clippy_lints/src/needless_pass_by_value.rs

   1 use rustc::hir::*;
   2 use rustc::hir::map::*;
   3 use rustc::hir::intravisit::FnKind;
   4 use rustc::lint::*;
   5 use rustc::ty::{self, RegionKind, 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, snippet_opt, span_lint_and_then};
  14 use utils::ptr::get_spans;
  15 use std::collections::{HashMap, HashSet};
  16 use std::borrow::Cow;
  17
  18 /// **What it does:** Checks for functions taking arguments by value, but not
  19 /// consuming them in its
  20 /// body.
  21 ///
  22 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  23 /// sometimes avoid
  24 /// unnecessary allocations.
  25 ///
  26 /// **Known problems:**
  27 /// * This lint suggests taking an argument by reference,
  28 /// however sometimes it is better to let users decide the argument type
  29 /// (by using `Borrow` trait, for example), depending on how the function is used.
  30 ///
  31 /// **Example:**
  32 /// ```rust
  33 /// fn foo(v: Vec<i32>) {
  34 ///     assert_eq!(v.len(), 42);
  35 /// }
  36 /// // should be
  37 /// fn foo(v: &[i32]) {
  38 ///     assert_eq!(v.len(), 42);
  39 /// }
  40 /// ```
  41 declare_lint! {
  42     pub NEEDLESS_PASS_BY_VALUE,
  43     Warn,
  44     "functions taking arguments by value, but not consuming them in its body"
  45 }
  46
  47 pub struct NeedlessPassByValue;
  48
  49 impl LintPass for NeedlessPassByValue {
  50     fn get_lints(&self) -> LintArray {
  51         lint_array![NEEDLESS_PASS_BY_VALUE]
  52     }
  53 }
  54
  55 macro_rules! need {
  56     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  57 }
  58
  59 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  60     fn check_fn(
  61         &mut self,
  62         cx: &LateContext<'a, 'tcx>,
  63         kind: FnKind<'tcx>,
  64         decl: &'tcx FnDecl,
  65         body: &'tcx Body,
  66         span: Span,
  67         node_id: NodeId,
  68     ) {
  69         if in_macro(span) {
  70             return;
  71         }
  72
  73         match kind {
  74             FnKind::ItemFn(.., attrs) => for a in attrs {
  75                 if_chain! {
  76                     if a.meta_item_list().is_some();
  77                     if let Some(name) = a.name();
  78                     if name == "proc_macro_derive";
  79                     then {
  80                         return;
  81                     }
  82                 }
  83             },
  84             FnKind::Method(..) => (),
  85             _ => return,
  86         }
  87
  88         // Exclude non-inherent impls
  89         if let Some(NodeItem(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
  90             if matches!(item.node, ItemImpl(_, _, _, _, Some(_), _, _) | ItemAutoImpl(..) |
  91                 ItemTrait(..))
  92             {
  93                 return;
  94             }
  95         }
  96
  97         // Allow `Borrow` or functions to be taken by value
  98         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
  99         let fn_traits = [
 100             need!(cx.tcx.lang_items().fn_trait()),
 101             need!(cx.tcx.lang_items().fn_once_trait()),
 102             need!(cx.tcx.lang_items().fn_mut_trait()),
 103         ];
 104
 105         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 106
 107         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
 108
 109         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 110             .filter(|p| !p.is_global())
 111             .filter_map(|pred| {
 112                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 113                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_regions() {
 114                         return None;
 115                     }
 116                     Some(poly_trait_ref)
 117                 } else {
 118                     None
 119                 }
 120             })
 121             .collect::<Vec<_>>();
 122
 123         // Collect moved variables and spans which will need dereferencings from the
 124         // function body.
 125         let MovedVariablesCtxt {
 126             moved_vars,
 127             spans_need_deref,
 128             ..
 129         } = {
 130             let mut ctx = MovedVariablesCtxt::new(cx);
 131             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 132             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 133                 .consume_body(body);
 134             ctx
 135         };
 136
 137         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 138         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 139
 140         for (idx, ((input, &ty), arg)) in decl.inputs
 141             .iter()
 142             .zip(fn_sig.inputs())
 143             .zip(&body.arguments)
 144             .enumerate()
 145         {
 146             // All spans generated from a proc-macro invocation are the same...
 147             if span == input.span {
 148                 return;
 149             }
 150
 151             // Ignore `self`s.
 152             if idx == 0 {
 153                 if let PatKind::Binding(_, _, name, ..) = arg.pat.node {
 154                     if name.node.as_str() == "self" {
 155                         continue;
 156                     }
 157                 }
 158             }
 159
 160             // * Exclude a type that is specifically bounded by `Borrow`.
 161             // * Exclude a type whose reference also fulfills its bound.
 162             //   (e.g. `std::convert::AsRef`, `serde::Serialize`)
 163             let (implements_borrow_trait, all_borrowable_trait) = {
 164                 let preds = preds
 165                     .iter()
 166                     .filter(|t| t.skip_binder().self_ty() == ty)
 167                     .collect::<Vec<_>>();
 168
 169                 (
 170                     preds.iter().any(|t| t.def_id() == borrow_trait),
 171                     !preds.is_empty() && preds.iter().all(|t| {
 172                         implements_trait(
 173                             cx,
 174                             cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
 175                             t.def_id(),
 176                             &t.skip_binder().input_types().skip(1).collect::<Vec<_>>(),
 177                         )
 178                     }),
 179                 )
 180             };
 181
 182             if_chain! {
 183                 if !is_self(arg);
 184                 if !ty.is_mutable_pointer();
 185                 if !is_copy(cx, ty);
 186                 if !fn_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 187                 if !implements_borrow_trait;
 188                 if !all_borrowable_trait;
 189
 190                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 191                 if !moved_vars.contains(&canonical_id);
 192                 then {
 193                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 194                         continue;
 195                     }
 196
 197                     // Dereference suggestion
 198                     let sugg = |db: &mut DiagnosticBuilder| {
 199                         let deref_span = spans_need_deref.get(&canonical_id);
 200                         if_chain! {
 201                             if match_type(cx, ty, &paths::VEC);
 202                             if let Some(clone_spans) =
 203                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 204                             if let TyPath(QPath::Resolved(_, ref path)) = input.node;
 205                             if let Some(elem_ty) = path.segments.iter()
 206                                 .find(|seg| seg.name == "Vec")
 207                                 .and_then(|ps| ps.parameters.as_ref())
 208                                 .map(|params| &params.types[0]);
 209                             then {
 210                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 211                                 db.span_suggestion(input.span,
 212                                                 "consider changing the type to",
 213                                                 slice_ty);
 214
 215                                 for (span, suggestion) in clone_spans {
 216                                     db.span_suggestion(
 217                                         span,
 218                                         &snippet_opt(cx, span)
 219                                             .map_or(
 220                                                 "change the call to".into(),
 221                                                 |x| Cow::from(format!("change `{}` to", x)),
 222                                             ),
 223                                         suggestion.into()
 224                                     );
 225                                 }
 226
 227                                 // cannot be destructured, no need for `*` suggestion
 228                                 assert!(deref_span.is_none());
 229                                 return;
 230                             }
 231                         }
 232
 233                         if match_type(cx, ty, &paths::STRING) {
 234                             if let Some(clone_spans) =
 235                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 236                                 db.span_suggestion(input.span, "consider changing the type to", "&str".to_string());
 237
 238                                 for (span, suggestion) in clone_spans {
 239                                     db.span_suggestion(
 240                                         span,
 241                                         &snippet_opt(cx, span)
 242                                             .map_or(
 243                                                 "change the call to".into(),
 244                                                 |x| Cow::from(format!("change `{}` to", x))
 245                                             ),
 246                                         suggestion.into(),
 247                                     );
 248                                 }
 249
 250                                 assert!(deref_span.is_none());
 251                                 return;
 252                             }
 253                         }
 254
 255                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 256
 257                         // Suggests adding `*` to dereference the added reference.
 258                         if let Some(deref_span) = deref_span {
 259                             spans.extend(
 260                                 deref_span
 261                                     .iter()
 262                                     .cloned()
 263                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 264                             );
 265                             spans.sort_by_key(|&(span, _)| span);
 266                         }
 267                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 268                     };
 269
 270                     span_lint_and_then(
 271                         cx,
 272                         NEEDLESS_PASS_BY_VALUE,
 273                         input.span,
 274                         "this argument is passed by value, but not consumed in the function body",
 275                         sugg,
 276                     );
 277                 }
 278             }
 279         }
 280     }
 281 }
 282
 283 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 284     cx: &'a LateContext<'a, 'tcx>,
 285     moved_vars: HashSet<NodeId>,
 286     /// Spans which need to be prefixed with `*` for dereferencing the
 287     /// suggested additional reference.
 288     spans_need_deref: HashMap<NodeId, HashSet<Span>>,
 289 }
 290
 291 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 292     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 293         Self {
 294             cx: cx,
 295             moved_vars: HashSet::new(),
 296             spans_need_deref: HashMap::new(),
 297         }
 298     }
 299
 300     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: mc::cmt<'tcx>) {
 301         let cmt = unwrap_downcast_or_interior(cmt);
 302
 303         if let mc::Categorization::Local(vid) = cmt.cat {
 304             self.moved_vars.insert(vid);
 305         }
 306     }
 307
 308     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>) {
 309         let cmt = unwrap_downcast_or_interior(cmt);
 310
 311         if let mc::Categorization::Local(vid) = cmt.cat {
 312             let mut id = matched_pat.id;
 313             loop {
 314                 let parent = self.cx.tcx.hir.get_parent_node(id);
 315                 if id == parent {
 316                     // no parent
 317                     return;
 318                 }
 319                 id = parent;
 320
 321                 if let Some(node) = self.cx.tcx.hir.find(id) {
 322                     match node {
 323                         map::Node::NodeExpr(e) => {
 324                             // `match` and `if let`
 325                             if let ExprMatch(ref c, ..) = e.node {
 326                                 self.spans_need_deref
 327                                     .entry(vid)
 328                                     .or_insert_with(HashSet::new)
 329                                     .insert(c.span);
 330                             }
 331                         },
 332
 333                         map::Node::NodeStmt(s) => {
 334                             // `let <pat> = x;`
 335                             if_chain! {
 336                                 if let StmtDecl(ref decl, _) = s.node;
 337                                 if let DeclLocal(ref local) = decl.node;
 338                                 then {
 339                                     self.spans_need_deref
 340                                         .entry(vid)
 341                                         .or_insert_with(HashSet::new)
 342                                         .insert(local.init
 343                                             .as_ref()
 344                                             .map(|e| e.span)
 345                                             .expect("`let` stmt without init aren't caught by match_pat"));
 346                                 }
 347                             }
 348                         },
 349
 350                         _ => {},
 351                     }
 352                 }
 353             }
 354         }
 355     }
 356 }
 357
 358 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 359     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 360         if let euv::ConsumeMode::Move(_) = mode {
 361             self.move_common(consume_id, consume_span, cmt);
 362         }
 363     }
 364
 365     fn matched_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::MatchMode) {
 366         if let euv::MatchMode::MovingMatch = mode {
 367             self.move_common(matched_pat.id, matched_pat.span, cmt);
 368         } else {
 369             self.non_moving_pat(matched_pat, cmt);
 370         }
 371     }
 372
 373     fn consume_pat(&mut self, consume_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 374         if let euv::ConsumeMode::Move(_) = mode {
 375             self.move_common(consume_pat.id, consume_pat.span, cmt);
 376         }
 377     }
 378
 379     fn borrow(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: ty::Region, _: ty::BorrowKind, _: euv::LoanCause) {}
 380
 381     fn mutate(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: euv::MutateMode) {}
 382
 383     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 384 }
 385
 386
 387 fn unwrap_downcast_or_interior(mut cmt: mc::cmt) -> mc::cmt {
 388     loop {
 389         match cmt.cat.clone() {
 390             mc::Categorization::Downcast(c, _) | mc::Categorization::Interior(c, _) => {
 391                 cmt = c;
 392             },
 393             _ => return cmt,
 394         }
 395     }
 396 }