src/librustc_mir/borrow_check/move_errors.rs

   1 use core::unicode::property::Pattern_White_Space;
   2
   3 use rustc::mir::*;
   4 use rustc::ty;
   5 use rustc_errors::{DiagnosticBuilder,Applicability};
   6 use syntax_pos::Span;
   7
   8 use crate::borrow_check::MirBorrowckCtxt;
   9 use crate::borrow_check::prefixes::PrefixSet;
  10 use crate::borrow_check::error_reporting::UseSpans;
  11 use crate::dataflow::move_paths::{
  12     IllegalMoveOrigin, IllegalMoveOriginKind,
  13     LookupResult, MoveError, MovePathIndex,
  14 };
  15
  16 // Often when desugaring a pattern match we may have many individual moves in
  17 // MIR that are all part of one operation from the user's point-of-view. For
  18 // example:
  19 //
  20 // let (x, y) = foo()
  21 //
  22 // would move x from the 0 field of some temporary, and y from the 1 field. We
  23 // group such errors together for cleaner error reporting.
  24 //
  25 // Errors are kept separate if they are from places with different parent move
  26 // paths. For example, this generates two errors:
  27 //
  28 // let (&x, &y) = (&String::new(), &String::new());
  29 #[derive(Debug)]
  30 enum GroupedMoveError<'tcx> {
  31     // Place expression can't be moved from,
  32     // e.g., match x[0] { s => (), } where x: &[String]
  33     MovesFromPlace {
  34         original_path: Place<'tcx>,
  35         span: Span,
  36         move_from: Place<'tcx>,
  37         kind: IllegalMoveOriginKind<'tcx>,
  38         binds_to: Vec<Local>,
  39     },
  40     // Part of a value expression can't be moved from,
  41     // e.g., match &String::new() { &x => (), }
  42     MovesFromValue {
  43         original_path: Place<'tcx>,
  44         span: Span,
  45         move_from: MovePathIndex,
  46         kind: IllegalMoveOriginKind<'tcx>,
  47         binds_to: Vec<Local>,
  48     },
  49     // Everything that isn't from pattern matching.
  50     OtherIllegalMove {
  51         original_path: Place<'tcx>,
  52         use_spans: UseSpans,
  53         kind: IllegalMoveOriginKind<'tcx>,
  54     },
  55 }
  56
  57 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
  58     pub(crate) fn report_move_errors(&mut self, move_errors: Vec<(Place<'tcx>, MoveError<'tcx>)>) {
  59         let grouped_errors = self.group_move_errors(move_errors);
  60         for error in grouped_errors {
  61             self.report(error);
  62         }
  63     }
  64
  65     fn group_move_errors(
  66         &self,
  67         errors: Vec<(Place<'tcx>, MoveError<'tcx>)>
  68     ) -> Vec<GroupedMoveError<'tcx>> {
  69         let mut grouped_errors = Vec::new();
  70         for (original_path, error) in errors {
  71             self.append_to_grouped_errors(&mut grouped_errors, original_path, error);
  72         }
  73         grouped_errors
  74     }
  75
  76     fn append_to_grouped_errors(
  77         &self,
  78         grouped_errors: &mut Vec<GroupedMoveError<'tcx>>,
  79         original_path: Place<'tcx>,
  80         error: MoveError<'tcx>,
  81     ) {
  82         match error {
  83             MoveError::UnionMove { .. } => {
  84                 unimplemented!("don't know how to report union move errors yet.")
  85             }
  86             MoveError::IllegalMove {
  87                 cannot_move_out_of: IllegalMoveOrigin { location, kind },
  88             } => {
  89                 // Note: that the only time we assign a place isn't a temporary
  90                 // to a user variable is when initializing it.
  91                 // If that ever stops being the case, then the ever initialized
  92                 // flow could be used.
  93                 if let Some(StatementKind::Assign(
  94                     Place::Base(PlaceBase::Local(local)),
  95                     box Rvalue::Use(Operand::Move(move_from)),
  96                 )) = self.body.basic_blocks()[location.block]
  97                     .statements
  98                     .get(location.statement_index)
  99                     .map(|stmt| &stmt.kind)
 100                 {
 101                     let local_decl = &self.body.local_decls[*local];
 102                     // opt_match_place is the
 103                     // match_span is the span of the expression being matched on
 104                     // match *x.y { ... }        match_place is Some(*x.y)
 105                     //       ^^^^                match_span is the span of *x.y
 106                     //
 107                     // opt_match_place is None for let [mut] x = ... statements,
 108                     // whether or not the right-hand side is a place expression
 109                     if let Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
 110                         opt_match_place: Some((ref opt_match_place, match_span)),
 111                         binding_mode: _,
 112                         opt_ty_info: _,
 113                         pat_span: _,
 114                     }))) = local_decl.is_user_variable
 115                     {
 116                         let stmt_source_info = self.body.source_info(location);
 117                         self.append_binding_error(
 118                             grouped_errors,
 119                             kind,
 120                             original_path,
 121                             move_from,
 122                             *local,
 123                             opt_match_place,
 124                             match_span,
 125                             stmt_source_info.span,
 126                         );
 127                         return;
 128                     }
 129                 }
 130
 131                 let move_spans = self.move_spans(&original_path, location);
 132                 grouped_errors.push(GroupedMoveError::OtherIllegalMove {
 133                     use_spans: move_spans,
 134                     original_path,
 135                     kind,
 136                 });
 137             }
 138         }
 139     }
 140
 141     fn append_binding_error(
 142         &self,
 143         grouped_errors: &mut Vec<GroupedMoveError<'tcx>>,
 144         kind: IllegalMoveOriginKind<'tcx>,
 145         original_path: Place<'tcx>,
 146         move_from: &Place<'tcx>,
 147         bind_to: Local,
 148         match_place: &Option<Place<'tcx>>,
 149         match_span: Span,
 150         statement_span: Span,
 151     ) {
 152         debug!(
 153             "append_binding_error(match_place={:?}, match_span={:?})",
 154             match_place, match_span
 155         );
 156
 157         let from_simple_let = match_place.is_none();
 158         let match_place = match_place.as_ref().unwrap_or(move_from);
 159
 160         match self.move_data.rev_lookup.find(match_place) {
 161             // Error with the match place
 162             LookupResult::Parent(_) => {
 163                 for ge in &mut *grouped_errors {
 164                     if let GroupedMoveError::MovesFromPlace { span, binds_to, .. } = ge {
 165                         if match_span == *span {
 166                             debug!("appending local({:?}) to list", bind_to);
 167                             if !binds_to.is_empty() {
 168                                 binds_to.push(bind_to);
 169                             }
 170                             return;
 171                         }
 172                     }
 173                 }
 174                 debug!("found a new move error location");
 175
 176                 // Don't need to point to x in let x = ... .
 177                 let (binds_to, span) = if from_simple_let {
 178                     (vec![], statement_span)
 179                 } else {
 180                     (vec![bind_to], match_span)
 181                 };
 182                 grouped_errors.push(GroupedMoveError::MovesFromPlace {
 183                     span,
 184                     move_from: match_place.clone(),
 185                     original_path,
 186                     kind,
 187                     binds_to,
 188                 });
 189             }
 190             // Error with the pattern
 191             LookupResult::Exact(_) => {
 192                 let mpi = match self.move_data.rev_lookup.find(move_from) {
 193                     LookupResult::Parent(Some(mpi)) => mpi,
 194                     // move_from should be a projection from match_place.
 195                     _ => unreachable!("Probably not unreachable..."),
 196                 };
 197                 for ge in &mut *grouped_errors {
 198                     if let GroupedMoveError::MovesFromValue {
 199                         span,
 200                         move_from: other_mpi,
 201                         binds_to,
 202                         ..
 203                     } = ge
 204                     {
 205                         if match_span == *span && mpi == *other_mpi {
 206                             debug!("appending local({:?}) to list", bind_to);
 207                             binds_to.push(bind_to);
 208                             return;
 209                         }
 210                     }
 211                 }
 212                 debug!("found a new move error location");
 213                 grouped_errors.push(GroupedMoveError::MovesFromValue {
 214                     span: match_span,
 215                     move_from: mpi,
 216                     original_path,
 217                     kind,
 218                     binds_to: vec![bind_to],
 219                 });
 220             }
 221         };
 222     }
 223
 224     fn report(&mut self, error: GroupedMoveError<'tcx>) {
 225         let (mut err, err_span) = {
 226             let (span, original_path, kind): (Span, &Place<'tcx>, &IllegalMoveOriginKind<'_>) =
 227                 match error {
 228                     GroupedMoveError::MovesFromPlace { span, ref original_path, ref kind, .. } |
 229                     GroupedMoveError::MovesFromValue { span, ref original_path, ref kind, .. } => {
 230                         (span, original_path, kind)
 231                     }
 232                     GroupedMoveError::OtherIllegalMove {
 233                         use_spans,
 234                         ref original_path,
 235                         ref kind
 236                     } => {
 237                         (use_spans.args_or_use(), original_path, kind)
 238                     },
 239                 };
 240             debug!("report: original_path={:?} span={:?}, kind={:?} \
 241                    original_path.is_upvar_field_projection={:?}", original_path, span, kind,
 242                    self.is_upvar_field_projection(original_path));
 243             (
 244                 match kind {
 245                     IllegalMoveOriginKind::Static => {
 246                         self.report_cannot_move_from_static(original_path, span)
 247                     }
 248                     IllegalMoveOriginKind::BorrowedContent { target_place } => {
 249                         self.report_cannot_move_from_borrowed_content(
 250                             original_path,
 251                             target_place,
 252                             span,
 253                         )
 254                     }
 255                     IllegalMoveOriginKind::InteriorOfTypeWithDestructor { container_ty: ty } => {
 256                         self.cannot_move_out_of_interior_of_drop(span, ty)
 257                     }
 258                     IllegalMoveOriginKind::InteriorOfSliceOrArray { ty, is_index } =>
 259                         self.cannot_move_out_of_interior_noncopy(
 260                             span, ty, Some(*is_index),
 261                         ),
 262                 },
 263                 span,
 264             )
 265         };
 266
 267         self.add_move_hints(error, &mut err, err_span);
 268         err.buffer(&mut self.errors_buffer);
 269     }
 270
 271     fn report_cannot_move_from_static(
 272         &mut self,
 273         place: &Place<'tcx>,
 274         span: Span
 275     ) -> DiagnosticBuilder<'a> {
 276         let mut base_static = place;
 277         loop {
 278             match base_static {
 279                 Place::Base(_) => break,
 280                 Place::Projection(box Projection { base, .. }) => base_static = base,
 281             }
 282         }
 283
 284         let description = if let Place::Base(_) = place {
 285             format!("static item `{}`", self.describe_place(place).unwrap())
 286         } else {
 287             format!(
 288                 "`{:?}` as `{:?}` is a static item",
 289                 self.describe_place(place).unwrap(),
 290                 self.describe_place(base_static).unwrap(),
 291             )
 292         };
 293
 294         self.cannot_move_out_of(span, &description)
 295     }
 296
 297     fn report_cannot_move_from_borrowed_content(
 298         &mut self,
 299         move_place: &Place<'tcx>,
 300         deref_target_place: &Place<'tcx>,
 301         span: Span,
 302     ) -> DiagnosticBuilder<'a> {
 303         // Inspect the type of the content behind the
 304         // borrow to provide feedback about why this
 305         // was a move rather than a copy.
 306         let ty = deref_target_place.ty(self.body, self.infcx.tcx).ty;
 307         let upvar_field = self.prefixes(&move_place, PrefixSet::All)
 308             .find_map(|p| self.is_upvar_field_projection(p));
 309
 310         let deref_base = match deref_target_place {
 311             Place::Projection(box Projection { base, elem: ProjectionElem::Deref }) => base,
 312             _ => bug!("deref_target_place is not a deref projection"),
 313         };
 314
 315         if let Place::Base(PlaceBase::Local(local)) = *deref_base {
 316             let decl = &self.body.local_decls[local];
 317             if decl.is_ref_for_guard() {
 318                 let mut err = self.cannot_move_out_of(
 319                     span,
 320                     &format!("`{}` in pattern guard", decl.name.unwrap()),
 321                 );
 322                 err.note(
 323                     "variables bound in patterns cannot be moved from \
 324                      until after the end of the pattern guard");
 325                 return err;
 326             }
 327         }
 328
 329         debug!("report: ty={:?}", ty);
 330         let mut err = match ty.sty {
 331             ty::Array(..) | ty::Slice(..) =>
 332                 self.cannot_move_out_of_interior_noncopy(span, ty, None),
 333             ty::Closure(def_id, closure_substs)
 334                 if def_id == self.mir_def_id && upvar_field.is_some()
 335             => {
 336                 let closure_kind_ty = closure_substs.closure_kind_ty(def_id, self.infcx.tcx);
 337                 let closure_kind = closure_kind_ty.to_opt_closure_kind();
 338                 let capture_description = match closure_kind {
 339                     Some(ty::ClosureKind::Fn) => {
 340                         "captured variable in an `Fn` closure"
 341                     }
 342                     Some(ty::ClosureKind::FnMut) => {
 343                         "captured variable in an `FnMut` closure"
 344                     }
 345                     Some(ty::ClosureKind::FnOnce) => {
 346                         bug!("closure kind does not match first argument type")
 347                     }
 348                     None => bug!("closure kind not inferred by borrowck"),
 349                 };
 350
 351                 let upvar = &self.upvars[upvar_field.unwrap().index()];
 352                 let upvar_hir_id = upvar.var_hir_id;
 353                 let upvar_name = upvar.name;
 354                 let upvar_span = self.infcx.tcx.hir().span(upvar_hir_id);
 355
 356                 let place_name = self.describe_place(move_place).unwrap();
 357
 358                 let place_description = if self.is_upvar_field_projection(move_place).is_some() {
 359                     format!("`{}`, a {}", place_name, capture_description)
 360                 } else {
 361                     format!(
 362                         "`{}`, as `{}` is a {}",
 363                         place_name,
 364                         upvar_name,
 365                         capture_description,
 366                     )
 367                 };
 368
 369                 debug!(
 370                     "report: closure_kind_ty={:?} closure_kind={:?} place_description={:?}",
 371                     closure_kind_ty, closure_kind, place_description,
 372                 );
 373
 374                 let mut diag = self.cannot_move_out_of(span, &place_description);
 375
 376                 diag.span_label(upvar_span, "captured outer variable");
 377
 378                 diag
 379             }
 380             _ => {
 381                 let source = self.borrowed_content_source(deref_base);
 382                 match (self.describe_place(move_place), source.describe_for_named_place()) {
 383                     (Some(place_desc), Some(source_desc)) => {
 384                         self.cannot_move_out_of(
 385                             span,
 386                             &format!("`{}` which is behind a {}", place_desc, source_desc),
 387                         )
 388                     }
 389                     (_, _) => {
 390                         self.cannot_move_out_of(
 391                             span,
 392                             &source.describe_for_unnamed_place(),
 393                         )
 394                     }
 395                 }
 396             },
 397         };
 398         let move_ty = format!(
 399             "{:?}",
 400             move_place.ty(self.body, self.infcx.tcx).ty,
 401         );
 402         let snippet = self.infcx.tcx.sess.source_map().span_to_snippet(span).unwrap();
 403         let is_option = move_ty.starts_with("std::option::Option");
 404         let is_result = move_ty.starts_with("std::result::Result");
 405         if  is_option || is_result {
 406             err.span_suggestion(
 407                 span,
 408                 &format!("consider borrowing the `{}`'s content", if is_option {
 409                     "Option"
 410                 } else {
 411                     "Result"
 412                 }),
 413                 format!("{}.as_ref()", snippet),
 414                 Applicability::MaybeIncorrect,
 415             );
 416         }
 417         err
 418     }
 419
 420     fn add_move_hints(
 421         &self,
 422         error: GroupedMoveError<'tcx>,
 423         err: &mut DiagnosticBuilder<'a>,
 424         span: Span,
 425     ) {
 426         let snippet = self.infcx.tcx.sess.source_map().span_to_snippet(span).unwrap();
 427         match error {
 428             GroupedMoveError::MovesFromPlace {
 429                 mut binds_to,
 430                 move_from,
 431                 ..
 432             } => {
 433                 err.span_suggestion(
 434                     span,
 435                     "consider borrowing here",
 436                     format!("&{}", snippet),
 437                     Applicability::Unspecified,
 438                 );
 439
 440                 if binds_to.is_empty() {
 441                     let place_ty = move_from.ty(self.body, self.infcx.tcx).ty;
 442                     let place_desc = match self.describe_place(&move_from) {
 443                         Some(desc) => format!("`{}`", desc),
 444                         None => format!("value"),
 445                     };
 446
 447                     self.note_type_does_not_implement_copy(
 448                         err,
 449                         &place_desc,
 450                         place_ty,
 451                         Some(span)
 452                     );
 453                 } else {
 454                     binds_to.sort();
 455                     binds_to.dedup();
 456
 457                     self.add_move_error_details(err, &binds_to);
 458                 }
 459             }
 460             GroupedMoveError::MovesFromValue { mut binds_to, .. } => {
 461                 binds_to.sort();
 462                 binds_to.dedup();
 463                 self.add_move_error_suggestions(err, &binds_to);
 464                 self.add_move_error_details(err, &binds_to);
 465             }
 466             // No binding. Nothing to suggest.
 467             GroupedMoveError::OtherIllegalMove { ref original_path, use_spans, .. } => {
 468                 let span = use_spans.var_or_use();
 469                 let place_ty = original_path.ty(self.body, self.infcx.tcx).ty;
 470                 let place_desc = match self.describe_place(original_path) {
 471                     Some(desc) => format!("`{}`", desc),
 472                     None => format!("value"),
 473                 };
 474                 self.note_type_does_not_implement_copy(
 475                     err,
 476                     &place_desc,
 477                     place_ty,
 478                     Some(span),
 479                 );
 480
 481                 use_spans.args_span_label(err, format!("move out of {} occurs here", place_desc));
 482                 use_spans.var_span_label(
 483                     err,
 484                     format!("move occurs due to use{}", use_spans.describe()),
 485                 );
 486             },
 487         }
 488     }
 489
 490     fn add_move_error_suggestions(
 491         &self,
 492         err: &mut DiagnosticBuilder<'a>,
 493         binds_to: &[Local],
 494     ) {
 495         let mut suggestions: Vec<(Span, &str, String)> = Vec::new();
 496         for local in binds_to {
 497             let bind_to = &self.body.local_decls[*local];
 498             if let Some(
 499                 ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
 500                     pat_span,
 501                     ..
 502                 }))
 503             ) = bind_to.is_user_variable {
 504                 let pat_snippet = self.infcx.tcx.sess.source_map()
 505                     .span_to_snippet(pat_span)
 506                     .unwrap();
 507                 if pat_snippet.starts_with('&') {
 508                     let pat_snippet = pat_snippet[1..].trim_start();
 509                     let suggestion;
 510                     let to_remove;
 511                     if pat_snippet.starts_with("mut")
 512                         && pat_snippet["mut".len()..].starts_with(Pattern_White_Space)
 513                     {
 514                         suggestion = pat_snippet["mut".len()..].trim_start();
 515                         to_remove = "&mut";
 516                     } else {
 517                         suggestion = pat_snippet;
 518                         to_remove = "&";
 519                     }
 520                     suggestions.push((
 521                         pat_span,
 522                         to_remove,
 523                         suggestion.to_owned(),
 524                     ));
 525                 }
 526             }
 527         }
 528         suggestions.sort_unstable_by_key(|&(span, _, _)| span);
 529         suggestions.dedup_by_key(|&mut (span, _, _)| span);
 530         for (span, to_remove, suggestion) in suggestions {
 531             err.span_suggestion(
 532                 span,
 533                 &format!("consider removing the `{}`", to_remove),
 534                 suggestion,
 535                 Applicability::MachineApplicable,
 536             );
 537         }
 538     }
 539
 540     fn add_move_error_details(
 541         &self,
 542         err: &mut DiagnosticBuilder<'a>,
 543         binds_to: &[Local],
 544     ) {
 545         let mut noncopy_var_spans = Vec::new();
 546         for (j, local) in binds_to.into_iter().enumerate() {
 547             let bind_to = &self.body.local_decls[*local];
 548             let binding_span = bind_to.source_info.span;
 549
 550             if j == 0 {
 551                 err.span_label(binding_span, "data moved here");
 552             } else {
 553                 err.span_label(binding_span, "...and here");
 554             }
 555
 556             if binds_to.len() == 1 {
 557                 self.note_type_does_not_implement_copy(
 558                     err,
 559                     &format!("`{}`", bind_to.name.unwrap()),
 560                     bind_to.ty,
 561                     Some(binding_span)
 562                 );
 563             } else {
 564                 noncopy_var_spans.push(binding_span);
 565             }
 566         }
 567
 568         if binds_to.len() > 1 {
 569             err.span_note(
 570                 noncopy_var_spans,
 571                 "move occurs because these variables have types that \
 572                     don't implement the `Copy` trait",
 573             );
 574         }
 575     }
 576 }