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::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
  13             snippet, span_lint_and_then};
  14 use std::collections::{HashMap, HashSet};
  15
  16 /// **What it does:** Checks for functions taking arguments by value, but not
  17 /// consuming them in its
  18 /// body.
  19 ///
  20 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  21 /// sometimes avoid
  22 /// unnecessary allocations.
  23 ///
  24 /// **Known problems:** Hopefully none.
  25 ///
  26 /// **Example:**
  27 /// ```rust
  28 /// fn foo(v: Vec<i32>) {
  29 ///     assert_eq!(v.len(), 42);
  30 /// }
  31 /// ```
  32 declare_lint! {
  33     pub NEEDLESS_PASS_BY_VALUE,
  34     Warn,
  35     "functions taking arguments by value, but not consuming them in its body"
  36 }
  37
  38 pub struct NeedlessPassByValue;
  39
  40 impl LintPass for NeedlessPassByValue {
  41     fn get_lints(&self) -> LintArray {
  42         lint_array![NEEDLESS_PASS_BY_VALUE]
  43     }
  44 }
  45
  46 macro_rules! need {
  47     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  48 }
  49
  50 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  51     fn check_fn(
  52         &mut self,
  53         cx: &LateContext<'a, 'tcx>,
  54         kind: FnKind<'tcx>,
  55         decl: &'tcx FnDecl,
  56         body: &'tcx Body,
  57         span: Span,
  58         node_id: NodeId,
  59     ) {
  60         if in_macro(span) {
  61             return;
  62         }
  63
  64         match kind {
  65             FnKind::ItemFn(.., attrs) => for a in attrs {
  66                 if_let_chain!{[
  67                     a.meta_item_list().is_some(),
  68                     let Some(name) = a.name(),
  69                     name == "proc_macro_derive",
  70                 ], {
  71                     return;
  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
  91         // function body.
  92         let MovedVariablesCtxt {
  93             moved_vars,
  94             spans_need_deref,
  95             ..
  96         } = {
  97             let mut ctx = MovedVariablesCtxt::new(cx);
  98             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
  99             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables).consume_body(body);
 100             ctx
 101         };
 102
 103         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 104         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 105
 106         for ((input, &ty), arg) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments) {
 107             // Determines whether `ty` implements `Borrow<U>` (U != ty) specifically.
 108             // This is needed due to the `Borrow<T> for T` blanket impl.
 109             let implements_borrow_trait = preds
 110                 .iter()
 111                 .filter_map(|pred| if let ty::Predicate::Trait(ref poly_trait_ref) = *pred {
 112                     Some(poly_trait_ref.skip_binder())
 113                 } else {
 114                     None
 115                 })
 116                 .filter(|tpred| tpred.def_id() == borrow_trait && tpred.self_ty() == ty)
 117                 .any(|tpred| {
 118                     tpred
 119                         .input_types()
 120                         .nth(1)
 121                         .expect("Borrow trait must have an parameter") != ty
 122                 });
 123
 124             if_let_chain! {[
 125                 !is_self(arg),
 126                 !ty.is_mutable_pointer(),
 127                 !is_copy(cx, ty),
 128                 !implements_trait(cx, ty, fn_trait, &[]),
 129                 !implements_trait(cx, ty, asref_trait, &[]),
 130                 !implements_borrow_trait,
 131
 132                 let PatKind::Binding(mode, defid, ..) = arg.pat.node,
 133                 !moved_vars.contains(&defid),
 134             ], {
 135                 // Note: `toplevel_ref_arg` warns if `BindByRef`
 136                 if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 137                     continue;
 138                 }
 139
 140                 // Suggestion logic
 141                 let sugg = |db: &mut DiagnosticBuilder| {
 142                     let deref_span = spans_need_deref.get(&defid);
 143                     if_let_chain! {[
 144                         match_type(cx, ty, &paths::VEC),
 145                         let TyPath(QPath::Resolved(_, ref path)) = input.node,
 146                         let Some(elem_ty) = path.segments.iter()
 147                             .find(|seg| seg.name == "Vec")
 148                             .map(|ps| &ps.parameters.types[0]),
 149                     ], {
 150                         let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 151                         db.span_suggestion(input.span,
 152                                         "consider changing the type to",
 153                                         slice_ty);
 154                         assert!(deref_span.is_none());
 155                         return; // `Vec` and `String` cannot be destructured - no need for `*` suggestion
 156                     }}
 157
 158                     if match_type(cx, ty, &paths::STRING) {
 159                         db.span_suggestion(input.span,
 160                                            "consider changing the type to",
 161                                            "&str".to_string());
 162                         assert!(deref_span.is_none());
 163                         return;
 164                     }
 165
 166                     let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 167
 168                     // Suggests adding `*` to dereference the added reference.
 169                     if let Some(deref_span) = deref_span {
 170                         spans.extend(deref_span.iter().cloned()
 171                                      .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))));
 172                         spans.sort_by_key(|&(span, _)| span);
 173                     }
 174                     multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 175                 };
 176
 177                 span_lint_and_then(cx,
 178                           NEEDLESS_PASS_BY_VALUE,
 179                           input.span,
 180                           "this argument is passed by value, but not consumed in the function body",
 181                           sugg);
 182             }}
 183         }
 184     }
 185 }
 186
 187 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 188     cx: &'a LateContext<'a, 'tcx>,
 189     moved_vars: HashSet<DefId>,
 190     /// Spans which need to be prefixed with `*` for dereferencing the
 191     /// suggested additional
 192     /// reference.
 193     spans_need_deref: HashMap<DefId, HashSet<Span>>,
 194 }
 195
 196 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 197     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 198         Self {
 199             cx: cx,
 200             moved_vars: HashSet::new(),
 201             spans_need_deref: HashMap::new(),
 202         }
 203     }
 204
 205     fn move_common(&mut self, _consume_id: NodeId, _span: Span, 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                 self.moved_vars.insert(def_id);
 213         }}
 214     }
 215
 216     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>) {
 217         let cmt = unwrap_downcast_or_interior(cmt);
 218
 219         if_let_chain! {[
 220             let mc::Categorization::Local(vid) = cmt.cat,
 221             let Some(def_id) = self.cx.tcx.hir.opt_local_def_id(vid),
 222         ], {
 223             let mut id = matched_pat.id;
 224             loop {
 225                 let parent = self.cx.tcx.hir.get_parent_node(id);
 226                 if id == parent {
 227                     // no parent
 228                     return;
 229                 }
 230                 id = parent;
 231
 232                 if let Some(node) = self.cx.tcx.hir.find(id) {
 233                     match node {
 234                         map::Node::NodeExpr(e) => {
 235                             // `match` and `if let`
 236                             if let ExprMatch(ref c, ..) = e.node {
 237                                 self.spans_need_deref
 238                                     .entry(def_id)
 239                                     .or_insert_with(HashSet::new)
 240                                     .insert(c.span);
 241                             }
 242                         }
 243
 244                         map::Node::NodeStmt(s) => {
 245                             // `let <pat> = x;`
 246                             if_let_chain! {[
 247                                 let StmtDecl(ref decl, _) = s.node,
 248                                 let DeclLocal(ref local) = decl.node,
 249                             ], {
 250                                 self.spans_need_deref
 251                                     .entry(def_id)
 252                                     .or_insert_with(HashSet::new)
 253                                     .insert(local.init
 254                                         .as_ref()
 255                                         .map(|e| e.span)
 256                                         .expect("`let` stmt without init aren't caught by match_pat"));
 257                             }}
 258                         }
 259
 260                         _ => {}
 261                     }
 262                 }
 263             }
 264         }}
 265     }
 266 }
 267
 268 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 269     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 270         if let euv::ConsumeMode::Move(_) = mode {
 271             self.move_common(consume_id, consume_span, cmt);
 272         }
 273     }
 274
 275     fn matched_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::MatchMode) {
 276         if let euv::MatchMode::MovingMatch = mode {
 277             self.move_common(matched_pat.id, matched_pat.span, cmt);
 278         } else {
 279             self.non_moving_pat(matched_pat, cmt);
 280         }
 281     }
 282
 283     fn consume_pat(&mut self, consume_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 284         if let euv::ConsumeMode::Move(_) = mode {
 285             self.move_common(consume_pat.id, consume_pat.span, cmt);
 286         }
 287     }
 288
 289     fn borrow(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: ty::Region, _: ty::BorrowKind, _: euv::LoanCause) {}
 290
 291     fn mutate(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: euv::MutateMode) {}
 292
 293     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 294 }
 295
 296
 297 fn unwrap_downcast_or_interior(mut cmt: mc::cmt) -> mc::cmt {
 298     loop {
 299         match cmt.cat.clone() {
 300             mc::Categorization::Downcast(c, _) | mc::Categorization::Interior(c, _) => {
 301                 cmt = c;
 302             },
 303             _ => return cmt,
 304         }
 305     }
 306 }