clippy_lints/src/needless_pass_by_value.rs

   1 use rustc::hir::*;
   2 use rustc::hir::map::*;
   3 use rustc::hir::intravisit::FnKind;
   4 use rustc::lint::*;
   5 use rustc::ty::{self, RegionKind, TypeFoldable};
   6 use rustc::traits;
   7 use rustc::middle::expr_use_visitor as euv;
   8 use rustc::middle::mem_categorization as mc;
   9 use rustc_target::spec::abi::Abi;
  10 use syntax::ast::NodeId;
  11 use syntax_pos::Span;
  12 use syntax::errors::DiagnosticBuilder;
  13 use crate::utils::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
  14             snippet, snippet_opt, span_lint_and_then};
  15 use crate::utils::ptr::get_spans;
  16 use std::collections::{HashMap, HashSet};
  17 use std::borrow::Cow;
  18
  19 /// **What it does:** Checks for functions taking arguments by value, but not
  20 /// consuming them in its
  21 /// body.
  22 ///
  23 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  24 /// sometimes avoid
  25 /// unnecessary allocations.
  26 ///
  27 /// **Known problems:**
  28 /// * This lint suggests taking an argument by reference,
  29 /// however sometimes it is better to let users decide the argument type
  30 /// (by using `Borrow` trait, for example), depending on how the function is used.
  31 ///
  32 /// **Example:**
  33 /// ```rust
  34 /// fn foo(v: Vec<i32>) {
  35 ///     assert_eq!(v.len(), 42);
  36 /// }
  37 /// // should be
  38 /// fn foo(v: &[i32]) {
  39 ///     assert_eq!(v.len(), 42);
  40 /// }
  41 /// ```
  42 declare_clippy_lint! {
  43     pub NEEDLESS_PASS_BY_VALUE,
  44     style,
  45     "functions taking arguments by value, but not consuming them in its body"
  46 }
  47
  48 pub struct NeedlessPassByValue;
  49
  50 impl LintPass for NeedlessPassByValue {
  51     fn get_lints(&self) -> LintArray {
  52         lint_array![NEEDLESS_PASS_BY_VALUE]
  53     }
  54 }
  55
  56 macro_rules! need {
  57     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  58 }
  59
  60 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  61     fn check_fn(
  62         &mut self,
  63         cx: &LateContext<'a, 'tcx>,
  64         kind: FnKind<'tcx>,
  65         decl: &'tcx FnDecl,
  66         body: &'tcx Body,
  67         span: Span,
  68         node_id: NodeId,
  69     ) {
  70         if in_macro(span) {
  71             return;
  72         }
  73
  74         match kind {
  75             FnKind::ItemFn(.., header, _, attrs) => {
  76                 if header.abi != Abi::Rust {
  77                     return;
  78                 }
  79                 for a in attrs {
  80                     if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
  81                         return;
  82                     }
  83                 }
  84             },
  85             FnKind::Method(..) => (),
  86             _ => return,
  87         }
  88
  89         // Exclude non-inherent impls
  90         if let Some(NodeItem(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
  91             if matches!(item.node, ItemImpl(_, _, _, _, Some(_), _, _) |
  92                 ItemTrait(..))
  93             {
  94                 return;
  95             }
  96         }
  97
  98         // Allow `Borrow` or functions to be taken by value
  99         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 100         let whitelisted_traits = [
 101             need!(cx.tcx.lang_items().fn_trait()),
 102             need!(cx.tcx.lang_items().fn_once_trait()),
 103             need!(cx.tcx.lang_items().fn_mut_trait()),
 104             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT))
 105         ];
 106
 107         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 108
 109         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
 110
 111         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 112             .filter(|p| !p.is_global())
 113             .filter_map(|pred| {
 114                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 115                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_regions() {
 116                         return None;
 117                     }
 118                     Some(poly_trait_ref)
 119                 } else {
 120                     None
 121                 }
 122             })
 123             .collect::<Vec<_>>();
 124
 125         // Collect moved variables and spans which will need dereferencings from the
 126         // function body.
 127         let MovedVariablesCtxt {
 128             moved_vars,
 129             spans_need_deref,
 130             ..
 131         } = {
 132             let mut ctx = MovedVariablesCtxt::new(cx);
 133             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 134             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 135                 .consume_body(body);
 136             ctx
 137         };
 138
 139         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 140         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 141
 142         for (idx, ((input, &ty), arg)) in decl.inputs
 143             .iter()
 144             .zip(fn_sig.inputs())
 145             .zip(&body.arguments)
 146             .enumerate()
 147         {
 148             // All spans generated from a proc-macro invocation are the same...
 149             if span == input.span {
 150                 return;
 151             }
 152
 153             // Ignore `self`s.
 154             if idx == 0 {
 155                 if let PatKind::Binding(_, _, name, ..) = arg.pat.node {
 156                     if name.node.as_str() == "self" {
 157                         continue;
 158                     }
 159                 }
 160             }
 161
 162             // * Exclude a type that is specifically bounded by `Borrow`.
 163             // * Exclude a type whose reference also fulfills its bound.
 164             //   (e.g. `std::convert::AsRef`, `serde::Serialize`)
 165             let (implements_borrow_trait, all_borrowable_trait) = {
 166                 let preds = preds
 167                     .iter()
 168                     .filter(|t| t.skip_binder().self_ty() == ty)
 169                     .collect::<Vec<_>>();
 170
 171                 (
 172                     preds.iter().any(|t| t.def_id() == borrow_trait),
 173                     !preds.is_empty() && preds.iter().all(|t| {
 174                         implements_trait(
 175                             cx,
 176                             cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
 177                             t.def_id(),
 178                             &t.skip_binder()
 179                                 .input_types()
 180                                 .skip(1)
 181                                 .map(|ty| ty.into())
 182                                 .collect::<Vec<_>>(),
 183                         )
 184                     }),
 185                 )
 186             };
 187
 188             if_chain! {
 189                 if !is_self(arg);
 190                 if !ty.is_mutable_pointer();
 191                 if !is_copy(cx, ty);
 192                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 193                 if !implements_borrow_trait;
 194                 if !all_borrowable_trait;
 195
 196                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 197                 if !moved_vars.contains(&canonical_id);
 198                 then {
 199                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 200                         continue;
 201                     }
 202
 203                     // Dereference suggestion
 204                     let sugg = |db: &mut DiagnosticBuilder| {
 205                         if let ty::TypeVariants::TyAdt(def, ..) = ty.sty {
 206                             if let Some(span) = cx.tcx.hir.span_if_local(def.did) {
 207                                 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
 208                                     db.span_help(span, "consider marking this type as Copy");
 209                                 }
 210                             }
 211                         }
 212
 213                         let deref_span = spans_need_deref.get(&canonical_id);
 214                         if_chain! {
 215                             if match_type(cx, ty, &paths::VEC);
 216                             if let Some(clone_spans) =
 217                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 218                             if let TyPath(QPath::Resolved(_, ref path)) = input.node;
 219                             if let Some(elem_ty) = path.segments.iter()
 220                                 .find(|seg| seg.name == "Vec")
 221                                 .and_then(|ps| ps.args.as_ref())
 222                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 223                                     GenericArg::Type(ty) => Some(ty),
 224                                     GenericArg::Lifetime(_) => None,
 225                                 }).unwrap());
 226                             then {
 227                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 228                                 db.span_suggestion(input.span,
 229                                                 "consider changing the type to",
 230                                                 slice_ty);
 231
 232                                 for (span, suggestion) in clone_spans {
 233                                     db.span_suggestion(
 234                                         span,
 235                                         &snippet_opt(cx, span)
 236                                             .map_or(
 237                                                 "change the call to".into(),
 238                                                 |x| Cow::from(format!("change `{}` to", x)),
 239                                             ),
 240                                         suggestion.into()
 241                                     );
 242                                 }
 243
 244                                 // cannot be destructured, no need for `*` suggestion
 245                                 assert!(deref_span.is_none());
 246                                 return;
 247                             }
 248                         }
 249
 250                         if match_type(cx, ty, &paths::STRING) {
 251                             if let Some(clone_spans) =
 252                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 253                                 db.span_suggestion(input.span, "consider changing the type to", "&str".to_string());
 254
 255                                 for (span, suggestion) in clone_spans {
 256                                     db.span_suggestion(
 257                                         span,
 258                                         &snippet_opt(cx, span)
 259                                             .map_or(
 260                                                 "change the call to".into(),
 261                                                 |x| Cow::from(format!("change `{}` to", x))
 262                                             ),
 263                                         suggestion.into(),
 264                                     );
 265                                 }
 266
 267                                 assert!(deref_span.is_none());
 268                                 return;
 269                             }
 270                         }
 271
 272                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 273
 274                         // Suggests adding `*` to dereference the added reference.
 275                         if let Some(deref_span) = deref_span {
 276                             spans.extend(
 277                                 deref_span
 278                                     .iter()
 279                                     .cloned()
 280                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 281                             );
 282                             spans.sort_by_key(|&(span, _)| span);
 283                         }
 284                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 285                     };
 286
 287                     span_lint_and_then(
 288                         cx,
 289                         NEEDLESS_PASS_BY_VALUE,
 290                         input.span,
 291                         "this argument is passed by value, but not consumed in the function body",
 292                         sugg,
 293                     );
 294                 }
 295             }
 296         }
 297     }
 298 }
 299
 300 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 301     cx: &'a LateContext<'a, 'tcx>,
 302     moved_vars: HashSet<NodeId>,
 303     /// Spans which need to be prefixed with `*` for dereferencing the
 304     /// suggested additional reference.
 305     spans_need_deref: HashMap<NodeId, HashSet<Span>>,
 306 }
 307
 308 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 309     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 310         Self {
 311             cx,
 312             moved_vars: HashSet::new(),
 313             spans_need_deref: HashMap::new(),
 314         }
 315     }
 316
 317     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 318         let cmt = unwrap_downcast_or_interior(cmt);
 319
 320         if let mc::Categorization::Local(vid) = cmt.cat {
 321             self.moved_vars.insert(vid);
 322         }
 323     }
 324
 325     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 326         let cmt = unwrap_downcast_or_interior(cmt);
 327
 328         if let mc::Categorization::Local(vid) = cmt.cat {
 329             let mut id = matched_pat.id;
 330             loop {
 331                 let parent = self.cx.tcx.hir.get_parent_node(id);
 332                 if id == parent {
 333                     // no parent
 334                     return;
 335                 }
 336                 id = parent;
 337
 338                 if let Some(node) = self.cx.tcx.hir.find(id) {
 339                     match node {
 340                         map::Node::NodeExpr(e) => {
 341                             // `match` and `if let`
 342                             if let ExprMatch(ref c, ..) = e.node {
 343                                 self.spans_need_deref
 344                                     .entry(vid)
 345                                     .or_insert_with(HashSet::new)
 346                                     .insert(c.span);
 347                             }
 348                         },
 349
 350                         map::Node::NodeStmt(s) => {
 351                             // `let <pat> = x;`
 352                             if_chain! {
 353                                 if let StmtDecl(ref decl, _) = s.node;
 354                                 if let DeclLocal(ref local) = decl.node;
 355                                 then {
 356                                     self.spans_need_deref
 357                                         .entry(vid)
 358                                         .or_insert_with(HashSet::new)
 359                                         .insert(local.init
 360                                             .as_ref()
 361                                             .map(|e| e.span)
 362                                             .expect("`let` stmt without init aren't caught by match_pat"));
 363                                 }
 364                             }
 365                         },
 366
 367                         _ => {},
 368                     }
 369                 }
 370             }
 371         }
 372     }
 373 }
 374
 375 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 376     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 377         if let euv::ConsumeMode::Move(_) = mode {
 378             self.move_common(consume_id, consume_span, cmt);
 379         }
 380     }
 381
 382     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 383         if let euv::MatchMode::MovingMatch = mode {
 384             self.move_common(matched_pat.id, matched_pat.span, cmt);
 385         } else {
 386             self.non_moving_pat(matched_pat, cmt);
 387         }
 388     }
 389
 390     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 391         if let euv::ConsumeMode::Move(_) = mode {
 392             self.move_common(consume_pat.id, consume_pat.span, cmt);
 393         }
 394     }
 395
 396     fn borrow(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: ty::Region, _: ty::BorrowKind, _: euv::LoanCause) {}
 397
 398     fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 399
 400     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 401 }
 402
 403
 404 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 405     loop {
 406         match cmt.cat {
 407             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 408                 cmt = c;
 409             },
 410             _ => return (*cmt).clone(),
 411         }
 412     };
 413 }