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