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::abi::Abi;
  10 use syntax::ast::NodeId;
  11 use syntax_pos::Span;
  12 use syntax::errors::DiagnosticBuilder;
  13 use utils::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
  14             snippet, snippet_opt, span_lint_and_then};
  15 use utils::ptr::get_spans;
  16 use std::collections::{HashMap, HashSet};
  17 use std::borrow::Cow;
  18
  19 /// **What it does:** Checks for functions taking arguments by value, but not
  20 /// consuming them in its
  21 /// body.
  22 ///
  23 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  24 /// sometimes avoid
  25 /// unnecessary allocations.
  26 ///
  27 /// **Known problems:**
  28 /// * This lint suggests taking an argument by reference,
  29 /// however sometimes it is better to let users decide the argument type
  30 /// (by using `Borrow` trait, for example), depending on how the function is used.
  31 ///
  32 /// **Example:**
  33 /// ```rust
  34 /// fn foo(v: Vec<i32>) {
  35 ///     assert_eq!(v.len(), 42);
  36 /// }
  37 /// // should be
  38 /// fn foo(v: &[i32]) {
  39 ///     assert_eq!(v.len(), 42);
  40 /// }
  41 /// ```
  42 declare_lint! {
  43     pub NEEDLESS_PASS_BY_VALUE,
  44     Warn,
  45     "functions taking arguments by value, but not consuming them in its body"
  46 }
  47
  48 pub struct NeedlessPassByValue;
  49
  50 impl LintPass for NeedlessPassByValue {
  51     fn get_lints(&self) -> LintArray {
  52         lint_array![NEEDLESS_PASS_BY_VALUE]
  53     }
  54 }
  55
  56 macro_rules! need {
  57     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  58 }
  59
  60 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  61     fn check_fn(
  62         &mut self,
  63         cx: &LateContext<'a, 'tcx>,
  64         kind: FnKind<'tcx>,
  65         decl: &'tcx FnDecl,
  66         body: &'tcx Body,
  67         span: Span,
  68         node_id: NodeId,
  69     ) {
  70         if in_macro(span) {
  71             return;
  72         }
  73
  74         match kind {
  75             FnKind::ItemFn(.., abi, _, attrs) => {
  76                 if abi != Abi::Rust {
  77                     return;
  78                 }
  79                 for a in attrs {
  80                     if_chain! {
  81                         if a.meta_item_list().is_some();
  82                         if let Some(name) = a.name();
  83                         if name == "proc_macro_derive";
  84                         then {
  85                             return;
  86                         }
  87                     }
  88                 }
  89             },
  90             FnKind::Method(..) => (),
  91             _ => return,
  92         }
  93
  94         // Exclude non-inherent impls
  95         if let Some(NodeItem(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
  96             if matches!(item.node, ItemImpl(_, _, _, _, Some(_), _, _) |
  97                 ItemTrait(..))
  98             {
  99                 return;
 100             }
 101         }
 102
 103         // Allow `Borrow` or functions to be taken by value
 104         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 105         let fn_traits = [
 106             need!(cx.tcx.lang_items().fn_trait()),
 107             need!(cx.tcx.lang_items().fn_once_trait()),
 108             need!(cx.tcx.lang_items().fn_mut_trait()),
 109         ];
 110
 111         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 112
 113         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
 114
 115         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 116             .filter(|p| !p.is_global())
 117             .filter_map(|pred| {
 118                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 119                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_regions() {
 120                         return None;
 121                     }
 122                     Some(poly_trait_ref)
 123                 } else {
 124                     None
 125                 }
 126             })
 127             .collect::<Vec<_>>();
 128
 129         // Collect moved variables and spans which will need dereferencings from the
 130         // function body.
 131         let MovedVariablesCtxt {
 132             moved_vars,
 133             spans_need_deref,
 134             ..
 135         } = {
 136             let mut ctx = MovedVariablesCtxt::new(cx);
 137             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 138             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 139                 .consume_body(body);
 140             ctx
 141         };
 142
 143         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 144         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 145
 146         for (idx, ((input, &ty), arg)) in decl.inputs
 147             .iter()
 148             .zip(fn_sig.inputs())
 149             .zip(&body.arguments)
 150             .enumerate()
 151         {
 152             // All spans generated from a proc-macro invocation are the same...
 153             if span == input.span {
 154                 return;
 155             }
 156
 157             // Ignore `self`s.
 158             if idx == 0 {
 159                 if let PatKind::Binding(_, _, name, ..) = arg.pat.node {
 160                     if name.node.as_str() == "self" {
 161                         continue;
 162                     }
 163                 }
 164             }
 165
 166             // * Exclude a type that is specifically bounded by `Borrow`.
 167             // * Exclude a type whose reference also fulfills its bound.
 168             //   (e.g. `std::convert::AsRef`, `serde::Serialize`)
 169             let (implements_borrow_trait, all_borrowable_trait) = {
 170                 let preds = preds
 171                     .iter()
 172                     .filter(|t| t.skip_binder().self_ty() == ty)
 173                     .collect::<Vec<_>>();
 174
 175                 (
 176                     preds.iter().any(|t| t.def_id() == borrow_trait),
 177                     !preds.is_empty() && preds.iter().all(|t| {
 178                         implements_trait(
 179                             cx,
 180                             cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
 181                             t.def_id(),
 182                             &t.skip_binder().input_types().skip(1).collect::<Vec<_>>(),
 183                         )
 184                     }),
 185                 )
 186             };
 187
 188             if_chain! {
 189                 if !is_self(arg);
 190                 if !ty.is_mutable_pointer();
 191                 if !is_copy(cx, ty);
 192                 if !fn_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 193                 if !implements_borrow_trait;
 194                 if !all_borrowable_trait;
 195
 196                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 197                 if !moved_vars.contains(&canonical_id);
 198                 then {
 199                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 200                         continue;
 201                     }
 202
 203                     // Dereference suggestion
 204                     let sugg = |db: &mut DiagnosticBuilder| {
 205                         let deref_span = spans_need_deref.get(&canonical_id);
 206                         if_chain! {
 207                             if match_type(cx, ty, &paths::VEC);
 208                             if let Some(clone_spans) =
 209                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 210                             if let TyPath(QPath::Resolved(_, ref path)) = input.node;
 211                             if let Some(elem_ty) = path.segments.iter()
 212                                 .find(|seg| seg.name == "Vec")
 213                                 .and_then(|ps| ps.parameters.as_ref())
 214                                 .map(|params| &params.types[0]);
 215                             then {
 216                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 217                                 db.span_suggestion(input.span,
 218                                                 "consider changing the type to",
 219                                                 slice_ty);
 220
 221                                 for (span, suggestion) in clone_spans {
 222                                     db.span_suggestion(
 223                                         span,
 224                                         &snippet_opt(cx, span)
 225                                             .map_or(
 226                                                 "change the call to".into(),
 227                                                 |x| Cow::from(format!("change `{}` to", x)),
 228                                             ),
 229                                         suggestion.into()
 230                                     );
 231                                 }
 232
 233                                 // cannot be destructured, no need for `*` suggestion
 234                                 assert!(deref_span.is_none());
 235                                 return;
 236                             }
 237                         }
 238
 239                         if match_type(cx, ty, &paths::STRING) {
 240                             if let Some(clone_spans) =
 241                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 242                                 db.span_suggestion(input.span, "consider changing the type to", "&str".to_string());
 243
 244                                 for (span, suggestion) in clone_spans {
 245                                     db.span_suggestion(
 246                                         span,
 247                                         &snippet_opt(cx, span)
 248                                             .map_or(
 249                                                 "change the call to".into(),
 250                                                 |x| Cow::from(format!("change `{}` to", x))
 251                                             ),
 252                                         suggestion.into(),
 253                                     );
 254                                 }
 255
 256                                 assert!(deref_span.is_none());
 257                                 return;
 258                             }
 259                         }
 260
 261                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 262
 263                         // Suggests adding `*` to dereference the added reference.
 264                         if let Some(deref_span) = deref_span {
 265                             spans.extend(
 266                                 deref_span
 267                                     .iter()
 268                                     .cloned()
 269                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 270                             );
 271                             spans.sort_by_key(|&(span, _)| span);
 272                         }
 273                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 274                     };
 275
 276                     span_lint_and_then(
 277                         cx,
 278                         NEEDLESS_PASS_BY_VALUE,
 279                         input.span,
 280                         "this argument is passed by value, but not consumed in the function body",
 281                         sugg,
 282                     );
 283                 }
 284             }
 285         }
 286     }
 287 }
 288
 289 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 290     cx: &'a LateContext<'a, 'tcx>,
 291     moved_vars: HashSet<NodeId>,
 292     /// Spans which need to be prefixed with `*` for dereferencing the
 293     /// suggested additional reference.
 294     spans_need_deref: HashMap<NodeId, HashSet<Span>>,
 295 }
 296
 297 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 298     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 299         Self {
 300             cx: cx,
 301             moved_vars: HashSet::new(),
 302             spans_need_deref: HashMap::new(),
 303         }
 304     }
 305
 306     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: mc::cmt<'tcx>) {
 307         let cmt = unwrap_downcast_or_interior(cmt);
 308
 309         if let mc::Categorization::Local(vid) = cmt.cat {
 310             self.moved_vars.insert(vid);
 311         }
 312     }
 313
 314     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>) {
 315         let cmt = unwrap_downcast_or_interior(cmt);
 316
 317         if let mc::Categorization::Local(vid) = cmt.cat {
 318             let mut id = matched_pat.id;
 319             loop {
 320                 let parent = self.cx.tcx.hir.get_parent_node(id);
 321                 if id == parent {
 322                     // no parent
 323                     return;
 324                 }
 325                 id = parent;
 326
 327                 if let Some(node) = self.cx.tcx.hir.find(id) {
 328                     match node {
 329                         map::Node::NodeExpr(e) => {
 330                             // `match` and `if let`
 331                             if let ExprMatch(ref c, ..) = e.node {
 332                                 self.spans_need_deref
 333                                     .entry(vid)
 334                                     .or_insert_with(HashSet::new)
 335                                     .insert(c.span);
 336                             }
 337                         },
 338
 339                         map::Node::NodeStmt(s) => {
 340                             // `let <pat> = x;`
 341                             if_chain! {
 342                                 if let StmtDecl(ref decl, _) = s.node;
 343                                 if let DeclLocal(ref local) = decl.node;
 344                                 then {
 345                                     self.spans_need_deref
 346                                         .entry(vid)
 347                                         .or_insert_with(HashSet::new)
 348                                         .insert(local.init
 349                                             .as_ref()
 350                                             .map(|e| e.span)
 351                                             .expect("`let` stmt without init aren't caught by match_pat"));
 352                                 }
 353                             }
 354                         },
 355
 356                         _ => {},
 357                     }
 358                 }
 359             }
 360         }
 361     }
 362 }
 363
 364 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 365     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 366         if let euv::ConsumeMode::Move(_) = mode {
 367             self.move_common(consume_id, consume_span, cmt);
 368         }
 369     }
 370
 371     fn matched_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::MatchMode) {
 372         if let euv::MatchMode::MovingMatch = mode {
 373             self.move_common(matched_pat.id, matched_pat.span, cmt);
 374         } else {
 375             self.non_moving_pat(matched_pat, cmt);
 376         }
 377     }
 378
 379     fn consume_pat(&mut self, consume_pat: &Pat, cmt: mc::cmt<'tcx>, mode: euv::ConsumeMode) {
 380         if let euv::ConsumeMode::Move(_) = mode {
 381             self.move_common(consume_pat.id, consume_pat.span, cmt);
 382         }
 383     }
 384
 385     fn borrow(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: ty::Region, _: ty::BorrowKind, _: euv::LoanCause) {}
 386
 387     fn mutate(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: euv::MutateMode) {}
 388
 389     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 390 }
 391
 392
 393 fn unwrap_downcast_or_interior(mut cmt: mc::cmt) -> mc::cmt {
 394     loop {
 395         match cmt.cat.clone() {
 396             mc::Categorization::Downcast(c, _) | mc::Categorization::Interior(c, _) => {
 397                 cmt = c;
 398             },
 399             _ => return cmt,
 400         }
 401     }
 402 }