clippy_lints/src/needless_pass_by_value.rs

   1 use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then};
   2 use clippy_utils::ptr::get_spans;
   3 use clippy_utils::source::{snippet, snippet_opt};
   4 use clippy_utils::ty::{implements_trait, is_copy, is_type_diagnostic_item};
   5 use clippy_utils::{get_trait_def_id, is_self, paths};
   6 use if_chain::if_chain;
   7 use rustc_ast::ast::Attribute;
   8 use rustc_data_structures::fx::FxHashSet;
   9 use rustc_errors::{Applicability, DiagnosticBuilder};
  10 use rustc_hir::intravisit::FnKind;
  11 use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Node, PatKind, QPath, TyKind};
  12 use rustc_hir::{HirIdMap, HirIdSet};
  13 use rustc_infer::infer::TyCtxtInferExt;
  14 use rustc_lint::{LateContext, LateLintPass};
  15 use rustc_middle::mir::FakeReadCause;
  16 use rustc_middle::ty::{self, TypeFoldable};
  17 use rustc_session::{declare_lint_pass, declare_tool_lint};
  18 use rustc_span::symbol::kw;
  19 use rustc_span::{sym, Span};
  20 use rustc_target::spec::abi::Abi;
  21 use rustc_trait_selection::traits;
  22 use rustc_trait_selection::traits::misc::can_type_implement_copy;
  23 use rustc_typeck::expr_use_visitor as euv;
  24 use std::borrow::Cow;
  25
  26 declare_clippy_lint! {
  27     /// **What it does:** Checks for functions taking arguments by value, but not
  28     /// consuming them in its
  29     /// body.
  30     ///
  31     /// **Why is this bad?** Taking arguments by reference is more flexible and can
  32     /// sometimes avoid
  33     /// unnecessary allocations.
  34     ///
  35     /// **Known problems:**
  36     /// * This lint suggests taking an argument by reference,
  37     /// however sometimes it is better to let users decide the argument type
  38     /// (by using `Borrow` trait, for example), depending on how the function is used.
  39     ///
  40     /// **Example:**
  41     /// ```rust
  42     /// fn foo(v: Vec<i32>) {
  43     ///     assert_eq!(v.len(), 42);
  44     /// }
  45     /// ```
  46     /// should be
  47     /// ```rust
  48     /// fn foo(v: &[i32]) {
  49     ///     assert_eq!(v.len(), 42);
  50     /// }
  51     /// ```
  52     pub NEEDLESS_PASS_BY_VALUE,
  53     pedantic,
  54     "functions taking arguments by value, but not consuming them in its body"
  55 }
  56
  57 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  58
  59 macro_rules! need {
  60     ($e: expr) => {
  61         if let Some(x) = $e {
  62             x
  63         } else {
  64             return;
  65         }
  66     };
  67 }
  68
  69 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
  70     #[allow(clippy::too_many_lines)]
  71     fn check_fn(
  72         &mut self,
  73         cx: &LateContext<'tcx>,
  74         kind: FnKind<'tcx>,
  75         decl: &'tcx FnDecl<'_>,
  76         body: &'tcx Body<'_>,
  77         span: Span,
  78         hir_id: HirId,
  79     ) {
  80         if span.from_expansion() {
  81             return;
  82         }
  83
  84         match kind {
  85             FnKind::ItemFn(.., header, _) => {
  86                 let attrs = cx.tcx.hir().attrs(hir_id);
  87                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  88                     return;
  89                 }
  90             },
  91             FnKind::Method(..) => (),
  92             FnKind::Closure => return,
  93         }
  94
  95         // Exclude non-inherent impls
  96         if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
  97             if matches!(
  98                 item.kind,
  99                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
 100             ) {
 101                 return;
 102             }
 103         }
 104
 105         // Allow `Borrow` or functions to be taken by value
 106         let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
 107         let allowed_traits = [
 108             need!(cx.tcx.lang_items().fn_trait()),
 109             need!(cx.tcx.lang_items().fn_once_trait()),
 110             need!(cx.tcx.lang_items().fn_mut_trait()),
 111             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 112         ];
 113
 114         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 115
 116         let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
 117
 118         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
 119             .filter(|p| !p.is_global())
 120             .filter_map(|obligation| {
 121                 // Note that we do not want to deal with qualified predicates here.
 122                 match obligation.predicate.kind().no_bound_vars() {
 123                     Some(ty::PredicateKind::Trait(pred, _)) if pred.def_id() != sized_trait => Some(pred),
 124                     _ => None,
 125                 }
 126             })
 127             .collect::<Vec<_>>();
 128
 129         // Collect moved variables and spans which will need dereferencings from the
 130         // function body.
 131         let MovedVariablesCtxt {
 132             moved_vars,
 133             spans_need_deref,
 134             ..
 135         } = {
 136             let mut ctx = MovedVariablesCtxt::default();
 137             cx.tcx.infer_ctxt().enter(|infcx| {
 138                 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
 139                     .consume_body(body);
 140             });
 141             ctx
 142         };
 143
 144         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 145         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
 146
 147         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 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(.., ident, _) = arg.pat.kind {
 156                     if ident.name == kw::SelfLower {
 157                         continue;
 158                     }
 159                 }
 160             }
 161
 162             //
 163             // * Exclude a type that is specifically bounded by `Borrow`.
 164             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 165             //   `serde::Serialize`)
 166             let (implements_borrow_trait, all_borrowable_trait) = {
 167                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
 168
 169                 (
 170                     preds.iter().any(|t| t.def_id() == borrow_trait),
 171                     !preds.is_empty() && {
 172                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
 173                         preds.iter().all(|t| {
 174                             let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
 175                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
 176                         })
 177                     },
 178                 )
 179             };
 180
 181             if_chain! {
 182                 if !is_self(arg);
 183                 if !ty.is_mutable_ptr();
 184                 if !is_copy(cx, ty);
 185                 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 186                 if !implements_borrow_trait;
 187                 if !all_borrowable_trait;
 188
 189                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
 190                 if !moved_vars.contains(&canonical_id);
 191                 then {
 192                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 193                         continue;
 194                     }
 195
 196                     // Dereference suggestion
 197                     let sugg = |diag: &mut DiagnosticBuilder<'_>| {
 198                         if let ty::Adt(def, ..) = ty.kind() {
 199                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 200                                 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
 201                                     diag.span_help(span, "consider marking this type as `Copy`");
 202                                 }
 203                             }
 204                         }
 205
 206                         let deref_span = spans_need_deref.get(&canonical_id);
 207                         if_chain! {
 208                             if is_type_diagnostic_item(cx, ty, sym::vec_type);
 209                             if let Some(clone_spans) =
 210                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 211                             if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
 212                             if let Some(elem_ty) = path.segments.iter()
 213                                 .find(|seg| seg.ident.name == sym::Vec)
 214                                 .and_then(|ps| ps.args.as_ref())
 215                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 216                                     GenericArg::Type(ty) => Some(ty),
 217                                     _ => None,
 218                                 }).unwrap());
 219                             then {
 220                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 221                                 diag.span_suggestion(
 222                                     input.span,
 223                                     "consider changing the type to",
 224                                     slice_ty,
 225                                     Applicability::Unspecified,
 226                                 );
 227
 228                                 for (span, suggestion) in clone_spans {
 229                                     diag.span_suggestion(
 230                                         span,
 231                                         &snippet_opt(cx, span)
 232                                             .map_or(
 233                                                 "change the call to".into(),
 234                                                 |x| Cow::from(format!("change `{}` to", x)),
 235                                             ),
 236                                         suggestion.into(),
 237                                         Applicability::Unspecified,
 238                                     );
 239                                 }
 240
 241                                 // cannot be destructured, no need for `*` suggestion
 242                                 assert!(deref_span.is_none());
 243                                 return;
 244                             }
 245                         }
 246
 247                         if is_type_diagnostic_item(cx, ty, sym::string_type) {
 248                             if let Some(clone_spans) =
 249                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 250                                 diag.span_suggestion(
 251                                     input.span,
 252                                     "consider changing the type to",
 253                                     "&str".to_string(),
 254                                     Applicability::Unspecified,
 255                                 );
 256
 257                                 for (span, suggestion) in clone_spans {
 258                                     diag.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                                         Applicability::Unspecified,
 267                                     );
 268                                 }
 269
 270                                 assert!(deref_span.is_none());
 271                                 return;
 272                             }
 273                         }
 274
 275                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 276
 277                         // Suggests adding `*` to dereference the added reference.
 278                         if let Some(deref_span) = deref_span {
 279                             spans.extend(
 280                                 deref_span
 281                                     .iter()
 282                                     .copied()
 283                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 284                             );
 285                             spans.sort_by_key(|&(span, _)| span);
 286                         }
 287                         multispan_sugg(diag, "consider taking a reference instead", spans);
 288                     };
 289
 290                     span_lint_and_then(
 291                         cx,
 292                         NEEDLESS_PASS_BY_VALUE,
 293                         input.span,
 294                         "this argument is passed by value, but not consumed in the function body",
 295                         sugg,
 296                     );
 297                 }
 298             }
 299         }
 300     }
 301 }
 302
 303 /// Functions marked with these attributes must have the exact signature.
 304 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 305     attrs.iter().any(|attr| {
 306         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
 307             .iter()
 308             .any(|&allow| attr.has_name(allow))
 309     })
 310 }
 311
 312 #[derive(Default)]
 313 struct MovedVariablesCtxt {
 314     moved_vars: HirIdSet,
 315     /// Spans which need to be prefixed with `*` for dereferencing the
 316     /// suggested additional reference.
 317     spans_need_deref: HirIdMap<FxHashSet<Span>>,
 318 }
 319
 320 impl MovedVariablesCtxt {
 321     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
 322         if let euv::PlaceBase::Local(vid) = cmt.place.base {
 323             self.moved_vars.insert(vid);
 324         }
 325     }
 326 }
 327
 328 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 329     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId, mode: euv::ConsumeMode) {
 330         if let euv::ConsumeMode::Move = mode {
 331             self.move_common(cmt);
 332         }
 333     }
 334
 335     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
 336
 337     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
 338
 339     fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}
 340 }