src/librustc_mir/borrow_check/mutability_errors.rs

   1 use rustc::hir;
   2 use rustc::hir::Node;
   3 use rustc::mir::{self, BindingForm, Constant, ClearCrossCrate, Local, Location, Mir};
   4 use rustc::mir::{Mutability, Operand, Place, Projection, ProjectionElem, Static, Terminator};
   5 use rustc::mir::TerminatorKind;
   6 use rustc::ty::{self, Const, DefIdTree, TyS, TyKind, TyCtxt};
   7 use rustc_data_structures::indexed_vec::Idx;
   8 use syntax_pos::Span;
   9 use syntax_pos::symbol::keywords;
  10
  11 use crate::dataflow::move_paths::InitLocation;
  12 use crate::borrow_check::MirBorrowckCtxt;
  13 use crate::util::borrowck_errors::{BorrowckErrors, Origin};
  14 use crate::util::collect_writes::FindAssignments;
  15 use crate::util::suggest_ref_mut;
  16 use rustc_errors::Applicability;
  17
  18 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
  19 pub(super) enum AccessKind {
  20     MutableBorrow,
  21     Mutate,
  22     Move,
  23 }
  24
  25 impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
  26     pub(super) fn report_mutability_error(
  27         &mut self,
  28         access_place: &Place<'tcx>,
  29         span: Span,
  30         the_place_err: &Place<'tcx>,
  31         error_access: AccessKind,
  32         location: Location,
  33     ) {
  34         debug!(
  35             "report_mutability_error(\
  36                 access_place={:?}, span={:?}, the_place_err={:?}, error_access={:?}, location={:?},\
  37             )",
  38             access_place, span, the_place_err, error_access, location,
  39         );
  40
  41         let mut err;
  42         let item_msg;
  43         let reason;
  44         let access_place_desc = self.describe_place(access_place);
  45         debug!("report_mutability_error: access_place_desc={:?}", access_place_desc);
  46
  47         match the_place_err {
  48             Place::Local(local) => {
  49                 item_msg = format!("`{}`", access_place_desc.unwrap());
  50                 if let Place::Local(_) = access_place {
  51                     reason = ", as it is not declared as mutable".to_string();
  52                 } else {
  53                     let name = self.mir.local_decls[*local]
  54                         .name
  55                         .expect("immutable unnamed local");
  56                     reason = format!(", as `{}` is not declared as mutable", name);
  57                 }
  58             }
  59
  60             Place::Projection(box Projection {
  61                 base,
  62                 elem: ProjectionElem::Field(upvar_index, _),
  63             }) => {
  64                 debug_assert!(is_closure_or_generator(
  65                     base.ty(self.mir, self.infcx.tcx).to_ty(self.infcx.tcx)
  66                 ));
  67
  68                 item_msg = format!("`{}`", access_place_desc.unwrap());
  69                 if access_place.is_upvar_field_projection(self.mir, &self.infcx.tcx).is_some() {
  70                     reason = ", as it is not declared as mutable".to_string();
  71                 } else {
  72                     let name = self.mir.upvar_decls[upvar_index.index()].debug_name;
  73                     reason = format!(", as `{}` is not declared as mutable", name);
  74                 }
  75             }
  76
  77             Place::Projection(box Projection {
  78                 base,
  79                 elem: ProjectionElem::Deref,
  80             }) => {
  81                 if *base == Place::Local(Local::new(1)) && !self.mir.upvar_decls.is_empty() {
  82                     item_msg = format!("`{}`", access_place_desc.unwrap());
  83                     debug_assert!(self.mir.local_decls[Local::new(1)].ty.is_region_ptr());
  84                     debug_assert!(is_closure_or_generator(
  85                         the_place_err.ty(self.mir, self.infcx.tcx).to_ty(self.infcx.tcx)
  86                     ));
  87
  88                     reason = if access_place.is_upvar_field_projection(self.mir,
  89                                                                        &self.infcx.tcx).is_some() {
  90                         ", as it is a captured variable in a `Fn` closure".to_string()
  91                     } else {
  92                         ", as `Fn` closures cannot mutate their captured variables".to_string()
  93                     }
  94                 } else if {
  95                     if let Place::Local(local) = *base {
  96                         if let Some(ClearCrossCrate::Set(BindingForm::RefForGuard))
  97                             = self.mir.local_decls[local].is_user_variable {
  98                                 true
  99                         } else {
 100                             false
 101                         }
 102                     } else {
 103                         false
 104                     }
 105                 } {
 106                     item_msg = format!("`{}`", access_place_desc.unwrap());
 107                     reason = ", as it is immutable for the pattern guard".to_string();
 108                 } else {
 109                     let pointer_type =
 110                         if base.ty(self.mir, self.infcx.tcx).to_ty(self.infcx.tcx).is_region_ptr() {
 111                             "`&` reference"
 112                         } else {
 113                             "`*const` pointer"
 114                         };
 115                     if let Some(desc) = access_place_desc {
 116                         item_msg = format!("`{}`", desc);
 117                         reason = match error_access {
 118                             AccessKind::Move |
 119                             AccessKind::Mutate => format!(" which is behind a {}", pointer_type),
 120                             AccessKind::MutableBorrow => {
 121                                 format!(", as it is behind a {}", pointer_type)
 122                             }
 123                         }
 124                     } else {
 125                         item_msg = format!("data in a {}", pointer_type);
 126                         reason = String::new();
 127                     }
 128                 }
 129             }
 130
 131             Place::Promoted(_) => unreachable!(),
 132
 133             Place::Static(box Static { def_id, ty: _ }) => {
 134                 if let Place::Static(_) = access_place {
 135                     item_msg = format!("immutable static item `{}`", access_place_desc.unwrap());
 136                     reason = String::new();
 137                 } else {
 138                     item_msg = format!("`{}`", access_place_desc.unwrap());
 139                     let static_name = &self.infcx.tcx.item_name(*def_id);
 140                     reason = format!(", as `{}` is an immutable static item", static_name);
 141                 }
 142             }
 143
 144             Place::Projection(box Projection {
 145                 base: _,
 146                 elem: ProjectionElem::Index(_),
 147             })
 148             | Place::Projection(box Projection {
 149                 base: _,
 150                 elem: ProjectionElem::ConstantIndex { .. },
 151             })
 152             | Place::Projection(box Projection {
 153                 base: _,
 154                 elem: ProjectionElem::Subslice { .. },
 155             })
 156             | Place::Projection(box Projection {
 157                 base: _,
 158                 elem: ProjectionElem::Downcast(..),
 159             }) => bug!("Unexpected immutable place."),
 160         }
 161
 162         debug!("report_mutability_error: item_msg={:?}, reason={:?}", item_msg, reason);
 163
 164         // `act` and `acted_on` are strings that let us abstract over
 165         // the verbs used in some diagnostic messages.
 166         let act;
 167         let acted_on;
 168
 169         let span = match error_access {
 170             AccessKind::Move => {
 171                 err = self.infcx.tcx
 172                     .cannot_move_out_of(span, &(item_msg + &reason), Origin::Mir);
 173                 err.span_label(span, "cannot move");
 174                 err.buffer(&mut self.errors_buffer);
 175                 return;
 176             }
 177             AccessKind::Mutate => {
 178                 err = self.infcx.tcx
 179                     .cannot_assign(span, &(item_msg + &reason), Origin::Mir);
 180                 act = "assign";
 181                 acted_on = "written";
 182                 span
 183             }
 184             AccessKind::MutableBorrow => {
 185                 act = "borrow as mutable";
 186                 acted_on = "borrowed as mutable";
 187
 188                 let borrow_spans = self.borrow_spans(span, location);
 189                 let borrow_span = borrow_spans.args_or_use();
 190                 err = self.infcx.tcx.cannot_borrow_path_as_mutable_because(
 191                     borrow_span,
 192                     &item_msg,
 193                     &reason,
 194                     Origin::Mir,
 195                 );
 196                 borrow_spans.var_span_label(
 197                     &mut err,
 198                     format!(
 199                         "mutable borrow occurs due to use of `{}` in closure",
 200                         // always Some() if the message is printed.
 201                         self.describe_place(access_place).unwrap_or_default(),
 202                     )
 203                 );
 204                 borrow_span
 205             }
 206         };
 207
 208         debug!("report_mutability_error: act={:?}, acted_on={:?}", act, acted_on);
 209
 210         match the_place_err {
 211             // Suggest making an existing shared borrow in a struct definition a mutable borrow.
 212             //
 213             // This is applicable when we have a deref of a field access to a deref of a local -
 214             // something like `*((*_1).0`. The local that we get will be a reference to the
 215             // struct we've got a field access of (it must be a reference since there's a deref
 216             // after the field access).
 217             Place::Projection(box Projection {
 218                 base: Place::Projection(box Projection {
 219                     base: Place::Projection(box Projection {
 220                         base,
 221                         elem: ProjectionElem::Deref,
 222                     }),
 223                     elem: ProjectionElem::Field(field, _),
 224                 }),
 225                 elem: ProjectionElem::Deref,
 226             }) => {
 227                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 228
 229                 if let Some((span, message)) = annotate_struct_field(
 230                     self.infcx.tcx,
 231                     base.ty(self.mir, self.infcx.tcx).to_ty(self.infcx.tcx),
 232                     field,
 233                 ) {
 234                     err.span_suggestion(
 235                         span,
 236                         "consider changing this to be mutable",
 237                         message,
 238                         Applicability::MaybeIncorrect,
 239                     );
 240                 }
 241             },
 242
 243             // Suggest removing a `&mut` from the use of a mutable reference.
 244             Place::Local(local)
 245                 if {
 246                     self.mir.local_decls.get(*local).map(|local_decl| {
 247                         if let ClearCrossCrate::Set(
 248                             mir::BindingForm::ImplicitSelf(kind)
 249                         ) = local_decl.is_user_variable.as_ref().unwrap() {
 250                             // Check if the user variable is a `&mut self` and we can therefore
 251                             // suggest removing the `&mut`.
 252                             //
 253                             // Deliberately fall into this case for all implicit self types,
 254                             // so that we don't fall in to the next case with them.
 255                             *kind == mir::ImplicitSelfKind::MutRef
 256                         } else if Some(keywords::SelfLower.name()) == local_decl.name {
 257                             // Otherwise, check if the name is the self kewyord - in which case
 258                             // we have an explicit self. Do the same thing in this case and check
 259                             // for a `self: &mut Self` to suggest removing the `&mut`.
 260                             if let ty::TyKind::Ref(
 261                                 _, _, hir::Mutability::MutMutable
 262                             ) = local_decl.ty.sty {
 263                                 true
 264                             } else {
 265                                 false
 266                             }
 267                         } else {
 268                             false
 269                         }
 270                     }).unwrap_or(false)
 271                 } =>
 272             {
 273                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 274                 err.span_label(span, "try removing `&mut` here");
 275             },
 276
 277             // We want to suggest users use `let mut` for local (user
 278             // variable) mutations...
 279             Place::Local(local) if self.mir.local_decls[*local].can_be_made_mutable() => {
 280                 // ... but it doesn't make sense to suggest it on
 281                 // variables that are `ref x`, `ref mut x`, `&self`,
 282                 // or `&mut self` (such variables are simply not
 283                 // mutable).
 284                 let local_decl = &self.mir.local_decls[*local];
 285                 assert_eq!(local_decl.mutability, Mutability::Not);
 286
 287                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 288                 err.span_suggestion(
 289                     local_decl.source_info.span,
 290                     "consider changing this to be mutable",
 291                     format!("mut {}", local_decl.name.unwrap()),
 292                     Applicability::MachineApplicable,
 293                 );
 294             }
 295
 296             // Also suggest adding mut for upvars
 297             Place::Projection(box Projection {
 298                 base,
 299                 elem: ProjectionElem::Field(upvar_index, _),
 300             }) => {
 301                 debug_assert!(is_closure_or_generator(
 302                     base.ty(self.mir, self.infcx.tcx).to_ty(self.infcx.tcx)
 303                 ));
 304
 305                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 306
 307                 let upvar_hir_id = self.mir.upvar_decls[upvar_index.index()]
 308                     .var_hir_id
 309                     .assert_crate_local();
 310                 let upvar_node_id = self.infcx.tcx.hir().hir_to_node_id(upvar_hir_id);
 311                 if let Some(Node::Binding(pat)) = self.infcx.tcx.hir().find(upvar_node_id) {
 312                     if let hir::PatKind::Binding(
 313                         hir::BindingAnnotation::Unannotated,
 314                         _,
 315                         _,
 316                         upvar_ident,
 317                         _,
 318                     ) = pat.node
 319                     {
 320                         err.span_suggestion(
 321                             upvar_ident.span,
 322                             "consider changing this to be mutable",
 323                             format!("mut {}", upvar_ident.name),
 324                             Applicability::MachineApplicable,
 325                         );
 326                     }
 327                 }
 328             }
 329
 330             // complete hack to approximate old AST-borrowck
 331             // diagnostic: if the span starts with a mutable borrow of
 332             // a local variable, then just suggest the user remove it.
 333             Place::Local(_)
 334                 if {
 335                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 336                         snippet.starts_with("&mut ")
 337                     } else {
 338                         false
 339                     }
 340                 } =>
 341             {
 342                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 343                 err.span_label(span, "try removing `&mut` here");
 344             }
 345
 346             Place::Projection(box Projection {
 347                 base: Place::Local(local),
 348                 elem: ProjectionElem::Deref,
 349             }) if {
 350                 if let Some(ClearCrossCrate::Set(BindingForm::RefForGuard)) =
 351                     self.mir.local_decls[*local].is_user_variable
 352                 {
 353                     true
 354                 } else {
 355                     false
 356                 }
 357             } =>
 358             {
 359                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 360                 err.note(
 361                     "variables bound in patterns are immutable until the end of the pattern guard",
 362                 );
 363             }
 364
 365             // We want to point out when a `&` can be readily replaced
 366             // with an `&mut`.
 367             //
 368             // FIXME: can this case be generalized to work for an
 369             // arbitrary base for the projection?
 370             Place::Projection(box Projection {
 371                 base: Place::Local(local),
 372                 elem: ProjectionElem::Deref,
 373             }) if self.mir.local_decls[*local].is_user_variable.is_some() =>
 374             {
 375                 let local_decl = &self.mir.local_decls[*local];
 376                 let suggestion = match local_decl.is_user_variable.as_ref().unwrap() {
 377                     ClearCrossCrate::Set(mir::BindingForm::ImplicitSelf(_)) => {
 378                         Some(suggest_ampmut_self(self.infcx.tcx, local_decl))
 379                     }
 380
 381                     ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
 382                         binding_mode: ty::BindingMode::BindByValue(_),
 383                         opt_ty_info,
 384                         ..
 385                     })) => Some(suggest_ampmut(
 386                         self.infcx.tcx,
 387                         self.mir,
 388                         *local,
 389                         local_decl,
 390                         *opt_ty_info,
 391                     )),
 392
 393                     ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
 394                         binding_mode: ty::BindingMode::BindByReference(_),
 395                         ..
 396                     })) => {
 397                         let pattern_span = local_decl.source_info.span;
 398                         suggest_ref_mut(self.infcx.tcx, pattern_span)
 399                             .map(|replacement| (pattern_span, replacement))
 400                     }
 401
 402                     ClearCrossCrate::Set(mir::BindingForm::RefForGuard) => unreachable!(),
 403
 404                     ClearCrossCrate::Clear => bug!("saw cleared local state"),
 405                 };
 406
 407                 let (pointer_sigil, pointer_desc) = if local_decl.ty.is_region_ptr() {
 408                     ("&", "reference")
 409                 } else {
 410                     ("*const", "pointer")
 411                 };
 412
 413                 if let Some((err_help_span, suggested_code)) = suggestion {
 414                     err.span_suggestion(
 415                         err_help_span,
 416                         &format!("consider changing this to be a mutable {}", pointer_desc),
 417                         suggested_code,
 418                         Applicability::MachineApplicable,
 419                     );
 420                 }
 421
 422                 if let Some(name) = local_decl.name {
 423                     err.span_label(
 424                         span,
 425                         format!(
 426                             "`{NAME}` is a `{SIGIL}` {DESC}, \
 427                              so the data it refers to cannot be {ACTED_ON}",
 428                             NAME = name,
 429                             SIGIL = pointer_sigil,
 430                             DESC = pointer_desc,
 431                             ACTED_ON = acted_on
 432                         ),
 433                     );
 434                 } else {
 435                     err.span_label(
 436                         span,
 437                         format!(
 438                             "cannot {ACT} through `{SIGIL}` {DESC}",
 439                             ACT = act,
 440                             SIGIL = pointer_sigil,
 441                             DESC = pointer_desc
 442                         ),
 443                     );
 444                 }
 445             }
 446
 447             Place::Projection(box Projection {
 448                 base,
 449                 elem: ProjectionElem::Deref,
 450             }) if *base == Place::Local(Local::new(1)) && !self.mir.upvar_decls.is_empty() =>
 451             {
 452                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 453                 err.span_help(
 454                     self.mir.span,
 455                     "consider changing this to accept closures that implement `FnMut`"
 456                 );
 457             }
 458
 459             Place::Projection(box Projection {
 460                 base: Place::Local(local),
 461                 elem: ProjectionElem::Deref,
 462             })  if error_access == AccessKind::MutableBorrow => {
 463                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 464
 465                 let mpi = self.move_data.rev_lookup.find_local(*local);
 466                 for i in self.move_data.init_path_map[mpi].iter() {
 467                     if let InitLocation::Statement(location) = self.move_data.inits[*i].location {
 468                         if let Some(
 469                             Terminator {
 470                                 kind: TerminatorKind::Call {
 471                                     func: Operand::Constant(box Constant {
 472                                         literal: ty::LazyConst::Evaluated(Const {
 473                                             ty: &TyS {
 474                                                 sty: TyKind::FnDef(id, substs),
 475                                                 ..
 476                                             },
 477                                             ..
 478                                         }),
 479                                         ..
 480                                     }),
 481                                     ..
 482                                 },
 483                                 ..
 484                             }
 485                         ) = &self.mir.basic_blocks()[location.block].terminator {
 486                             let index_trait = self.infcx.tcx.lang_items().index_trait();
 487                             if self.infcx.tcx.parent(id) == index_trait {
 488                                 let mut found = false;
 489                                 self.infcx.tcx.for_each_relevant_impl(
 490                                     self.infcx.tcx.lang_items().index_mut_trait().unwrap(),
 491                                     substs.type_at(0),
 492                                     |_relevant_impl| {
 493                                         found = true;
 494                                     }
 495                                 );
 496
 497                                 let extra = if found {
 498                                     String::new()
 499                                 } else {
 500                                     format!(", but it is not implemented for `{}`",
 501                                             substs.type_at(0))
 502                                 };
 503
 504                                 err.help(
 505                                     &format!(
 506                                         "trait `IndexMut` is required to modify indexed content{}",
 507                                          extra,
 508                                     ),
 509                                 );
 510                             }
 511                         }
 512                     }
 513                 }
 514             }
 515
 516             _ => {
 517                 err.span_label(span, format!("cannot {ACT}", ACT = act));
 518             }
 519         }
 520
 521         err.buffer(&mut self.errors_buffer);
 522     }
 523 }
 524
 525 fn suggest_ampmut_self<'cx, 'gcx, 'tcx>(
 526     tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 527     local_decl: &mir::LocalDecl<'tcx>,
 528 ) -> (Span, String) {
 529     let sp = local_decl.source_info.span;
 530     (sp, match tcx.sess.source_map().span_to_snippet(sp) {
 531         Ok(snippet) => {
 532             let lt_pos = snippet.find('\'');
 533             if let Some(lt_pos) = lt_pos {
 534                 format!("&{}mut self", &snippet[lt_pos..snippet.len() - 4])
 535             } else {
 536                 "&mut self".to_string()
 537             }
 538         }
 539         _ => "&mut self".to_string()
 540     })
 541 }
 542
 543 // When we want to suggest a user change a local variable to be a `&mut`, there
 544 // are three potential "obvious" things to highlight:
 545 //
 546 // let ident [: Type] [= RightHandSideExpression];
 547 //     ^^^^^    ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^
 548 //     (1.)     (2.)              (3.)
 549 //
 550 // We can always fallback on highlighting the first. But chances are good that
 551 // the user experience will be better if we highlight one of the others if possible;
 552 // for example, if the RHS is present and the Type is not, then the type is going to
 553 // be inferred *from* the RHS, which means we should highlight that (and suggest
 554 // that they borrow the RHS mutably).
 555 //
 556 // This implementation attempts to emulate AST-borrowck prioritization
 557 // by trying (3.), then (2.) and finally falling back on (1.).
 558 fn suggest_ampmut<'cx, 'gcx, 'tcx>(
 559     tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 560     mir: &Mir<'tcx>,
 561     local: Local,
 562     local_decl: &mir::LocalDecl<'tcx>,
 563     opt_ty_info: Option<Span>,
 564 ) -> (Span, String) {
 565     let locations = mir.find_assignments(local);
 566     if !locations.is_empty() {
 567         let assignment_rhs_span = mir.source_info(locations[0]).span;
 568         if let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span) {
 569             if let (true, Some(ws_pos)) = (
 570                 src.starts_with("&'"),
 571                 src.find(|c: char| -> bool { c.is_whitespace() }),
 572             ) {
 573                 let lt_name = &src[1..ws_pos];
 574                 let ty = &src[ws_pos..];
 575                 return (assignment_rhs_span, format!("&{} mut {}", lt_name, ty));
 576             } else if src.starts_with('&') {
 577                 let borrowed_expr = &src[1..];
 578                 return (assignment_rhs_span, format!("&mut {}", borrowed_expr));
 579             }
 580         }
 581     }
 582
 583     let highlight_span = match opt_ty_info {
 584         // if this is a variable binding with an explicit type,
 585         // try to highlight that for the suggestion.
 586         Some(ty_span) => ty_span,
 587
 588         // otherwise, just highlight the span associated with
 589         // the (MIR) LocalDecl.
 590         None => local_decl.source_info.span,
 591     };
 592
 593     if let Ok(src) = tcx.sess.source_map().span_to_snippet(highlight_span) {
 594         if let (true, Some(ws_pos)) = (
 595             src.starts_with("&'"),
 596             src.find(|c: char| -> bool { c.is_whitespace() }),
 597         ) {
 598             let lt_name = &src[1..ws_pos];
 599             let ty = &src[ws_pos..];
 600             return (highlight_span, format!("&{} mut{}", lt_name, ty));
 601         }
 602     }
 603
 604     let ty_mut = local_decl.ty.builtin_deref(true).unwrap();
 605     assert_eq!(ty_mut.mutbl, hir::MutImmutable);
 606     (highlight_span,
 607      if local_decl.ty.is_region_ptr() {
 608          format!("&mut {}", ty_mut.ty)
 609      } else {
 610          format!("*mut {}", ty_mut.ty)
 611      })
 612 }
 613
 614 fn is_closure_or_generator(ty: ty::Ty<'_>) -> bool {
 615     ty.is_closure() || ty.is_generator()
 616 }
 617
 618 /// Adds a suggestion to a struct definition given a field access to a local.
 619 /// This function expects the local to be a reference to a struct in order to produce a suggestion.
 620 ///
 621 /// ```text
 622 /// LL |     s: &'a String
 623 ///    |        ---------- use `&'a mut String` here to make mutable
 624 /// ```
 625 fn annotate_struct_field(
 626     tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 627     ty: ty::Ty<'tcx>,
 628     field: &mir::Field,
 629 ) -> Option<(Span, String)> {
 630     // Expect our local to be a reference to a struct of some kind.
 631     if let ty::TyKind::Ref(_, ty, _) = ty.sty {
 632         if let ty::TyKind::Adt(def, _) = ty.sty {
 633             let field = def.all_fields().nth(field.index())?;
 634             // Use the HIR types to construct the diagnostic message.
 635             let node_id = tcx.hir().as_local_node_id(field.did)?;
 636             let node = tcx.hir().find(node_id)?;
 637             // Now we're dealing with the actual struct that we're going to suggest a change to,
 638             // we can expect a field that is an immutable reference to a type.
 639             if let hir::Node::Field(field) = node {
 640                 if let hir::TyKind::Rptr(lifetime, hir::MutTy {
 641                     mutbl: hir::Mutability::MutImmutable,
 642                     ref ty
 643                 }) = field.ty.node {
 644                     // Get the snippets in two parts - the named lifetime (if there is one) and
 645                     // type being referenced, that way we can reconstruct the snippet without loss
 646                     // of detail.
 647                     let type_snippet = tcx.sess.source_map().span_to_snippet(ty.span).ok()?;
 648                     let lifetime_snippet = if !lifetime.is_elided() {
 649                         format!("{} ", tcx.sess.source_map().span_to_snippet(lifetime.span).ok()?)
 650                     } else {
 651                         String::new()
 652                     };
 653
 654                     return Some((
 655                         field.ty.span,
 656                         format!(
 657                             "&{}mut {}",
 658                             lifetime_snippet, &*type_snippet,
 659                         ),
 660                     ));
 661                 }
 662             }
 663         }
 664     }
 665
 666     None
 667 }