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, is_type_lang_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, Diagnostic};
  10 use rustc_hir::intravisit::FnKind;
  11 use rustc_hir::{
  12     BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Mutability, Node, PatKind, QPath, TyKind,
  13 };
  14 use rustc_hir::{HirIdMap, HirIdSet, LangItem};
  15 use rustc_hir_typeck::expr_use_visitor as euv;
  16 use rustc_infer::infer::TyCtxtInferExt;
  17 use rustc_lint::{LateContext, LateLintPass};
  18 use rustc_middle::mir::FakeReadCause;
  19 use rustc_middle::ty::{self, TypeVisitable};
  20 use rustc_session::{declare_lint_pass, declare_tool_lint};
  21 use rustc_span::symbol::kw;
  22 use rustc_span::{sym, Span};
  23 use rustc_target::spec::abi::Abi;
  24 use rustc_trait_selection::traits;
  25 use rustc_trait_selection::traits::misc::can_type_implement_copy;
  26 use std::borrow::Cow;
  27
  28 declare_clippy_lint! {
  29     /// ### What it does
  30     /// Checks for functions taking arguments by value, but not
  31     /// consuming them in its
  32     /// body.
  33     ///
  34     /// ### Why is this bad?
  35     /// Taking arguments by reference is more flexible and can
  36     /// sometimes avoid
  37     /// unnecessary allocations.
  38     ///
  39     /// ### Known problems
  40     /// * This lint suggests taking an argument by reference,
  41     /// however sometimes it is better to let users decide the argument type
  42     /// (by using `Borrow` trait, for example), depending on how the function is used.
  43     ///
  44     /// ### Example
  45     /// ```rust
  46     /// fn foo(v: Vec<i32>) {
  47     ///     assert_eq!(v.len(), 42);
  48     /// }
  49     /// ```
  50     /// should be
  51     /// ```rust
  52     /// fn foo(v: &[i32]) {
  53     ///     assert_eq!(v.len(), 42);
  54     /// }
  55     /// ```
  56     #[clippy::version = "pre 1.29.0"]
  57     pub NEEDLESS_PASS_BY_VALUE,
  58     pedantic,
  59     "functions taking arguments by value, but not consuming them in its body"
  60 }
  61
  62 declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
  63
  64 macro_rules! need {
  65     ($e: expr) => {
  66         if let Some(x) = $e {
  67             x
  68         } else {
  69             return;
  70         }
  71     };
  72 }
  73
  74 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
  75     #[expect(clippy::too_many_lines)]
  76     fn check_fn(
  77         &mut self,
  78         cx: &LateContext<'tcx>,
  79         kind: FnKind<'tcx>,
  80         decl: &'tcx FnDecl<'_>,
  81         body: &'tcx Body<'_>,
  82         span: Span,
  83         hir_id: HirId,
  84     ) {
  85         if span.from_expansion() {
  86             return;
  87         }
  88
  89         match kind {
  90             FnKind::ItemFn(.., header) => {
  91                 let attrs = cx.tcx.hir().attrs(hir_id);
  92                 if header.abi != Abi::Rust || requires_exact_signature(attrs) {
  93                     return;
  94                 }
  95             },
  96             FnKind::Method(..) => (),
  97             FnKind::Closure => return,
  98         }
  99
 100         // Exclude non-inherent impls
 101         if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
 102             if matches!(
 103                 item.kind,
 104                 ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
 105             ) {
 106                 return;
 107             }
 108         }
 109
 110         // Allow `Borrow` or functions to be taken by value
 111         let allowed_traits = [
 112             need!(cx.tcx.lang_items().fn_trait()),
 113             need!(cx.tcx.lang_items().fn_once_trait()),
 114             need!(cx.tcx.lang_items().fn_mut_trait()),
 115             need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
 116         ];
 117
 118         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
 119
 120         let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
 121
 122         let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
 123             .filter(|p| !p.is_global())
 124             .filter_map(|obligation| {
 125                 // Note that we do not want to deal with qualified predicates here.
 126                 match obligation.predicate.kind().no_bound_vars() {
 127                     Some(ty::PredicateKind::Trait(pred)) if pred.def_id() != sized_trait => Some(pred),
 128                     _ => None,
 129                 }
 130             })
 131             .collect::<Vec<_>>();
 132
 133         // Collect moved variables and spans which will need dereferencings from the
 134         // function body.
 135         let MovedVariablesCtxt {
 136             moved_vars,
 137             spans_need_deref,
 138             ..
 139         } = {
 140             let mut ctx = MovedVariablesCtxt::default();
 141             let infcx = cx.tcx.infer_ctxt().build();
 142             euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results()).consume_body(body);
 143             ctx
 144         };
 145
 146         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 147         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
 148
 149         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 150             // All spans generated from a proc-macro invocation are the same...
 151             if span == input.span {
 152                 return;
 153             }
 154
 155             // Ignore `self`s.
 156             if idx == 0 {
 157                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 158                     if ident.name == kw::SelfLower {
 159                         continue;
 160                     }
 161                 }
 162             }
 163
 164             //
 165             // * Exclude a type that is specifically bounded by `Borrow`.
 166             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 167             //   `serde::Serialize`)
 168             let (implements_borrow_trait, all_borrowable_trait) = {
 169                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
 170
 171                 (
 172                     preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())),
 173                     !preds.is_empty() && {
 174                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_erased, ty);
 175                         preds.iter().all(|t| {
 176                             let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
 177                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
 178                         })
 179                     },
 180                 )
 181             };
 182
 183             if_chain! {
 184                 if !is_self(arg);
 185                 if !ty.is_mutable_ptr();
 186                 if !is_copy(cx, ty);
 187                 if ty.is_sized(cx.tcx, cx.param_env);
 188                 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 189                 if !implements_borrow_trait;
 190                 if !all_borrowable_trait;
 191
 192                 if let PatKind::Binding(BindingAnnotation(_, Mutability::Not), canonical_id, ..) = arg.pat.kind;
 193                 if !moved_vars.contains(&canonical_id);
 194                 then {
 195                     // Dereference suggestion
 196                     let sugg = |diag: &mut Diagnostic| {
 197                         if let ty::Adt(def, ..) = ty.kind() {
 198                             if let Some(span) = cx.tcx.hir().span_if_local(def.did()) {
 199                                 if can_type_implement_copy(
 200                                     cx.tcx,
 201                                     cx.param_env,
 202                                     ty,
 203                                     traits::ObligationCause::dummy_with_span(span),
 204                                 ).is_ok() {
 205                                     diag.span_help(span, "consider marking this type as `Copy`");
 206                                 }
 207                             }
 208                         }
 209
 210                         let deref_span = spans_need_deref.get(&canonical_id);
 211                         if_chain! {
 212                             if is_type_diagnostic_item(cx, ty, sym::Vec);
 213                             if let Some(clone_spans) =
 214                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 215                             if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
 216                             if let Some(elem_ty) = path.segments.iter()
 217                                 .find(|seg| seg.ident.name == sym::Vec)
 218                                 .and_then(|ps| ps.args.as_ref())
 219                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 220                                     GenericArg::Type(ty) => Some(ty),
 221                                     _ => None,
 222                                 }).unwrap());
 223                             then {
 224                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 225                                 diag.span_suggestion(
 226                                     input.span,
 227                                     "consider changing the type to",
 228                                     slice_ty,
 229                                     Applicability::Unspecified,
 230                                 );
 231
 232                                 for (span, suggestion) in clone_spans {
 233                                     diag.span_suggestion(
 234                                         span,
 235                                         snippet_opt(cx, span)
 236                                             .map_or(
 237                                                 "change the call to".into(),
 238                                                 |x| Cow::from(format!("change `{x}` to")),
 239                                             )
 240                                             .as_ref(),
 241                                         suggestion,
 242                                         Applicability::Unspecified,
 243                                     );
 244                                 }
 245
 246                                 // cannot be destructured, no need for `*` suggestion
 247                                 assert!(deref_span.is_none());
 248                                 return;
 249                             }
 250                         }
 251
 252                         if is_type_lang_item(cx, ty, LangItem::String) {
 253                             if let Some(clone_spans) =
 254                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 255                                 diag.span_suggestion(
 256                                     input.span,
 257                                     "consider changing the type to",
 258                                     "&str",
 259                                     Applicability::Unspecified,
 260                                 );
 261
 262                                 for (span, suggestion) in clone_spans {
 263                                     diag.span_suggestion(
 264                                         span,
 265                                         snippet_opt(cx, span)
 266                                             .map_or(
 267                                                 "change the call to".into(),
 268                                                 |x| Cow::from(format!("change `{x}` to"))
 269                                             )
 270                                             .as_ref(),
 271                                         suggestion,
 272                                         Applicability::Unspecified,
 273                                     );
 274                                 }
 275
 276                                 assert!(deref_span.is_none());
 277                                 return;
 278                             }
 279                         }
 280
 281                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 282
 283                         // Suggests adding `*` to dereference the added reference.
 284                         if let Some(deref_span) = deref_span {
 285                             spans.extend(
 286                                 deref_span
 287                                     .iter()
 288                                     .copied()
 289                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 290                             );
 291                             spans.sort_by_key(|&(span, _)| span);
 292                         }
 293                         multispan_sugg(diag, "consider taking a reference instead", spans);
 294                     };
 295
 296                     span_lint_and_then(
 297                         cx,
 298                         NEEDLESS_PASS_BY_VALUE,
 299                         input.span,
 300                         "this argument is passed by value, but not consumed in the function body",
 301                         sugg,
 302                     );
 303                 }
 304             }
 305         }
 306     }
 307 }
 308
 309 /// Functions marked with these attributes must have the exact signature.
 310 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 311     attrs.iter().any(|attr| {
 312         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
 313             .iter()
 314             .any(|&allow| attr.has_name(allow))
 315     })
 316 }
 317
 318 #[derive(Default)]
 319 struct MovedVariablesCtxt {
 320     moved_vars: HirIdSet,
 321     /// Spans which need to be prefixed with `*` for dereferencing the
 322     /// suggested additional reference.
 323     spans_need_deref: HirIdMap<FxHashSet<Span>>,
 324 }
 325
 326 impl MovedVariablesCtxt {
 327     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
 328         if let euv::PlaceBase::Local(vid) = cmt.place.base {
 329             self.moved_vars.insert(vid);
 330         }
 331     }
 332 }
 333
 334 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 335     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) {
 336         self.move_common(cmt);
 337     }
 338
 339     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
 340
 341     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
 342
 343     fn fake_read(
 344         &mut self,
 345         _: &rustc_hir_typeck::expr_use_visitor::PlaceWithHirId<'tcx>,
 346         _: FakeReadCause,
 347         _: HirId,
 348     ) {
 349     }
 350 }