clippy_lints/src/needless_pass_by_value.rs

   1 use matches::matches;
   2 use rustc::hir::*;
   3 use rustc::hir::intravisit::FnKind;
   4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   5 use rustc::{declare_tool_lint, lint_array};
   6 use if_chain::if_chain;
   7 use rustc::ty::{self, RegionKind, TypeFoldable};
   8 use rustc::traits;
   9 use rustc::middle::expr_use_visitor as euv;
  10 use rustc::middle::mem_categorization as mc;
  11 use rustc_target::spec::abi::Abi;
  12 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  13 use syntax::ast::NodeId;
  14 use syntax_pos::Span;
  15 use 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
  21 /// **What it does:** Checks for functions taking arguments by value, but not
  22 /// consuming them in its
  23 /// body.
  24 ///
  25 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  26 /// sometimes avoid
  27 /// unnecessary allocations.
  28 ///
  29 /// **Known problems:**
  30 /// * This lint suggests taking an argument by reference,
  31 /// however sometimes it is better to let users decide the argument type
  32 /// (by using `Borrow` trait, for example), depending on how the function is used.
  33 ///
  34 /// **Example:**
  35 /// ```rust
  36 /// fn foo(v: Vec<i32>) {
  37 ///     assert_eq!(v.len(), 42);
  38 /// }
  39 /// // should be
  40 /// fn foo(v: &[i32]) {
  41 ///     assert_eq!(v.len(), 42);
  42 /// }
  43 /// ```
  44 declare_clippy_lint! {
  45     pub NEEDLESS_PASS_BY_VALUE,
  46     style,
  47     "functions taking arguments by value, but not consuming them in its body"
  48 }
  49
  50 pub struct NeedlessPassByValue;
  51
  52 impl LintPass for NeedlessPassByValue {
  53     fn get_lints(&self) -> LintArray {
  54         lint_array![NEEDLESS_PASS_BY_VALUE]
  55     }
  56 }
  57
  58 macro_rules! need {
  59     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  60 }
  61
  62 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  63     fn check_fn(
  64         &mut self,
  65         cx: &LateContext<'a, 'tcx>,
  66         kind: FnKind<'tcx>,
  67         decl: &'tcx FnDecl,
  68         body: &'tcx Body,
  69         span: Span,
  70         node_id: NodeId,
  71     ) {
  72         if in_macro(span) {
  73             return;
  74         }
  75
  76         match kind {
  77             FnKind::ItemFn(.., header, _, attrs) => {
  78                 if header.abi != Abi::Rust {
  79                     return;
  80                 }
  81                 for a in attrs {
  82                     if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
  83                         return;
  84                     }
  85                 }
  86             },
  87             FnKind::Method(..) => (),
  88             _ => return,
  89         }
  90
  91         // Exclude non-inherent impls
  92         if let Some(Node::Item(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
  93             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
  94                 ItemKind::Trait(..))
  95             {
  96                 return;
  97             }
  98         }
  99
 100         // Allow `Borrow` or functions to be taken by value
 101         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 102         let whitelisted_traits = [
 103             need!(cx.tcx.lang_items().fn_trait()),
 104             need!(cx.tcx.lang_items().fn_once_trait()),
 105             need!(cx.tcx.lang_items().fn_mut_trait()),
 106             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT))
 107         ];
 108
 109         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 110
 111         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
 112
 113         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 114             .filter(|p| !p.is_global())
 115             .filter_map(|pred| {
 116                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 117                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_regions() {
 118                         return None;
 119                     }
 120                     Some(poly_trait_ref)
 121                 } else {
 122                     None
 123                 }
 124             })
 125             .collect::<Vec<_>>();
 126
 127         // Collect moved variables and spans which will need dereferencings from the
 128         // function body.
 129         let MovedVariablesCtxt {
 130             moved_vars,
 131             spans_need_deref,
 132             ..
 133         } = {
 134             let mut ctx = MovedVariablesCtxt::new(cx);
 135             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 136             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 137                 .consume_body(body);
 138             ctx
 139         };
 140
 141         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 142         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 143
 144         for (idx, ((input, &ty), arg)) in decl.inputs
 145             .iter()
 146             .zip(fn_sig.inputs())
 147             .zip(&body.arguments)
 148             .enumerate()
 149         {
 150             // All spans generated from a proc-macro invocation are the same...
 151             if span == input.span {
 152                 return;
 153             }
 154
 155             // Ignore `self`s.
 156             if idx == 0 {
 157                 if let PatKind::Binding(_, _, ident, ..) = arg.pat.node {
 158                     if ident.as_str() == "self" {
 159                         continue;
 160                     }
 161                 }
 162             }
 163
 164             // * Exclude a type that is specifically bounded by `Borrow`.
 165             // * Exclude a type whose reference also fulfills its bound.
 166             //   (e.g. `std::convert::AsRef`, `serde::Serialize`)
 167             let (implements_borrow_trait, all_borrowable_trait) = {
 168                 let preds = preds
 169                     .iter()
 170                     .filter(|t| t.skip_binder().self_ty() == ty)
 171                     .collect::<Vec<_>>();
 172
 173                 (
 174                     preds.iter().any(|t| t.def_id() == borrow_trait),
 175                     !preds.is_empty() && preds.iter().all(|t| {
 176                         implements_trait(
 177                             cx,
 178                             cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
 179                             t.def_id(),
 180                             &t.skip_binder()
 181                                 .input_types()
 182                                 .skip(1)
 183                                 .map(|ty| ty.into())
 184                                 .collect::<Vec<_>>(),
 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(input.span,
 231                                                 "consider changing the type to",
 232                                                 slice_ty);
 233
 234                                 for (span, suggestion) in clone_spans {
 235                                     db.span_suggestion(
 236                                         span,
 237                                         &snippet_opt(cx, span)
 238                                             .map_or(
 239                                                 "change the call to".into(),
 240                                                 |x| Cow::from(format!("change `{}` to", x)),
 241                                             ),
 242                                         suggestion.into()
 243                                     );
 244                                 }
 245
 246                                 // cannot be destructured, no need for `*` suggestion
 247                                 assert!(deref_span.is_none());
 248                                 return;
 249                             }
 250                         }
 251
 252                         if match_type(cx, ty, &paths::STRING) {
 253                             if let Some(clone_spans) =
 254                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 255                                 db.span_suggestion(input.span, "consider changing the type to", "&str".to_string());
 256
 257                                 for (span, suggestion) in clone_spans {
 258                                     db.span_suggestion(
 259                                         span,
 260                                         &snippet_opt(cx, span)
 261                                             .map_or(
 262                                                 "change the call to".into(),
 263                                                 |x| Cow::from(format!("change `{}` to", x))
 264                                             ),
 265                                         suggestion.into(),
 266                                     );
 267                                 }
 268
 269                                 assert!(deref_span.is_none());
 270                                 return;
 271                             }
 272                         }
 273
 274                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 275
 276                         // Suggests adding `*` to dereference the added reference.
 277                         if let Some(deref_span) = deref_span {
 278                             spans.extend(
 279                                 deref_span
 280                                     .iter()
 281                                     .cloned()
 282                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 283                             );
 284                             spans.sort_by_key(|&(span, _)| span);
 285                         }
 286                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 287                     };
 288
 289                     span_lint_and_then(
 290                         cx,
 291                         NEEDLESS_PASS_BY_VALUE,
 292                         input.span,
 293                         "this argument is passed by value, but not consumed in the function body",
 294                         sugg,
 295                     );
 296                 }
 297             }
 298         }
 299     }
 300 }
 301
 302 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 303     cx: &'a LateContext<'a, 'tcx>,
 304     moved_vars: FxHashSet<NodeId>,
 305     /// Spans which need to be prefixed with `*` for dereferencing the
 306     /// suggested additional reference.
 307     spans_need_deref: FxHashMap<NodeId, FxHashSet<Span>>,
 308 }
 309
 310 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 311     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 312         Self {
 313             cx,
 314             moved_vars: FxHashSet::default(),
 315             spans_need_deref: FxHashMap::default(),
 316         }
 317     }
 318
 319     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 320         let cmt = unwrap_downcast_or_interior(cmt);
 321
 322         if let mc::Categorization::Local(vid) = cmt.cat {
 323             self.moved_vars.insert(vid);
 324         }
 325     }
 326
 327     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 328         let cmt = unwrap_downcast_or_interior(cmt);
 329
 330         if let mc::Categorization::Local(vid) = cmt.cat {
 331             let mut id = matched_pat.id;
 332             loop {
 333                 let parent = self.cx.tcx.hir.get_parent_node(id);
 334                 if id == parent {
 335                     // no parent
 336                     return;
 337                 }
 338                 id = parent;
 339
 340                 if let Some(node) = self.cx.tcx.hir.find(id) {
 341                     match node {
 342                         Node::Expr(e) => {
 343                             // `match` and `if let`
 344                             if let ExprKind::Match(ref c, ..) = e.node {
 345                                 self.spans_need_deref
 346                                     .entry(vid)
 347                                     .or_insert_with(FxHashSet::default)
 348                                     .insert(c.span);
 349                             }
 350                         },
 351
 352                         Node::Stmt(s) => {
 353                             // `let <pat> = x;`
 354                             if_chain! {
 355                                 if let StmtKind::Decl(ref decl, _) = s.node;
 356                                 if let DeclKind::Local(ref local) = decl.node;
 357                                 then {
 358                                     self.spans_need_deref
 359                                         .entry(vid)
 360                                         .or_insert_with(FxHashSet::default)
 361                                         .insert(local.init
 362                                             .as_ref()
 363                                             .map(|e| e.span)
 364                                             .expect("`let` stmt without init aren't caught by match_pat"));
 365                                 }
 366                             }
 367                         },
 368
 369                         _ => {},
 370                     }
 371                 }
 372             }
 373         }
 374     }
 375 }
 376
 377 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 378     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 379         if let euv::ConsumeMode::Move(_) = mode {
 380             self.move_common(consume_id, consume_span, cmt);
 381         }
 382     }
 383
 384     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 385         if let euv::MatchMode::MovingMatch = mode {
 386             self.move_common(matched_pat.id, matched_pat.span, cmt);
 387         } else {
 388             self.non_moving_pat(matched_pat, cmt);
 389         }
 390     }
 391
 392     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 393         if let euv::ConsumeMode::Move(_) = mode {
 394             self.move_common(consume_pat.id, consume_pat.span, cmt);
 395         }
 396     }
 397
 398     fn borrow(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: ty::Region<'_>, _: ty::BorrowKind, _: euv::LoanCause) {}
 399
 400     fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 401
 402     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 403 }
 404
 405
 406 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 407     loop {
 408         match cmt.cat {
 409             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 410                 cmt = c;
 411             },
 412             _ => return (*cmt).clone(),
 413         }
 414     };
 415 }