to recreate
  70         // the context each time.
  71         let mut v = EscapeDelegate {
  72             tcx: cx.tcx,
  73             set: NodeSet(),
  74             infcx: &infcx,
  75             target: TargetDataLayout::parse(cx.sess()),
  76             too_large_for_stack: self.too_large_for_stack,
  77         };
  78
  79         {
  80             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  81             vis.walk_fn(decl, body);
  82         }
  83
  84         for node in v.set {
  85             span_lint(cx,
  86                       BOXED_LOCAL,
  87                       cx.tcx.map.span(node),
  88                       "local variable doesn't need to be boxed here");
  89         }
  90     }
  91 }
  92
  93 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx, 'gcx> {
  94     fn consume(&mut self, _: NodeId, _: Span, cmt: cmt<'tcx>, mode: ConsumeMode) {
  95         if let Categorization::Local(lid) = cmt.cat {
  96             if self.set.contains(&lid) {
  97                 if let Move(DirectRefMove) = mode {
  98                     // moved out or in. clearly can't be localized
  99                     self.set.remove(&lid);
 100                 }
 101             }
 102         }
 103     }
 104     fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
 105     fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
 106         let map = &self.tcx.map;
 107         if map.is_argument(consume_pat.id) {
 108             // Skip closure arguments
 109             if let Some(NodeExpr(..)) = map.find(map.get_parent_node(consume_pat.id)) {
 110                 return;
 111             }
 112             if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 113                 self.set.insert(consume_pat.id);
 114             }
 115             return;
 116         }
 117         if let Categorization::Rvalue(..) = cmt.cat {
 118             if let Some(NodeStmt(st)) = map.find(map.get_parent_node(cmt.id)) {
 119                 if let StmtDecl(ref decl, _) = st.node {
 120                     if let DeclLocal(ref loc) = decl.node {
 121                         if let Some(ref ex) = loc.init {
 122                             if let ExprBox(..) = ex.node {
 123                                 if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 124                                     // let x = box (...)
 125                                     self.set.insert(consume_pat.id);
 126                                 }
 127                                 // TODO Box::new
 128                                 // TODO vec![]
 129                                 // TODO "foo".to_owned() and friends
 130                             }
 131                         }
 132                     }
 133                 }
 134             }
 135         }
 136         if let Categorization::Local(lid) = cmt.cat {
 137             if self.set.contains(&lid) {
 138                 // let y = x where x is known
 139                 // remove x, insert y
 140                 self.set.insert(consume_pat.id);
 141                 self.set.remove(&lid);
 142             }
 143         }
 144
 145     }
 146     fn borrow(&mut self, borrow_id: NodeId, _: Span, cmt: cmt<'tcx>, _: &ty::Region, _: ty::BorrowKind,
 147               loan_cause: LoanCause) {
 148         use rustc::ty::adjustment::Adjust;
 149
 150         if let Categorization::Local(lid) = cmt.cat {
 151             if self.set.contains(&lid) {
 152                 if let Some(&Adjust::DerefRef { autoderefs, .. }) = self.tcx
 153                                                                         .tables
 154                                                                         .borrow()
 155                                                                         .adjustments
 156                                                                         .get(&borrow_id)
 157                                                                         .map(|a| &a.kind) {
 158                     if LoanCause::AutoRef == loan_cause {
 159                         // x.foo()
 160                         if autoderefs == 0 {
 161                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 162                         }
 163                     } else {
 164                         span_bug!(cmt.span, "Unknown adjusted AutoRef");
 165                     }
 166                 } else if LoanCause::AddrOf == loan_cause {
 167                     // &x
 168                     if let Some(&Adjust::DerefRef { autoderefs, .. }) = self.tcx
 169                                                                             .tables
 170                                                                             .borrow()
 171                                                                             .adjustments
 172                                                                             .get(&self.tcx
 173                                                                             .map
 174                                                                             .get_parent_node(borrow_id))
 175                                                                             .map(|a| &a.kind) {
 176                         if autoderefs <= 1 {
 177                             // foo(&x) where no extra autoreffing is happening
 178                             self.set.remove(&lid);
 179                         }
 180                     }
 181
 182                 } else if LoanCause::MatchDiscriminant == loan_cause {
 183                     self.set.remove(&lid); // `match x` can move
 184                 }
 185                 // do nothing for matches, etc. These can't escape
 186             }
 187         }
 188     }
 189     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 190     fn mutate(&mut self, _: NodeId, _: Span, _: cmt<'tcx>, _: MutateMode) {}
 191 }
 192
 193 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> EscapeDelegate<'a, 'tcx, 'gcx> {
 194     fn is_large_box(&self, ty: ty::Ty<'gcx>) -> bool {
 195         // Large types need to be boxed to avoid stack
 196         // overflows.
 197         match ty.sty {
 198             ty::TyBox(inner) => {
 199                 if let Ok(layout) = inner.layout(self.infcx) {
 200                     let size = layout.size(&self.target);
 201                     size.bytes() > self.too_large_for_stack
 202                 } else {
 203                     false
 204                 }
 205             },
 206             _ => false,
 207         }
 208     }
 209 }