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::{
   5     implements_trait, implements_trait_with_env, is_copy, is_type_diagnostic_item, is_type_lang_item,
   6 };
   7 use clippy_utils::{get_trait_def_id, is_self, paths};
   8 use if_chain::if_chain;
   9 use rustc_ast::ast::Attribute;
  10 use rustc_data_structures::fx::FxHashSet;
  11 use rustc_errors::{Applicability, Diagnostic};
  12 use rustc_hir::intravisit::FnKind;
  13 use rustc_hir::{
  14     BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Mutability, Node, PatKind, QPath, TyKind,
  15 };
  16 use rustc_hir::{HirIdMap, HirIdSet, LangItem};
  17 use rustc_hir_typeck::expr_use_visitor as euv;
  18 use rustc_infer::infer::TyCtxtInferExt;
  19 use rustc_lint::{LateContext, LateLintPass};
  20 use rustc_middle::mir::FakeReadCause;
  21 use rustc_middle::ty::{self, TypeVisitable};
  22 use rustc_session::{declare_lint_pass, declare_tool_lint};
  23 use rustc_span::symbol::kw;
  24 use rustc_span::{sym, Span};
  25 use rustc_target::spec::abi::Abi;
  26 use rustc_trait_selection::traits;
  27 use rustc_trait_selection::traits::misc::type_allowed_to_implement_copy;
  28 use std::borrow::Cow;
  29
  30 declare_clippy_lint! {
  31     /// ### What it does
  32     /// Checks for functions taking arguments by value, but not
  33     /// consuming them in its
  34     /// body.
  35     ///
  36     /// ### Why is this bad?
  37     /// Taking arguments by reference is more flexible and can
  38     /// sometimes avoid
  39     /// unnecessary allocations.
  40     ///
  41     /// ### Known problems
  42     /// * This lint suggests taking an argument by reference,
  43     /// however sometimes it is better to let users decide the argument type
  44     /// (by using `Borrow` trait, for example), depending on how the function is used.
  45     ///
  46     /// ### Example
  47     /// ```rust
  48     /// fn foo(v: Vec<i32>) {
  49     ///     assert_eq!(v.len(), 42);
  50     /// }
  51     /// ```
  52     /// should be
  53     /// ```rust
  54     /// fn foo(v: &[i32]) {
  55     ///     assert_eq!(v.len(), 42);
  56     /// }
  57     /// ```
  58     #[clippy::version = "pre 1.29.0"]
  59     pub NEEDLESS_PASS_BY_VALUE,
  60     pedantic,
  61     "functions taking arguments by value, but not consuming them in its body"
  62 }
  63
  64 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  65
  66 macro_rules! need {
  67     ($e: expr) => {
  68         if let Some(x) = $e {
  69             x
  70         } else {
  71             return;
  72         }
  73     };
  74 }
  75
  76 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
  77     #[expect(clippy::too_many_lines)]
  78     fn check_fn(
  79         &mut self,
  80         cx: &LateContext<'tcx>,
  81         kind: FnKind<'tcx>,
  82         decl: &'tcx FnDecl<'_>,
  83         body: &'tcx Body<'_>,
  84         span: Span,
  85         hir_id: HirId,
  86     ) {
  87         if span.from_expansion() {
  88             return;
  89         }
  90
  91         match kind {
  92             FnKind::ItemFn(.., header) => {
  93                 let attrs = cx.tcx.hir().attrs(hir_id);
  94                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  95                     return;
  96                 }
  97             },
  98             FnKind::Method(..) => (),
  99             FnKind::Closure => return,
 100         }
 101
 102         // Exclude non-inherent impls
 103         if let Some(Node::Item(item)) = cx.tcx.hir().find_parent(hir_id) {
 104             if matches!(
 105                 item.kind,
 106                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
 107             ) {
 108                 return;
 109             }
 110         }
 111
 112         // Allow `Borrow` or functions to be taken by value
 113         let allowed_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(hir_id);
 123
 124         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
 125             .filter(|p| !p.is_global())
 126             .filter_map(|obligation| {
 127                 // Note that we do not want to deal with qualified predicates here.
 128                 match obligation.predicate.kind().no_bound_vars() {
 129                     Some(ty::PredicateKind::Clause(ty::Clause::Trait(pred))) if pred.def_id() != sized_trait => {
 130                         Some(pred)
 131                     },
 132                     _ => None,
 133                 }
 134             })
 135             .collect::<Vec<_>>();
 136
 137         // Collect moved variables and spans which will need dereferencings from the
 138         // function body.
 139         let MovedVariablesCtxt {
 140             moved_vars,
 141             spans_need_deref,
 142             ..
 143         } = {
 144             let mut ctx = MovedVariablesCtxt::default();
 145             let infcx = cx.tcx.infer_ctxt().build();
 146             euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body);
 147             ctx
 148         };
 149
 150         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 151         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
 152
 153         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 154             // All spans generated from a proc-macro invocation are the same...
 155             if span == input.span {
 156                 return;
 157             }
 158
 159             // Ignore `self`s.
 160             if idx == 0 {
 161                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 162                     if ident.name == kw::SelfLower {
 163                         continue;
 164                     }
 165                 }
 166             }
 167
 168             //
 169             // * Exclude a type that is specifically bounded by `Borrow`.
 170             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 171             //   `serde::Serialize`)
 172             let (implements_borrow_trait, all_borrowable_trait) = {
 173                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
 174
 175                 (
 176                     preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())),
 177                     !preds.is_empty() && {
 178                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_erased, ty);
 179                         preds.iter().all(|t| {
 180                             let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
 181                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
 182                         })
 183                     },
 184                 )
 185             };
 186
 187             if_chain! {
 188                 if !is_self(arg);
 189                 if !ty.is_mutable_ptr();
 190                 if !is_copy(cx, ty);
 191                 if ty.is_sized(cx.tcx, cx.param_env);
 192                 if !allowed_traits.iter().any(|&t| implements_trait_with_env(cx.tcx, cx.param_env, ty, t, [None]));
 193                 if !implements_borrow_trait;
 194                 if !all_borrowable_trait;
 195
 196                 if let PatKind::Binding(BindingAnnotation(_, Mutability::Not), canonical_id, ..) = arg.pat.kind;
 197                 if !moved_vars.contains(&canonical_id);
 198                 then {
 199                     // Dereference suggestion
 200                     let sugg = |diag: &mut Diagnostic| {
 201                         if let ty::Adt(def, ..) = ty.kind() {
 202                             if let Some(span) = cx.tcx.hir().span_if_local(def.did()) {
 203                                 if type_allowed_to_implement_copy(
 204                                     cx.tcx,
 205                                     cx.param_env,
 206                                     ty,
 207                                     traits::ObligationCause::dummy_with_span(span),
 208                                 ).is_ok() {
 209                                     diag.span_help(span, "consider marking this type as `Copy`");
 210                                 }
 211                             }
 212                         }
 213
 214                         let deref_span = spans_need_deref.get(&canonical_id);
 215                         if_chain! {
 216                             if is_type_diagnostic_item(cx, ty, sym::Vec);
 217                             if let Some(clone_spans) =
 218                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 219                             if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
 220                             if let Some(elem_ty) = path.segments.iter()
 221                                 .find(|seg| seg.ident.name == sym::Vec)
 222                                 .and_then(|ps| ps.args.as_ref())
 223                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 224                                     GenericArg::Type(ty) => Some(ty),
 225                                     _ => None,
 226                                 }).unwrap());
 227                             then {
 228                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 229                                 diag.span_suggestion(
 230                                     input.span,
 231                                     "consider changing the type to",
 232                                     slice_ty,
 233                                     Applicability::Unspecified,
 234                                 );
 235
 236                                 for (span, suggestion) in clone_spans {
 237                                     diag.span_suggestion(
 238                                         span,
 239                                         snippet_opt(cx, span)
 240                                             .map_or(
 241                                                 "change the call to".into(),
 242                                                 |x| Cow::from(format!("change `{x}` to")),
 243                                             )
 244                                             .as_ref(),
 245                                         suggestion,
 246                                         Applicability::Unspecified,
 247                                     );
 248                                 }
 249
 250                                 // cannot be destructured, no need for `*` suggestion
 251                                 assert!(deref_span.is_none());
 252                                 return;
 253                             }
 254                         }
 255
 256                         if is_type_lang_item(cx, ty, LangItem::String) {
 257                             if let Some(clone_spans) =
 258                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 259                                 diag.span_suggestion(
 260                                     input.span,
 261                                     "consider changing the type to",
 262                                     "&str",
 263                                     Applicability::Unspecified,
 264                                 );
 265
 266                                 for (span, suggestion) in clone_spans {
 267                                     diag.span_suggestion(
 268                                         span,
 269                                         snippet_opt(cx, span)
 270                                             .map_or(
 271                                                 "change the call to".into(),
 272                                                 |x| Cow::from(format!("change `{x}` to"))
 273                                             )
 274                                             .as_ref(),
 275                                         suggestion,
 276                                         Applicability::Unspecified,
 277                                     );
 278                                 }
 279
 280                                 assert!(deref_span.is_none());
 281                                 return;
 282                             }
 283                         }
 284
 285                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 286
 287                         // Suggests adding `*` to dereference the added reference.
 288                         if let Some(deref_span) = deref_span {
 289                             spans.extend(
 290                                 deref_span
 291                                     .iter()
 292                                     .copied()
 293                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 294                             );
 295                             spans.sort_by_key(|&(span, _)| span);
 296                         }
 297                         multispan_sugg(diag, "consider taking a reference instead", spans);
 298                     };
 299
 300                     span_lint_and_then(
 301                         cx,
 302                         NEEDLESS_PASS_BY_VALUE,
 303                         input.span,
 304                         "this argument is passed by value, but not consumed in the function body",
 305                         sugg,
 306                     );
 307                 }
 308             }
 309         }
 310     }
 311 }
 312
 313 /// Functions marked with these attributes must have the exact signature.
 314 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 315     attrs.iter().any(|attr| {
 316         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
 317             .iter()
 318             .any(|&allow| attr.has_name(allow))
 319     })
 320 }
 321
 322 #[derive(Default)]
 323 struct MovedVariablesCtxt {
 324     moved_vars: HirIdSet,
 325     /// Spans which need to be prefixed with `*` for dereferencing the
 326     /// suggested additional reference.
 327     spans_need_deref: HirIdMap<FxHashSet<Span>>,
 328 }
 329
 330 impl MovedVariablesCtxt {
 331     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
 332         if let euv::PlaceBase::Local(vid) = cmt.place.base {
 333             self.moved_vars.insert(vid);
 334         }
 335     }
 336 }
 337
 338 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 339     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) {
 340         self.move_common(cmt);
 341     }
 342
 343     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
 344
 345     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
 346
 347     fn fake_read(&mut self, _: &rustc_hir_typeck::expr_use_visitor::PlaceWithHirId<'tcx>, _: FakeReadCause, _: HirId) {}
 348 }