clippy_lints/src/needless_pass_by_value.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10
  11 use matches::matches;
  12 use crate::rustc::hir::*;
  13 use crate::rustc::hir::intravisit::FnKind;
  14 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  15 use crate::rustc::{declare_tool_lint, lint_array};
  16 use if_chain::if_chain;
  17 use crate::rustc::ty::{self, RegionKind, TypeFoldable};
  18 use crate::rustc::traits;
  19 use crate::rustc::middle::expr_use_visitor as euv;
  20 use crate::rustc::middle::mem_categorization as mc;
  21 use crate::rustc_target::spec::abi::Abi;
  22 use crate::rustc_data_structures::fx::{FxHashMap, FxHashSet};
  23 use crate::syntax::ast::NodeId;
  24 use crate::syntax_pos::Span;
  25 use crate::syntax::errors::DiagnosticBuilder;
  26 use crate::utils::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
  27             snippet, snippet_opt, span_lint_and_then};
  28 use crate::utils::ptr::get_spans;
  29 use std::borrow::Cow;
  30 use crate::rustc_errors::Applicability;
  31
  32 /// **What it does:** Checks for functions taking arguments by value, but not
  33 /// consuming them in its
  34 /// body.
  35 ///
  36 /// **Why is this bad?** Taking arguments by reference is more flexible and can
  37 /// sometimes avoid
  38 /// unnecessary allocations.
  39 ///
  40 /// **Known problems:**
  41 /// * This lint suggests taking an argument by reference,
  42 /// however sometimes it is better to let users decide the argument type
  43 /// (by using `Borrow` trait, for example), depending on how the function is used.
  44 ///
  45 /// **Example:**
  46 /// ```rust
  47 /// fn foo(v: Vec<i32>) {
  48 ///     assert_eq!(v.len(), 42);
  49 /// }
  50 /// // should be
  51 /// fn foo(v: &[i32]) {
  52 ///     assert_eq!(v.len(), 42);
  53 /// }
  54 /// ```
  55 declare_clippy_lint! {
  56     pub NEEDLESS_PASS_BY_VALUE,
  57     pedantic,
  58     "functions taking arguments by value, but not consuming them in its body"
  59 }
  60
  61 pub struct NeedlessPassByValue;
  62
  63 impl LintPass for NeedlessPassByValue {
  64     fn get_lints(&self) -> LintArray {
  65         lint_array![NEEDLESS_PASS_BY_VALUE]
  66     }
  67 }
  68
  69 macro_rules! need {
  70     ($e: expr) => { if let Some(x) = $e { x } else { return; } };
  71 }
  72
  73 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
  74     fn check_fn(
  75         &mut self,
  76         cx: &LateContext<'a, 'tcx>,
  77         kind: FnKind<'tcx>,
  78         decl: &'tcx FnDecl,
  79         body: &'tcx Body,
  80         span: Span,
  81         node_id: NodeId,
  82     ) {
  83         if in_macro(span) {
  84             return;
  85         }
  86
  87         match kind {
  88             FnKind::ItemFn(.., header, _, attrs) => {
  89                 if header.abi != Abi::Rust {
  90                     return;
  91                 }
  92                 for a in attrs {
  93                     if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
  94                         return;
  95                     }
  96                 }
  97             },
  98             FnKind::Method(..) => (),
  99             _ => return,
 100         }
 101
 102         // Exclude non-inherent impls
 103         if let Some(Node::Item(item)) = cx.tcx.hir.find(cx.tcx.hir.get_parent_node(node_id)) {
 104             if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
 105                 ItemKind::Trait(..))
 106             {
 107                 return;
 108             }
 109         }
 110
 111         // Allow `Borrow` or functions to be taken by value
 112         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 113         let whitelisted_traits = [
 114             need!(cx.tcx.lang_items().fn_trait()),
 115             need!(cx.tcx.lang_items().fn_once_trait()),
 116             need!(cx.tcx.lang_items().fn_mut_trait()),
 117             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT))
 118         ];
 119
 120         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 121
 122         let fn_def_id = cx.tcx.hir.local_def_id(node_id);
 123
 124         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
 125             .filter(|p| !p.is_global())
 126             .filter_map(|pred| {
 127                 if let ty::Predicate::Trait(poly_trait_ref) = pred {
 128                     if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars() {
 129                         return None;
 130                     }
 131                     Some(poly_trait_ref)
 132                 } else {
 133                     None
 134                 }
 135             })
 136             .collect::<Vec<_>>();
 137
 138         // Collect moved variables and spans which will need dereferencings from the
 139         // function body.
 140         let MovedVariablesCtxt {
 141             moved_vars,
 142             spans_need_deref,
 143             ..
 144         } = {
 145             let mut ctx = MovedVariablesCtxt::new(cx);
 146             let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
 147             euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
 148                 .consume_body(body);
 149             ctx
 150         };
 151
 152         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 153         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 154
 155         for (idx, ((input, &ty), arg)) in decl.inputs
 156             .iter()
 157             .zip(fn_sig.inputs())
 158             .zip(&body.arguments)
 159             .enumerate()
 160         {
 161             // All spans generated from a proc-macro invocation are the same...
 162             if span == input.span {
 163                 return;
 164             }
 165
 166             // Ignore `self`s.
 167             if idx == 0 {
 168                 if let PatKind::Binding(_, _, ident, ..) = arg.pat.node {
 169                     if ident.as_str() == "self" {
 170                         continue;
 171                     }
 172                 }
 173             }
 174
 175             // * Exclude a type that is specifically bounded by `Borrow`.
 176             // * Exclude a type whose reference also fulfills its bound.
 177             //   (e.g. `std::convert::AsRef`, `serde::Serialize`)
 178             let (implements_borrow_trait, all_borrowable_trait) = {
 179                 let preds = preds
 180                     .iter()
 181                     .filter(|t| t.skip_binder().self_ty() == ty)
 182                     .collect::<Vec<_>>();
 183
 184                 (
 185                     preds.iter().any(|t| t.def_id() == borrow_trait),
 186                     !preds.is_empty() && preds.iter().all(|t| {
 187                         let ty_params = &t.skip_binder().trait_ref.substs.iter().skip(1)
 188                             .cloned()
 189                             .collect::<Vec<_>>();
 190                         implements_trait(
 191                             cx,
 192                             cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty),
 193                             t.def_id(),
 194                             ty_params
 195                         )
 196                     }),
 197                 )
 198             };
 199
 200             if_chain! {
 201                 if !is_self(arg);
 202                 if !ty.is_mutable_pointer();
 203                 if !is_copy(cx, ty);
 204                 if !whitelisted_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 205                 if !implements_borrow_trait;
 206                 if !all_borrowable_trait;
 207
 208                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node;
 209                 if !moved_vars.contains(&canonical_id);
 210                 then {
 211                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 212                         continue;
 213                     }
 214
 215                     // Dereference suggestion
 216                     let sugg = |db: &mut DiagnosticBuilder<'_>| {
 217                         if let ty::TyKind::Adt(def, ..) = ty.sty {
 218                             if let Some(span) = cx.tcx.hir.span_if_local(def.did) {
 219                                 if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
 220                                     db.span_help(span, "consider marking this type as Copy");
 221                                 }
 222                             }
 223                         }
 224
 225                         let deref_span = spans_need_deref.get(&canonical_id);
 226                         if_chain! {
 227                             if match_type(cx, ty, &paths::VEC);
 228                             if let Some(clone_spans) =
 229                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 230                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
 231                             if let Some(elem_ty) = path.segments.iter()
 232                                 .find(|seg| seg.ident.name == "Vec")
 233                                 .and_then(|ps| ps.args.as_ref())
 234                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 235                                     GenericArg::Type(ty) => Some(ty),
 236                                     GenericArg::Lifetime(_) => None,
 237                                 }).unwrap());
 238                             then {
 239                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 240                                 db.span_suggestion_with_applicability(
 241                                     input.span,
 242                                     "consider changing the type to",
 243                                     slice_ty,
 244                                     Applicability::Unspecified,
 245                                 );
 246
 247                                 for (span, suggestion) in clone_spans {
 248                                     db.span_suggestion_with_applicability(
 249                                         span,
 250                                         &snippet_opt(cx, span)
 251                                             .map_or(
 252                                                 "change the call to".into(),
 253                                                 |x| Cow::from(format!("change `{}` to", x)),
 254                                             ),
 255                                         suggestion.into(),
 256                                         Applicability::Unspecified,
 257                                     );
 258                                 }
 259
 260                                 // cannot be destructured, no need for `*` suggestion
 261                                 assert!(deref_span.is_none());
 262                                 return;
 263                             }
 264                         }
 265
 266                         if match_type(cx, ty, &paths::STRING) {
 267                             if let Some(clone_spans) =
 268                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 269                                 db.span_suggestion_with_applicability(
 270                                     input.span,
 271                                     "consider changing the type to",
 272                                     "&str".to_string(),
 273                                     Applicability::Unspecified,
 274                                 );
 275
 276                                 for (span, suggestion) in clone_spans {
 277                                     db.span_suggestion_with_applicability(
 278                                         span,
 279                                         &snippet_opt(cx, span)
 280                                             .map_or(
 281                                                 "change the call to".into(),
 282                                                 |x| Cow::from(format!("change `{}` to", x))
 283                                             ),
 284                                         suggestion.into(),
 285                                         Applicability::Unspecified,
 286                                     );
 287                                 }
 288
 289                                 assert!(deref_span.is_none());
 290                                 return;
 291                             }
 292                         }
 293
 294                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 295
 296                         // Suggests adding `*` to dereference the added reference.
 297                         if let Some(deref_span) = deref_span {
 298                             spans.extend(
 299                                 deref_span
 300                                     .iter()
 301                                     .cloned()
 302                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 303                             );
 304                             spans.sort_by_key(|&(span, _)| span);
 305                         }
 306                         multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
 307                     };
 308
 309                     span_lint_and_then(
 310                         cx,
 311                         NEEDLESS_PASS_BY_VALUE,
 312                         input.span,
 313                         "this argument is passed by value, but not consumed in the function body",
 314                         sugg,
 315                     );
 316                 }
 317             }
 318         }
 319     }
 320 }
 321
 322 struct MovedVariablesCtxt<'a, 'tcx: 'a> {
 323     cx: &'a LateContext<'a, 'tcx>,
 324     moved_vars: FxHashSet<NodeId>,
 325     /// Spans which need to be prefixed with `*` for dereferencing the
 326     /// suggested additional reference.
 327     spans_need_deref: FxHashMap<NodeId, FxHashSet<Span>>,
 328 }
 329
 330 impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
 331     fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
 332         Self {
 333             cx,
 334             moved_vars: FxHashSet::default(),
 335             spans_need_deref: FxHashMap::default(),
 336         }
 337     }
 338
 339     fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
 340         let cmt = unwrap_downcast_or_interior(cmt);
 341
 342         if let mc::Categorization::Local(vid) = cmt.cat {
 343             self.moved_vars.insert(vid);
 344         }
 345     }
 346
 347     fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
 348         let cmt = unwrap_downcast_or_interior(cmt);
 349
 350         if let mc::Categorization::Local(vid) = cmt.cat {
 351             let mut id = matched_pat.id;
 352             loop {
 353                 let parent = self.cx.tcx.hir.get_parent_node(id);
 354                 if id == parent {
 355                     // no parent
 356                     return;
 357                 }
 358                 id = parent;
 359
 360                 if let Some(node) = self.cx.tcx.hir.find(id) {
 361                     match node {
 362                         Node::Expr(e) => {
 363                             // `match` and `if let`
 364                             if let ExprKind::Match(ref c, ..) = e.node {
 365                                 self.spans_need_deref
 366                                     .entry(vid)
 367                                     .or_insert_with(FxHashSet::default)
 368                                     .insert(c.span);
 369                             }
 370                         },
 371
 372                         Node::Stmt(s) => {
 373                             // `let <pat> = x;`
 374                             if_chain! {
 375                                 if let StmtKind::Decl(ref decl, _) = s.node;
 376                                 if let DeclKind::Local(ref local) = decl.node;
 377                                 then {
 378                                     self.spans_need_deref
 379                                         .entry(vid)
 380                                         .or_insert_with(FxHashSet::default)
 381                                         .insert(local.init
 382                                             .as_ref()
 383                                             .map(|e| e.span)
 384                                             .expect("`let` stmt without init aren't caught by match_pat"));
 385                                 }
 386                             }
 387                         },
 388
 389                         _ => {},
 390                     }
 391                 }
 392             }
 393         }
 394     }
 395 }
 396
 397 impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
 398     fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 399         if let euv::ConsumeMode::Move(_) = mode {
 400             self.move_common(consume_id, consume_span, cmt);
 401         }
 402     }
 403
 404     fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
 405         if let euv::MatchMode::MovingMatch = mode {
 406             self.move_common(matched_pat.id, matched_pat.span, cmt);
 407         } else {
 408             self.non_moving_pat(matched_pat, cmt);
 409         }
 410     }
 411
 412     fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
 413         if let euv::ConsumeMode::Move(_) = mode {
 414             self.move_common(consume_pat.id, consume_pat.span, cmt);
 415         }
 416     }
 417
 418     fn borrow(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: ty::Region<'_>, _: ty::BorrowKind, _: euv::LoanCause) {}
 419
 420     fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
 421
 422     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
 423 }
 424
 425
 426 fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
 427     loop {
 428         match cmt.cat {
 429             mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
 430                 cmt = c;
 431             },
 432             _ => return (*cmt).clone(),
 433         }
 434     };
 435 }