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_or_desugar, is_copy, is_self, match_type, multispan_sugg, paths,
   4     snippet, 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_lint_pass, declare_tool_lint};
  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::ast::Attribute;
  21 use syntax::errors::DiagnosticBuilder;
  22 use syntax_pos::Span;
  23
  24 declare_clippy_lint! {
  25     /// **What it does:** Checks for functions taking arguments by value, but not
  26     /// consuming them in its
  27     /// body.
  28     ///
  29     /// **Why is this bad?** Taking arguments by reference is more flexible and can
  30     /// sometimes avoid
  31     /// unnecessary allocations.
  32     ///
  33     /// **Known problems:**
  34     /// * This lint suggests taking an argument by reference,
  35     /// however sometimes it is better to let users decide the argument type
  36     /// (by using `Borrow` trait, for example), depending on how the function is used.
  37     ///
  38     /// **Example:**
  39     /// ```rust
  40     /// fn foo(v: Vec<i32>) {
  41     ///     assert_eq!(v.len(), 42);
  42     /// }
  43     /// // should be
  44     /// fn foo(v: &[i32]) {
  45     ///     assert_eq!(v.len(), 42);
  46     /// }
  47     /// ```
  48     pub NEEDLESS_PASS_BY_VALUE,
  49     pedantic,
  50     "functions taking arguments by value, but not consuming them in its body"
  51 }
  52
  53 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  54
  55 macro_rules! need {
  56     ($e: expr) => {
  57         if let Some(x) = $e {
  58             x
  59         } else {
  60             return;
  61         }
  62     };
  63 }
  64
  65 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  66     #[allow(clippy::too_many_lines)]
  67     fn check_fn(
  68         &mut self,
  69         cx: &LateContext<'a, 'tcx>,
  70         kind: FnKind<'tcx>,
  71         decl: &'tcx FnDecl,
  72         body: &'tcx Body,
  73         span: Span,
  74         hir_id: HirId,
  75     ) {
  76         if in_macro_or_desugar(span) {
  77             return;
  78         }
  79
  80         match kind {
  81             FnKind::ItemFn(.., header, _, attrs) => {
  82                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  83                     return;
  84                 }
  85             },
  86             FnKind::Method(..) => (),
  87             _ => return,
  88         }
  89
  90         // Exclude non-inherent impls
  91         if let Some(Node::Item(item)) = cx
  92             .tcx
  93             .hir()
  94             .find_by_hir_id(cx.tcx.hir().get_parent_node_by_hir_id(hir_id))
  95         {
  96             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
  97                 ItemKind::Trait(..))
  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 whitelisted_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             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 110         ];
 111
 112         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 113
 114         let fn_def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
 115
 116         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 117             .filter(|p| !p.is_global())
 118             .filter_map(|pred| {
 119                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 120                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
 121                     {
 122                         return None;
 123                     }
 124                     Some(poly_trait_ref)
 125                 } else {
 126                     None
 127                 }
 128             })
 129             .collect::<Vec<_>>();
 130
 131         // Collect moved variables and spans which will need dereferencings from the
 132         // function body.
 133         let MovedVariablesCtxt {
 134             moved_vars,
 135             spans_need_deref,
 136             ..
 137         } = {
 138             let mut ctx = MovedVariablesCtxt::new(cx);
 139             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 140             euv::ExprUseVisitor::new(
 141                 &mut ctx,
 142                 cx.tcx,
 143                 fn_def_id,
 144                 cx.param_env,
 145                 region_scope_tree,
 146                 cx.tables,
 147                 None,
 148             )
 149             .consume_body(body);
 150             ctx
 151         };
 152
 153         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 154         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 155
 156         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments).enumerate() {
 157             // All spans generated from a proc-macro invocation are the same...
 158             if span == input.span {
 159                 return;
 160             }
 161
 162             // Ignore `self`s.
 163             if idx == 0 {
 164                 if let PatKind::Binding(.., ident, _) = arg.pat.node {
 165                     if ident.as_str() == "self" {
 166                         continue;
 167                     }
 168                 }
 169             }
 170
 171             //
 172             // * Exclude a type that is specifically bounded by `Borrow`.
 173             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 174             //   `serde::Serialize`)
 175             let (implements_borrow_trait, all_borrowable_trait) = {
 176                 let preds = preds
 177                     .iter()
 178                     .filter(|t| t.skip_binder().self_ty() == ty)
 179                     .collect::<Vec<_>>();
 180
 181                 (
 182                     preds.iter().any(|t| t.def_id() == borrow_trait),
 183                     !preds.is_empty()
 184                         && preds.iter().all(|t| {
 185                             let ty_params = &t
 186                                 .skip_binder()
 187                                 .trait_ref
 188                                 .substs
 189                                 .iter()
 190                                 .skip(1)
 191                                 .cloned()
 192                                 .collect::<Vec<_>>();
 193                             implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
 194                         }),
 195                 )
 196             };
 197
 198             if_chain! {
 199                 if !is_self(arg);
 200                 if !ty.is_mutable_pointer();
 201                 if !is_copy(cx, ty);
 202                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 203                 if !implements_borrow_trait;
 204                 if !all_borrowable_trait;
 205
 206                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 207                 if !moved_vars.contains(&canonical_id);
 208                 then {
 209                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 210                         continue;
 211                     }
 212
 213                     // Dereference suggestion
 214                     let sugg = |db: &mut DiagnosticBuilder<'_>| {
 215                         if let ty::Adt(def, ..) = ty.sty {
 216                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 217                                 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
 218                                     db.span_help(span, "consider marking this type as Copy");
 219                                 }
 220                             }
 221                         }
 222
 223                         let deref_span = spans_need_deref.get(&canonical_id);
 224                         if_chain! {
 225                             if match_type(cx, ty, &paths::VEC);
 226                             if let Some(clone_spans) =
 227                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 228                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
 229                             if let Some(elem_ty) = path.segments.iter()
 230                                 .find(|seg| seg.ident.name == sym!(Vec))
 231                                 .and_then(|ps| ps.args.as_ref())
 232                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 233                                     GenericArg::Type(ty) => Some(ty),
 234                                     _ => None,
 235                                 }).unwrap());
 236                             then {
 237                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 238                                 db.span_suggestion(
 239                                     input.span,
 240                                     "consider changing the type to",
 241                                     slice_ty,
 242                                     Applicability::Unspecified,
 243                                 );
 244
 245                                 for (span, suggestion) in clone_spans {
 246                                     db.span_suggestion(
 247                                         span,
 248                                         &snippet_opt(cx, span)
 249                                             .map_or(
 250                                                 "change the call to".into(),
 251                                                 |x| Cow::from(format!("change `{}` to", x)),
 252                                             ),
 253                                         suggestion.into(),
 254                                         Applicability::Unspecified,
 255                                     );
 256                                 }
 257
 258                                 // cannot be destructured, no need for `*` suggestion
 259                                 assert!(deref_span.is_none());
 260                                 return;
 261                             }
 262                         }
 263
 264                         if match_type(cx, ty, &paths::STRING) {
 265                             if let Some(clone_spans) =
 266                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 267                                 db.span_suggestion(
 268                                     input.span,
 269                                     "consider changing the type to",
 270                                     "&str".to_string(),
 271                                     Applicability::Unspecified,
 272                                 );
 273
 274                                 for (span, suggestion) in clone_spans {
 275                                     db.span_suggestion(
 276                                         span,
 277                                         &snippet_opt(cx, span)
 278                                             .map_or(
 279                                                 "change the call to".into(),
 280                                                 |x| Cow::from(format!("change `{}` to", x))
 281                                             ),
 282                                         suggestion.into(),
 283                                         Applicability::Unspecified,
 284                                     );
 285                                 }
 286
 287                                 assert!(deref_span.is_none());
 288                                 return;
 289                             }
 290                         }
 291
 292                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 293
 294                         // Suggests adding `*` to dereference the added reference.
 295                         if let Some(deref_span) = deref_span {
 296                             spans.extend(
 297                                 deref_span
 298                                     .iter()
 299                                     .cloned()
 300                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 301                             );
 302                             spans.sort_by_key(|&(span, _)| span);
 303                         }
 304                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 305                     };
 306
 307                     span_lint_and_then(
 308                         cx,
 309                         NEEDLESS_PASS_BY_VALUE,
 310                         input.span,
 311                         "this argument is passed by value, but not consumed in the function body",
 312                         sugg,
 313                     );
 314                 }
 315             }
 316         }
 317     }
 318 }
 319
 320 /// Functions marked with these attributes must have the exact signature.
 321 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 322     attrs.iter().any(|attr| {
 323         [sym!(proc_macro), sym!(proc_macro_attribute), sym!(proc_macro_derive)]
 324             .iter()
 325             .any(|&allow| attr.check_name(allow))
 326     })
 327 }
 328
 329 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 330     cx: &'a LateContext<'a, 'tcx>,
 331     moved_vars: FxHashSet<HirId>,
 332     /// Spans which need to be prefixed with `*` for dereferencing the
 333     /// suggested additional reference.
 334     spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
 335 }
 336
 337 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 338     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 339         Self {
 340             cx,
 341             moved_vars: FxHashSet::default(),
 342             spans_need_deref: FxHashMap::default(),
 343         }
 344     }
 345
 346     fn move_common(&mut self, _consume_id: HirId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 347         let cmt = unwrap_downcast_or_interior(cmt);
 348
 349         if let mc::Categorization::Local(vid) = cmt.cat {
 350             self.moved_vars.insert(vid);
 351         }
 352     }
 353
 354     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 355         let cmt = unwrap_downcast_or_interior(cmt);
 356
 357         if let mc::Categorization::Local(vid) = cmt.cat {
 358             let mut id = matched_pat.hir_id;
 359             loop {
 360                 let parent = self.cx.tcx.hir().get_parent_node_by_hir_id(id);
 361                 if id == parent {
 362                     // no parent
 363                     return;
 364                 }
 365                 id = parent;
 366
 367                 if let Some(node) = self.cx.tcx.hir().find_by_hir_id(id) {
 368                     match node {
 369                         Node::Expr(e) => {
 370                             // `match` and `if let`
 371                             if let ExprKind::Match(ref c, ..) = e.node {
 372                                 self.spans_need_deref
 373                                     .entry(vid)
 374                                     .or_insert_with(FxHashSet::default)
 375                                     .insert(c.span);
 376                             }
 377                         },
 378
 379                         Node::Stmt(s) => {
 380                             // `let <pat> = x;`
 381                             if_chain! {
 382                                 if let StmtKind::Local(ref local) = s.node;
 383                                 then {
 384                                     self.spans_need_deref
 385                                         .entry(vid)
 386                                         .or_insert_with(FxHashSet::default)
 387                                         .insert(local.init
 388                                             .as_ref()
 389                                             .map(|e| e.span)
 390                                             .expect("`let` stmt without init aren't caught by match_pat"));
 391                                 }
 392                             }
 393                         },
 394
 395                         _ => {},
 396                     }
 397                 }
 398             }
 399         }
 400     }
 401 }
 402
 403 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 404     fn consume(&mut self, consume_id: HirId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 405         if let euv::ConsumeMode::Move(_) = mode {
 406             self.move_common(consume_id, consume_span, cmt);
 407         }
 408     }
 409
 410     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 411         if let euv::MatchMode::MovingMatch = mode {
 412             self.move_common(matched_pat.hir_id, matched_pat.span, cmt);
 413         } else {
 414             self.non_moving_pat(matched_pat, cmt);
 415         }
 416     }
 417
 418     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 419         if let euv::ConsumeMode::Move(_) = mode {
 420             self.move_common(consume_pat.hir_id, consume_pat.span, cmt);
 421         }
 422     }
 423
 424     fn borrow(
 425         &mut self,
 426         _: HirId,
 427         _: Span,
 428         _: &mc::cmt_<'tcx>,
 429         _: ty::Region<'_>,
 430         _: ty::BorrowKind,
 431         _: euv::LoanCause,
 432     ) {
 433     }
 434
 435     fn mutate(&mut self, _: HirId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 436
 437     fn decl_without_init(&mut self, _: HirId, _: Span) {}
 438 }
 439
 440 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 441     loop {
 442         match cmt.cat {
 443             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 444                 cmt = c;
 445             },
 446             _ => return (*cmt).clone(),
 447         }
 448     }
 449 }