clippy_lints/src/needless_pass_by_value.rs

   1 use crate::utils::ptr::get_spans;
   2 use crate::utils::{get_trait_def_id, is_self, multispan_sugg, paths, snippet, snippet_opt, span_lint_and_then};
   3 use clippy_utils::ty::{implements_trait, is_copy, is_type_diagnostic_item};
   4 use if_chain::if_chain;
   5 use rustc_ast::ast::Attribute;
   6 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   7 use rustc_errors::{Applicability, DiagnosticBuilder};
   8 use rustc_hir::intravisit::FnKind;
   9 use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Node, PatKind, QPath, TyKind};
  10 use rustc_infer::infer::TyCtxtInferExt;
  11 use rustc_lint::{LateContext, LateLintPass};
  12 use rustc_middle::ty::{self, TypeFoldable};
  13 use rustc_session::{declare_lint_pass, declare_tool_lint};
  14 use rustc_span::symbol::kw;
  15 use rustc_span::{sym, Span};
  16 use rustc_target::spec::abi::Abi;
  17 use rustc_trait_selection::traits;
  18 use rustc_trait_selection::traits::misc::can_type_implement_copy;
  19 use rustc_typeck::expr_use_visitor as euv;
  20 use std::borrow::Cow;
  21
  22 declare_clippy_lint! {
  23     /// **What it does:** Checks for functions taking arguments by value, but not
  24     /// consuming them in its
  25     /// body.
  26     ///
  27     /// **Why is this bad?** Taking arguments by reference is more flexible and can
  28     /// sometimes avoid
  29     /// unnecessary allocations.
  30     ///
  31     /// **Known problems:**
  32     /// * This lint suggests taking an argument by reference,
  33     /// however sometimes it is better to let users decide the argument type
  34     /// (by using `Borrow` trait, for example), depending on how the function is used.
  35     ///
  36     /// **Example:**
  37     /// ```rust
  38     /// fn foo(v: Vec<i32>) {
  39     ///     assert_eq!(v.len(), 42);
  40     /// }
  41     /// ```
  42     /// should be
  43     /// ```rust
  44     /// fn foo(v: &[i32]) {
  45     ///     assert_eq!(v.len(), 42);
  46     /// }
  47     /// ```
  48     pub NEEDLESS_PASS_BY_VALUE,
  49     pedantic,
  50     "functions taking arguments by value, but not consuming them in its body"
  51 }
  52
  53 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  54
  55 macro_rules! need {
  56     ($e: expr) => {
  57         if let Some(x) = $e {
  58             x
  59         } else {
  60             return;
  61         }
  62     };
  63 }
  64
  65 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
  66     #[allow(clippy::too_many_lines)]
  67     fn check_fn(
  68         &mut self,
  69         cx: &LateContext<'tcx>,
  70         kind: FnKind<'tcx>,
  71         decl: &'tcx FnDecl<'_>,
  72         body: &'tcx Body<'_>,
  73         span: Span,
  74         hir_id: HirId,
  75     ) {
  76         if span.from_expansion() {
  77             return;
  78         }
  79
  80         match kind {
  81             FnKind::ItemFn(.., header, _) => {
  82                 let attrs = cx.tcx.hir().attrs(hir_id);
  83                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  84                     return;
  85                 }
  86             },
  87             FnKind::Method(..) => (),
  88             FnKind::Closure => 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(hir_id)) {
  93             if matches!(
  94                 item.kind,
  95                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | 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 allowed_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(hir_id);
 113
 114         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
 115             .filter(|p| !p.is_global())
 116             .filter_map(|obligation| {
 117                 // Note that we do not want to deal with qualified predicates here.
 118                 match obligation.predicate.kind().no_bound_vars() {
 119                     Some(ty::PredicateKind::Trait(pred, _)) if pred.def_id() != sized_trait => Some(pred),
 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::default();
 133             cx.tcx.infer_ctxt().enter(|infcx| {
 134                 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
 135                     .consume_body(body);
 136             });
 137             ctx
 138         };
 139
 140         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 141         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
 142
 143         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 144             // All spans generated from a proc-macro invocation are the same...
 145             if span == input.span {
 146                 return;
 147             }
 148
 149             // Ignore `self`s.
 150             if idx == 0 {
 151                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 152                     if ident.name == kw::SelfLower {
 153                         continue;
 154                     }
 155                 }
 156             }
 157
 158             //
 159             // * Exclude a type that is specifically bounded by `Borrow`.
 160             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 161             //   `serde::Serialize`)
 162             let (implements_borrow_trait, all_borrowable_trait) = {
 163                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
 164
 165                 (
 166                     preds.iter().any(|t| t.def_id() == borrow_trait),
 167                     !preds.is_empty() && {
 168                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
 169                         preds.iter().all(|t| {
 170                             let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
 171                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
 172                         })
 173                     },
 174                 )
 175             };
 176
 177             if_chain! {
 178                 if !is_self(arg);
 179                 if !ty.is_mutable_ptr();
 180                 if !is_copy(cx, ty);
 181                 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 182                 if !implements_borrow_trait;
 183                 if !all_borrowable_trait;
 184
 185                 if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
 186                 if !moved_vars.contains(&canonical_id);
 187                 then {
 188                     if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
 189                         continue;
 190                     }
 191
 192                     // Dereference suggestion
 193                     let sugg = |diag: &mut DiagnosticBuilder<'_>| {
 194                         if let ty::Adt(def, ..) = ty.kind() {
 195                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
 196                                 if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
 197                                     diag.span_help(span, "consider marking this type as `Copy`");
 198                                 }
 199                             }
 200                         }
 201
 202                         let deref_span = spans_need_deref.get(&canonical_id);
 203                         if_chain! {
 204                             if is_type_diagnostic_item(cx, ty, sym::vec_type);
 205                             if let Some(clone_spans) =
 206                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 207                             if let TyKind::Path(QPath::Resolved(_, ref path)) = input.kind;
 208                             if let Some(elem_ty) = path.segments.iter()
 209                                 .find(|seg| seg.ident.name == sym::Vec)
 210                                 .and_then(|ps| ps.args.as_ref())
 211                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 212                                     GenericArg::Type(ty) => Some(ty),
 213                                     _ => None,
 214                                 }).unwrap());
 215                             then {
 216                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 217                                 diag.span_suggestion(
 218                                     input.span,
 219                                     "consider changing the type to",
 220                                     slice_ty,
 221                                     Applicability::Unspecified,
 222                                 );
 223
 224                                 for (span, suggestion) in clone_spans {
 225                                     diag.span_suggestion(
 226                                         span,
 227                                         &snippet_opt(cx, span)
 228                                             .map_or(
 229                                                 "change the call to".into(),
 230                                                 |x| Cow::from(format!("change `{}` to", x)),
 231                                             ),
 232                                         suggestion.into(),
 233                                         Applicability::Unspecified,
 234                                     );
 235                                 }
 236
 237                                 // cannot be destructured, no need for `*` suggestion
 238                                 assert!(deref_span.is_none());
 239                                 return;
 240                             }
 241                         }
 242
 243                         if is_type_diagnostic_item(cx, ty, sym::string_type) {
 244                             if let Some(clone_spans) =
 245                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 246                                 diag.span_suggestion(
 247                                     input.span,
 248                                     "consider changing the type to",
 249                                     "&str".to_string(),
 250                                     Applicability::Unspecified,
 251                                 );
 252
 253                                 for (span, suggestion) in clone_spans {
 254                                     diag.span_suggestion(
 255                                         span,
 256                                         &snippet_opt(cx, span)
 257                                             .map_or(
 258                                                 "change the call to".into(),
 259                                                 |x| Cow::from(format!("change `{}` to", x))
 260                                             ),
 261                                         suggestion.into(),
 262                                         Applicability::Unspecified,
 263                                     );
 264                                 }
 265
 266                                 assert!(deref_span.is_none());
 267                                 return;
 268                             }
 269                         }
 270
 271                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 272
 273                         // Suggests adding `*` to dereference the added reference.
 274                         if let Some(deref_span) = deref_span {
 275                             spans.extend(
 276                                 deref_span
 277                                     .iter()
 278                                     .cloned()
 279                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 280                             );
 281                             spans.sort_by_key(|&(span, _)| span);
 282                         }
 283                         multispan_sugg(diag, "consider taking a reference instead", spans);
 284                     };
 285
 286                     span_lint_and_then(
 287                         cx,
 288                         NEEDLESS_PASS_BY_VALUE,
 289                         input.span,
 290                         "this argument is passed by value, but not consumed in the function body",
 291                         sugg,
 292                     );
 293                 }
 294             }
 295         }
 296     }
 297 }
 298
 299 /// Functions marked with these attributes must have the exact signature.
 300 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 301     attrs.iter().any(|attr| {
 302         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
 303             .iter()
 304             .any(|&allow| attr.has_name(allow))
 305     })
 306 }
 307
 308 #[derive(Default)]
 309 struct MovedVariablesCtxt {
 310     moved_vars: FxHashSet<HirId>,
 311     /// Spans which need to be prefixed with `*` for dereferencing the
 312     /// suggested additional reference.
 313     spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
 314 }
 315
 316 impl MovedVariablesCtxt {
 317     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
 318         if let euv::PlaceBase::Local(vid) = cmt.place.base {
 319             self.moved_vars.insert(vid);
 320         }
 321     }
 322 }
 323
 324 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 325     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId, mode: euv::ConsumeMode) {
 326         if let euv::ConsumeMode::Move = mode {
 327             self.move_common(cmt);
 328         }
 329     }
 330
 331     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
 332
 333     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
 334 }