compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

   1 use rustc_errors::{Applicability, Diagnostic};
   2 use rustc_hir as hir;
   3 use rustc_hir::intravisit::Visitor;
   4 use rustc_hir::Node;
   5 use rustc_middle::hir::map::Map;
   6 use rustc_middle::mir::{Mutability, Place, PlaceRef, ProjectionElem};
   7 use rustc_middle::ty::{self, Ty, TyCtxt};
   8 use rustc_middle::{
   9     hir::place::PlaceBase,
  10     mir::{self, BindingForm, ClearCrossCrate, Local, LocalDecl, LocalInfo, LocalKind, Location},
  11 };
  12 use rustc_span::source_map::DesugaringKind;
  13 use rustc_span::symbol::{kw, Symbol};
  14 use rustc_span::{sym, BytePos, Span};
  15
  16 use crate::diagnostics::BorrowedContentSource;
  17 use crate::MirBorrowckCtxt;
  18 use rustc_const_eval::util::collect_writes::FindAssignments;
  19
  20 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
  21 pub(crate) enum AccessKind {
  22     MutableBorrow,
  23     Mutate,
  24 }
  25
  26 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
  27     pub(crate) fn report_mutability_error(
  28         &mut self,
  29         access_place: Place<'tcx>,
  30         span: Span,
  31         the_place_err: PlaceRef<'tcx>,
  32         error_access: AccessKind,
  33         location: Location,
  34     ) {
  35         debug!(
  36             "report_mutability_error(\
  37                 access_place={:?}, span={:?}, the_place_err={:?}, error_access={:?}, location={:?},\
  38             )",
  39             access_place, span, the_place_err, error_access, location,
  40         );
  41
  42         let mut err;
  43         let item_msg;
  44         let reason;
  45         let mut opt_source = None;
  46         let access_place_desc = self.describe_any_place(access_place.as_ref());
  47         debug!("report_mutability_error: access_place_desc={:?}", access_place_desc);
  48
  49         match the_place_err {
  50             PlaceRef { local, projection: [] } => {
  51                 item_msg = access_place_desc;
  52                 if access_place.as_local().is_some() {
  53                     reason = ", as it is not declared as mutable".to_string();
  54                 } else {
  55                     let name = self.local_names[local].expect("immutable unnamed local");
  56                     reason = format!(", as `{name}` is not declared as mutable");
  57                 }
  58             }
  59
  60             PlaceRef {
  61                 local,
  62                 projection: [proj_base @ .., ProjectionElem::Field(upvar_index, _)],
  63             } => {
  64                 debug_assert!(is_closure_or_generator(
  65                     Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty
  66                 ));
  67
  68                 let imm_borrow_derefed = self.upvars[upvar_index.index()]
  69                     .place
  70                     .place
  71                     .deref_tys()
  72                     .any(|ty| matches!(ty.kind(), ty::Ref(.., hir::Mutability::Not)));
  73
  74                 // If the place is immutable then:
  75                 //
  76                 // - Either we deref an immutable ref to get to our final place.
  77                 //    - We don't capture derefs of raw ptrs
  78                 // - Or the final place is immut because the root variable of the capture
  79                 //   isn't marked mut and we should suggest that to the user.
  80                 if imm_borrow_derefed {
  81                     // If we deref an immutable ref then the suggestion here doesn't help.
  82                     return;
  83                 } else {
  84                     item_msg = access_place_desc;
  85                     if self.is_upvar_field_projection(access_place.as_ref()).is_some() {
  86                         reason = ", as it is not declared as mutable".to_string();
  87                     } else {
  88                         let name = self.upvars[upvar_index.index()].place.to_string(self.infcx.tcx);
  89                         reason = format!(", as `{name}` is not declared as mutable");
  90                     }
  91                 }
  92             }
  93
  94             PlaceRef { local, projection: [ProjectionElem::Deref] }
  95                 if self.body.local_decls[local].is_ref_for_guard() =>
  96             {
  97                 item_msg = access_place_desc;
  98                 reason = ", as it is immutable for the pattern guard".to_string();
  99             }
 100             PlaceRef { local, projection: [ProjectionElem::Deref] }
 101                 if self.body.local_decls[local].is_ref_to_static() =>
 102             {
 103                 if access_place.projection.len() == 1 {
 104                     item_msg = format!("immutable static item {access_place_desc}");
 105                     reason = String::new();
 106                 } else {
 107                     item_msg = access_place_desc;
 108                     let local_info = &self.body.local_decls[local].local_info;
 109                     if let Some(box LocalInfo::StaticRef { def_id, .. }) = *local_info {
 110                         let static_name = &self.infcx.tcx.item_name(def_id);
 111                         reason = format!(", as `{static_name}` is an immutable static item");
 112                     } else {
 113                         bug!("is_ref_to_static return true, but not ref to static?");
 114                     }
 115                 }
 116             }
 117             PlaceRef { local: _, projection: [proj_base @ .., ProjectionElem::Deref] } => {
 118                 if the_place_err.local == ty::CAPTURE_STRUCT_LOCAL
 119                     && proj_base.is_empty()
 120                     && !self.upvars.is_empty()
 121                 {
 122                     item_msg = access_place_desc;
 123                     debug_assert!(
 124                         self.body.local_decls[ty::CAPTURE_STRUCT_LOCAL].ty.is_region_ptr()
 125                     );
 126                     debug_assert!(is_closure_or_generator(
 127                         Place::ty_from(
 128                             the_place_err.local,
 129                             the_place_err.projection,
 130                             self.body,
 131                             self.infcx.tcx
 132                         )
 133                         .ty
 134                     ));
 135
 136                     reason = if self.is_upvar_field_projection(access_place.as_ref()).is_some() {
 137                         ", as it is a captured variable in a `Fn` closure".to_string()
 138                     } else {
 139                         ", as `Fn` closures cannot mutate their captured variables".to_string()
 140                     }
 141                 } else {
 142                     let source = self.borrowed_content_source(PlaceRef {
 143                         local: the_place_err.local,
 144                         projection: proj_base,
 145                     });
 146                     let pointer_type = source.describe_for_immutable_place(self.infcx.tcx);
 147                     opt_source = Some(source);
 148                     if let Some(desc) = self.describe_place(access_place.as_ref()) {
 149                         item_msg = format!("`{desc}`");
 150                         reason = match error_access {
 151                             AccessKind::Mutate => format!(", which is behind {pointer_type}"),
 152                             AccessKind::MutableBorrow => {
 153                                 format!(", as it is behind {pointer_type}")
 154                             }
 155                         }
 156                     } else {
 157                         item_msg = format!("data in {pointer_type}");
 158                         reason = String::new();
 159                     }
 160                 }
 161             }
 162
 163             PlaceRef {
 164                 local: _,
 165                 projection:
 166                     [
 167                         ..,
 168                         ProjectionElem::Index(_)
 169                         | ProjectionElem::ConstantIndex { .. }
 170                         | ProjectionElem::OpaqueCast { .. }
 171                         | ProjectionElem::Subslice { .. }
 172                         | ProjectionElem::Downcast(..),
 173                     ],
 174             } => bug!("Unexpected immutable place."),
 175         }
 176
 177         debug!("report_mutability_error: item_msg={:?}, reason={:?}", item_msg, reason);
 178
 179         // `act` and `acted_on` are strings that let us abstract over
 180         // the verbs used in some diagnostic messages.
 181         let act;
 182         let acted_on;
 183         let mut suggest = true;
 184         let mut mut_error = None;
 185         let mut count = 1;
 186
 187         let span = match error_access {
 188             AccessKind::Mutate => {
 189                 err = self.cannot_assign(span, &(item_msg + &reason));
 190                 act = "assign";
 191                 acted_on = "written";
 192                 span
 193             }
 194             AccessKind::MutableBorrow => {
 195                 act = "borrow as mutable";
 196                 acted_on = "borrowed as mutable";
 197
 198                 let borrow_spans = self.borrow_spans(span, location);
 199                 let borrow_span = borrow_spans.args_or_use();
 200                 match the_place_err {
 201                     PlaceRef { local, projection: [] }
 202                         if self.body.local_decls[local].can_be_made_mutable() =>
 203                     {
 204                         let span = self.body.local_decls[local].source_info.span;
 205                         mut_error = Some(span);
 206                         if let Some((buffer, c)) = self.get_buffered_mut_error(span) {
 207                             // We've encountered a second (or more) attempt to mutably borrow an
 208                             // immutable binding, so the likely problem is with the binding
 209                             // declaration, not the use. We collect these in a single diagnostic
 210                             // and make the binding the primary span of the error.
 211                             err = buffer;
 212                             count = c + 1;
 213                             if count == 2 {
 214                                 err.replace_span_with(span, false);
 215                                 err.span_label(span, "not mutable");
 216                             }
 217                             suggest = false;
 218                         } else {
 219                             err = self.cannot_borrow_path_as_mutable_because(
 220                                 borrow_span,
 221                                 &item_msg,
 222                                 &reason,
 223                             );
 224                         }
 225                     }
 226                     _ => {
 227                         err = self.cannot_borrow_path_as_mutable_because(
 228                             borrow_span,
 229                             &item_msg,
 230                             &reason,
 231                         );
 232                     }
 233                 }
 234                 if suggest {
 235                     borrow_spans.var_span_label(
 236                         &mut err,
 237                         format!(
 238                             "mutable borrow occurs due to use of {} in closure",
 239                             self.describe_any_place(access_place.as_ref()),
 240                         ),
 241                         "mutable",
 242                     );
 243                 }
 244                 borrow_span
 245             }
 246         };
 247
 248         debug!("report_mutability_error: act={:?}, acted_on={:?}", act, acted_on);
 249
 250         match the_place_err {
 251             // Suggest making an existing shared borrow in a struct definition a mutable borrow.
 252             //
 253             // This is applicable when we have a deref of a field access to a deref of a local -
 254             // something like `*((*_1).0`. The local that we get will be a reference to the
 255             // struct we've got a field access of (it must be a reference since there's a deref
 256             // after the field access).
 257             PlaceRef {
 258                 local,
 259                 projection:
 260                     [
 261                         proj_base @ ..,
 262                         ProjectionElem::Deref,
 263                         ProjectionElem::Field(field, _),
 264                         ProjectionElem::Deref,
 265                     ],
 266             } => {
 267                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 268
 269                 if let Some(span) = get_mut_span_in_struct_field(
 270                     self.infcx.tcx,
 271                     Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty,
 272                     *field,
 273                 ) {
 274                     err.span_suggestion_verbose(
 275                         span,
 276                         "consider changing this to be mutable",
 277                         " mut ",
 278                         Applicability::MaybeIncorrect,
 279                     );
 280                 }
 281             }
 282
 283             // Suggest removing a `&mut` from the use of a mutable reference.
 284             PlaceRef { local, projection: [] }
 285                 if self
 286                     .body
 287                     .local_decls
 288                     .get(local)
 289                     .map(|l| mut_borrow_of_mutable_ref(l, self.local_names[local]))
 290                     .unwrap_or(false) =>
 291             {
 292                 let decl = &self.body.local_decls[local];
 293                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 294                 if let Some(mir::Statement {
 295                     source_info,
 296                     kind:
 297                         mir::StatementKind::Assign(box (
 298                             _,
 299                             mir::Rvalue::Ref(
 300                                 _,
 301                                 mir::BorrowKind::Mut { allow_two_phase_borrow: false },
 302                                 _,
 303                             ),
 304                         )),
 305                     ..
 306                 }) = &self.body[location.block].statements.get(location.statement_index)
 307                 {
 308                     match decl.local_info {
 309                         Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
 310                             mir::VarBindingForm {
 311                                 binding_mode: ty::BindingMode::BindByValue(Mutability::Not),
 312                                 opt_ty_info: Some(sp),
 313                                 opt_match_place: _,
 314                                 pat_span: _,
 315                             },
 316                         )))) => {
 317                             if suggest {
 318                                 err.span_note(sp, "the binding is already a mutable borrow");
 319                             }
 320                         }
 321                         _ => {
 322                             err.span_note(
 323                                 decl.source_info.span,
 324                                 "the binding is already a mutable borrow",
 325                             );
 326                         }
 327                     }
 328                     if let Ok(snippet) =
 329                         self.infcx.tcx.sess.source_map().span_to_snippet(source_info.span)
 330                     {
 331                         if snippet.starts_with("&mut ") {
 332                             // We don't have access to the HIR to get accurate spans, but we can
 333                             // give a best effort structured suggestion.
 334                             err.span_suggestion_verbose(
 335                                 source_info.span.with_hi(source_info.span.lo() + BytePos(5)),
 336                                 "try removing `&mut` here",
 337                                 "",
 338                                 Applicability::MachineApplicable,
 339                             );
 340                         } else {
 341                             // This can occur with things like `(&mut self).foo()`.
 342                             err.span_help(source_info.span, "try removing `&mut` here");
 343                         }
 344                     } else {
 345                         err.span_help(source_info.span, "try removing `&mut` here");
 346                     }
 347                 } else if decl.mutability == Mutability::Not
 348                     && !matches!(
 349                         decl.local_info,
 350                         Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::ImplicitSelf(
 351                             hir::ImplicitSelfKind::MutRef
 352                         ))))
 353                     )
 354                 {
 355                     err.span_suggestion_verbose(
 356                         decl.source_info.span.shrink_to_lo(),
 357                         "consider making the binding mutable",
 358                         "mut ",
 359                         Applicability::MachineApplicable,
 360                     );
 361                 }
 362             }
 363
 364             // We want to suggest users use `let mut` for local (user
 365             // variable) mutations...
 366             PlaceRef { local, projection: [] }
 367                 if self.body.local_decls[local].can_be_made_mutable() =>
 368             {
 369                 // ... but it doesn't make sense to suggest it on
 370                 // variables that are `ref x`, `ref mut x`, `&self`,
 371                 // or `&mut self` (such variables are simply not
 372                 // mutable).
 373                 let local_decl = &self.body.local_decls[local];
 374                 assert_eq!(local_decl.mutability, Mutability::Not);
 375
 376                 if count < 10 {
 377                     err.span_label(span, format!("cannot {act}"));
 378                 }
 379                 if suggest {
 380                     err.span_suggestion_verbose(
 381                         local_decl.source_info.span.shrink_to_lo(),
 382                         "consider changing this to be mutable",
 383                         "mut ".to_string(),
 384                         Applicability::MachineApplicable,
 385                     );
 386                     let tcx = self.infcx.tcx;
 387                     if let ty::Closure(id, _) = *the_place_err.ty(self.body, tcx).ty.kind() {
 388                         self.show_mutating_upvar(tcx, id.expect_local(), the_place_err, &mut err);
 389                     }
 390                 }
 391             }
 392
 393             // Also suggest adding mut for upvars
 394             PlaceRef {
 395                 local,
 396                 projection: [proj_base @ .., ProjectionElem::Field(upvar_index, _)],
 397             } => {
 398                 debug_assert!(is_closure_or_generator(
 399                     Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty
 400                 ));
 401
 402                 let captured_place = &self.upvars[upvar_index.index()].place;
 403
 404                 err.span_label(span, format!("cannot {act}"));
 405
 406                 let upvar_hir_id = captured_place.get_root_variable();
 407
 408                 if let Some(Node::Pat(pat)) = self.infcx.tcx.hir().find(upvar_hir_id)
 409                     && let hir::PatKind::Binding(
 410                         hir::BindingAnnotation::NONE,
 411                         _,
 412                         upvar_ident,
 413                         _,
 414                     ) = pat.kind
 415                 {
 416                     err.span_suggestion(
 417                         upvar_ident.span,
 418                         "consider changing this to be mutable",
 419                         format!("mut {}", upvar_ident.name),
 420                         Applicability::MachineApplicable,
 421                     );
 422                 }
 423
 424                 let tcx = self.infcx.tcx;
 425                 if let ty::Ref(_, ty, Mutability::Mut) = the_place_err.ty(self.body, tcx).ty.kind()
 426                     && let ty::Closure(id, _) = *ty.kind()
 427                 {
 428                     self.show_mutating_upvar(tcx, id.expect_local(), the_place_err, &mut err);
 429                 }
 430             }
 431
 432             // complete hack to approximate old AST-borrowck
 433             // diagnostic: if the span starts with a mutable borrow of
 434             // a local variable, then just suggest the user remove it.
 435             PlaceRef { local: _, projection: [] }
 436                 if self
 437                     .infcx
 438                     .tcx
 439                     .sess
 440                     .source_map()
 441                     .span_to_snippet(span)
 442                     .map_or(false, |snippet| snippet.starts_with("&mut ")) =>
 443             {
 444                 err.span_label(span, format!("cannot {act}"));
 445                 err.span_suggestion(
 446                     span,
 447                     "try removing `&mut` here",
 448                     "",
 449                     Applicability::MaybeIncorrect,
 450                 );
 451             }
 452
 453             PlaceRef { local, projection: [ProjectionElem::Deref] }
 454                 if self.body.local_decls[local].is_ref_for_guard() =>
 455             {
 456                 err.span_label(span, format!("cannot {act}"));
 457                 err.note(
 458                     "variables bound in patterns are immutable until the end of the pattern guard",
 459                 );
 460             }
 461
 462             // We want to point out when a `&` can be readily replaced
 463             // with an `&mut`.
 464             //
 465             // FIXME: can this case be generalized to work for an
 466             // arbitrary base for the projection?
 467             PlaceRef { local, projection: [ProjectionElem::Deref] }
 468                 if self.body.local_decls[local].is_user_variable() =>
 469             {
 470                 let local_decl = &self.body.local_decls[local];
 471
 472                 let (pointer_sigil, pointer_desc) = if local_decl.ty.is_region_ptr() {
 473                     ("&", "reference")
 474                 } else {
 475                     ("*const", "pointer")
 476                 };
 477
 478                 match self.local_names[local] {
 479                     Some(name) if !local_decl.from_compiler_desugaring() => {
 480                         let label = match local_decl.local_info.as_deref().unwrap() {
 481                             LocalInfo::User(ClearCrossCrate::Set(
 482                                 mir::BindingForm::ImplicitSelf(_),
 483                             )) => {
 484                                 let (span, suggestion) =
 485                                     suggest_ampmut_self(self.infcx.tcx, local_decl);
 486                                 Some((true, span, suggestion))
 487                             }
 488
 489                             LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
 490                                 mir::VarBindingForm {
 491                                     binding_mode: ty::BindingMode::BindByValue(_),
 492                                     opt_ty_info,
 493                                     ..
 494                                 },
 495                             ))) => {
 496                                 // check if the RHS is from desugaring
 497                                 let opt_assignment_rhs_span =
 498                                     self.body.find_assignments(local).first().map(|&location| {
 499                                         if let Some(mir::Statement {
 500                                             source_info: _,
 501                                             kind:
 502                                                 mir::StatementKind::Assign(box (
 503                                                     _,
 504                                                     mir::Rvalue::Use(mir::Operand::Copy(place)),
 505                                                 )),
 506                                         }) = self.body[location.block]
 507                                             .statements
 508                                             .get(location.statement_index)
 509                                         {
 510                                             self.body.local_decls[place.local].source_info.span
 511                                         } else {
 512                                             self.body.source_info(location).span
 513                                         }
 514                                     });
 515                                 match opt_assignment_rhs_span.and_then(|s| s.desugaring_kind()) {
 516                                     // on for loops, RHS points to the iterator part
 517                                     Some(DesugaringKind::ForLoop) => {
 518                                         self.suggest_similar_mut_method_for_for_loop(&mut err);
 519                                         err.span_label(opt_assignment_rhs_span.unwrap(), format!(
 520                                             "this iterator yields `{pointer_sigil}` {pointer_desc}s",
 521                                         ));
 522                                         None
 523                                     }
 524                                     // don't create labels for compiler-generated spans
 525                                     Some(_) => None,
 526                                     None => {
 527                                         let label = if name != kw::SelfLower {
 528                                             suggest_ampmut(
 529                                                 self.infcx.tcx,
 530                                                 local_decl,
 531                                                 opt_assignment_rhs_span,
 532                                                 *opt_ty_info,
 533                                             )
 534                                         } else {
 535                                             match local_decl.local_info.as_deref() {
 536                                                 Some(LocalInfo::User(ClearCrossCrate::Set(
 537                                                     mir::BindingForm::Var(mir::VarBindingForm {
 538                                                         opt_ty_info: None,
 539                                                         ..
 540                                                     }),
 541                                                 ))) => {
 542                                                     let (span, sugg) = suggest_ampmut_self(
 543                                                         self.infcx.tcx,
 544                                                         local_decl,
 545                                                     );
 546                                                     (true, span, sugg)
 547                                                 }
 548                                                 // explicit self (eg `self: &'a Self`)
 549                                                 _ => suggest_ampmut(
 550                                                     self.infcx.tcx,
 551                                                     local_decl,
 552                                                     opt_assignment_rhs_span,
 553                                                     *opt_ty_info,
 554                                                 ),
 555                                             }
 556                                         };
 557                                         Some(label)
 558                                     }
 559                                 }
 560                             }
 561
 562                             LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
 563                                 mir::VarBindingForm {
 564                                     binding_mode: ty::BindingMode::BindByReference(_),
 565                                     ..
 566                                 },
 567                             ))) => {
 568                                 let pattern_span = local_decl.source_info.span;
 569                                 suggest_ref_mut(self.infcx.tcx, pattern_span)
 570                                     .map(|replacement| (true, pattern_span, replacement))
 571                             }
 572
 573                             LocalInfo::User(ClearCrossCrate::Clear) => {
 574                                 bug!("saw cleared local state")
 575                             }
 576
 577                             _ => unreachable!(),
 578                         };
 579
 580                         match label {
 581                             Some((true, err_help_span, suggested_code)) => {
 582                                 let (is_trait_sig, local_trait) = self.is_error_in_trait(local);
 583                                 if !is_trait_sig {
 584                                     err.span_suggestion_verbose(
 585                                         err_help_span,
 586                                         &format!(
 587                                             "consider changing this to be a mutable {pointer_desc}"
 588                                         ),
 589                                         suggested_code,
 590                                         Applicability::MachineApplicable,
 591                                     );
 592                                 } else if let Some(x) = local_trait {
 593                                     err.span_suggestion_verbose(
 594                                         x,
 595                                         &format!(
 596                                             "consider changing that to be a mutable {pointer_desc}"
 597                                         ),
 598                                         suggested_code,
 599                                         Applicability::MachineApplicable,
 600                                     );
 601                                 }
 602                             }
 603                             Some((false, err_label_span, message)) => {
 604                                 err.span_label(
 605                                     err_label_span,
 606                                     &format!(
 607                                         "consider changing this binding's type to be: `{message}`"
 608                                     ),
 609                                 );
 610                             }
 611                             None => {}
 612                         }
 613                         err.span_label(
 614                             span,
 615                             format!(
 616                                 "`{name}` is a `{pointer_sigil}` {pointer_desc}, \
 617                                  so the data it refers to cannot be {acted_on}",
 618                             ),
 619                         );
 620                     }
 621                     _ => {
 622                         err.span_label(
 623                             span,
 624                             format!("cannot {act} through `{pointer_sigil}` {pointer_desc}"),
 625                         );
 626                     }
 627                 }
 628             }
 629
 630             PlaceRef { local, projection: [ProjectionElem::Deref] }
 631                 if local == ty::CAPTURE_STRUCT_LOCAL && !self.upvars.is_empty() =>
 632             {
 633                 self.expected_fn_found_fn_mut_call(&mut err, span, act);
 634             }
 635
 636             PlaceRef { local: _, projection: [.., ProjectionElem::Deref] } => {
 637                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 638
 639                 match opt_source {
 640                     Some(BorrowedContentSource::OverloadedDeref(ty)) => {
 641                         err.help(&format!(
 642                             "trait `DerefMut` is required to modify through a dereference, \
 643                              but it is not implemented for `{ty}`",
 644                         ));
 645                     }
 646                     Some(BorrowedContentSource::OverloadedIndex(ty)) => {
 647                         err.help(&format!(
 648                             "trait `IndexMut` is required to modify indexed content, \
 649                              but it is not implemented for `{ty}`",
 650                         ));
 651                         self.suggest_map_index_mut_alternatives(ty, &mut err, span);
 652                     }
 653                     _ => (),
 654                 }
 655             }
 656
 657             _ => {
 658                 err.span_label(span, format!("cannot {act}"));
 659             }
 660         }
 661
 662         if let Some(span) = mut_error {
 663             self.buffer_mut_error(span, err, count);
 664         } else {
 665             self.buffer_error(err);
 666         }
 667     }
 668
 669     fn suggest_map_index_mut_alternatives(&self, ty: Ty<'tcx>, err: &mut Diagnostic, span: Span) {
 670         let Some(adt) = ty.ty_adt_def() else { return };
 671         let did = adt.did();
 672         if self.infcx.tcx.is_diagnostic_item(sym::HashMap, did)
 673             || self.infcx.tcx.is_diagnostic_item(sym::BTreeMap, did)
 674         {
 675             struct V<'a, 'tcx> {
 676                 assign_span: Span,
 677                 err: &'a mut Diagnostic,
 678                 ty: Ty<'tcx>,
 679                 suggested: bool,
 680             }
 681             impl<'a, 'tcx> Visitor<'tcx> for V<'a, 'tcx> {
 682                 fn visit_stmt(&mut self, stmt: &'tcx hir::Stmt<'tcx>) {
 683                     hir::intravisit::walk_stmt(self, stmt);
 684                     let expr = match stmt.kind {
 685                         hir::StmtKind::Semi(expr) | hir::StmtKind::Expr(expr) => expr,
 686                         hir::StmtKind::Local(hir::Local { init: Some(expr), .. }) => expr,
 687                         _ => {
 688                             return;
 689                         }
 690                     };
 691                     if let hir::ExprKind::Assign(place, rv, _sp) = expr.kind
 692                         && let hir::ExprKind::Index(val, index) = place.kind
 693                         && (expr.span == self.assign_span || place.span == self.assign_span)
 694                     {
 695                         // val[index] = rv;
 696                         // ---------- place
 697                         self.err.multipart_suggestions(
 698                             &format!(
 699                                 "to modify a `{}`, use `.get_mut()`, `.insert()` or the entry API",
 700                                 self.ty,
 701                             ),
 702                             vec![
 703                                 vec![ // val.insert(index, rv);
 704                                     (
 705                                         val.span.shrink_to_hi().with_hi(index.span.lo()),
 706                                         ".insert(".to_string(),
 707                                     ),
 708                                     (
 709                                         index.span.shrink_to_hi().with_hi(rv.span.lo()),
 710                                         ", ".to_string(),
 711                                     ),
 712                                     (rv.span.shrink_to_hi(), ")".to_string()),
 713                                 ],
 714                                 vec![ // val.get_mut(index).map(|v| { *v = rv; });
 715                                     (
 716                                         val.span.shrink_to_hi().with_hi(index.span.lo()),
 717                                         ".get_mut(".to_string(),
 718                                     ),
 719                                     (
 720                                         index.span.shrink_to_hi().with_hi(place.span.hi()),
 721                                         ").map(|val| { *val".to_string(),
 722                                     ),
 723                                     (
 724                                         rv.span.shrink_to_hi(),
 725                                         "; })".to_string(),
 726                                     ),
 727                                 ],
 728                                 vec![ // let x = val.entry(index).or_insert(rv);
 729                                     (val.span.shrink_to_lo(), "let val = ".to_string()),
 730                                     (
 731                                         val.span.shrink_to_hi().with_hi(index.span.lo()),
 732                                         ".entry(".to_string(),
 733                                     ),
 734                                     (
 735                                         index.span.shrink_to_hi().with_hi(rv.span.lo()),
 736                                         ").or_insert(".to_string(),
 737                                     ),
 738                                     (rv.span.shrink_to_hi(), ")".to_string()),
 739                                 ],
 740                             ],
 741                             Applicability::MachineApplicable,
 742                         );
 743                         self.suggested = true;
 744                     } else if let hir::ExprKind::MethodCall(_path, receiver, _, sp) = expr.kind
 745                         && let hir::ExprKind::Index(val, index) = receiver.kind
 746                         && expr.span == self.assign_span
 747                     {
 748                         // val[index].path(args..);
 749                         self.err.multipart_suggestion(
 750                             &format!("to modify a `{}` use `.get_mut()`", self.ty),
 751                             vec![
 752                                 (
 753                                     val.span.shrink_to_hi().with_hi(index.span.lo()),
 754                                     ".get_mut(".to_string(),
 755                                 ),
 756                                 (
 757                                     index.span.shrink_to_hi().with_hi(receiver.span.hi()),
 758                                     ").map(|val| val".to_string(),
 759                                 ),
 760                                 (sp.shrink_to_hi(), ")".to_string()),
 761                             ],
 762                             Applicability::MachineApplicable,
 763                         );
 764                         self.suggested = true;
 765                     }
 766                 }
 767             }
 768             let hir_map = self.infcx.tcx.hir();
 769             let def_id = self.body.source.def_id();
 770             let hir_id = hir_map.local_def_id_to_hir_id(def_id.as_local().unwrap());
 771             let node = hir_map.find(hir_id);
 772             let Some(hir::Node::Item(item)) = node else { return; };
 773             let hir::ItemKind::Fn(.., body_id) = item.kind else { return; };
 774             let body = self.infcx.tcx.hir().body(body_id);
 775             let mut v = V { assign_span: span, err, ty, suggested: false };
 776             v.visit_body(body);
 777             if !v.suggested {
 778                 err.help(&format!(
 779                     "to modify a `{ty}`, use `.get_mut()`, `.insert()` or the entry API",
 780                 ));
 781             }
 782         }
 783     }
 784
 785     /// User cannot make signature of a trait mutable without changing the
 786     /// trait. So we find if this error belongs to a trait and if so we move
 787     /// suggestion to the trait or disable it if it is out of scope of this crate
 788     fn is_error_in_trait(&self, local: Local) -> (bool, Option<Span>) {
 789         if self.body.local_kind(local) != LocalKind::Arg {
 790             return (false, None);
 791         }
 792         let hir_map = self.infcx.tcx.hir();
 793         let my_def = self.body.source.def_id();
 794         let my_hir = hir_map.local_def_id_to_hir_id(my_def.as_local().unwrap());
 795         let Some(td) =
 796             self.infcx.tcx.impl_of_method(my_def).and_then(|x| self.infcx.tcx.trait_id_of_impl(x))
 797         else {
 798             return (false, None);
 799         };
 800         (
 801             true,
 802             td.as_local().and_then(|tld| match hir_map.find_by_def_id(tld) {
 803                 Some(Node::Item(hir::Item {
 804                     kind: hir::ItemKind::Trait(_, _, _, _, items),
 805                     ..
 806                 })) => {
 807                     let mut f_in_trait_opt = None;
 808                     for hir::TraitItemRef { id: fi, kind: k, .. } in *items {
 809                         let hi = fi.hir_id();
 810                         if !matches!(k, hir::AssocItemKind::Fn { .. }) {
 811                             continue;
 812                         }
 813                         if hir_map.name(hi) != hir_map.name(my_hir) {
 814                             continue;
 815                         }
 816                         f_in_trait_opt = Some(hi);
 817                         break;
 818                     }
 819                     f_in_trait_opt.and_then(|f_in_trait| match hir_map.find(f_in_trait) {
 820                         Some(Node::TraitItem(hir::TraitItem {
 821                             kind:
 822                                 hir::TraitItemKind::Fn(
 823                                     hir::FnSig { decl: hir::FnDecl { inputs, .. }, .. },
 824                                     _,
 825                                 ),
 826                             ..
 827                         })) => {
 828                             let hir::Ty { span, .. } = inputs[local.index() - 1];
 829                             Some(span)
 830                         }
 831                         _ => None,
 832                     })
 833                 }
 834                 _ => None,
 835             }),
 836         )
 837     }
 838
 839     // point to span of upvar making closure call require mutable borrow
 840     fn show_mutating_upvar(
 841         &self,
 842         tcx: TyCtxt<'_>,
 843         closure_local_def_id: hir::def_id::LocalDefId,
 844         the_place_err: PlaceRef<'tcx>,
 845         err: &mut Diagnostic,
 846     ) {
 847         let tables = tcx.typeck(closure_local_def_id);
 848         let closure_hir_id = tcx.hir().local_def_id_to_hir_id(closure_local_def_id);
 849         if let Some((span, closure_kind_origin)) =
 850             &tables.closure_kind_origins().get(closure_hir_id)
 851         {
 852             let reason = if let PlaceBase::Upvar(upvar_id) = closure_kind_origin.base {
 853                 let upvar = ty::place_to_string_for_capture(tcx, closure_kind_origin);
 854                 let root_hir_id = upvar_id.var_path.hir_id;
 855                 // we have an origin for this closure kind starting at this root variable so it's safe to unwrap here
 856                 let captured_places =
 857                     tables.closure_min_captures[&closure_local_def_id].get(&root_hir_id).unwrap();
 858
 859                 let origin_projection = closure_kind_origin
 860                     .projections
 861                     .iter()
 862                     .map(|proj| proj.kind)
 863                     .collect::<Vec<_>>();
 864                 let mut capture_reason = String::new();
 865                 for captured_place in captured_places {
 866                     let captured_place_kinds = captured_place
 867                         .place
 868                         .projections
 869                         .iter()
 870                         .map(|proj| proj.kind)
 871                         .collect::<Vec<_>>();
 872                     if rustc_middle::ty::is_ancestor_or_same_capture(
 873                         &captured_place_kinds,
 874                         &origin_projection,
 875                     ) {
 876                         match captured_place.info.capture_kind {
 877                             ty::UpvarCapture::ByRef(
 878                                 ty::BorrowKind::MutBorrow | ty::BorrowKind::UniqueImmBorrow,
 879                             ) => {
 880                                 capture_reason = format!("mutable borrow of `{upvar}`");
 881                             }
 882                             ty::UpvarCapture::ByValue => {
 883                                 capture_reason = format!("possible mutation of `{upvar}`");
 884                             }
 885                             _ => bug!("upvar `{upvar}` borrowed, but not mutably"),
 886                         }
 887                         break;
 888                     }
 889                 }
 890                 if capture_reason.is_empty() {
 891                     bug!("upvar `{upvar}` borrowed, but cannot find reason");
 892                 }
 893                 capture_reason
 894             } else {
 895                 bug!("not an upvar")
 896             };
 897             err.span_label(
 898                 *span,
 899                 format!(
 900                     "calling `{}` requires mutable binding due to {}",
 901                     self.describe_place(the_place_err).unwrap(),
 902                     reason
 903                 ),
 904             );
 905         }
 906     }
 907
 908     // Attempt to search similar mutable associated items for suggestion.
 909     // In the future, attempt in all path but initially for RHS of for_loop
 910     fn suggest_similar_mut_method_for_for_loop(&self, err: &mut Diagnostic) {
 911         use hir::{
 912             BodyId, Expr,
 913             ExprKind::{Block, Call, DropTemps, Match, MethodCall},
 914             HirId, ImplItem, ImplItemKind, Item, ItemKind,
 915         };
 916
 917         fn maybe_body_id_of_fn(hir_map: Map<'_>, id: HirId) -> Option<BodyId> {
 918             match hir_map.find(id) {
 919                 Some(Node::Item(Item { kind: ItemKind::Fn(_, _, body_id), .. }))
 920                 | Some(Node::ImplItem(ImplItem { kind: ImplItemKind::Fn(_, body_id), .. })) => {
 921                     Some(*body_id)
 922                 }
 923                 _ => None,
 924             }
 925         }
 926         let hir_map = self.infcx.tcx.hir();
 927         let mir_body_hir_id = self.mir_hir_id();
 928         if let Some(fn_body_id) = maybe_body_id_of_fn(hir_map, mir_body_hir_id) {
 929             if let Block(
 930                 hir::Block {
 931                     expr:
 932                         Some(Expr {
 933                             kind:
 934                                 DropTemps(Expr {
 935                                     kind:
 936                                         Match(
 937                                             Expr {
 938                                                 kind:
 939                                                     Call(
 940                                                         _,
 941                                                         [
 942                                                             Expr {
 943                                                                 kind:
 944                                                                     MethodCall(path_segment, _, _, span),
 945                                                                 hir_id,
 946                                                                 ..
 947                                                             },
 948                                                             ..,
 949                                                         ],
 950                                                     ),
 951                                                 ..
 952                                             },
 953                                             ..,
 954                                         ),
 955                                     ..
 956                                 }),
 957                             ..
 958                         }),
 959                     ..
 960                 },
 961                 _,
 962             ) = hir_map.body(fn_body_id).value.kind
 963             {
 964                 let opt_suggestions = self
 965                     .infcx
 966                     .tcx
 967                     .typeck(path_segment.hir_id.owner.def_id)
 968                     .type_dependent_def_id(*hir_id)
 969                     .and_then(|def_id| self.infcx.tcx.impl_of_method(def_id))
 970                     .map(|def_id| self.infcx.tcx.associated_items(def_id))
 971                     .map(|assoc_items| {
 972                         assoc_items
 973                             .in_definition_order()
 974                             .map(|assoc_item_def| assoc_item_def.ident(self.infcx.tcx))
 975                             .filter(|&ident| {
 976                                 let original_method_ident = path_segment.ident;
 977                                 original_method_ident != ident
 978                                     && ident
 979                                         .as_str()
 980                                         .starts_with(&original_method_ident.name.to_string())
 981                             })
 982                             .map(|ident| format!("{ident}()"))
 983                             .peekable()
 984                     });
 985
 986                 if let Some(mut suggestions) = opt_suggestions
 987                     && suggestions.peek().is_some()
 988                 {
 989                     err.span_suggestions(
 990                         *span,
 991                         "use mutable method",
 992                         suggestions,
 993                         Applicability::MaybeIncorrect,
 994                     );
 995                 }
 996             }
 997         };
 998     }
 999
1000     /// Targeted error when encountering an `FnMut` closure where an `Fn` closure was expected.
1001     fn expected_fn_found_fn_mut_call(&self, err: &mut Diagnostic, sp: Span, act: &str) {
1002         err.span_label(sp, format!("cannot {act}"));
1003
1004         let hir = self.infcx.tcx.hir();
1005         let closure_id = self.mir_hir_id();
1006         let closure_span = self.infcx.tcx.def_span(self.mir_def_id());
1007         let fn_call_id = hir.get_parent_node(closure_id);
1008         let node = hir.get(fn_call_id);
1009         let def_id = hir.enclosing_body_owner(fn_call_id);
1010         let mut look_at_return = true;
1011         // If we can detect the expression to be an `fn` call where the closure was an argument,
1012         // we point at the `fn` definition argument...
1013         if let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Call(func, args), .. }) = node {
1014             let arg_pos = args
1015                 .iter()
1016                 .enumerate()
1017                 .filter(|(_, arg)| arg.hir_id == closure_id)
1018                 .map(|(pos, _)| pos)
1019                 .next();
1020             let tables = self.infcx.tcx.typeck(def_id);
1021             if let Some(ty::FnDef(def_id, _)) =
1022                 tables.node_type_opt(func.hir_id).as_ref().map(|ty| ty.kind())
1023             {
1024                 let arg = match hir.get_if_local(*def_id) {
1025                     Some(
1026                         hir::Node::Item(hir::Item {
1027                             ident, kind: hir::ItemKind::Fn(sig, ..), ..
1028                         })
1029                         | hir::Node::TraitItem(hir::TraitItem {
1030                             ident,
1031                             kind: hir::TraitItemKind::Fn(sig, _),
1032                             ..
1033                         })
1034                         | hir::Node::ImplItem(hir::ImplItem {
1035                             ident,
1036                             kind: hir::ImplItemKind::Fn(sig, _),
1037                             ..
1038                         }),
1039                     ) => Some(
1040                         arg_pos
1041                             .and_then(|pos| {
1042                                 sig.decl.inputs.get(
1043                                     pos + if sig.decl.implicit_self.has_implicit_self() {
1044                                         1
1045                                     } else {
1046                                         0
1047                                     },
1048                                 )
1049                             })
1050                             .map(|arg| arg.span)
1051                             .unwrap_or(ident.span),
1052                     ),
1053                     _ => None,
1054                 };
1055                 if let Some(span) = arg {
1056                     err.span_label(span, "change this to accept `FnMut` instead of `Fn`");
1057                     err.span_label(func.span, "expects `Fn` instead of `FnMut`");
1058                     err.span_label(closure_span, "in this closure");
1059                     look_at_return = false;
1060                 }
1061             }
1062         }
1063
1064         if look_at_return && hir.get_return_block(closure_id).is_some() {
1065             // ...otherwise we are probably in the tail expression of the function, point at the
1066             // return type.
1067             match hir.get_by_def_id(hir.get_parent_item(fn_call_id).def_id) {
1068                 hir::Node::Item(hir::Item { ident, kind: hir::ItemKind::Fn(sig, ..), .. })
1069                 | hir::Node::TraitItem(hir::TraitItem {
1070                     ident,
1071                     kind: hir::TraitItemKind::Fn(sig, _),
1072                     ..
1073                 })
1074                 | hir::Node::ImplItem(hir::ImplItem {
1075                     ident,
1076                     kind: hir::ImplItemKind::Fn(sig, _),
1077                     ..
1078                 }) => {
1079                     err.span_label(ident.span, "");
1080                     err.span_label(
1081                         sig.decl.output.span(),
1082                         "change this to return `FnMut` instead of `Fn`",
1083                     );
1084                     err.span_label(closure_span, "in this closure");
1085                 }
1086                 _ => {}
1087             }
1088         }
1089     }
1090 }
1091
1092 fn mut_borrow_of_mutable_ref(local_decl: &LocalDecl<'_>, local_name: Option<Symbol>) -> bool {
1093     debug!("local_info: {:?}, ty.kind(): {:?}", local_decl.local_info, local_decl.ty.kind());
1094
1095     match local_decl.local_info.as_deref() {
1096         // Check if mutably borrowing a mutable reference.
1097         Some(LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
1098             mir::VarBindingForm {
1099                 binding_mode: ty::BindingMode::BindByValue(Mutability::Not), ..
1100             },
1101         )))) => matches!(local_decl.ty.kind(), ty::Ref(_, _, hir::Mutability::Mut)),
1102         Some(LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::ImplicitSelf(kind)))) => {
1103             // Check if the user variable is a `&mut self` and we can therefore
1104             // suggest removing the `&mut`.
1105             //
1106             // Deliberately fall into this case for all implicit self types,
1107             // so that we don't fall in to the next case with them.
1108             *kind == hir::ImplicitSelfKind::MutRef
1109         }
1110         _ if Some(kw::SelfLower) == local_name => {
1111             // Otherwise, check if the name is the `self` keyword - in which case
1112             // we have an explicit self. Do the same thing in this case and check
1113             // for a `self: &mut Self` to suggest removing the `&mut`.
1114             matches!(local_decl.ty.kind(), ty::Ref(_, _, hir::Mutability::Mut))
1115         }
1116         _ => false,
1117     }
1118 }
1119
1120 fn suggest_ampmut_self<'tcx>(
1121     tcx: TyCtxt<'tcx>,
1122     local_decl: &mir::LocalDecl<'tcx>,
1123 ) -> (Span, String) {
1124     let sp = local_decl.source_info.span;
1125     (
1126         sp,
1127         match tcx.sess.source_map().span_to_snippet(sp) {
1128             Ok(snippet) => {
1129                 let lt_pos = snippet.find('\'');
1130                 if let Some(lt_pos) = lt_pos {
1131                     format!("&{}mut self", &snippet[lt_pos..snippet.len() - 4])
1132                 } else {
1133                     "&mut self".to_string()
1134                 }
1135             }
1136             _ => "&mut self".to_string(),
1137         },
1138     )
1139 }
1140
1141 // When we want to suggest a user change a local variable to be a `&mut`, there
1142 // are three potential "obvious" things to highlight:
1143 //
1144 // let ident [: Type] [= RightHandSideExpression];
1145 //     ^^^^^    ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^
1146 //     (1.)     (2.)              (3.)
1147 //
1148 // We can always fallback on highlighting the first. But chances are good that
1149 // the user experience will be better if we highlight one of the others if possible;
1150 // for example, if the RHS is present and the Type is not, then the type is going to
1151 // be inferred *from* the RHS, which means we should highlight that (and suggest
1152 // that they borrow the RHS mutably).
1153 //
1154 // This implementation attempts to emulate AST-borrowck prioritization
1155 // by trying (3.), then (2.) and finally falling back on (1.).
1156 fn suggest_ampmut<'tcx>(
1157     tcx: TyCtxt<'tcx>,
1158     local_decl: &mir::LocalDecl<'tcx>,
1159     opt_assignment_rhs_span: Option<Span>,
1160     opt_ty_info: Option<Span>,
1161 ) -> (bool, Span, String) {
1162     if let Some(assignment_rhs_span) = opt_assignment_rhs_span
1163         && let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span)
1164     {
1165         let is_mutbl = |ty: &str| -> bool {
1166             if let Some(rest) = ty.strip_prefix("mut") {
1167                 match rest.chars().next() {
1168                     // e.g. `&mut x`
1169                     Some(c) if c.is_whitespace() => true,
1170                     // e.g. `&mut(x)`
1171                     Some('(') => true,
1172                     // e.g. `&mut{x}`
1173                     Some('{') => true,
1174                     // e.g. `&mutablevar`
1175                     _ => false,
1176                 }
1177             } else {
1178                 false
1179             }
1180         };
1181         if let (true, Some(ws_pos)) = (src.starts_with("&'"), src.find(char::is_whitespace)) {
1182             let lt_name = &src[1..ws_pos];
1183             let ty = src[ws_pos..].trim_start();
1184             if !is_mutbl(ty) {
1185                 return (true, assignment_rhs_span, format!("&{lt_name} mut {ty}"));
1186             }
1187         } else if let Some(stripped) = src.strip_prefix('&') {
1188             let stripped = stripped.trim_start();
1189             if !is_mutbl(stripped) {
1190                 return (true, assignment_rhs_span, format!("&mut {stripped}"));
1191             }
1192         }
1193     }
1194
1195     let (suggestability, highlight_span) = match opt_ty_info {
1196         // if this is a variable binding with an explicit type,
1197         // try to highlight that for the suggestion.
1198         Some(ty_span) => (true, ty_span),
1199
1200         // otherwise, just highlight the span associated with
1201         // the (MIR) LocalDecl.
1202         None => (false, local_decl.source_info.span),
1203     };
1204
1205     if let Ok(src) = tcx.sess.source_map().span_to_snippet(highlight_span)
1206         && let (true, Some(ws_pos)) = (src.starts_with("&'"), src.find(char::is_whitespace))
1207     {
1208         let lt_name = &src[1..ws_pos];
1209         let ty = &src[ws_pos..];
1210         return (true, highlight_span, format!("&{} mut{}", lt_name, ty));
1211     }
1212
1213     let ty_mut = local_decl.ty.builtin_deref(true).unwrap();
1214     assert_eq!(ty_mut.mutbl, hir::Mutability::Not);
1215     (
1216         suggestability,
1217         highlight_span,
1218         if local_decl.ty.is_region_ptr() {
1219             format!("&mut {}", ty_mut.ty)
1220         } else {
1221             format!("*mut {}", ty_mut.ty)
1222         },
1223     )
1224 }
1225
1226 fn is_closure_or_generator(ty: Ty<'_>) -> bool {
1227     ty.is_closure() || ty.is_generator()
1228 }
1229
1230 /// Given a field that needs to be mutable, returns a span where the " mut " could go.
1231 /// This function expects the local to be a reference to a struct in order to produce a span.
1232 ///
1233 /// ```text
1234 /// LL |     s: &'a   String
1235 ///    |           ^^^ returns a span taking up the space here
1236 /// ```
1237 fn get_mut_span_in_struct_field<'tcx>(
1238     tcx: TyCtxt<'tcx>,
1239     ty: Ty<'tcx>,
1240     field: mir::Field,
1241 ) -> Option<Span> {
1242     // Expect our local to be a reference to a struct of some kind.
1243     if let ty::Ref(_, ty, _) = ty.kind()
1244         && let ty::Adt(def, _) = ty.kind()
1245         && let field = def.all_fields().nth(field.index())?
1246         // Use the HIR types to construct the diagnostic message.
1247         && let node = tcx.hir().find_by_def_id(field.did.as_local()?)?
1248         // Now we're dealing with the actual struct that we're going to suggest a change to,
1249         // we can expect a field that is an immutable reference to a type.
1250         && let hir::Node::Field(field) = node
1251         && let hir::TyKind::Ref(lt, hir::MutTy { mutbl: hir::Mutability::Not, ty }) = field.ty.kind
1252     {
1253         return Some(lt.ident.span.between(ty.span));
1254     }
1255
1256     None
1257 }
1258
1259 /// If possible, suggest replacing `ref` with `ref mut`.
1260 fn suggest_ref_mut(tcx: TyCtxt<'_>, binding_span: Span) -> Option<String> {
1261     let hi_src = tcx.sess.source_map().span_to_snippet(binding_span).ok()?;
1262     if hi_src.starts_with("ref") && hi_src["ref".len()..].starts_with(rustc_lexer::is_whitespace) {
1263         let replacement = format!("ref mut{}", &hi_src["ref".len()..]);
1264         Some(replacement)
1265     } else {
1266         None
1267     }
1268 }