src/librustc_mir/borrow_check/diagnostics/move_errors.rs

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