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