clippy_lints/src/needless_pass_by_value.rs

   1 use matches::matches;
   2 use crate::rustc::hir::*;
   3 use crate::rustc::hir::intravisit::FnKind;
   4 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   5 use crate::rustc::{declare_tool_lint, lint_array};
   6 use if_chain::if_chain;
   7 use crate::rustc::ty::{self, RegionKind, TypeFoldable};
   8 use crate::rustc::traits;
   9 use crate::rustc::middle::expr_use_visitor as euv;
  10 use crate::rustc::middle::mem_categorization as mc;
  11 use crate::rustc_target::spec::abi::Abi;
  12 use crate::rustc_data_structures::fx::{FxHashMap, FxHashSet};
  13 use crate::syntax::ast::NodeId;
  14 use crate::syntax_pos::Span;
  15 use crate::syntax::errors::DiagnosticBuilder;
  16 use crate::utils::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
  17             snippet, snippet_opt, span_lint_and_then};
  18 use crate::utils::ptr::get_spans;
  19 use std::borrow::Cow;
  20 use crate::rustc_errors::Applicability;
  21
  22 /// **What it does:** Checks for functions taking arguments by value, but not
  23 /// consuming them in its
  24 /// body.
  25 ///
  26 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  27 /// sometimes avoid
  28 /// unnecessary allocations.
  29 ///
  30 /// **Known problems:**
  31 /// * This lint suggests taking an argument by reference,
  32 /// however sometimes it is better to let users decide the argument type
  33 /// (by using `Borrow` trait, for example), depending on how the function is used.
  34 ///
  35 /// **Example:**
  36 /// ```rust
  37 /// fn foo(v: Vec<i32>) {
  38 ///     assert_eq!(v.len(), 42);
  39 /// }
  40 /// // should be
  41 /// fn foo(v: &[i32]) {
  42 ///     assert_eq!(v.len(), 42);
  43 /// }
  44 /// ```
  45 declare_clippy_lint! {
  46     pub NEEDLESS_PASS_BY_VALUE,
  47     style,
  48     "functions taking arguments by value, but not consuming them in its body"
  49 }
  50
  51 pub struct NeedlessPassByValue;
  52
  53 impl LintPass for NeedlessPassByValue {
  54     fn get_lints(&self) -> LintArray {
  55         lint_array![NEEDLESS_PASS_BY_VALUE]
  56     }
  57 }
  58
  59 macro_rules! need {
  60     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  61 }
  62
  63 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  64     fn check_fn(
  65         &mut self,
  66         cx: &LateContext<'a, 'tcx>,
  67         kind: FnKind<'tcx>,
  68         decl: &'tcx FnDecl,
  69         body: &'tcx Body,
  70         span: Span,
  71         node_id: NodeId,
  72     ) {
  73         if in_macro(span) {
  74             return;
  75         }
  76
  77         match kind {
  78             FnKind::ItemFn(.., header, _, attrs) => {
  79                 if header.abi != Abi::Rust {
  80                     return;
  81                 }
  82                 for a in attrs {
  83                     if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
  84                         return;
  85                     }
  86                 }
  87             },
  88             FnKind::Method(..) => (),
  89             _ => return,
  90         }
  91
  92         // Exclude non-inherent impls
  93         if let Some(Node::Item(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
  94             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
  95                 ItemKind::Trait(..))
  96             {
  97                 return;
  98             }
  99         }
 100
 101         // Allow `Borrow` or functions to be taken by value
 102         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 103         let whitelisted_traits = [
 104             need!(cx.tcx.lang_items().fn_trait()),
 105             need!(cx.tcx.lang_items().fn_once_trait()),
 106             need!(cx.tcx.lang_items().fn_mut_trait()),
 107             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT))
 108         ];
 109
 110         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 111
 112         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
 113
 114         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 115             .filter(|p| !p.is_global())
 116             .filter_map(|pred| {
 117                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 118                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_regions() {
 119                         return None;
 120                     }
 121                     Some(poly_trait_ref)
 122                 } else {
 123                     None
 124                 }
 125             })
 126             .collect::<Vec<_>>();
 127
 128         // Collect moved variables and spans which will need dereferencings from the
 129         // function body.
 130         let MovedVariablesCtxt {
 131             moved_vars,
 132             spans_need_deref,
 133             ..
 134         } = {
 135             let mut ctx = MovedVariablesCtxt::new(cx);
 136             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 137             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 138                 .consume_body(body);
 139             ctx
 140         };
 141
 142         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 143         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 144
 145         for (idx, ((input, &ty), arg)) in decl.inputs
 146             .iter()
 147             .zip(fn_sig.inputs())
 148             .zip(&body.arguments)
 149             .enumerate()
 150         {
 151             // All spans generated from a proc-macro invocation are the same...
 152             if span == input.span {
 153                 return;
 154             }
 155
 156             // Ignore `self`s.
 157             if idx == 0 {
 158                 if let PatKind::Binding(_, _, ident, ..) = arg.pat.node {
 159                     if ident.as_str() == "self" {
 160                         continue;
 161                     }
 162                 }
 163             }
 164
 165             // * Exclude a type that is specifically bounded by `Borrow`.
 166             // * Exclude a type whose reference also fulfills its bound.
 167             //   (e.g. `std::convert::AsRef`, `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() && preds.iter().all(|t| {
 177                         let ty_params = &t.skip_binder().trait_ref.substs.iter().skip(1)
 178                             .cloned()
 179                             .collect::<Vec<_>>();
 180                         implements_trait(
 181                             cx,
 182                             cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
 183                             t.def_id(),
 184                             ty_params
 185                         )
 186                     }),
 187                 )
 188             };
 189
 190             if_chain! {
 191                 if !is_self(arg);
 192                 if !ty.is_mutable_pointer();
 193                 if !is_copy(cx, ty);
 194                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 195                 if !implements_borrow_trait;
 196                 if !all_borrowable_trait;
 197
 198                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 199                 if !moved_vars.contains(&canonical_id);
 200                 then {
 201                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 202                         continue;
 203                     }
 204
 205                     // Dereference suggestion
 206                     let sugg = |db: &mut DiagnosticBuilder<'_>| {
 207                         if let ty::TyKind::Adt(def, ..) = ty.sty {
 208                             if let Some(span) = cx.tcx.hir.span_if_local(def.did) {
 209                                 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
 210                                     db.span_help(span, "consider marking this type as Copy");
 211                                 }
 212                             }
 213                         }
 214
 215                         let deref_span = spans_need_deref.get(&canonical_id);
 216                         if_chain! {
 217                             if match_type(cx, ty, &paths::VEC);
 218                             if let Some(clone_spans) =
 219                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 220                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
 221                             if let Some(elem_ty) = path.segments.iter()
 222                                 .find(|seg| seg.ident.name == "Vec")
 223                                 .and_then(|ps| ps.args.as_ref())
 224                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 225                                     GenericArg::Type(ty) => Some(ty),
 226                                     GenericArg::Lifetime(_) => None,
 227                                 }).unwrap());
 228                             then {
 229                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 230                                 db.span_suggestion_with_applicability(
 231                                     input.span,
 232                                     "consider changing the type to",
 233                                     slice_ty,
 234                                     Applicability::Unspecified,
 235                                 );
 236
 237                                 for (span, suggestion) in clone_spans {
 238                                     db.span_suggestion_with_applicability(
 239                                         span,
 240                                         &snippet_opt(cx, span)
 241                                             .map_or(
 242                                                 "change the call to".into(),
 243                                                 |x| Cow::from(format!("change `{}` to", x)),
 244                                             ),
 245                                         suggestion.into(),
 246                                         Applicability::Unspecified,
 247                                     );
 248                                 }
 249
 250                                 // cannot be destructured, no need for `*` suggestion
 251                                 assert!(deref_span.is_none());
 252                                 return;
 253                             }
 254                         }
 255
 256                         if match_type(cx, ty, &paths::STRING) {
 257                             if let Some(clone_spans) =
 258                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 259                                 db.span_suggestion_with_applicability(
 260                                     input.span,
 261                                     "consider changing the type to",
 262                                     "&str".to_string(),
 263                                     Applicability::Unspecified,
 264                                 );
 265
 266                                 for (span, suggestion) in clone_spans {
 267                                     db.span_suggestion_with_applicability(
 268                                         span,
 269                                         &snippet_opt(cx, span)
 270                                             .map_or(
 271                                                 "change the call to".into(),
 272                                                 |x| Cow::from(format!("change `{}` to", x))
 273                                             ),
 274                                         suggestion.into(),
 275                                         Applicability::Unspecified,
 276                                     );
 277                                 }
 278
 279                                 assert!(deref_span.is_none());
 280                                 return;
 281                             }
 282                         }
 283
 284                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 285
 286                         // Suggests adding `*` to dereference the added reference.
 287                         if let Some(deref_span) = deref_span {
 288                             spans.extend(
 289                                 deref_span
 290                                     .iter()
 291                                     .cloned()
 292                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 293                             );
 294                             spans.sort_by_key(|&(span, _)| span);
 295                         }
 296                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 297                     };
 298
 299                     span_lint_and_then(
 300                         cx,
 301                         NEEDLESS_PASS_BY_VALUE,
 302                         input.span,
 303                         "this argument is passed by value, but not consumed in the function body",
 304                         sugg,
 305                     );
 306                 }
 307             }
 308         }
 309     }
 310 }
 311
 312 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 313     cx: &'a LateContext<'a, 'tcx>,
 314     moved_vars: FxHashSet<NodeId>,
 315     /// Spans which need to be prefixed with `*` for dereferencing the
 316     /// suggested additional reference.
 317     spans_need_deref: FxHashMap<NodeId, FxHashSet<Span>>,
 318 }
 319
 320 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 321     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 322         Self {
 323             cx,
 324             moved_vars: FxHashSet::default(),
 325             spans_need_deref: FxHashMap::default(),
 326         }
 327     }
 328
 329     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 330         let cmt = unwrap_downcast_or_interior(cmt);
 331
 332         if let mc::Categorization::Local(vid) = cmt.cat {
 333             self.moved_vars.insert(vid);
 334         }
 335     }
 336
 337     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 338         let cmt = unwrap_downcast_or_interior(cmt);
 339
 340         if let mc::Categorization::Local(vid) = cmt.cat {
 341             let mut id = matched_pat.id;
 342             loop {
 343                 let parent = self.cx.tcx.hir.get_parent_node(id);
 344                 if id == parent {
 345                     // no parent
 346                     return;
 347                 }
 348                 id = parent;
 349
 350                 if let Some(node) = self.cx.tcx.hir.find(id) {
 351                     match node {
 352                         Node::Expr(e) => {
 353                             // `match` and `if let`
 354                             if let ExprKind::Match(ref c, ..) = e.node {
 355                                 self.spans_need_deref
 356                                     .entry(vid)
 357                                     .or_insert_with(FxHashSet::default)
 358                                     .insert(c.span);
 359                             }
 360                         },
 361
 362                         Node::Stmt(s) => {
 363                             // `let <pat> = x;`
 364                             if_chain! {
 365                                 if let StmtKind::Decl(ref decl, _) = s.node;
 366                                 if let DeclKind::Local(ref local) = decl.node;
 367                                 then {
 368                                     self.spans_need_deref
 369                                         .entry(vid)
 370                                         .or_insert_with(FxHashSet::default)
 371                                         .insert(local.init
 372                                             .as_ref()
 373                                             .map(|e| e.span)
 374                                             .expect("`let` stmt without init aren't caught by match_pat"));
 375                                 }
 376                             }
 377                         },
 378
 379                         _ => {},
 380                     }
 381                 }
 382             }
 383         }
 384     }
 385 }
 386
 387 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 388     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 389         if let euv::ConsumeMode::Move(_) = mode {
 390             self.move_common(consume_id, consume_span, cmt);
 391         }
 392     }
 393
 394     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 395         if let euv::MatchMode::MovingMatch = mode {
 396             self.move_common(matched_pat.id, matched_pat.span, cmt);
 397         } else {
 398             self.non_moving_pat(matched_pat, cmt);
 399         }
 400     }
 401
 402     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 403         if let euv::ConsumeMode::Move(_) = mode {
 404             self.move_common(consume_pat.id, consume_pat.span, cmt);
 405         }
 406     }
 407
 408     fn borrow(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: ty::Region<'_>, _: ty::BorrowKind, _: euv::LoanCause) {}
 409
 410     fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 411
 412     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 413 }
 414
 415
 416 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 417     loop {
 418         match cmt.cat {
 419             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 420                 cmt = c;
 421             },
 422             _ => return (*cmt).clone(),
 423         }
 424     };
 425 }