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
  61 macro_rules! need {
  62     ($e: expr) => {
  63         if let Some(x) = $e {
  64             x
  65         } else {
  66             return;
  67         }
  68     };
  69 }
  70
  71 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  72     fn check_fn(
  73         &mut self,
  74         cx: &LateContext<'a, 'tcx>,
  75         kind: FnKind<'tcx>,
  76         decl: &'tcx FnDecl,
  77         body: &'tcx Body,
  78         span: Span,
  79         node_id: NodeId,
  80     ) {
  81         if in_macro(span) {
  82             return;
  83         }
  84
  85         match kind {
  86             FnKind::ItemFn(.., header, _, attrs) => {
  87                 if header.abi != Abi::Rust {
  88                     return;
  89                 }
  90                 for a in attrs {
  91                     if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
  92                         return;
  93                     }
  94                 }
  95             },
  96             FnKind::Method(..) => (),
  97             _ => return,
  98         }
  99
 100         // Exclude non-inherent impls
 101         if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(node_id)) {
 102             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
 103                 ItemKind::Trait(..))
 104             {
 105                 return;
 106             }
 107         }
 108
 109         // Allow `Borrow` or functions to be taken by value
 110         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 111         let whitelisted_traits = [
 112             need!(cx.tcx.lang_items().fn_trait()),
 113             need!(cx.tcx.lang_items().fn_once_trait()),
 114             need!(cx.tcx.lang_items().fn_mut_trait()),
 115             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 116         ];
 117
 118         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 119
 120         let fn_def_id = cx.tcx.hir().local_def_id(node_id);
 121
 122         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 123             .filter(|p| !p.is_global())
 124             .filter_map(|pred| {
 125                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 126                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
 127                     {
 128                         return None;
 129                     }
 130                     Some(poly_trait_ref)
 131                 } else {
 132                     None
 133                 }
 134             })
 135             .collect::<Vec<_>>();
 136
 137         // Collect moved variables and spans which will need dereferencings from the
 138         // function body.
 139         let MovedVariablesCtxt {
 140             moved_vars,
 141             spans_need_deref,
 142             ..
 143         } = {
 144             let mut ctx = MovedVariablesCtxt::new(cx);
 145             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 146             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 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.arguments).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.node {
 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::ReErased, ty), t.def_id(), ty_params)
 192                         }),
 193                 )
 194             };
 195
 196             if_chain! {
 197                 if !is_self(arg);
 198                 if !ty.is_mutable_pointer();
 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.node;
 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.sty {
 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 match_type(cx, ty, &paths::VEC);
 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.node;
 227                             if let Some(elem_ty) = path.segments.iter()
 228                                 .find(|seg| seg.ident.name == "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                                     GenericArg::Lifetime(_) => None,
 233                                 }).unwrap());
 234                             then {
 235                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 236                                 db.span_suggestion_with_applicability(
 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_with_applicability(
 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_with_applicability(
 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_with_applicability(
 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 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 319     cx: &'a LateContext<'a, 'tcx>,
 320     moved_vars: FxHashSet<NodeId>,
 321     /// Spans which need to be prefixed with `*` for dereferencing the
 322     /// suggested additional reference.
 323     spans_need_deref: FxHashMap<NodeId, FxHashSet<Span>>,
 324 }
 325
 326 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 327     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 328         Self {
 329             cx,
 330             moved_vars: FxHashSet::default(),
 331             spans_need_deref: FxHashMap::default(),
 332         }
 333     }
 334
 335     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 336         let cmt = unwrap_downcast_or_interior(cmt);
 337
 338         if let mc::Categorization::Local(vid) = cmt.cat {
 339             self.moved_vars.insert(vid);
 340         }
 341     }
 342
 343     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 344         let cmt = unwrap_downcast_or_interior(cmt);
 345
 346         if let mc::Categorization::Local(vid) = cmt.cat {
 347             let mut id = matched_pat.id;
 348             loop {
 349                 let parent = self.cx.tcx.hir().get_parent_node(id);
 350                 if id == parent {
 351                     // no parent
 352                     return;
 353                 }
 354                 id = parent;
 355
 356                 if let Some(node) = self.cx.tcx.hir().find(id) {
 357                     match node {
 358                         Node::Expr(e) => {
 359                             // `match` and `if let`
 360                             if let ExprKind::Match(ref c, ..) = e.node {
 361                                 self.spans_need_deref
 362                                     .entry(vid)
 363                                     .or_insert_with(FxHashSet::default)
 364                                     .insert(c.span);
 365                             }
 366                         },
 367
 368                         Node::Stmt(s) => {
 369                             // `let <pat> = x;`
 370                             if_chain! {
 371                                 if let StmtKind::Decl(ref decl, _) = s.node;
 372                                 if let DeclKind::Local(ref local) = decl.node;
 373                                 then {
 374                                     self.spans_need_deref
 375                                         .entry(vid)
 376                                         .or_insert_with(FxHashSet::default)
 377                                         .insert(local.init
 378                                             .as_ref()
 379                                             .map(|e| e.span)
 380                                             .expect("`let` stmt without init aren't caught by match_pat"));
 381                                 }
 382                             }
 383                         },
 384
 385                         _ => {},
 386                     }
 387                 }
 388             }
 389         }
 390     }
 391 }
 392
 393 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 394     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 395         if let euv::ConsumeMode::Move(_) = mode {
 396             self.move_common(consume_id, consume_span, cmt);
 397         }
 398     }
 399
 400     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 401         if let euv::MatchMode::MovingMatch = mode {
 402             self.move_common(matched_pat.id, matched_pat.span, cmt);
 403         } else {
 404             self.non_moving_pat(matched_pat, cmt);
 405         }
 406     }
 407
 408     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 409         if let euv::ConsumeMode::Move(_) = mode {
 410             self.move_common(consume_pat.id, consume_pat.span, cmt);
 411         }
 412     }
 413
 414     fn borrow(
 415         &mut self,
 416         _: NodeId,
 417         _: Span,
 418         _: &mc::cmt_<'tcx>,
 419         _: ty::Region<'_>,
 420         _: ty::BorrowKind,
 421         _: euv::LoanCause,
 422     ) {
 423     }
 424
 425     fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 426
 427     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 428 }
 429
 430 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 431     loop {
 432         match cmt.cat {
 433             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 434                 cmt = c;
 435             },
 436             _ => return (*cmt).clone(),
 437         }
 438     }
 439 }