clippy_lints/src/needless_pass_by_value.rs

   1 use crate::utils::ptr::get_spans;
   2 use crate::utils::{
   3     get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths, snippet,
   4     snippet_opt, span_lint_and_then,
   5 };
   6 use if_chain::if_chain;
   7 use matches::matches;
   8 use rustc::hir::intravisit::FnKind;
   9 use rustc::hir::*;
  10 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  11 use rustc::middle::expr_use_visitor as euv;
  12 use rustc::middle::mem_categorization as mc;
  13 use rustc::traits;
  14 use rustc::ty::{self, RegionKind, TypeFoldable};
  15 use rustc::{declare_tool_lint, lint_array};
  16 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  17 use rustc_errors::Applicability;
  18 use rustc_target::spec::abi::Abi;
  19 use std::borrow::Cow;
  20 use syntax::errors::DiagnosticBuilder;
  21 use syntax_pos::Span;
  22
  23 declare_clippy_lint! {
  24     /// **What it does:** Checks for functions taking arguments by value, but not
  25     /// consuming them in its
  26     /// body.
  27     ///
  28     /// **Why is this bad?** Taking arguments by reference is more flexible and can
  29     /// sometimes avoid
  30     /// unnecessary allocations.
  31     ///
  32     /// **Known problems:**
  33     /// * This lint suggests taking an argument by reference,
  34     /// however sometimes it is better to let users decide the argument type
  35     /// (by using `Borrow` trait, for example), depending on how the function is used.
  36     ///
  37     /// **Example:**
  38     /// ```rust
  39     /// fn foo(v: Vec<i32>) {
  40     ///     assert_eq!(v.len(), 42);
  41     /// }
  42     /// // should be
  43     /// fn foo(v: &[i32]) {
  44     ///     assert_eq!(v.len(), 42);
  45     /// }
  46     /// ```
  47     pub NEEDLESS_PASS_BY_VALUE,
  48     pedantic,
  49     "functions taking arguments by value, but not consuming them in its body"
  50 }
  51
  52 pub struct NeedlessPassByValue;
  53
  54 impl LintPass for NeedlessPassByValue {
  55     fn get_lints(&self) -> LintArray {
  56         lint_array![NEEDLESS_PASS_BY_VALUE]
  57     }
  58
  59     fn name(&self) -> &'static str {
  60         "NeedlessPassByValue"
  61     }
  62 }
  63
  64 macro_rules! need {
  65     ($e: expr) => {
  66         if let Some(x) = $e {
  67             x
  68         } else {
  69             return;
  70         }
  71     };
  72 }
  73
  74 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  75     #[allow(clippy::too_many_lines)]
  76     fn check_fn(
  77         &mut self,
  78         cx: &LateContext<'a, 'tcx>,
  79         kind: FnKind<'tcx>,
  80         decl: &'tcx FnDecl,
  81         body: &'tcx Body,
  82         span: Span,
  83         hir_id: HirId,
  84     ) {
  85         if in_macro(span) {
  86             return;
  87         }
  88
  89         match kind {
  90             FnKind::ItemFn(.., header, _, attrs) => {
  91                 if header.abi != Abi::Rust {
  92                     return;
  93                 }
  94                 for a in attrs {
  95                     if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
  96                         return;
  97                     }
  98                 }
  99             },
 100             FnKind::Method(..) => (),
 101             _ => return,
 102         }
 103
 104         // Exclude non-inherent impls
 105         if let Some(Node::Item(item)) = cx
 106             .tcx
 107             .hir()
 108             .find_by_hir_id(cx.tcx.hir().get_parent_node_by_hir_id(hir_id))
 109         {
 110             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
 111                 ItemKind::Trait(..))
 112             {
 113                 return;
 114             }
 115         }
 116
 117         // Allow `Borrow` or functions to be taken by value
 118         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 119         let whitelisted_traits = [
 120             need!(cx.tcx.lang_items().fn_trait()),
 121             need!(cx.tcx.lang_items().fn_once_trait()),
 122             need!(cx.tcx.lang_items().fn_mut_trait()),
 123             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 124         ];
 125
 126         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 127
 128         let fn_def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
 129
 130         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 131             .filter(|p| !p.is_global())
 132             .filter_map(|pred| {
 133                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 134                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
 135                     {
 136                         return None;
 137                     }
 138                     Some(poly_trait_ref)
 139                 } else {
 140                     None
 141                 }
 142             })
 143             .collect::<Vec<_>>();
 144
 145         // Collect moved variables and spans which will need dereferencings from the
 146         // function body.
 147         let MovedVariablesCtxt {
 148             moved_vars,
 149             spans_need_deref,
 150             ..
 151         } = {
 152             let mut ctx = MovedVariablesCtxt::new(cx);
 153             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 154             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 155                 .consume_body(body);
 156             ctx
 157         };
 158
 159         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 160         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 161
 162         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments).enumerate() {
 163             // All spans generated from a proc-macro invocation are the same...
 164             if span == input.span {
 165                 return;
 166             }
 167
 168             // Ignore `self`s.
 169             if idx == 0 {
 170                 if let PatKind::Binding(.., ident, _) = arg.pat.node {
 171                     if ident.as_str() == "self" {
 172                         continue;
 173                     }
 174                 }
 175             }
 176
 177             //
 178             // * Exclude a type that is specifically bounded by `Borrow`.
 179             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 180             //   `serde::Serialize`)
 181             let (implements_borrow_trait, all_borrowable_trait) = {
 182                 let preds = preds
 183                     .iter()
 184                     .filter(|t| t.skip_binder().self_ty() == ty)
 185                     .collect::<Vec<_>>();
 186
 187                 (
 188                     preds.iter().any(|t| t.def_id() == borrow_trait),
 189                     !preds.is_empty()
 190                         && preds.iter().all(|t| {
 191                             let ty_params = &t
 192                                 .skip_binder()
 193                                 .trait_ref
 194                                 .substs
 195                                 .iter()
 196                                 .skip(1)
 197                                 .cloned()
 198                                 .collect::<Vec<_>>();
 199                             implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
 200                         }),
 201                 )
 202             };
 203
 204             if_chain! {
 205                 if !is_self(arg);
 206                 if !ty.is_mutable_pointer();
 207                 if !is_copy(cx, ty);
 208                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 209                 if !implements_borrow_trait;
 210                 if !all_borrowable_trait;
 211
 212                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 213                 if !moved_vars.contains(&canonical_id);
 214                 then {
 215                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 216                         continue;
 217                     }
 218
 219                     // Dereference suggestion
 220                     let sugg = |db: &mut DiagnosticBuilder<'_>| {
 221                         if let ty::Adt(def, ..) = ty.sty {
 222                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 223                                 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
 224                                     db.span_help(span, "consider marking this type as Copy");
 225                                 }
 226                             }
 227                         }
 228
 229                         let deref_span = spans_need_deref.get(&canonical_id);
 230                         if_chain! {
 231                             if match_type(cx, ty, &paths::VEC);
 232                             if let Some(clone_spans) =
 233                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 234                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
 235                             if let Some(elem_ty) = path.segments.iter()
 236                                 .find(|seg| seg.ident.name == "Vec")
 237                                 .and_then(|ps| ps.args.as_ref())
 238                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 239                                     GenericArg::Type(ty) => Some(ty),
 240                                     _ => None,
 241                                 }).unwrap());
 242                             then {
 243                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 244                                 db.span_suggestion(
 245                                     input.span,
 246                                     "consider changing the type to",
 247                                     slice_ty,
 248                                     Applicability::Unspecified,
 249                                 );
 250
 251                                 for (span, suggestion) in clone_spans {
 252                                     db.span_suggestion(
 253                                         span,
 254                                         &snippet_opt(cx, span)
 255                                             .map_or(
 256                                                 "change the call to".into(),
 257                                                 |x| Cow::from(format!("change `{}` to", x)),
 258                                             ),
 259                                         suggestion.into(),
 260                                         Applicability::Unspecified,
 261                                     );
 262                                 }
 263
 264                                 // cannot be destructured, no need for `*` suggestion
 265                                 assert!(deref_span.is_none());
 266                                 return;
 267                             }
 268                         }
 269
 270                         if match_type(cx, ty, &paths::STRING) {
 271                             if let Some(clone_spans) =
 272                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 273                                 db.span_suggestion(
 274                                     input.span,
 275                                     "consider changing the type to",
 276                                     "&str".to_string(),
 277                                     Applicability::Unspecified,
 278                                 );
 279
 280                                 for (span, suggestion) in clone_spans {
 281                                     db.span_suggestion(
 282                                         span,
 283                                         &snippet_opt(cx, span)
 284                                             .map_or(
 285                                                 "change the call to".into(),
 286                                                 |x| Cow::from(format!("change `{}` to", x))
 287                                             ),
 288                                         suggestion.into(),
 289                                         Applicability::Unspecified,
 290                                     );
 291                                 }
 292
 293                                 assert!(deref_span.is_none());
 294                                 return;
 295                             }
 296                         }
 297
 298                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 299
 300                         // Suggests adding `*` to dereference the added reference.
 301                         if let Some(deref_span) = deref_span {
 302                             spans.extend(
 303                                 deref_span
 304                                     .iter()
 305                                     .cloned()
 306                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 307                             );
 308                             spans.sort_by_key(|&(span, _)| span);
 309                         }
 310                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 311                     };
 312
 313                     span_lint_and_then(
 314                         cx,
 315                         NEEDLESS_PASS_BY_VALUE,
 316                         input.span,
 317                         "this argument is passed by value, but not consumed in the function body",
 318                         sugg,
 319                     );
 320                 }
 321             }
 322         }
 323     }
 324 }
 325
 326 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 327     cx: &'a LateContext<'a, 'tcx>,
 328     moved_vars: FxHashSet<HirId>,
 329     /// Spans which need to be prefixed with `*` for dereferencing the
 330     /// suggested additional reference.
 331     spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
 332 }
 333
 334 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 335     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 336         Self {
 337             cx,
 338             moved_vars: FxHashSet::default(),
 339             spans_need_deref: FxHashMap::default(),
 340         }
 341     }
 342
 343     fn move_common(&mut self, _consume_id: HirId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 344         let cmt = unwrap_downcast_or_interior(cmt);
 345
 346         if let mc::Categorization::Local(vid) = cmt.cat {
 347             self.moved_vars.insert(vid);
 348         }
 349     }
 350
 351     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 352         let cmt = unwrap_downcast_or_interior(cmt);
 353
 354         if let mc::Categorization::Local(vid) = cmt.cat {
 355             let mut id = matched_pat.hir_id;
 356             loop {
 357                 let parent = self.cx.tcx.hir().get_parent_node_by_hir_id(id);
 358                 if id == parent {
 359                     // no parent
 360                     return;
 361                 }
 362                 id = parent;
 363
 364                 if let Some(node) = self.cx.tcx.hir().find_by_hir_id(id) {
 365                     match node {
 366                         Node::Expr(e) => {
 367                             // `match` and `if let`
 368                             if let ExprKind::Match(ref c, ..) = e.node {
 369                                 self.spans_need_deref
 370                                     .entry(vid)
 371                                     .or_insert_with(FxHashSet::default)
 372                                     .insert(c.span);
 373                             }
 374                         },
 375
 376                         Node::Stmt(s) => {
 377                             // `let <pat> = x;`
 378                             if_chain! {
 379                                 if let StmtKind::Local(ref local) = s.node;
 380                                 then {
 381                                     self.spans_need_deref
 382                                         .entry(vid)
 383                                         .or_insert_with(FxHashSet::default)
 384                                         .insert(local.init
 385                                             .as_ref()
 386                                             .map(|e| e.span)
 387                                             .expect("`let` stmt without init aren't caught by match_pat"));
 388                                 }
 389                             }
 390                         },
 391
 392                         _ => {},
 393                     }
 394                 }
 395             }
 396         }
 397     }
 398 }
 399
 400 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 401     fn consume(&mut self, consume_id: HirId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 402         if let euv::ConsumeMode::Move(_) = mode {
 403             self.move_common(consume_id, consume_span, cmt);
 404         }
 405     }
 406
 407     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 408         if let euv::MatchMode::MovingMatch = mode {
 409             self.move_common(matched_pat.hir_id, matched_pat.span, cmt);
 410         } else {
 411             self.non_moving_pat(matched_pat, cmt);
 412         }
 413     }
 414
 415     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 416         if let euv::ConsumeMode::Move(_) = mode {
 417             self.move_common(consume_pat.hir_id, consume_pat.span, cmt);
 418         }
 419     }
 420
 421     fn borrow(
 422         &mut self,
 423         _: HirId,
 424         _: Span,
 425         _: &mc::cmt_<'tcx>,
 426         _: ty::Region<'_>,
 427         _: ty::BorrowKind,
 428         _: euv::LoanCause,
 429     ) {
 430     }
 431
 432     fn mutate(&mut self, _: HirId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 433
 434     fn decl_without_init(&mut self, _: HirId, _: Span) {}
 435 }
 436
 437 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 438     loop {
 439         match cmt.cat {
 440             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 441                 cmt = c;
 442             },
 443             _ => return (*cmt).clone(),
 444         }
 445     }
 446 }