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 macro_rules! need {
  45     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  46 }
  47
  48 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  49     fn check_fn(
  50         &mut self,
  51         cx: &LateContext<'a, 'tcx>,
  52         kind: FnKind<'tcx>,
  53         decl: &'tcx FnDecl,
  54         body: &'tcx Body,
  55         span: Span,
  56         node_id: NodeId
  57     ) {
  58         if in_macro(cx, span) {
  59             return;
  60         }
  61
  62         if !matches!(kind, FnKind::ItemFn(..)) {
  63             return;
  64         }
  65
  66         // Allows these to be passed by value.
  67         let fn_trait = need!(cx.tcx.lang_items.fn_trait());
  68         let asref_trait = need!(get_trait_def_id(cx, &paths::ASREF_TRAIT));
  69         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
  70
  71         let preds: Vec<ty::Predicate> = {
  72             let parameter_env = ty::ParameterEnvironment::for_item(cx.tcx, node_id);
  73             traits::elaborate_predicates(cx.tcx, parameter_env.caller_bounds.clone())
  74                 .filter(|p| !p.is_global())
  75                 .collect()
  76         };
  77
  78         // Collect moved variables and spans which will need dereferencings from the function body.
  79         let MovedVariablesCtxt { moved_vars, spans_need_deref, .. } = {
  80             let mut ctx = MovedVariablesCtxt::new(cx);
  81             let infcx = cx.tcx.borrowck_fake_infer_ctxt(body.id());
  82             {
  83                 let mut v = euv::ExprUseVisitor::new(&mut ctx, &infcx);
  84                 v.consume_body(body);
  85             }
  86             ctx
  87         };
  88
  89         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
  90         let param_env = ty::ParameterEnvironment::for_item(cx.tcx, node_id);
  91         let fn_sig = cx.tcx.item_type(fn_def_id).fn_sig();
  92         let fn_sig = cx.tcx.liberate_late_bound_regions(param_env.free_id_outlive, &fn_sig);
  93
  94         for ((input, ty), arg) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments) {
  95
  96             // Determines whether `ty` implements `Borrow<U>` (U != ty) specifically.
  97             // This is needed due to the `Borrow<T> for T` blanket impl.
  98             let implements_borrow_trait = preds.iter()
  99                 .filter_map(|pred| if let ty::Predicate::Trait(ref poly_trait_ref) = *pred {
 100                     Some(poly_trait_ref.skip_binder())
 101                 } else {
 102                     None
 103                 })
 104                 .filter(|tpred| tpred.def_id() == borrow_trait && &tpred.self_ty() == ty)
 105                 .any(|tpred| &tpred.input_types().nth(1).expect("Borrow trait must have an parameter") != ty);
 106
 107             if_let_chain! {[
 108                 !is_self(arg),
 109                 !ty.is_mutable_pointer(),
 110                 !is_copy(cx, ty, node_id),
 111                 !implements_trait(cx, ty, fn_trait, &[], Some(node_id)),
 112                 !implements_trait(cx, ty, asref_trait, &[], Some(node_id)),
 113                 !implements_borrow_trait,
 114
 115                 let PatKind::Binding(mode, defid, ..) = arg.pat.node,
 116                 !moved_vars.contains(&defid),
 117             ], {
 118                 // Note: `toplevel_ref_arg` warns if `BindByRef`
 119                 let m = match mode {
 120                     BindingMode::BindByRef(m) | BindingMode::BindByValue(m) => m,
 121                 };
 122                 if m == Mutability::MutMutable {
 123                     continue;
 124                 }
 125
 126                 // Suggestion logic
 127                 let sugg = |db: &mut DiagnosticBuilder| {
 128                     let deref_span = spans_need_deref.get(&defid);
 129                     if_let_chain! {[
 130                         match_type(cx, ty, &paths::VEC),
 131                         let TyPath(QPath::Resolved(_, ref path)) = input.node,
 132                         let Some(elem_ty) = path.segments.iter()
 133                             .find(|seg| &*seg.name.as_str() == "Vec")
 134                             .map(|ps| ps.parameters.types()[0]),
 135                     ], {
 136                         let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 137                         db.span_suggestion(input.span,
 138                                         &format!("consider changing the type to `{}`", slice_ty),
 139                                         slice_ty);
 140                         assert!(deref_span.is_none());
 141                         return; // `Vec` and `String` cannot be destructured - no need for `*` suggestion
 142                     }}
 143
 144                     if match_type(cx, ty, &paths::STRING) {
 145                         db.span_suggestion(input.span,
 146                                            "consider changing the type to `&str`",
 147                                            "&str".to_string());
 148                         assert!(deref_span.is_none());
 149                         return;
 150                     }
 151
 152                     let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 153
 154                     // Suggests adding `*` to dereference the added reference.
 155                     if let Some(deref_span) = deref_span {
 156                         spans.extend(deref_span.iter().cloned()
 157                                      .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))));
 158                         spans.sort_by_key(|&(span, _)| span);
 159                     }
 160                     multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 161                 };
 162
 163                 span_lint_and_then(cx,
 164                           NEEDLESS_PASS_BY_VALUE,
 165                           input.span,
 166                           "this argument is passed by value, but not consumed in the function body",
 167                           sugg);
 168             }}
 169         }
 170     }
 171 }
 172
 173 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 174     cx: &'a LateContext<'a, 'tcx>,
 175     moved_vars: HashSet<DefId>,
 176     /// Spans which need to be prefixed with `*` for dereferencing the suggested additional
 177     /// reference.
 178     spans_need_deref: HashMap<DefId, HashSet<Span>>,
 179 }
 180
 181 impl<'a, 'tcx: 'a> MovedVariablesCtxt<'a, 'tcx> {
 182     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 183         MovedVariablesCtxt {
 184             cx: cx,
 185             moved_vars: HashSet::new(),
 186             spans_need_deref: HashMap::new(),
 187         }
 188     }
 189
 190     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: mc::cmt<'tcx>) {
 191         let cmt = unwrap_downcast_or_interior(cmt);
 192
 193         if_let_chain! {[
 194             let mc::Categorization::Local(vid) = cmt.cat,
 195             let Some(def_id) = self.cx.tcx.hir.opt_local_def_id(vid),
 196         ], {
 197                 self.moved_vars.insert(def_id);
 198         }}
 199     }
 200
 201     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>) {
 202         let cmt = unwrap_downcast_or_interior(cmt);
 203
 204         if_let_chain! {[
 205             let mc::Categorization::Local(vid) = cmt.cat,
 206             let Some(def_id) = self.cx.tcx.hir.opt_local_def_id(vid),
 207         ], {
 208             let mut id = matched_pat.id;
 209             loop {
 210                 let parent = self.cx.tcx.hir.get_parent_node(id);
 211                 if id == parent {
 212                     // no parent
 213                     return;
 214                 }
 215                 id = parent;
 216
 217                 if let Some(node) = self.cx.tcx.hir.find(id) {
 218                     match node {
 219                         map::Node::NodeExpr(e) => {
 220                             // `match` and `if let`
 221                             if let ExprMatch(ref c, ..) = e.node {
 222                                 self.spans_need_deref
 223                                     .entry(def_id)
 224                                     .or_insert_with(HashSet::new)
 225                                     .insert(c.span);
 226                             }
 227                         }
 228
 229                         map::Node::NodeStmt(s) => {
 230                             // `let <pat> = x;`
 231                             if_let_chain! {[
 232                                 let StmtDecl(ref decl, _) = s.node,
 233                                 let DeclLocal(ref local) = decl.node,
 234                             ], {
 235                                 self.spans_need_deref
 236                                     .entry(def_id)
 237                                     .or_insert_with(HashSet::new)
 238                                     .insert(local.init
 239                                         .as_ref()
 240                                         .map(|e| e.span)
 241                                         .expect("`let` stmt without init aren't caught by match_pat"));
 242                             }}
 243                         }
 244
 245                         _ => {}
 246                     }
 247                 }
 248             }
 249         }}
 250     }
 251 }
 252
 253 impl<'a, 'tcx: 'a> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 254     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 255         if let euv::ConsumeMode::Move(_) = mode {
 256             self.move_common(consume_id, consume_span, cmt);
 257         }
 258     }
 259
 260     fn matched_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::MatchMode) {
 261         if let euv::MatchMode::MovingMatch = mode {
 262             self.move_common(matched_pat.id, matched_pat.span, cmt);
 263         } else {
 264             self.non_moving_pat(matched_pat, cmt);
 265         }
 266     }
 267
 268     fn consume_pat(&mut self, consume_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 269         if let euv::ConsumeMode::Move(_) = mode {
 270             self.move_common(consume_pat.id, consume_pat.span, cmt);
 271         }
 272     }
 273
 274     fn borrow(
 275         &mut self,
 276         _: NodeId,
 277         _: Span,
 278         _: mc::cmt<'tcx>,
 279         _: &'tcx ty::Region,
 280         _: ty::BorrowKind,
 281         _: euv::LoanCause
 282     ) {
 283     }
 284
 285     fn mutate(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: euv::MutateMode) {}
 286
 287     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 288 }
 289
 290
 291 fn unwrap_downcast_or_interior(mut cmt: mc::cmt) -> mc::cmt {
 292     loop {
 293         match cmt.cat.clone() {
 294             mc::Categorization::Downcast(c, _) |
 295             mc::Categorization::Interior(c, _) => {
 296                 cmt = c;
 297             },
 298             _ => return cmt,
 299         }
 300     }
 301 }