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::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 pub struct NeedlessPassByValue;
  54
  55 impl LintPass for NeedlessPassByValue {
  56     fn get_lints(&self) -> LintArray {
  57         lint_array![NEEDLESS_PASS_BY_VALUE]
  58     }
  59
  60     fn name(&self) -> &'static str {
  61         "NeedlessPassByValue"
  62     }
  63 }
  64
  65 macro_rules! need {
  66     ($e: expr) => {
  67         if let Some(x) = $e {
  68             x
  69         } else {
  70             return;
  71         }
  72     };
  73 }
  74
  75 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  76     #[allow(clippy::too_many_lines)]
  77     fn check_fn(
  78         &mut self,
  79         cx: &LateContext<'a, 'tcx>,
  80         kind: FnKind<'tcx>,
  81         decl: &'tcx FnDecl,
  82         body: &'tcx Body,
  83         span: Span,
  84         hir_id: HirId,
  85     ) {
  86         if in_macro(span) {
  87             return;
  88         }
  89
  90         match kind {
  91             FnKind::ItemFn(.., header, _, attrs) => {
  92                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  93                     return;
  94                 }
  95             },
  96             FnKind::Method(..) => (),
  97             _ => return,
  98         }
  99
 100         // Exclude non-inherent impls
 101         if let Some(Node::Item(item)) = cx
 102             .tcx
 103             .hir()
 104             .find_by_hir_id(cx.tcx.hir().get_parent_node_by_hir_id(hir_id))
 105         {
 106             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
 107                 ItemKind::Trait(..))
 108             {
 109                 return;
 110             }
 111         }
 112
 113         // Allow `Borrow` or functions to be taken by value
 114         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 115         let whitelisted_traits = [
 116             need!(cx.tcx.lang_items().fn_trait()),
 117             need!(cx.tcx.lang_items().fn_once_trait()),
 118             need!(cx.tcx.lang_items().fn_mut_trait()),
 119             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 120         ];
 121
 122         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 123
 124         let fn_def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
 125
 126         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 127             .filter(|p| !p.is_global())
 128             .filter_map(|pred| {
 129                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 130                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
 131                     {
 132                         return None;
 133                     }
 134                     Some(poly_trait_ref)
 135                 } else {
 136                     None
 137                 }
 138             })
 139             .collect::<Vec<_>>();
 140
 141         // Collect moved variables and spans which will need dereferencings from the
 142         // function body.
 143         let MovedVariablesCtxt {
 144             moved_vars,
 145             spans_need_deref,
 146             ..
 147         } = {
 148             let mut ctx = MovedVariablesCtxt::new(cx);
 149             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 150             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 151                 .consume_body(body);
 152             ctx
 153         };
 154
 155         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 156         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 157
 158         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments).enumerate() {
 159             // All spans generated from a proc-macro invocation are the same...
 160             if span == input.span {
 161                 return;
 162             }
 163
 164             // Ignore `self`s.
 165             if idx == 0 {
 166                 if let PatKind::Binding(.., ident, _) = arg.pat.node {
 167                     if ident.as_str() == "self" {
 168                         continue;
 169                     }
 170                 }
 171             }
 172
 173             //
 174             // * Exclude a type that is specifically bounded by `Borrow`.
 175             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 176             //   `serde::Serialize`)
 177             let (implements_borrow_trait, all_borrowable_trait) = {
 178                 let preds = preds
 179                     .iter()
 180                     .filter(|t| t.skip_binder().self_ty() == ty)
 181                     .collect::<Vec<_>>();
 182
 183                 (
 184                     preds.iter().any(|t| t.def_id() == borrow_trait),
 185                     !preds.is_empty()
 186                         && preds.iter().all(|t| {
 187                             let ty_params = &t
 188                                 .skip_binder()
 189                                 .trait_ref
 190                                 .substs
 191                                 .iter()
 192                                 .skip(1)
 193                                 .cloned()
 194                                 .collect::<Vec<_>>();
 195                             implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReEmpty, ty), t.def_id(), ty_params)
 196                         }),
 197                 )
 198             };
 199
 200             if_chain! {
 201                 if !is_self(arg);
 202                 if !ty.is_mutable_pointer();
 203                 if !is_copy(cx, ty);
 204                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 205                 if !implements_borrow_trait;
 206                 if !all_borrowable_trait;
 207
 208                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 209                 if !moved_vars.contains(&canonical_id);
 210                 then {
 211                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 212                         continue;
 213                     }
 214
 215                     // Dereference suggestion
 216                     let sugg = |db: &mut DiagnosticBuilder<'_>| {
 217                         if let ty::Adt(def, ..) = ty.sty {
 218                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 219                                 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
 220                                     db.span_help(span, "consider marking this type as Copy");
 221                                 }
 222                             }
 223                         }
 224
 225                         let deref_span = spans_need_deref.get(&canonical_id);
 226                         if_chain! {
 227                             if match_type(cx, ty, &paths::VEC);
 228                             if let Some(clone_spans) =
 229                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 230                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
 231                             if let Some(elem_ty) = path.segments.iter()
 232                                 .find(|seg| seg.ident.name == "Vec")
 233                                 .and_then(|ps| ps.args.as_ref())
 234                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 235                                     GenericArg::Type(ty) => Some(ty),
 236                                     _ => None,
 237                                 }).unwrap());
 238                             then {
 239                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 240                                 db.span_suggestion(
 241                                     input.span,
 242                                     "consider changing the type to",
 243                                     slice_ty,
 244                                     Applicability::Unspecified,
 245                                 );
 246
 247                                 for (span, suggestion) in clone_spans {
 248                                     db.span_suggestion(
 249                                         span,
 250                                         &snippet_opt(cx, span)
 251                                             .map_or(
 252                                                 "change the call to".into(),
 253                                                 |x| Cow::from(format!("change `{}` to", x)),
 254                                             ),
 255                                         suggestion.into(),
 256                                         Applicability::Unspecified,
 257                                     );
 258                                 }
 259
 260                                 // cannot be destructured, no need for `*` suggestion
 261                                 assert!(deref_span.is_none());
 262                                 return;
 263                             }
 264                         }
 265
 266                         if match_type(cx, ty, &paths::STRING) {
 267                             if let Some(clone_spans) =
 268                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 269                                 db.span_suggestion(
 270                                     input.span,
 271                                     "consider changing the type to",
 272                                     "&str".to_string(),
 273                                     Applicability::Unspecified,
 274                                 );
 275
 276                                 for (span, suggestion) in clone_spans {
 277                                     db.span_suggestion(
 278                                         span,
 279                                         &snippet_opt(cx, span)
 280                                             .map_or(
 281                                                 "change the call to".into(),
 282                                                 |x| Cow::from(format!("change `{}` to", x))
 283                                             ),
 284                                         suggestion.into(),
 285                                         Applicability::Unspecified,
 286                                     );
 287                                 }
 288
 289                                 assert!(deref_span.is_none());
 290                                 return;
 291                             }
 292                         }
 293
 294                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 295
 296                         // Suggests adding `*` to dereference the added reference.
 297                         if let Some(deref_span) = deref_span {
 298                             spans.extend(
 299                                 deref_span
 300                                     .iter()
 301                                     .cloned()
 302                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 303                             );
 304                             spans.sort_by_key(|&(span, _)| span);
 305                         }
 306                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 307                     };
 308
 309                     span_lint_and_then(
 310                         cx,
 311                         NEEDLESS_PASS_BY_VALUE,
 312                         input.span,
 313                         "this argument is passed by value, but not consumed in the function body",
 314                         sugg,
 315                     );
 316                 }
 317             }
 318         }
 319     }
 320 }
 321
 322 /// Functions marked with these attributes must have the exact signature.
 323 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 324     attrs.iter().any(|attr| {
 325         ["proc_macro", "proc_macro_attribute", "proc_macro_derive"]
 326             .iter()
 327             .any(|&allow| attr.check_name(allow))
 328     })
 329 }
 330
 331 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 332     cx: &'a LateContext<'a, 'tcx>,
 333     moved_vars: FxHashSet<HirId>,
 334     /// Spans which need to be prefixed with `*` for dereferencing the
 335     /// suggested additional reference.
 336     spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
 337 }
 338
 339 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 340     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 341         Self {
 342             cx,
 343             moved_vars: FxHashSet::default(),
 344             spans_need_deref: FxHashMap::default(),
 345         }
 346     }
 347
 348     fn move_common(&mut self, _consume_id: HirId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 349         let cmt = unwrap_downcast_or_interior(cmt);
 350
 351         if let mc::Categorization::Local(vid) = cmt.cat {
 352             self.moved_vars.insert(vid);
 353         }
 354     }
 355
 356     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 357         let cmt = unwrap_downcast_or_interior(cmt);
 358
 359         if let mc::Categorization::Local(vid) = cmt.cat {
 360             let mut id = matched_pat.hir_id;
 361             loop {
 362                 let parent = self.cx.tcx.hir().get_parent_node_by_hir_id(id);
 363                 if id == parent {
 364                     // no parent
 365                     return;
 366                 }
 367                 id = parent;
 368
 369                 if let Some(node) = self.cx.tcx.hir().find_by_hir_id(id) {
 370                     match node {
 371                         Node::Expr(e) => {
 372                             // `match` and `if let`
 373                             if let ExprKind::Match(ref c, ..) = e.node {
 374                                 self.spans_need_deref
 375                                     .entry(vid)
 376                                     .or_insert_with(FxHashSet::default)
 377                                     .insert(c.span);
 378                             }
 379                         },
 380
 381                         Node::Stmt(s) => {
 382                             // `let <pat> = x;`
 383                             if_chain! {
 384                                 if let StmtKind::Local(ref local) = s.node;
 385                                 then {
 386                                     self.spans_need_deref
 387                                         .entry(vid)
 388                                         .or_insert_with(FxHashSet::default)
 389                                         .insert(local.init
 390                                             .as_ref()
 391                                             .map(|e| e.span)
 392                                             .expect("`let` stmt without init aren't caught by match_pat"));
 393                                 }
 394                             }
 395                         },
 396
 397                         _ => {},
 398                     }
 399                 }
 400             }
 401         }
 402     }
 403 }
 404
 405 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 406     fn consume(&mut self, consume_id: HirId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 407         if let euv::ConsumeMode::Move(_) = mode {
 408             self.move_common(consume_id, consume_span, cmt);
 409         }
 410     }
 411
 412     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 413         if let euv::MatchMode::MovingMatch = mode {
 414             self.move_common(matched_pat.hir_id, matched_pat.span, cmt);
 415         } else {
 416             self.non_moving_pat(matched_pat, cmt);
 417         }
 418     }
 419
 420     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 421         if let euv::ConsumeMode::Move(_) = mode {
 422             self.move_common(consume_pat.hir_id, consume_pat.span, cmt);
 423         }
 424     }
 425
 426     fn borrow(
 427         &mut self,
 428         _: HirId,
 429         _: Span,
 430         _: &mc::cmt_<'tcx>,
 431         _: ty::Region<'_>,
 432         _: ty::BorrowKind,
 433         _: euv::LoanCause,
 434     ) {
 435     }
 436
 437     fn mutate(&mut self, _: HirId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 438
 439     fn decl_without_init(&mut self, _: HirId, _: Span) {}
 440 }
 441
 442 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 443     loop {
 444         match cmt.cat {
 445             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 446                 cmt = c;
 447             },
 448             _ => return (*cmt).clone(),
 449         }
 450     }
 451 }