clippy_lints/src/needless_pass_by_value.rs

   1 use rustc::hir::*;
   2 use rustc::hir::intravisit::FnKind;
   3 use rustc::hir::def_id::DefId;
   4 use rustc::lint::*;
   5 use rustc::ty::{self, TypeFoldable};
   6 use rustc::traits;
   7 use rustc::middle::expr_use_visitor as euv;
   8 use rustc::middle::mem_categorization as mc;
   9 use syntax::ast::NodeId;
  10 use syntax_pos::Span;
  11 use syntax::errors::DiagnosticBuilder;
  12 use utils::{in_macro, is_self, is_copy, implements_trait, get_trait_def_id, match_type, snippet, span_lint_and_then,
  13             multispan_sugg, paths};
  14 use std::collections::{HashSet, HashMap};
  15
  16 /// **What it does:** Checks for functions taking arguments by value, but not consuming them in its
  17 /// body.
  18 ///
  19 /// **Why is this bad?** Taking arguments by reference is more flexible and can sometimes avoid
  20 /// unnecessary allocations.
  21 ///
  22 /// **Known problems:** Hopefully none.
  23 ///
  24 /// **Example:**
  25 /// ```rust
  26 /// fn foo(v: Vec<i32>) {
  27 ///     assert_eq!(v.len(), 42);
  28 /// }
  29 /// ```
  30 declare_lint! {
  31     pub NEEDLESS_PASS_BY_VALUE,
  32     Warn,
  33     "functions taking arguments by value, but not consuming them in its body"
  34 }
  35
  36 pub struct NeedlessPassByValue;
  37
  38 impl LintPass for NeedlessPassByValue {
  39     fn get_lints(&self) -> LintArray {
  40         lint_array![NEEDLESS_PASS_BY_VALUE]
  41     }
  42 }
  43
  44 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  45     fn check_fn(
  46         &mut self,
  47         cx: &LateContext<'a, 'tcx>,
  48         kind: FnKind<'tcx>,
  49         decl: &'tcx FnDecl,
  50         body: &'tcx Body,
  51         span: Span,
  52         node_id: NodeId
  53     ) {
  54         if in_macro(cx, span) {
  55             return;
  56         }
  57
  58         if !matches!(kind, FnKind::ItemFn(..)) {
  59             return;
  60         }
  61
  62         // Allows these to be passed by value.
  63         let fn_trait = cx.tcx.lang_items.fn_trait().expect("failed to find `Fn` trait");
  64         let asref_trait = get_trait_def_id(cx, &paths::ASREF_TRAIT).expect("failed to find `AsRef` trait");
  65         let borrow_trait = get_trait_def_id(cx, &paths::BORROW_TRAIT).expect("failed to find `Borrow` trait");
  66
  67         let preds: Vec<ty::Predicate> = {
  68             let parameter_env = ty::ParameterEnvironment::for_item(cx.tcx, node_id);
  69             traits::elaborate_predicates(cx.tcx, parameter_env.caller_bounds.clone())
  70                 .filter(|p| !p.is_global())
  71                 .collect()
  72         };
  73
  74         // Collect moved variables and spans which will need dereferencings from the function body.
  75         let MovedVariablesCtxt { moved_vars, spans_need_deref, .. } = {
  76             let mut ctx = MovedVariablesCtxt::new(cx);
  77             let infcx = cx.tcx.borrowck_fake_infer_ctxt(body.id());
  78             {
  79                 let mut v = euv::ExprUseVisitor::new(&mut ctx, &infcx);
  80                 v.consume_body(body);
  81             }
  82             ctx
  83         };
  84
  85         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
  86         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, node_id);
  87         let fn_sig = cx.tcx.item_type(fn_def_id).fn_sig();
  88         let fn_sig = cx.tcx.liberate_late_bound_regions(param_env.free_id_outlive, fn_sig);
  89
  90         for ((input, ty), arg) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments) {
  91
  92             // Determines whether `ty` implements `Borrow<U>` (U != ty) specifically.
  93             // This is needed due to the `Borrow<T> for T` blanket impl.
  94             let implements_borrow_trait = preds.iter()
  95                 .filter_map(|pred| if let ty::Predicate::Trait(ref poly_trait_ref) = *pred {
  96                     Some(poly_trait_ref.skip_binder())
  97                 } else {
  98                     None
  99                 })
 100                 .filter(|tpred| tpred.def_id() == borrow_trait && &tpred.self_ty() == ty)
 101                 .any(|tpred| &tpred.input_types().nth(1).expect("Borrow trait must have an parameter") != ty);
 102
 103             if_let_chain! {[
 104                 !is_self(arg),
 105                 !ty.is_mutable_pointer(),
 106                 !is_copy(cx, ty, node_id),
 107                 !implements_trait(cx, ty, fn_trait, &[], Some(node_id)),
 108                 !implements_trait(cx, ty, asref_trait, &[], Some(node_id)),
 109                 !implements_borrow_trait,
 110
 111                 let PatKind::Binding(mode, defid, ..) = arg.pat.node,
 112                 !moved_vars.contains(&defid),
 113             ], {
 114                 // Note: `toplevel_ref_arg` warns if `BindByRef`
 115                 let m = match mode {
 116                     BindingMode::BindByRef(m) | BindingMode::BindByValue(m) => m,
 117                 };
 118                 if m == Mutability::MutMutable {
 119                     continue;
 120                 }
 121
 122                 // Suggestion logic
 123                 let sugg = |db: &mut DiagnosticBuilder| {
 124                     if_let_chain! {[
 125                         match_type(cx, ty, &paths::VEC),
 126                         let TyPath(QPath::Resolved(_, ref path)) = input.node,
 127                         let Some(elem_ty) = path.segments.iter()
 128                             .find(|seg| &*seg.name.as_str() == "Vec")
 129                             .map(|ps| ps.parameters.types()[0]),
 130                     ], {
 131                         let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 132                         db.span_suggestion(input.span,
 133                                         &format!("consider changing the type to `{}`", slice_ty),
 134                                         slice_ty);
 135                         return; // `Vec` and `String` cannot be destructured - no need for `*` suggestion
 136                     }}
 137
 138                     if match_type(cx, ty, &paths::STRING) {
 139                         db.span_suggestion(input.span,
 140                                            "consider changing the type to `&str`",
 141                                            "&str".to_string());
 142                         return;
 143                     }
 144
 145                     let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 146
 147                     // Suggests adding `*` to dereference the added reference.
 148                     if let Some(deref_span) = spans_need_deref.get(&defid) {
 149                         spans.extend(deref_span.iter().cloned()
 150                                      .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))));
 151                         spans.sort_by_key(|&(span, _)| span);
 152                     }
 153                     multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 154                 };
 155
 156                 span_lint_and_then(cx,
 157                           NEEDLESS_PASS_BY_VALUE,
 158                           input.span,
 159                           "this argument is passed by value, but not consumed in the function body",
 160                           sugg);
 161             }}
 162         }
 163     }
 164 }
 165
 166 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 167     cx: &'a LateContext<'a, 'tcx>,
 168     moved_vars: HashSet<DefId>,
 169     /// Spans which need to be prefixed with `*` for dereferencing the suggested additional
 170     /// reference.
 171     spans_need_deref: HashMap<DefId, HashSet<Span>>,
 172 }
 173
 174 impl<'a, 'tcx: 'a> MovedVariablesCtxt<'a, 'tcx> {
 175     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 176         MovedVariablesCtxt {
 177             cx: cx,
 178             moved_vars: HashSet::new(),
 179             spans_need_deref: HashMap::new(),
 180         }
 181     }
 182
 183     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: mc::cmt<'tcx>) {
 184         let cmt = unwrap_downcast_or_interior(cmt);
 185
 186         if_let_chain! {[
 187             let mc::Categorization::Local(vid) = cmt.cat,
 188             let Some(def_id) = self.cx.tcx.hir.opt_local_def_id(vid),
 189         ], {
 190                 self.moved_vars.insert(def_id);
 191         }}
 192     }
 193
 194     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>) {
 195         let cmt = unwrap_downcast_or_interior(cmt);
 196
 197         if_let_chain! {[
 198             let mc::Categorization::Local(vid) = cmt.cat,
 199             let Some(def_id) = self.cx.tcx.hir.opt_local_def_id(vid),
 200         ], {
 201             let mut id = matched_pat.id;
 202             loop {
 203                 let parent = self.cx.tcx.hir.get_parent_node(id);
 204                 if id == parent {
 205                     // no parent
 206                     return;
 207                 }
 208                 id = parent;
 209
 210                 if let Some(node) = self.cx.tcx.hir.find(id) {
 211                     match node {
 212                         map::Node::NodeExpr(e) => {
 213                             // `match` and `if let`
 214                             if let ExprMatch(ref c, ..) = e.node {
 215                                 self.spans_need_deref
 216                                     .entry(def_id)
 217                                     .or_insert_with(HashSet::new)
 218                                     .insert(c.span);
 219                             }
 220                         }
 221
 222                         map::Node::NodeStmt(s) => {
 223                             // `let <pat> = x;`
 224                             if_let_chain! {[
 225                                 let StmtDecl(ref decl, _) = s.node,
 226                                 let DeclLocal(ref local) = decl.node,
 227                             ], {
 228                                 self.spans_need_deref
 229                                     .entry(def_id)
 230                                     .or_insert_with(HashSet::new)
 231                                     .insert(local.init
 232                                         .as_ref()
 233                                         .map(|e| e.span)
 234                                         .expect("`let` stmt without init aren't caught by match_pat"));
 235                             }}
 236                         }
 237
 238                         _ => {}
 239                     }
 240                 }
 241             }
 242         }}
 243     }
 244 }
 245
 246 impl<'a, 'tcx: 'a> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 247     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 248         if let euv::ConsumeMode::Move(_) = mode {
 249             self.move_common(consume_id, consume_span, cmt);
 250         }
 251     }
 252
 253     fn matched_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::MatchMode) {
 254         if let euv::MatchMode::MovingMatch = mode {
 255             self.move_common(matched_pat.id, matched_pat.span, cmt);
 256         } else {
 257             self.non_moving_pat(matched_pat, cmt);
 258         }
 259     }
 260
 261     fn consume_pat(&mut self, consume_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 262         if let euv::ConsumeMode::Move(_) = mode {
 263             self.move_common(consume_pat.id, consume_pat.span, cmt);
 264         }
 265     }
 266
 267     fn borrow(
 268         &mut self,
 269         _: NodeId,
 270         _: Span,
 271         _: mc::cmt<'tcx>,
 272         _: &'tcx ty::Region,
 273         _: ty::BorrowKind,
 274         _: euv::LoanCause
 275     ) {
 276     }
 277
 278     fn mutate(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: euv::MutateMode) {}
 279
 280     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 281 }
 282
 283
 284 fn unwrap_downcast_or_interior(mut cmt: mc::cmt) -> mc::cmt {
 285     loop {
 286         match cmt.cat.clone() {
 287             mc::Categorization::Downcast(c, _) |
 288             mc::Categorization::Interior(c, _) => {
 289                 cmt = c;
 290             },
 291             _ => return cmt,
 292         }
 293     }
 294 }