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