clippy_lints/src/dereference.rs

   1 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_hir_and_then};
   2 use clippy_utils::source::{snippet_with_applicability, snippet_with_context};
   3 use clippy_utils::sugg::has_enclosing_paren;
   4 use clippy_utils::ty::{expr_sig, is_copy, peel_mid_ty_refs, ty_sig, variant_of_res};
   5 use clippy_utils::{
   6     fn_def_id, get_parent_expr, get_parent_expr_for_hir, is_lint_allowed, meets_msrv, msrvs, path_to_local,
   7     walk_to_expr_usage,
   8 };
   9 use rustc_ast::util::parser::{PREC_POSTFIX, PREC_PREFIX};
  10 use rustc_data_structures::fx::FxIndexMap;
  11 use rustc_errors::Applicability;
  12 use rustc_hir::intravisit::{walk_ty, Visitor};
  13 use rustc_hir::{
  14     self as hir, def_id::DefId, BindingAnnotation, Body, BodyId, BorrowKind, Closure, Expr, ExprKind, FnRetTy,
  15     GenericArg, HirId, ImplItem, ImplItemKind, Item, ItemKind, Local, MatchSource, Mutability, Node, Pat, PatKind,
  16     Path, QPath, TraitItem, TraitItemKind, TyKind, UnOp,
  17 };
  18 use rustc_index::bit_set::BitSet;
  19 use rustc_infer::infer::TyCtxtInferExt;
  20 use rustc_lint::{LateContext, LateLintPass};
  21 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
  22 use rustc_middle::ty::{
  23     self, subst::Subst, Binder, BoundVariableKind, EarlyBinder, FnSig, GenericArgKind, List, ParamTy, PredicateKind,
  24     ProjectionPredicate, Ty, TyCtxt, TypeVisitable, TypeckResults,
  25 };
  26 use rustc_semver::RustcVersion;
  27 use rustc_session::{declare_tool_lint, impl_lint_pass};
  28 use rustc_span::{symbol::sym, Span, Symbol, DUMMY_SP};
  29 use rustc_trait_selection::infer::InferCtxtExt as _;
  30 use rustc_trait_selection::traits::{query::evaluate_obligation::InferCtxtExt as _, Obligation, ObligationCause};
  31 use std::collections::VecDeque;
  32
  33 declare_clippy_lint! {
  34     /// ### What it does
  35     /// Checks for explicit `deref()` or `deref_mut()` method calls.
  36     ///
  37     /// ### Why is this bad?
  38     /// Dereferencing by `&*x` or `&mut *x` is clearer and more concise,
  39     /// when not part of a method chain.
  40     ///
  41     /// ### Example
  42     /// ```rust
  43     /// use std::ops::Deref;
  44     /// let a: &mut String = &mut String::from("foo");
  45     /// let b: &str = a.deref();
  46     /// ```
  47     ///
  48     /// Use instead:
  49     /// ```rust
  50     /// let a: &mut String = &mut String::from("foo");
  51     /// let b = &*a;
  52     /// ```
  53     ///
  54     /// This lint excludes:
  55     /// ```rust,ignore
  56     /// let _ = d.unwrap().deref();
  57     /// ```
  58     #[clippy::version = "1.44.0"]
  59     pub EXPLICIT_DEREF_METHODS,
  60     pedantic,
  61     "Explicit use of deref or deref_mut method while not in a method chain."
  62 }
  63
  64 declare_clippy_lint! {
  65     /// ### What it does
  66     /// Checks for address of operations (`&`) that are going to
  67     /// be dereferenced immediately by the compiler.
  68     ///
  69     /// ### Why is this bad?
  70     /// Suggests that the receiver of the expression borrows
  71     /// the expression.
  72     ///
  73     /// ### Example
  74     /// ```rust
  75     /// fn fun(_a: &i32) {}
  76     ///
  77     /// let x: &i32 = &&&&&&5;
  78     /// fun(&x);
  79     /// ```
  80     ///
  81     /// Use instead:
  82     /// ```rust
  83     /// # fn fun(_a: &i32) {}
  84     /// let x: &i32 = &5;
  85     /// fun(x);
  86     /// ```
  87     #[clippy::version = "pre 1.29.0"]
  88     pub NEEDLESS_BORROW,
  89     style,
  90     "taking a reference that is going to be automatically dereferenced"
  91 }
  92
  93 declare_clippy_lint! {
  94     /// ### What it does
  95     /// Checks for `ref` bindings which create a reference to a reference.
  96     ///
  97     /// ### Why is this bad?
  98     /// The address-of operator at the use site is clearer about the need for a reference.
  99     ///
 100     /// ### Example
 101     /// ```rust
 102     /// let x = Some("");
 103     /// if let Some(ref x) = x {
 104     ///     // use `x` here
 105     /// }
 106     /// ```
 107     ///
 108     /// Use instead:
 109     /// ```rust
 110     /// let x = Some("");
 111     /// if let Some(x) = x {
 112     ///     // use `&x` here
 113     /// }
 114     /// ```
 115     #[clippy::version = "1.54.0"]
 116     pub REF_BINDING_TO_REFERENCE,
 117     pedantic,
 118     "`ref` binding to a reference"
 119 }
 120
 121 declare_clippy_lint! {
 122     /// ### What it does
 123     /// Checks for dereferencing expressions which would be covered by auto-deref.
 124     ///
 125     /// ### Why is this bad?
 126     /// This unnecessarily complicates the code.
 127     ///
 128     /// ### Example
 129     /// ```rust
 130     /// let x = String::new();
 131     /// let y: &str = &*x;
 132     /// ```
 133     /// Use instead:
 134     /// ```rust
 135     /// let x = String::new();
 136     /// let y: &str = &x;
 137     /// ```
 138     #[clippy::version = "1.60.0"]
 139     pub EXPLICIT_AUTO_DEREF,
 140     complexity,
 141     "dereferencing when the compiler would automatically dereference"
 142 }
 143
 144 impl_lint_pass!(Dereferencing => [
 145     EXPLICIT_DEREF_METHODS,
 146     NEEDLESS_BORROW,
 147     REF_BINDING_TO_REFERENCE,
 148     EXPLICIT_AUTO_DEREF,
 149 ]);
 150
 151 #[derive(Default)]
 152 pub struct Dereferencing {
 153     state: Option<(State, StateData)>,
 154
 155     // While parsing a `deref` method call in ufcs form, the path to the function is itself an
 156     // expression. This is to store the id of that expression so it can be skipped when
 157     // `check_expr` is called for it.
 158     skip_expr: Option<HirId>,
 159
 160     /// The body the first local was found in. Used to emit lints when the traversal of the body has
 161     /// been finished. Note we can't lint at the end of every body as they can be nested within each
 162     /// other.
 163     current_body: Option<BodyId>,
 164
 165     /// The list of locals currently being checked by the lint.
 166     /// If the value is `None`, then the binding has been seen as a ref pattern, but is not linted.
 167     /// This is needed for or patterns where one of the branches can be linted, but another can not
 168     /// be.
 169     ///
 170     /// e.g. `m!(x) | Foo::Bar(ref x)`
 171     ref_locals: FxIndexMap<HirId, Option<RefPat>>,
 172
 173     // `IntoIterator` for arrays requires Rust 1.53.
 174     msrv: Option<RustcVersion>,
 175 }
 176
 177 impl Dereferencing {
 178     #[must_use]
 179     pub fn new(msrv: Option<RustcVersion>) -> Self {
 180         Self {
 181             msrv,
 182             ..Dereferencing::default()
 183         }
 184     }
 185 }
 186
 187 struct StateData {
 188     /// Span of the top level expression
 189     span: Span,
 190     hir_id: HirId,
 191     position: Position,
 192 }
 193
 194 struct DerefedBorrow {
 195     count: usize,
 196     msg: &'static str,
 197     snip_expr: Option<HirId>,
 198 }
 199
 200 enum State {
 201     // Any number of deref method calls.
 202     DerefMethod {
 203         // The number of calls in a sequence which changed the referenced type
 204         ty_changed_count: usize,
 205         is_final_ufcs: bool,
 206         /// The required mutability
 207         target_mut: Mutability,
 208     },
 209     DerefedBorrow(DerefedBorrow),
 210     ExplicitDeref {
 211         mutability: Option<Mutability>,
 212     },
 213     ExplicitDerefField {
 214         name: Symbol,
 215     },
 216     Reborrow {
 217         mutability: Mutability,
 218     },
 219     Borrow {
 220         mutability: Mutability,
 221     },
 222 }
 223
 224 // A reference operation considered by this lint pass
 225 enum RefOp {
 226     Method(Mutability),
 227     Deref,
 228     AddrOf(Mutability),
 229 }
 230
 231 struct RefPat {
 232     /// Whether every usage of the binding is dereferenced.
 233     always_deref: bool,
 234     /// The spans of all the ref bindings for this local.
 235     spans: Vec<Span>,
 236     /// The applicability of this suggestion.
 237     app: Applicability,
 238     /// All the replacements which need to be made.
 239     replacements: Vec<(Span, String)>,
 240     /// The [`HirId`] that the lint should be emitted at.
 241     hir_id: HirId,
 242 }
 243
 244 impl<'tcx> LateLintPass<'tcx> for Dereferencing {
 245     #[expect(clippy::too_many_lines)]
 246     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 247         // Skip path expressions from deref calls. e.g. `Deref::deref(e)`
 248         if Some(expr.hir_id) == self.skip_expr.take() {
 249             return;
 250         }
 251
 252         if let Some(local) = path_to_local(expr) {
 253             self.check_local_usage(cx, expr, local);
 254         }
 255
 256         // Stop processing sub expressions when a macro call is seen
 257         if expr.span.from_expansion() {
 258             if let Some((state, data)) = self.state.take() {
 259                 report(cx, expr, state, data);
 260             }
 261             return;
 262         }
 263
 264         let typeck = cx.typeck_results();
 265         let (kind, sub_expr) = if let Some(x) = try_parse_ref_op(cx.tcx, typeck, expr) {
 266             x
 267         } else {
 268             // The whole chain of reference operations has been seen
 269             if let Some((state, data)) = self.state.take() {
 270                 report(cx, expr, state, data);
 271             }
 272             return;
 273         };
 274
 275         match (self.state.take(), kind) {
 276             (None, kind) => {
 277                 let expr_ty = typeck.expr_ty(expr);
 278                 let (position, adjustments) = walk_parents(cx, expr, self.msrv);
 279
 280                 match kind {
 281                     RefOp::Deref => {
 282                         if let Position::FieldAccess(name) = position
 283                             && !ty_contains_field(typeck.expr_ty(sub_expr), name)
 284                         {
 285                             self.state = Some((
 286                                 State::ExplicitDerefField { name },
 287                                 StateData { span: expr.span, hir_id: expr.hir_id, position },
 288                             ));
 289                         } else if position.is_deref_stable() {
 290                             self.state = Some((
 291                                 State::ExplicitDeref { mutability: None },
 292                                 StateData { span: expr.span, hir_id: expr.hir_id, position },
 293                             ));
 294                         }
 295                     }
 296                     RefOp::Method(target_mut)
 297                         if !is_lint_allowed(cx, EXPLICIT_DEREF_METHODS, expr.hir_id)
 298                             && position.lint_explicit_deref() =>
 299                     {
 300                         self.state = Some((
 301                             State::DerefMethod {
 302                                 ty_changed_count: if deref_method_same_type(expr_ty, typeck.expr_ty(sub_expr)) {
 303                                     0
 304                                 } else {
 305                                     1
 306                                 },
 307                                 is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
 308                                 target_mut,
 309                             },
 310                             StateData {
 311                                 span: expr.span,
 312                                 hir_id: expr.hir_id,
 313                                 position
 314                             },
 315                         ));
 316                     },
 317                     RefOp::AddrOf(mutability) => {
 318                         // Find the number of times the borrow is auto-derefed.
 319                         let mut iter = adjustments.iter();
 320                         let mut deref_count = 0usize;
 321                         let next_adjust = loop {
 322                             match iter.next() {
 323                                 Some(adjust) => {
 324                                     if !matches!(adjust.kind, Adjust::Deref(_)) {
 325                                         break Some(adjust);
 326                                     } else if !adjust.target.is_ref() {
 327                                         deref_count += 1;
 328                                         break iter.next();
 329                                     }
 330                                     deref_count += 1;
 331                                 },
 332                                 None => break None,
 333                             };
 334                         };
 335
 336                         // Determine the required number of references before any can be removed. In all cases the
 337                         // reference made by the current expression will be removed. After that there are four cases to
 338                         // handle.
 339                         //
 340                         // 1. Auto-borrow will trigger in the current position, so no further references are required.
 341                         // 2. Auto-deref ends at a reference, or the underlying type, so one extra needs to be left to
 342                         //    handle the automatically inserted re-borrow.
 343                         // 3. Auto-deref hits a user-defined `Deref` impl, so at least one reference needs to exist to
 344                         //    start auto-deref.
 345                         // 4. If the chain of non-user-defined derefs ends with a mutable re-borrow, and re-borrow
 346                         //    adjustments will not be inserted automatically, then leave one further reference to avoid
 347                         //    moving a mutable borrow.
 348                         //    e.g.
 349                         //        fn foo<T>(x: &mut Option<&mut T>, y: &mut T) {
 350                         //            let x = match x {
 351                         //                // Removing the borrow will cause `x` to be moved
 352                         //                Some(x) => &mut *x,
 353                         //                None => y
 354                         //            };
 355                         //        }
 356                         let deref_msg =
 357                             "this expression creates a reference which is immediately dereferenced by the compiler";
 358                         let borrow_msg = "this expression borrows a value the compiler would automatically borrow";
 359                         let impl_msg = "the borrowed expression implements the required traits";
 360
 361                         let (required_refs, msg, snip_expr) = if position.can_auto_borrow() {
 362                             (1, if deref_count == 1 { borrow_msg } else { deref_msg }, None)
 363                         } else if let Position::ImplArg(hir_id) = position {
 364                             (0, impl_msg, Some(hir_id))
 365                         } else if let Some(&Adjust::Borrow(AutoBorrow::Ref(_, mutability))) =
 366                             next_adjust.map(|a| &a.kind)
 367                         {
 368                             if matches!(mutability, AutoBorrowMutability::Mut { .. }) && !position.is_reborrow_stable()
 369                             {
 370                                 (3, deref_msg, None)
 371                             } else {
 372                                 (2, deref_msg, None)
 373                             }
 374                         } else {
 375                             (2, deref_msg, None)
 376                         };
 377
 378                         if deref_count >= required_refs {
 379                             self.state = Some((
 380                                 State::DerefedBorrow(DerefedBorrow {
 381                                     // One of the required refs is for the current borrow expression, the remaining ones
 382                                     // can't be removed without breaking the code. See earlier comment.
 383                                     count: deref_count - required_refs,
 384                                     msg,
 385                                     snip_expr,
 386                                 }),
 387                                 StateData { span: expr.span, hir_id: expr.hir_id, position },
 388                             ));
 389                         } else if position.is_deref_stable()
 390                             // Auto-deref doesn't combine with other adjustments
 391                             && next_adjust.map_or(true, |a| matches!(a.kind, Adjust::Deref(_) | Adjust::Borrow(_)))
 392                             && iter.all(|a| matches!(a.kind, Adjust::Deref(_) | Adjust::Borrow(_)))
 393                         {
 394                             self.state = Some((
 395                                 State::Borrow { mutability },
 396                                 StateData {
 397                                     span: expr.span,
 398                                     hir_id: expr.hir_id,
 399                                     position
 400                                 },
 401                             ));
 402                         }
 403                     },
 404                     RefOp::Method(..) => (),
 405                 }
 406             },
 407             (
 408                 Some((
 409                     State::DerefMethod {
 410                         target_mut,
 411                         ty_changed_count,
 412                         ..
 413                     },
 414                     data,
 415                 )),
 416                 RefOp::Method(_),
 417             ) => {
 418                 self.state = Some((
 419                     State::DerefMethod {
 420                         ty_changed_count: if deref_method_same_type(typeck.expr_ty(expr), typeck.expr_ty(sub_expr)) {
 421                             ty_changed_count
 422                         } else {
 423                             ty_changed_count + 1
 424                         },
 425                         is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
 426                         target_mut,
 427                     },
 428                     data,
 429                 ));
 430             },
 431             (Some((State::DerefedBorrow(state), data)), RefOp::AddrOf(_)) if state.count != 0 => {
 432                 self.state = Some((
 433                     State::DerefedBorrow(DerefedBorrow {
 434                         count: state.count - 1,
 435                         ..state
 436                     }),
 437                     data,
 438                 ));
 439             },
 440             (Some((State::DerefedBorrow(state), data)), RefOp::AddrOf(mutability)) => {
 441                 let position = data.position;
 442                 report(cx, expr, State::DerefedBorrow(state), data);
 443                 if position.is_deref_stable() {
 444                     self.state = Some((
 445                         State::Borrow { mutability },
 446                         StateData {
 447                             span: expr.span,
 448                             hir_id: expr.hir_id,
 449                             position,
 450                         },
 451                     ));
 452                 }
 453             },
 454             (Some((State::DerefedBorrow(state), data)), RefOp::Deref) => {
 455                 let position = data.position;
 456                 report(cx, expr, State::DerefedBorrow(state), data);
 457                 if let Position::FieldAccess(name) = position
 458                     && !ty_contains_field(typeck.expr_ty(sub_expr), name)
 459                 {
 460                     self.state = Some((
 461                         State::ExplicitDerefField { name },
 462                         StateData { span: expr.span, hir_id: expr.hir_id, position },
 463                     ));
 464                 } else if position.is_deref_stable() {
 465                     self.state = Some((
 466                         State::ExplicitDeref { mutability: None },
 467                         StateData { span: expr.span, hir_id: expr.hir_id, position },
 468                     ));
 469                 }
 470             },
 471
 472             (Some((State::Borrow { mutability }, data)), RefOp::Deref) => {
 473                 if typeck.expr_ty(sub_expr).is_ref() {
 474                     self.state = Some((State::Reborrow { mutability }, data));
 475                 } else {
 476                     self.state = Some((
 477                         State::ExplicitDeref {
 478                             mutability: Some(mutability),
 479                         },
 480                         data,
 481                     ));
 482                 }
 483             },
 484             (Some((State::Reborrow { mutability }, data)), RefOp::Deref) => {
 485                 self.state = Some((
 486                     State::ExplicitDeref {
 487                         mutability: Some(mutability),
 488                     },
 489                     data,
 490                 ));
 491             },
 492             (state @ Some((State::ExplicitDeref { .. }, _)), RefOp::Deref) => {
 493                 self.state = state;
 494             },
 495             (Some((State::ExplicitDerefField { name }, data)), RefOp::Deref)
 496                 if !ty_contains_field(typeck.expr_ty(sub_expr), name) =>
 497             {
 498                 self.state = Some((State::ExplicitDerefField { name }, data));
 499             },
 500
 501             (Some((state, data)), _) => report(cx, expr, state, data),
 502         }
 503     }
 504
 505     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
 506         if let PatKind::Binding(BindingAnnotation::REF, id, name, _) = pat.kind {
 507             if let Some(opt_prev_pat) = self.ref_locals.get_mut(&id) {
 508                 // This binding id has been seen before. Add this pattern to the list of changes.
 509                 if let Some(prev_pat) = opt_prev_pat {
 510                     if pat.span.from_expansion() {
 511                         // Doesn't match the context of the previous pattern. Can't lint here.
 512                         *opt_prev_pat = None;
 513                     } else {
 514                         prev_pat.spans.push(pat.span);
 515                         prev_pat.replacements.push((
 516                             pat.span,
 517                             snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut prev_pat.app)
 518                                 .0
 519                                 .into(),
 520                         ));
 521                     }
 522                 }
 523                 return;
 524             }
 525
 526             if_chain! {
 527                 if !pat.span.from_expansion();
 528                 if let ty::Ref(_, tam, _) = *cx.typeck_results().pat_ty(pat).kind();
 529                 // only lint immutable refs, because borrowed `&mut T` cannot be moved out
 530                 if let ty::Ref(_, _, Mutability::Not) = *tam.kind();
 531                 then {
 532                     let mut app = Applicability::MachineApplicable;
 533                     let snip = snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut app).0;
 534                     self.current_body = self.current_body.or(cx.enclosing_body);
 535                     self.ref_locals.insert(
 536                         id,
 537                         Some(RefPat {
 538                             always_deref: true,
 539                             spans: vec![pat.span],
 540                             app,
 541                             replacements: vec![(pat.span, snip.into())],
 542                             hir_id: pat.hir_id,
 543                         }),
 544                     );
 545                 }
 546             }
 547         }
 548     }
 549
 550     fn check_body_post(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
 551         if Some(body.id()) == self.current_body {
 552             for pat in self.ref_locals.drain(..).filter_map(|(_, x)| x) {
 553                 let replacements = pat.replacements;
 554                 let app = pat.app;
 555                 let lint = if pat.always_deref {
 556                     NEEDLESS_BORROW
 557                 } else {
 558                     REF_BINDING_TO_REFERENCE
 559                 };
 560                 span_lint_hir_and_then(
 561                     cx,
 562                     lint,
 563                     pat.hir_id,
 564                     pat.spans,
 565                     "this pattern creates a reference to a reference",
 566                     |diag| {
 567                         diag.multipart_suggestion("try this", replacements, app);
 568                     },
 569                 );
 570             }
 571             self.current_body = None;
 572         }
 573     }
 574
 575     extract_msrv_attr!(LateContext);
 576 }
 577
 578 fn try_parse_ref_op<'tcx>(
 579     tcx: TyCtxt<'tcx>,
 580     typeck: &'tcx TypeckResults<'_>,
 581     expr: &'tcx Expr<'_>,
 582 ) -> Option<(RefOp, &'tcx Expr<'tcx>)> {
 583     let (def_id, arg) = match expr.kind {
 584         ExprKind::MethodCall(_, arg, [], _) => (typeck.type_dependent_def_id(expr.hir_id)?, arg),
 585         ExprKind::Call(
 586             Expr {
 587                 kind: ExprKind::Path(path),
 588                 hir_id,
 589                 ..
 590             },
 591             [arg],
 592         ) => (typeck.qpath_res(path, *hir_id).opt_def_id()?, arg),
 593         ExprKind::Unary(UnOp::Deref, sub_expr) if !typeck.expr_ty(sub_expr).is_unsafe_ptr() => {
 594             return Some((RefOp::Deref, sub_expr));
 595         },
 596         ExprKind::AddrOf(BorrowKind::Ref, mutability, sub_expr) => return Some((RefOp::AddrOf(mutability), sub_expr)),
 597         _ => return None,
 598     };
 599     if tcx.is_diagnostic_item(sym::deref_method, def_id) {
 600         Some((RefOp::Method(Mutability::Not), arg))
 601     } else if tcx.trait_of_item(def_id)? == tcx.lang_items().deref_mut_trait()? {
 602         Some((RefOp::Method(Mutability::Mut), arg))
 603     } else {
 604         None
 605     }
 606 }
 607
 608 // Checks whether the type for a deref call actually changed the type, not just the mutability of
 609 // the reference.
 610 fn deref_method_same_type<'tcx>(result_ty: Ty<'tcx>, arg_ty: Ty<'tcx>) -> bool {
 611     match (result_ty.kind(), arg_ty.kind()) {
 612         (ty::Ref(_, result_ty, _), ty::Ref(_, arg_ty, _)) => result_ty == arg_ty,
 613
 614         // The result type for a deref method is always a reference
 615         // Not matching the previous pattern means the argument type is not a reference
 616         // This means that the type did change
 617         _ => false,
 618     }
 619 }
 620
 621 /// The position of an expression relative to it's parent.
 622 #[derive(Clone, Copy)]
 623 enum Position {
 624     MethodReceiver,
 625     /// The method is defined on a reference type. e.g. `impl Foo for &T`
 626     MethodReceiverRefImpl,
 627     Callee,
 628     ImplArg(HirId),
 629     FieldAccess(Symbol),
 630     Postfix,
 631     Deref,
 632     /// Any other location which will trigger auto-deref to a specific time.
 633     /// Contains the precedence of the parent expression and whether the target type is sized.
 634     DerefStable(i8, bool),
 635     /// Any other location which will trigger auto-reborrowing.
 636     /// Contains the precedence of the parent expression.
 637     ReborrowStable(i8),
 638     /// Contains the precedence of the parent expression.
 639     Other(i8),
 640 }
 641 impl Position {
 642     fn is_deref_stable(self) -> bool {
 643         matches!(self, Self::DerefStable(..))
 644     }
 645
 646     fn is_reborrow_stable(self) -> bool {
 647         matches!(self, Self::DerefStable(..) | Self::ReborrowStable(_))
 648     }
 649
 650     fn can_auto_borrow(self) -> bool {
 651         matches!(self, Self::MethodReceiver | Self::FieldAccess(_) | Self::Callee)
 652     }
 653
 654     fn lint_explicit_deref(self) -> bool {
 655         matches!(self, Self::Other(_) | Self::DerefStable(..) | Self::ReborrowStable(_))
 656     }
 657
 658     fn precedence(self) -> i8 {
 659         match self {
 660             Self::MethodReceiver
 661             | Self::MethodReceiverRefImpl
 662             | Self::Callee
 663             | Self::FieldAccess(_)
 664             | Self::Postfix => PREC_POSTFIX,
 665             Self::ImplArg(_) | Self::Deref => PREC_PREFIX,
 666             Self::DerefStable(p, _) | Self::ReborrowStable(p) | Self::Other(p) => p,
 667         }
 668     }
 669 }
 670
 671 /// Walks up the parent expressions attempting to determine both how stable the auto-deref result
 672 /// is, and which adjustments will be applied to it. Note this will not consider auto-borrow
 673 /// locations as those follow different rules.
 674 #[expect(clippy::too_many_lines)]
 675 fn walk_parents<'tcx>(
 676     cx: &LateContext<'tcx>,
 677     e: &'tcx Expr<'_>,
 678     msrv: Option<RustcVersion>,
 679 ) -> (Position, &'tcx [Adjustment<'tcx>]) {
 680     let mut adjustments = [].as_slice();
 681     let mut precedence = 0i8;
 682     let ctxt = e.span.ctxt();
 683     let position = walk_to_expr_usage(cx, e, &mut |parent, child_id| {
 684         // LocalTableInContext returns the wrong lifetime, so go use `expr_adjustments` instead.
 685         if adjustments.is_empty() && let Node::Expr(e) = cx.tcx.hir().get(child_id) {
 686             adjustments = cx.typeck_results().expr_adjustments(e);
 687         }
 688         match parent {
 689             Node::Local(Local { ty: Some(ty), span, .. }) if span.ctxt() == ctxt => {
 690                 Some(binding_ty_auto_deref_stability(cx, ty, precedence, List::empty()))
 691             },
 692             Node::Item(&Item {
 693                 kind: ItemKind::Static(..) | ItemKind::Const(..),
 694                 def_id,
 695                 span,
 696                 ..
 697             })
 698             | Node::TraitItem(&TraitItem {
 699                 kind: TraitItemKind::Const(..),
 700                 def_id,
 701                 span,
 702                 ..
 703             })
 704             | Node::ImplItem(&ImplItem {
 705                 kind: ImplItemKind::Const(..),
 706                 def_id,
 707                 span,
 708                 ..
 709             }) if span.ctxt() == ctxt => {
 710                 let ty = cx.tcx.type_of(def_id);
 711                 Some(ty_auto_deref_stability(cx, ty, precedence).position_for_result(cx))
 712             },
 713
 714             Node::Item(&Item {
 715                 kind: ItemKind::Fn(..),
 716                 def_id,
 717                 span,
 718                 ..
 719             })
 720             | Node::TraitItem(&TraitItem {
 721                 kind: TraitItemKind::Fn(..),
 722                 def_id,
 723                 span,
 724                 ..
 725             })
 726             | Node::ImplItem(&ImplItem {
 727                 kind: ImplItemKind::Fn(..),
 728                 def_id,
 729                 span,
 730                 ..
 731             }) if span.ctxt() == ctxt => {
 732                 let output = cx
 733                     .tcx
 734                     .erase_late_bound_regions(cx.tcx.fn_sig(def_id.to_def_id()).output());
 735                 Some(ty_auto_deref_stability(cx, output, precedence).position_for_result(cx))
 736             },
 737
 738             Node::ExprField(field) if field.span.ctxt() == ctxt => match get_parent_expr_for_hir(cx, field.hir_id) {
 739                 Some(Expr {
 740                     hir_id,
 741                     kind: ExprKind::Struct(path, ..),
 742                     ..
 743                 }) => variant_of_res(cx, cx.qpath_res(path, *hir_id))
 744                     .and_then(|variant| variant.fields.iter().find(|f| f.name == field.ident.name))
 745                     .map(|field_def| {
 746                         ty_auto_deref_stability(cx, cx.tcx.type_of(field_def.did), precedence).position_for_arg()
 747                     }),
 748                 _ => None,
 749             },
 750
 751             Node::Expr(parent) if parent.span.ctxt() == ctxt => match parent.kind {
 752                 ExprKind::Ret(_) => {
 753                     let owner_id = cx.tcx.hir().body_owner(cx.enclosing_body.unwrap());
 754                     Some(
 755                         if let Node::Expr(
 756                             closure_expr @ Expr {
 757                                 kind: ExprKind::Closure(closure),
 758                                 ..
 759                             },
 760                         ) = cx.tcx.hir().get(owner_id)
 761                         {
 762                             closure_result_position(cx, closure, cx.typeck_results().expr_ty(closure_expr), precedence)
 763                         } else {
 764                             let output = cx
 765                                 .tcx
 766                                 .erase_late_bound_regions(cx.tcx.fn_sig(cx.tcx.hir().local_def_id(owner_id)).output());
 767                             ty_auto_deref_stability(cx, output, precedence).position_for_result(cx)
 768                         },
 769                     )
 770                 },
 771                 ExprKind::Closure(closure) => Some(closure_result_position(
 772                     cx,
 773                     closure,
 774                     cx.typeck_results().expr_ty(parent),
 775                     precedence,
 776                 )),
 777                 ExprKind::Call(func, _) if func.hir_id == child_id => {
 778                     (child_id == e.hir_id).then_some(Position::Callee)
 779                 },
 780                 ExprKind::Call(func, args) => args
 781                     .iter()
 782                     .position(|arg| arg.hir_id == child_id)
 783                     .zip(expr_sig(cx, func))
 784                     .and_then(|(i, sig)| {
 785                         sig.input_with_hir(i).map(|(hir_ty, ty)| match hir_ty {
 786                             // Type inference for closures can depend on how they're called. Only go by the explicit
 787                             // types here.
 788                             Some(hir_ty) => binding_ty_auto_deref_stability(cx, hir_ty, precedence, ty.bound_vars()),
 789                             None => {
 790                                 if let ty::Param(param_ty) = ty.skip_binder().kind() {
 791                                     needless_borrow_impl_arg_position(cx, parent, i, *param_ty, e, precedence, msrv)
 792                                 } else {
 793                                     ty_auto_deref_stability(cx, cx.tcx.erase_late_bound_regions(ty), precedence)
 794                                         .position_for_arg()
 795                                 }
 796                             },
 797                         })
 798                     }),
 799                 ExprKind::MethodCall(_, receiver, args, _) => {
 800                     let id = cx.typeck_results().type_dependent_def_id(parent.hir_id).unwrap();
 801                     if receiver.hir_id == child_id {
 802                         // Check for calls to trait methods where the trait is implemented on a reference.
 803                         // Two cases need to be handled:
 804                         // * `self` methods on `&T` will never have auto-borrow
 805                         // * `&self` methods on `&T` can have auto-borrow, but `&self` methods on `T` will take
 806                         //   priority.
 807                         if e.hir_id != child_id {
 808                             return Some(Position::ReborrowStable(precedence))
 809                         } else if let Some(trait_id) = cx.tcx.trait_of_item(id)
 810                             && let arg_ty = cx.tcx.erase_regions(cx.typeck_results().expr_ty_adjusted(e))
 811                             && let ty::Ref(_, sub_ty, _) = *arg_ty.kind()
 812                             && let subs = match cx
 813                                 .typeck_results()
 814                                 .node_substs_opt(parent.hir_id)
 815                                 .and_then(|subs| subs.get(1..))
 816                             {
 817                                 Some(subs) => cx.tcx.mk_substs(subs.iter().copied()),
 818                                 None => cx.tcx.mk_substs(std::iter::empty::<ty::subst::GenericArg<'_>>()),
 819                             } && let impl_ty = if cx.tcx.fn_sig(id).skip_binder().inputs()[0].is_ref() {
 820                                 // Trait methods taking `&self`
 821                                 sub_ty
 822                             } else {
 823                                 // Trait methods taking `self`
 824                                 arg_ty
 825                             } && impl_ty.is_ref()
 826                             && cx.tcx.infer_ctxt().enter(|infcx|
 827                                 infcx
 828                                     .type_implements_trait(trait_id, impl_ty, subs, cx.param_env)
 829                                     .must_apply_modulo_regions()
 830                             )
 831                         {
 832                             return Some(Position::MethodReceiverRefImpl)
 833                         }
 834                         return Some(Position::MethodReceiver);
 835                     }
 836                     args.iter().position(|arg| arg.hir_id == child_id).map(|i| {
 837                         let ty = cx.tcx.fn_sig(id).skip_binder().inputs()[i + 1];
 838                         if let ty::Param(param_ty) = ty.kind() {
 839                             needless_borrow_impl_arg_position(cx, parent, i + 1, *param_ty, e, precedence, msrv)
 840                         } else {
 841                             ty_auto_deref_stability(
 842                                 cx,
 843                                 cx.tcx.erase_late_bound_regions(cx.tcx.fn_sig(id).input(i + 1)),
 844                                 precedence,
 845                             )
 846                             .position_for_arg()
 847                         }
 848                     })
 849                 },
 850                 ExprKind::Field(child, name) if child.hir_id == e.hir_id => Some(Position::FieldAccess(name.name)),
 851                 ExprKind::Unary(UnOp::Deref, child) if child.hir_id == e.hir_id => Some(Position::Deref),
 852                 ExprKind::Match(child, _, MatchSource::TryDesugar | MatchSource::AwaitDesugar)
 853                 | ExprKind::Index(child, _)
 854                     if child.hir_id == e.hir_id =>
 855                 {
 856                     Some(Position::Postfix)
 857                 },
 858                 _ if child_id == e.hir_id => {
 859                     precedence = parent.precedence().order();
 860                     None
 861                 },
 862                 _ => None,
 863             },
 864             _ => None,
 865         }
 866     })
 867     .unwrap_or(Position::Other(precedence));
 868     (position, adjustments)
 869 }
 870
 871 fn closure_result_position<'tcx>(
 872     cx: &LateContext<'tcx>,
 873     closure: &'tcx Closure<'_>,
 874     ty: Ty<'tcx>,
 875     precedence: i8,
 876 ) -> Position {
 877     match closure.fn_decl.output {
 878         FnRetTy::Return(hir_ty) => {
 879             if let Some(sig) = ty_sig(cx, ty)
 880                 && let Some(output) = sig.output()
 881             {
 882                 binding_ty_auto_deref_stability(cx, hir_ty, precedence, output.bound_vars())
 883             } else {
 884                 Position::Other(precedence)
 885             }
 886         },
 887         FnRetTy::DefaultReturn(_) => Position::Other(precedence),
 888     }
 889 }
 890
 891 // Checks the stability of auto-deref when assigned to a binding with the given explicit type.
 892 //
 893 // e.g.
 894 // let x = Box::new(Box::new(0u32));
 895 // let y1: &Box<_> = x.deref();
 896 // let y2: &Box<_> = &x;
 897 //
 898 // Here `y1` and `y2` would resolve to different types, so the type `&Box<_>` is not stable when
 899 // switching to auto-dereferencing.
 900 fn binding_ty_auto_deref_stability<'tcx>(
 901     cx: &LateContext<'tcx>,
 902     ty: &'tcx hir::Ty<'_>,
 903     precedence: i8,
 904     binder_args: &'tcx List<BoundVariableKind>,
 905 ) -> Position {
 906     let TyKind::Rptr(_, ty) = &ty.kind else {
 907         return Position::Other(precedence);
 908     };
 909     let mut ty = ty;
 910
 911     loop {
 912         break match ty.ty.kind {
 913             TyKind::Rptr(_, ref ref_ty) => {
 914                 ty = ref_ty;
 915                 continue;
 916             },
 917             TyKind::Path(
 918                 QPath::TypeRelative(_, path)
 919                 | QPath::Resolved(
 920                     _,
 921                     Path {
 922                         segments: [.., path], ..
 923                     },
 924                 ),
 925             ) => {
 926                 if let Some(args) = path.args
 927                     && args.args.iter().any(|arg| match arg {
 928                         GenericArg::Infer(_) => true,
 929                         GenericArg::Type(ty) => ty_contains_infer(ty),
 930                         _ => false,
 931                     })
 932                 {
 933                     Position::ReborrowStable(precedence)
 934                 } else {
 935                     Position::DerefStable(
 936                         precedence,
 937                         cx.tcx
 938                             .erase_late_bound_regions(Binder::bind_with_vars(
 939                                 cx.typeck_results().node_type(ty.ty.hir_id),
 940                                 binder_args,
 941                             ))
 942                             .is_sized(cx.tcx.at(DUMMY_SP), cx.param_env.without_caller_bounds()),
 943                     )
 944                 }
 945             },
 946             TyKind::Slice(_) => Position::DerefStable(precedence, false),
 947             TyKind::Array(..) | TyKind::Ptr(_) | TyKind::BareFn(_) => Position::DerefStable(precedence, true),
 948             TyKind::Never
 949             | TyKind::Tup(_)
 950             | TyKind::Path(_) => Position::DerefStable(
 951                 precedence,
 952                 cx.tcx
 953                     .erase_late_bound_regions(Binder::bind_with_vars(
 954                         cx.typeck_results().node_type(ty.ty.hir_id),
 955                         binder_args,
 956                     ))
 957                     .is_sized(cx.tcx.at(DUMMY_SP), cx.param_env.without_caller_bounds()),
 958             ),
 959             TyKind::OpaqueDef(..) | TyKind::Infer | TyKind::Typeof(..) | TyKind::TraitObject(..) | TyKind::Err => {
 960                 Position::ReborrowStable(precedence)
 961             },
 962         };
 963     }
 964 }
 965
 966 // Checks whether a type is inferred at some point.
 967 // e.g. `_`, `Box<_>`, `[_]`
 968 fn ty_contains_infer(ty: &hir::Ty<'_>) -> bool {
 969     struct V(bool);
 970     impl Visitor<'_> for V {
 971         fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
 972             if self.0
 973                 || matches!(
 974                     ty.kind,
 975                     TyKind::OpaqueDef(..) | TyKind::Infer | TyKind::Typeof(_) | TyKind::Err
 976                 )
 977             {
 978                 self.0 = true;
 979             } else {
 980                 walk_ty(self, ty);
 981             }
 982         }
 983
 984         fn visit_generic_arg(&mut self, arg: &GenericArg<'_>) {
 985             if self.0 || matches!(arg, GenericArg::Infer(_)) {
 986                 self.0 = true;
 987             } else if let GenericArg::Type(ty) = arg {
 988                 self.visit_ty(ty);
 989             }
 990         }
 991     }
 992     let mut v = V(false);
 993     v.visit_ty(ty);
 994     v.0
 995 }
 996
 997 // Checks whether:
 998 // * child is an expression of the form `&e` in an argument position requiring an `impl Trait`
 999 // * `e`'s type implements `Trait` and is copyable
1000 // If the conditions are met, returns `Some(Position::ImplArg(..))`; otherwise, returns `None`.
1001 //   The "is copyable" condition is to avoid the case where removing the `&` means `e` would have to
1002 // be moved, but it cannot be.
1003 fn needless_borrow_impl_arg_position<'tcx>(
1004     cx: &LateContext<'tcx>,
1005     parent: &Expr<'tcx>,
1006     arg_index: usize,
1007     param_ty: ParamTy,
1008     mut expr: &Expr<'tcx>,
1009     precedence: i8,
1010     msrv: Option<RustcVersion>,
1011 ) -> Position {
1012     let destruct_trait_def_id = cx.tcx.lang_items().destruct_trait();
1013     let sized_trait_def_id = cx.tcx.lang_items().sized_trait();
1014
1015     let Some(callee_def_id) = fn_def_id(cx, parent) else { return Position::Other(precedence) };
1016     let fn_sig = cx.tcx.fn_sig(callee_def_id).skip_binder();
1017     let substs_with_expr_ty = cx
1018         .typeck_results()
1019         .node_substs(if let ExprKind::Call(callee, _) = parent.kind {
1020             callee.hir_id
1021         } else {
1022             parent.hir_id
1023         });
1024
1025     let predicates = cx.tcx.param_env(callee_def_id).caller_bounds();
1026     let projection_predicates = predicates
1027         .iter()
1028         .filter_map(|predicate| {
1029             if let PredicateKind::Projection(projection_predicate) = predicate.kind().skip_binder() {
1030                 Some(projection_predicate)
1031             } else {
1032                 None
1033             }
1034         })
1035         .collect::<Vec<_>>();
1036
1037     let mut trait_with_ref_mut_self_method = false;
1038
1039     // If no traits were found, or only the `Destruct`, `Sized`, or `Any` traits were found, return.
1040     if predicates
1041         .iter()
1042         .filter_map(|predicate| {
1043             if let PredicateKind::Trait(trait_predicate) = predicate.kind().skip_binder()
1044                 && trait_predicate.trait_ref.self_ty() == param_ty.to_ty(cx.tcx)
1045             {
1046                 Some(trait_predicate.trait_ref.def_id)
1047             } else {
1048                 None
1049             }
1050         })
1051         .inspect(|trait_def_id| {
1052             trait_with_ref_mut_self_method |= has_ref_mut_self_method(cx, *trait_def_id);
1053         })
1054         .all(|trait_def_id| {
1055             Some(trait_def_id) == destruct_trait_def_id
1056                 || Some(trait_def_id) == sized_trait_def_id
1057                 || cx.tcx.is_diagnostic_item(sym::Any, trait_def_id)
1058         })
1059     {
1060         return Position::Other(precedence);
1061     }
1062
1063     // `substs_with_referent_ty` can be constructed outside of `check_referent` because the same
1064     // elements are modified each time `check_referent` is called.
1065     let mut substs_with_referent_ty = substs_with_expr_ty.to_vec();
1066
1067     let mut check_referent = |referent| {
1068         let referent_ty = cx.typeck_results().expr_ty(referent);
1069
1070         if !is_copy(cx, referent_ty) {
1071             return false;
1072         }
1073
1074         // https://github.com/rust-lang/rust-clippy/pull/9136#pullrequestreview-1037379321
1075         if trait_with_ref_mut_self_method && !matches!(referent_ty.kind(), ty::Ref(_, _, Mutability::Mut)) {
1076             return false;
1077         }
1078
1079         if !replace_types(
1080             cx,
1081             param_ty,
1082             referent_ty,
1083             fn_sig,
1084             arg_index,
1085             &projection_predicates,
1086             &mut substs_with_referent_ty,
1087         ) {
1088             return false;
1089         }
1090
1091         predicates.iter().all(|predicate| {
1092             if let PredicateKind::Trait(trait_predicate) = predicate.kind().skip_binder()
1093                 && cx.tcx.is_diagnostic_item(sym::IntoIterator, trait_predicate.trait_ref.def_id)
1094                 && let ty::Param(param_ty) = trait_predicate.self_ty().kind()
1095                 && let GenericArgKind::Type(ty) = substs_with_referent_ty[param_ty.index as usize].unpack()
1096                 && ty.is_array()
1097                 && !meets_msrv(msrv, msrvs::ARRAY_INTO_ITERATOR)
1098             {
1099                 return false;
1100             }
1101
1102             let predicate = EarlyBinder(predicate).subst(cx.tcx, &substs_with_referent_ty);
1103             let obligation = Obligation::new(ObligationCause::dummy(), cx.param_env, predicate);
1104             cx.tcx
1105                 .infer_ctxt()
1106                 .enter(|infcx| infcx.predicate_must_hold_modulo_regions(&obligation))
1107         })
1108     };
1109
1110     let mut needless_borrow = false;
1111     while let ExprKind::AddrOf(_, _, referent) = expr.kind {
1112         if !check_referent(referent) {
1113             break;
1114         }
1115         expr = referent;
1116         needless_borrow = true;
1117     }
1118
1119     if needless_borrow {
1120         Position::ImplArg(expr.hir_id)
1121     } else {
1122         Position::Other(precedence)
1123     }
1124 }
1125
1126 fn has_ref_mut_self_method(cx: &LateContext<'_>, trait_def_id: DefId) -> bool {
1127     cx.tcx
1128         .associated_items(trait_def_id)
1129         .in_definition_order()
1130         .any(|assoc_item| {
1131             if assoc_item.fn_has_self_parameter {
1132                 let self_ty = cx.tcx.fn_sig(assoc_item.def_id).skip_binder().inputs()[0];
1133                 matches!(self_ty.kind(), ty::Ref(_, _, Mutability::Mut))
1134             } else {
1135                 false
1136             }
1137         })
1138 }
1139
1140 // Iteratively replaces `param_ty` with `new_ty` in `substs`, and similarly for each resulting
1141 // projected type that is a type parameter. Returns `false` if replacing the types would have an
1142 // effect on the function signature beyond substituting `new_ty` for `param_ty`.
1143 // See: https://github.com/rust-lang/rust-clippy/pull/9136#discussion_r927212757
1144 fn replace_types<'tcx>(
1145     cx: &LateContext<'tcx>,
1146     param_ty: ParamTy,
1147     new_ty: Ty<'tcx>,
1148     fn_sig: FnSig<'tcx>,
1149     arg_index: usize,
1150     projection_predicates: &[ProjectionPredicate<'tcx>],
1151     substs: &mut [ty::GenericArg<'tcx>],
1152 ) -> bool {
1153     let mut replaced = BitSet::new_empty(substs.len());
1154
1155     let mut deque = VecDeque::with_capacity(substs.len());
1156     deque.push_back((param_ty, new_ty));
1157
1158     while let Some((param_ty, new_ty)) = deque.pop_front() {
1159         // If `replaced.is_empty()`, then `param_ty` and `new_ty` are those initially passed in.
1160         if !fn_sig
1161             .inputs_and_output
1162             .iter()
1163             .enumerate()
1164             .all(|(i, ty)| (replaced.is_empty() && i == arg_index) || !ty.contains(param_ty.to_ty(cx.tcx)))
1165         {
1166             return false;
1167         }
1168
1169         substs[param_ty.index as usize] = ty::GenericArg::from(new_ty);
1170
1171         // The `replaced.insert(...)` check provides some protection against infinite loops.
1172         if replaced.insert(param_ty.index) {
1173             for projection_predicate in projection_predicates {
1174                 if projection_predicate.projection_ty.self_ty() == param_ty.to_ty(cx.tcx)
1175                     && let Some(term_ty) = projection_predicate.term.ty()
1176                     && let ty::Param(term_param_ty) = term_ty.kind()
1177                 {
1178                     let item_def_id = projection_predicate.projection_ty.item_def_id;
1179                     let assoc_item = cx.tcx.associated_item(item_def_id);
1180                     let projection = cx.tcx
1181                         .mk_projection(assoc_item.def_id, cx.tcx.mk_substs_trait(new_ty, &[]));
1182
1183                     if let Ok(projected_ty) = cx.tcx.try_normalize_erasing_regions(cx.param_env, projection)
1184                         && substs[term_param_ty.index as usize] != ty::GenericArg::from(projected_ty)
1185                     {
1186                         deque.push_back((*term_param_ty, projected_ty));
1187                     }
1188                 }
1189             }
1190         }
1191     }
1192
1193     true
1194 }
1195
1196 struct TyPosition<'tcx> {
1197     position: Position,
1198     ty: Option<Ty<'tcx>>,
1199 }
1200 impl From<Position> for TyPosition<'_> {
1201     fn from(position: Position) -> Self {
1202         Self { position, ty: None }
1203     }
1204 }
1205 impl<'tcx> TyPosition<'tcx> {
1206     fn new_deref_stable_for_result(precedence: i8, ty: Ty<'tcx>) -> Self {
1207         Self {
1208             position: Position::ReborrowStable(precedence),
1209             ty: Some(ty),
1210         }
1211     }
1212     fn position_for_result(self, cx: &LateContext<'tcx>) -> Position {
1213         match (self.position, self.ty) {
1214             (Position::ReborrowStable(precedence), Some(ty)) => {
1215                 Position::DerefStable(precedence, ty.is_sized(cx.tcx.at(DUMMY_SP), cx.param_env))
1216             },
1217             (position, _) => position,
1218         }
1219     }
1220     fn position_for_arg(self) -> Position {
1221         self.position
1222     }
1223 }
1224
1225 // Checks whether a type is stable when switching to auto dereferencing,
1226 fn ty_auto_deref_stability<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, precedence: i8) -> TyPosition<'tcx> {
1227     let ty::Ref(_, mut ty, _) = *ty.kind() else {
1228         return Position::Other(precedence).into();
1229     };
1230
1231     loop {
1232         break match *ty.kind() {
1233             ty::Ref(_, ref_ty, _) => {
1234                 ty = ref_ty;
1235                 continue;
1236             },
1237             ty::Param(_) => TyPosition::new_deref_stable_for_result(precedence, ty),
1238             ty::Infer(_) | ty::Error(_) | ty::Bound(..) | ty::Opaque(..) | ty::Placeholder(_) | ty::Dynamic(..) => {
1239                 Position::ReborrowStable(precedence).into()
1240             },
1241             ty::Adt(..) if ty.has_placeholders() || ty.has_opaque_types() => {
1242                 Position::ReborrowStable(precedence).into()
1243             },
1244             ty::Adt(_, substs) if substs.has_param_types_or_consts() => {
1245                 TyPosition::new_deref_stable_for_result(precedence, ty)
1246             },
1247             ty::Bool
1248             | ty::Char
1249             | ty::Int(_)
1250             | ty::Uint(_)
1251             | ty::Array(..)
1252             | ty::Float(_)
1253             | ty::RawPtr(..)
1254             | ty::FnPtr(_) => Position::DerefStable(precedence, true).into(),
1255             ty::Str | ty::Slice(..) => Position::DerefStable(precedence, false).into(),
1256             ty::Adt(..)
1257             | ty::Foreign(_)
1258             | ty::FnDef(..)
1259             | ty::Generator(..)
1260             | ty::GeneratorWitness(..)
1261             | ty::Closure(..)
1262             | ty::Never
1263             | ty::Tuple(_)
1264             | ty::Projection(_) => Position::DerefStable(
1265                 precedence,
1266                 ty.is_sized(cx.tcx.at(DUMMY_SP), cx.param_env.without_caller_bounds()),
1267             )
1268             .into(),
1269         };
1270     }
1271 }
1272
1273 fn ty_contains_field(ty: Ty<'_>, name: Symbol) -> bool {
1274     if let ty::Adt(adt, _) = *ty.kind() {
1275         adt.is_struct() && adt.all_fields().any(|f| f.name == name)
1276     } else {
1277         false
1278     }
1279 }
1280
1281 #[expect(clippy::needless_pass_by_value, clippy::too_many_lines)]
1282 fn report<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, state: State, data: StateData) {
1283     match state {
1284         State::DerefMethod {
1285             ty_changed_count,
1286             is_final_ufcs,
1287             target_mut,
1288         } => {
1289             let mut app = Applicability::MachineApplicable;
1290             let (expr_str, expr_is_macro_call) = snippet_with_context(cx, expr.span, data.span.ctxt(), "..", &mut app);
1291             let ty = cx.typeck_results().expr_ty(expr);
1292             let (_, ref_count) = peel_mid_ty_refs(ty);
1293             let deref_str = if ty_changed_count >= ref_count && ref_count != 0 {
1294                 // a deref call changing &T -> &U requires two deref operators the first time
1295                 // this occurs. One to remove the reference, a second to call the deref impl.
1296                 "*".repeat(ty_changed_count + 1)
1297             } else {
1298                 "*".repeat(ty_changed_count)
1299             };
1300             let addr_of_str = if ty_changed_count < ref_count {
1301                 // Check if a reborrow from &mut T -> &T is required.
1302                 if target_mut == Mutability::Not && matches!(ty.kind(), ty::Ref(_, _, Mutability::Mut)) {
1303                     "&*"
1304                 } else {
1305                     ""
1306                 }
1307             } else if target_mut == Mutability::Mut {
1308                 "&mut "
1309             } else {
1310                 "&"
1311             };
1312
1313             let expr_str = if !expr_is_macro_call && is_final_ufcs && expr.precedence().order() < PREC_PREFIX {
1314                 format!("({})", expr_str)
1315             } else {
1316                 expr_str.into_owned()
1317             };
1318
1319             span_lint_and_sugg(
1320                 cx,
1321                 EXPLICIT_DEREF_METHODS,
1322                 data.span,
1323                 match target_mut {
1324                     Mutability::Not => "explicit `deref` method call",
1325                     Mutability::Mut => "explicit `deref_mut` method call",
1326                 },
1327                 "try this",
1328                 format!("{}{}{}", addr_of_str, deref_str, expr_str),
1329                 app,
1330             );
1331         },
1332         State::DerefedBorrow(state) => {
1333             let mut app = Applicability::MachineApplicable;
1334             let snip_expr = state.snip_expr.map_or(expr, |hir_id| cx.tcx.hir().expect_expr(hir_id));
1335             let (snip, snip_is_macro) = snippet_with_context(cx, snip_expr.span, data.span.ctxt(), "..", &mut app);
1336             span_lint_hir_and_then(cx, NEEDLESS_BORROW, data.hir_id, data.span, state.msg, |diag| {
1337                 let calls_field = matches!(expr.kind, ExprKind::Field(..)) && matches!(data.position, Position::Callee);
1338                 let sugg = if !snip_is_macro
1339                     && !has_enclosing_paren(&snip)
1340                     && (expr.precedence().order() < data.position.precedence() || calls_field)
1341                 {
1342                     format!("({})", snip)
1343                 } else {
1344                     snip.into()
1345                 };
1346                 diag.span_suggestion(data.span, "change this to", sugg, app);
1347             });
1348         },
1349         State::ExplicitDeref { mutability } => {
1350             if matches!(
1351                 expr.kind,
1352                 ExprKind::Block(..)
1353                     | ExprKind::ConstBlock(_)
1354                     | ExprKind::If(..)
1355                     | ExprKind::Loop(..)
1356                     | ExprKind::Match(..)
1357             ) && matches!(data.position, Position::DerefStable(_, true))
1358             {
1359                 // Rustc bug: auto deref doesn't work on block expression when targeting sized types.
1360                 return;
1361             }
1362
1363             let (prefix, precedence) = if let Some(mutability) = mutability
1364                 && !cx.typeck_results().expr_ty(expr).is_ref()
1365             {
1366                 let prefix = match mutability {
1367                     Mutability::Not => "&",
1368                     Mutability::Mut => "&mut ",
1369                 };
1370                 (prefix, 0)
1371             } else {
1372                 ("", data.position.precedence())
1373             };
1374             span_lint_hir_and_then(
1375                 cx,
1376                 EXPLICIT_AUTO_DEREF,
1377                 data.hir_id,
1378                 data.span,
1379                 "deref which would be done by auto-deref",
1380                 |diag| {
1381                     let mut app = Applicability::MachineApplicable;
1382                     let (snip, snip_is_macro) = snippet_with_context(cx, expr.span, data.span.ctxt(), "..", &mut app);
1383                     let sugg =
1384                         if !snip_is_macro && expr.precedence().order() < precedence && !has_enclosing_paren(&snip) {
1385                             format!("{}({})", prefix, snip)
1386                         } else {
1387                             format!("{}{}", prefix, snip)
1388                         };
1389                     diag.span_suggestion(data.span, "try this", sugg, app);
1390                 },
1391             );
1392         },
1393         State::ExplicitDerefField { .. } => {
1394             if matches!(
1395                 expr.kind,
1396                 ExprKind::Block(..)
1397                     | ExprKind::ConstBlock(_)
1398                     | ExprKind::If(..)
1399                     | ExprKind::Loop(..)
1400                     | ExprKind::Match(..)
1401             ) && matches!(data.position, Position::DerefStable(_, true))
1402             {
1403                 // Rustc bug: auto deref doesn't work on block expression when targeting sized types.
1404                 return;
1405             }
1406
1407             span_lint_hir_and_then(
1408                 cx,
1409                 EXPLICIT_AUTO_DEREF,
1410                 data.hir_id,
1411                 data.span,
1412                 "deref which would be done by auto-deref",
1413                 |diag| {
1414                     let mut app = Applicability::MachineApplicable;
1415                     let snip = snippet_with_context(cx, expr.span, data.span.ctxt(), "..", &mut app).0;
1416                     diag.span_suggestion(data.span, "try this", snip.into_owned(), app);
1417                 },
1418             );
1419         },
1420         State::Borrow { .. } | State::Reborrow { .. } => (),
1421     }
1422 }
1423
1424 impl Dereferencing {
1425     fn check_local_usage<'tcx>(&mut self, cx: &LateContext<'tcx>, e: &Expr<'tcx>, local: HirId) {
1426         if let Some(outer_pat) = self.ref_locals.get_mut(&local) {
1427             if let Some(pat) = outer_pat {
1428                 // Check for auto-deref
1429                 if !matches!(
1430                     cx.typeck_results().expr_adjustments(e),
1431                     [
1432                         Adjustment {
1433                             kind: Adjust::Deref(_),
1434                             ..
1435                         },
1436                         Adjustment {
1437                             kind: Adjust::Deref(_),
1438                             ..
1439                         },
1440                         ..
1441                     ]
1442                 ) {
1443                     match get_parent_expr(cx, e) {
1444                         // Field accesses are the same no matter the number of references.
1445                         Some(Expr {
1446                             kind: ExprKind::Field(..),
1447                             ..
1448                         }) => (),
1449                         Some(&Expr {
1450                             span,
1451                             kind: ExprKind::Unary(UnOp::Deref, _),
1452                             ..
1453                         }) if !span.from_expansion() => {
1454                             // Remove explicit deref.
1455                             let snip = snippet_with_context(cx, e.span, span.ctxt(), "..", &mut pat.app).0;
1456                             pat.replacements.push((span, snip.into()));
1457                         },
1458                         Some(parent) if !parent.span.from_expansion() => {
1459                             // Double reference might be needed at this point.
1460                             if parent.precedence().order() == PREC_POSTFIX {
1461                                 // Parentheses would be needed here, don't lint.
1462                                 *outer_pat = None;
1463                             } else {
1464                                 pat.always_deref = false;
1465                                 let snip = snippet_with_context(cx, e.span, parent.span.ctxt(), "..", &mut pat.app).0;
1466                                 pat.replacements.push((e.span, format!("&{}", snip)));
1467                             }
1468                         },
1469                         _ if !e.span.from_expansion() => {
1470                             // Double reference might be needed at this point.
1471                             pat.always_deref = false;
1472                             let snip = snippet_with_applicability(cx, e.span, "..", &mut pat.app);
1473                             pat.replacements.push((e.span, format!("&{}", snip)));
1474                         },
1475                         // Edge case for macros. The span of the identifier will usually match the context of the
1476                         // binding, but not if the identifier was created in a macro. e.g. `concat_idents` and proc
1477                         // macros
1478                         _ => *outer_pat = None,
1479                     }
1480                 }
1481             }
1482         }
1483     }
1484 }