compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

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