src/tools/clippy/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, 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};
  15 use rustc_infer::infer::TyCtxtInferExt;
  16 use rustc_lint::{LateContext, LateLintPass};
  17 use rustc_middle::mir::FakeReadCause;
  18 use rustc_middle::ty::{self, TypeVisitable};
  19 use rustc_session::{declare_lint_pass, declare_tool_lint};
  20 use rustc_span::symbol::kw;
  21 use rustc_span::{sym, Span};
  22 use rustc_target::spec::abi::Abi;
  23 use rustc_trait_selection::traits;
  24 use rustc_trait_selection::traits::misc::can_type_implement_copy;
  25 use rustc_hir_analysis::expr_use_visitor as euv;
  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             cx.tcx.infer_ctxt().enter(|infcx| {
 142                 euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
 143                     .consume_body(body);
 144             });
 145             ctx
 146         };
 147
 148         let fn_sig = cx.tcx.fn_sig(fn_def_id);
 149         let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
 150
 151         for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
 152             // All spans generated from a proc-macro invocation are the same...
 153             if span == input.span {
 154                 return;
 155             }
 156
 157             // Ignore `self`s.
 158             if idx == 0 {
 159                 if let PatKind::Binding(.., ident, _) = arg.pat.kind {
 160                     if ident.name == kw::SelfLower {
 161                         continue;
 162                     }
 163                 }
 164             }
 165
 166             //
 167             // * Exclude a type that is specifically bounded by `Borrow`.
 168             // * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
 169             //   `serde::Serialize`)
 170             let (implements_borrow_trait, all_borrowable_trait) = {
 171                 let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
 172
 173                 (
 174                     preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())),
 175                     !preds.is_empty() && {
 176                         let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_erased, ty);
 177                         preds.iter().all(|t| {
 178                             let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
 179                             implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
 180                         })
 181                     },
 182                 )
 183             };
 184
 185             if_chain! {
 186                 if !is_self(arg);
 187                 if !ty.is_mutable_ptr();
 188                 if !is_copy(cx, ty);
 189                 if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
 190                 if !implements_borrow_trait;
 191                 if !all_borrowable_trait;
 192
 193                 if let PatKind::Binding(BindingAnnotation(_, Mutability::Not), canonical_id, ..) = arg.pat.kind;
 194                 if !moved_vars.contains(&canonical_id);
 195                 then {
 196                     // Dereference suggestion
 197                     let sugg = |diag: &mut Diagnostic| {
 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(
 201                                     cx.tcx,
 202                                     cx.param_env,
 203                                     ty,
 204                                     traits::ObligationCause::dummy_with_span(span),
 205                                 ).is_ok() {
 206                                     diag.span_help(span, "consider marking this type as `Copy`");
 207                                 }
 208                             }
 209                         }
 210
 211                         let deref_span = spans_need_deref.get(&canonical_id);
 212                         if_chain! {
 213                             if is_type_diagnostic_item(cx, ty, sym::Vec);
 214                             if let Some(clone_spans) =
 215                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
 216                             if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
 217                             if let Some(elem_ty) = path.segments.iter()
 218                                 .find(|seg| seg.ident.name == sym::Vec)
 219                                 .and_then(|ps| ps.args.as_ref())
 220                                 .map(|params| params.args.iter().find_map(|arg| match arg {
 221                                     GenericArg::Type(ty) => Some(ty),
 222                                     _ => None,
 223                                 }).unwrap());
 224                             then {
 225                                 let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
 226                                 diag.span_suggestion(
 227                                     input.span,
 228                                     "consider changing the type to",
 229                                     slice_ty,
 230                                     Applicability::Unspecified,
 231                                 );
 232
 233                                 for (span, suggestion) in clone_spans {
 234                                     diag.span_suggestion(
 235                                         span,
 236                                         snippet_opt(cx, span)
 237                                             .map_or(
 238                                                 "change the call to".into(),
 239                                                 |x| Cow::from(format!("change `{}` to", x)),
 240                                             )
 241                                             .as_ref(),
 242                                         suggestion,
 243                                         Applicability::Unspecified,
 244                                     );
 245                                 }
 246
 247                                 // cannot be destructured, no need for `*` suggestion
 248                                 assert!(deref_span.is_none());
 249                                 return;
 250                             }
 251                         }
 252
 253                         if is_type_diagnostic_item(cx, ty, sym::String) {
 254                             if let Some(clone_spans) =
 255                                 get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
 256                                 diag.span_suggestion(
 257                                     input.span,
 258                                     "consider changing the type to",
 259                                     "&str",
 260                                     Applicability::Unspecified,
 261                                 );
 262
 263                                 for (span, suggestion) in clone_spans {
 264                                     diag.span_suggestion(
 265                                         span,
 266                                         snippet_opt(cx, span)
 267                                             .map_or(
 268                                                 "change the call to".into(),
 269                                                 |x| Cow::from(format!("change `{}` to", x))
 270                                             )
 271                                             .as_ref(),
 272                                         suggestion,
 273                                         Applicability::Unspecified,
 274                                     );
 275                                 }
 276
 277                                 assert!(deref_span.is_none());
 278                                 return;
 279                             }
 280                         }
 281
 282                         let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
 283
 284                         // Suggests adding `*` to dereference the added reference.
 285                         if let Some(deref_span) = deref_span {
 286                             spans.extend(
 287                                 deref_span
 288                                     .iter()
 289                                     .copied()
 290                                     .map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
 291                             );
 292                             spans.sort_by_key(|&(span, _)| span);
 293                         }
 294                         multispan_sugg(diag, "consider taking a reference instead", spans);
 295                     };
 296
 297                     span_lint_and_then(
 298                         cx,
 299                         NEEDLESS_PASS_BY_VALUE,
 300                         input.span,
 301                         "this argument is passed by value, but not consumed in the function body",
 302                         sugg,
 303                     );
 304                 }
 305             }
 306         }
 307     }
 308 }
 309
 310 /// Functions marked with these attributes must have the exact signature.
 311 fn requires_exact_signature(attrs: &[Attribute]) -> bool {
 312     attrs.iter().any(|attr| {
 313         [sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
 314             .iter()
 315             .any(|&allow| attr.has_name(allow))
 316     })
 317 }
 318
 319 #[derive(Default)]
 320 struct MovedVariablesCtxt {
 321     moved_vars: HirIdSet,
 322     /// Spans which need to be prefixed with `*` for dereferencing the
 323     /// suggested additional reference.
 324     spans_need_deref: HirIdMap<FxHashSet<Span>>,
 325 }
 326
 327 impl MovedVariablesCtxt {
 328     fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
 329         if let euv::PlaceBase::Local(vid) = cmt.place.base {
 330             self.moved_vars.insert(vid);
 331         }
 332     }
 333 }
 334
 335 impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
 336     fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) {
 337         self.move_common(cmt);
 338     }
 339
 340     fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
 341
 342     fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
 343
 344     fn fake_read(&mut self, _: &rustc_hir_analysis::expr_use_visitor::PlaceWithHirId<'tcx>, _: FakeReadCause, _: HirId) {}
 345 }