clippy_lints/src/needless_pass_by_value.rs

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