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