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<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  64     fn check_fn(
  65         &mut self,
  66         cx: &LateContext<'a, 'tcx>,
  67         _: visit::FnKind<'tcx>,
  68         decl: &'tcx FnDecl,
  69         body: &'tcx Expr,
  70         _: Span,
  71         id: NodeId,
  72     ) {
  73         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
  74
  75         let infcx = cx.tcx.borrowck_fake_infer_ctxt(param_env);
  76
  77         // we store the infcx because it is expensive to recreate
  78         // the context each time.
  79         let mut v = EscapeDelegate {
  80             tcx: cx.tcx,
  81             set: NodeSet(),
  82             infcx: &infcx,
  83             target: TargetDataLayout::parse(cx.sess()),
  84             too_large_for_stack: self.too_large_for_stack,
  85         };
  86
  87         {
  88             let mut vis = ExprUseVisitor::new(&mut v, &infcx);
  89             vis.walk_fn(decl, body);
  90         }
  91
  92         for node in v.set {
  93             span_lint(cx,
  94                       BOXED_LOCAL,
  95                       cx.tcx.map.span(node),
  96                       "local variable doesn't need to be boxed here");
  97         }
  98     }
  99 }
 100
 101 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx, 'gcx> {
 102     fn consume(&mut self, _: NodeId, _: Span, cmt: cmt<'tcx>, mode: ConsumeMode) {
 103         if let Categorization::Local(lid) = cmt.cat {
 104             if self.set.contains(&lid) {
 105                 if let Move(DirectRefMove) = mode {
 106                     // moved out or in. clearly can't be localized
 107                     self.set.remove(&lid);
 108                 }
 109             }
 110         }
 111     }
 112     fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
 113     fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
 114         let map = &self.tcx.map;
 115         if map.is_argument(consume_pat.id) {
 116             // Skip closure arguments
 117             if let Some(NodeExpr(..)) = map.find(map.get_parent_node(consume_pat.id)) {
 118                 return;
 119             }
 120             if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 121                 self.set.insert(consume_pat.id);
 122             }
 123             return;
 124         }
 125         if let Categorization::Rvalue(..) = cmt.cat {
 126             if let Some(NodeStmt(st)) = map.find(map.get_parent_node(cmt.id)) {
 127                 if let StmtDecl(ref decl, _) = st.node {
 128                     if let DeclLocal(ref loc) = decl.node {
 129                         if let Some(ref ex) = loc.init {
 130                             if let ExprBox(..) = ex.node {
 131                                 if is_non_trait_box(cmt.ty) && !self.is_large_box(cmt.ty) {
 132                                     // let x = box (...)
 133                                     self.set.insert(consume_pat.id);
 134                                 }
 135                                 // TODO Box::new
 136                                 // TODO vec![]
 137                                 // TODO "foo".to_owned() and friends
 138                             }
 139                         }
 140                     }
 141                 }
 142             }
 143         }
 144         if let Categorization::Local(lid) = cmt.cat {
 145             if self.set.contains(&lid) {
 146                 // let y = x where x is known
 147                 // remove x, insert y
 148                 self.set.insert(consume_pat.id);
 149                 self.set.remove(&lid);
 150             }
 151         }
 152
 153     }
 154     fn borrow(&mut self, borrow_id: NodeId, _: Span, cmt: cmt<'tcx>, _: &ty::Region, _: ty::BorrowKind,
 155               loan_cause: LoanCause) {
 156         use rustc::ty::adjustment::Adjust;
 157
 158         if let Categorization::Local(lid) = cmt.cat {
 159             if self.set.contains(&lid) {
 160                 if let Some(&Adjust::DerefRef { autoderefs, .. }) = self.tcx
 161                                                                         .tables
 162                                                                         .borrow()
 163                                                                         .adjustments
 164                                                                         .get(&borrow_id)
 165                                                                         .map(|a| &a.kind) {
 166                     if LoanCause::AutoRef == loan_cause {
 167                         // x.foo()
 168                         if autoderefs == 0 {
 169                             self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
 170                         }
 171                     } else {
 172                         span_bug!(cmt.span, "Unknown adjusted AutoRef");
 173                     }
 174                 } else if LoanCause::AddrOf == loan_cause {
 175                     // &x
 176                     if let Some(&Adjust::DerefRef { autoderefs, .. }) = self.tcx
 177                                                                             .tables
 178                                                                             .borrow()
 179                                                                             .adjustments
 180                                                                             .get(&self.tcx
 181                                                                             .map
 182                                                                             .get_parent_node(borrow_id))
 183                                                                             .map(|a| &a.kind) {
 184                         if autoderefs <= 1 {
 185                             // foo(&x) where no extra autoreffing is happening
 186                             self.set.remove(&lid);
 187                         }
 188                     }
 189
 190                 } else if LoanCause::MatchDiscriminant == loan_cause {
 191                     self.set.remove(&lid); // `match x` can move
 192                 }
 193                 // do nothing for matches, etc. These can't escape
 194             }
 195         }
 196     }
 197     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 198     fn mutate(&mut self, _: NodeId, _: Span, _: cmt<'tcx>, _: MutateMode) {}
 199 }
 200
 201 impl<'a, 'tcx: 'a+'gcx, 'gcx: 'a> EscapeDelegate<'a, 'tcx, 'gcx> {
 202     fn is_large_box(&self, ty: ty::Ty<'gcx>) -> bool {
 203         // Large types need to be boxed to avoid stack
 204         // overflows.
 205         match ty.sty {
 206             ty::TyBox(inner) => {
 207                 if let Ok(layout) = inner.layout(self.infcx) {
 208                     let size = layout.size(&self.target);
 209                     size.bytes() > self.too_large_for_stack
 210                 } else {
 211                     false
 212                 }
 213             },
 214             _ => false,
 215         }
 216     }
 217 }