src/librustc_typeck/check/method/suggest.rs

   1 //! Give useful errors and suggestions to users when an item can't be
   2 //! found or is otherwise invalid.
   3
   4 use crate::check::FnCtxt;
   5 use crate::middle::lang_items::FnOnceTraitLangItem;
   6 use crate::namespace::Namespace;
   7 use crate::util::nodemap::FxHashSet;
   8 use errors::{Applicability, DiagnosticBuilder};
   9 use rustc_data_structures::sync::Lrc;
  10 use rustc::hir::{self, ExprKind, Node, QPath};
  11 use rustc::hir::def::{Res, DefKind};
  12 use rustc::hir::def_id::{CRATE_DEF_INDEX, LOCAL_CRATE, DefId};
  13 use rustc::hir::map as hir_map;
  14 use rustc::hir::print;
  15 use rustc::infer::type_variable::TypeVariableOrigin;
  16 use rustc::traits::Obligation;
  17 use rustc::ty::{self, Ty, TyCtxt, ToPolyTraitRef, ToPredicate, TypeFoldable};
  18 use rustc::ty::print::with_crate_prefix;
  19 use syntax_pos::{Span, FileName};
  20 use syntax::ast;
  21 use syntax::util::lev_distance;
  22
  23 use std::cmp::Ordering;
  24
  25 use super::{MethodError, NoMatchData, CandidateSource};
  26 use super::probe::Mode;
  27
  28 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  29     fn is_fn_ty(&self, ty: Ty<'tcx>, span: Span) -> bool {
  30         let tcx = self.tcx;
  31         match ty.sty {
  32             // Not all of these (e.g., unsafe fns) implement `FnOnce`,
  33             // so we look for these beforehand.
  34             ty::Closure(..) |
  35             ty::FnDef(..) |
  36             ty::FnPtr(_) => true,
  37             // If it's not a simple function, look for things which implement `FnOnce`.
  38             _ => {
  39                 let fn_once = match tcx.lang_items().require(FnOnceTraitLangItem) {
  40                     Ok(fn_once) => fn_once,
  41                     Err(..) => return false,
  42                 };
  43
  44                 self.autoderef(span, ty).any(|(ty, _)| {
  45                     self.probe(|_| {
  46                         let fn_once_substs = tcx.mk_substs_trait(ty, &[
  47                             self.next_ty_var(TypeVariableOrigin::MiscVariable(span)).into()
  48                         ]);
  49                         let trait_ref = ty::TraitRef::new(fn_once, fn_once_substs);
  50                         let poly_trait_ref = trait_ref.to_poly_trait_ref();
  51                         let obligation =
  52                             Obligation::misc(span,
  53                                              self.body_id,
  54                                              self.param_env,
  55                                              poly_trait_ref.to_predicate());
  56                         self.predicate_may_hold(&obligation)
  57                     })
  58                 })
  59             }
  60         }
  61     }
  62
  63     pub fn report_method_error<'b>(
  64         &self,
  65         span: Span,
  66         rcvr_ty: Ty<'tcx>,
  67         item_name: ast::Ident,
  68         source: SelfSource<'b>,
  69         error: MethodError<'tcx>,
  70         args: Option<&'gcx [hir::Expr]>,
  71     ) {
  72         let orig_span = span;
  73         let mut span = span;
  74         // Avoid suggestions when we don't know what's going on.
  75         if rcvr_ty.references_error() {
  76             return;
  77         }
  78
  79         let report_candidates = |
  80             span: Span,
  81             err: &mut DiagnosticBuilder<'_>,
  82             mut sources: Vec<CandidateSource>,
  83         | {
  84             sources.sort();
  85             sources.dedup();
  86             // Dynamic limit to avoid hiding just one candidate, which is silly.
  87             let limit = if sources.len() == 5 { 5 } else { 4 };
  88
  89             for (idx, source) in sources.iter().take(limit).enumerate() {
  90                 match *source {
  91                     CandidateSource::ImplSource(impl_did) => {
  92                         // Provide the best span we can. Use the item, if local to crate, else
  93                         // the impl, if local to crate (item may be defaulted), else nothing.
  94                         let item = match self.associated_item(
  95                             impl_did,
  96                             item_name,
  97                             Namespace::Value,
  98                         ).or_else(|| {
  99                             let impl_trait_ref = self.tcx.impl_trait_ref(impl_did)?;
 100                             self.associated_item(
 101                                 impl_trait_ref.def_id,
 102                                 item_name,
 103                                 Namespace::Value,
 104                             )
 105                         }) {
 106                             Some(item) => item,
 107                             None => continue,
 108                         };
 109                         let note_span = self.tcx.hir().span_if_local(item.def_id).or_else(|| {
 110                             self.tcx.hir().span_if_local(impl_did)
 111                         });
 112
 113                         let impl_ty = self.impl_self_ty(span, impl_did).ty;
 114
 115                         let insertion = match self.tcx.impl_trait_ref(impl_did) {
 116                             None => String::new(),
 117                             Some(trait_ref) => {
 118                                 format!(" of the trait `{}`",
 119                                         self.tcx.def_path_str(trait_ref.def_id))
 120                             }
 121                         };
 122
 123                         let note_str = if sources.len() > 1 {
 124                             format!("candidate #{} is defined in an impl{} for the type `{}`",
 125                                     idx + 1,
 126                                     insertion,
 127                                     impl_ty)
 128                         } else {
 129                             format!("the candidate is defined in an impl{} for the type `{}`",
 130                                     insertion,
 131                                     impl_ty)
 132                         };
 133                         if let Some(note_span) = note_span {
 134                             // We have a span pointing to the method. Show note with snippet.
 135                             err.span_note(self.tcx.sess.source_map().def_span(note_span),
 136                                           &note_str);
 137                         } else {
 138                             err.note(&note_str);
 139                         }
 140                     }
 141                     CandidateSource::TraitSource(trait_did) => {
 142                         let item = match self.associated_item(
 143                             trait_did,
 144                             item_name,
 145                             Namespace::Value)
 146                         {
 147                             Some(item) => item,
 148                             None => continue,
 149                         };
 150                         let item_span = self.tcx.sess.source_map()
 151                             .def_span(self.tcx.def_span(item.def_id));
 152                         if sources.len() > 1 {
 153                             span_note!(err,
 154                                        item_span,
 155                                        "candidate #{} is defined in the trait `{}`",
 156                                        idx + 1,
 157                                        self.tcx.def_path_str(trait_did));
 158                         } else {
 159                             span_note!(err,
 160                                        item_span,
 161                                        "the candidate is defined in the trait `{}`",
 162                                        self.tcx.def_path_str(trait_did));
 163                         }
 164                         err.help(&format!("to disambiguate the method call, write `{}::{}({}{})` \
 165                                           instead",
 166                                           self.tcx.def_path_str(trait_did),
 167                                           item_name,
 168                                           if rcvr_ty.is_region_ptr() && args.is_some() {
 169                                               if rcvr_ty.is_mutable_pointer() {
 170                                                   "&mut "
 171                                               } else {
 172                                                   "&"
 173                                               }
 174                                           } else {
 175                                               ""
 176                                           },
 177                                           args.map(|arg| arg.iter()
 178                                               .map(|arg| print::to_string(print::NO_ANN,
 179                                                                           |s| s.print_expr(arg)))
 180                                               .collect::<Vec<_>>()
 181                                               .join(", ")).unwrap_or_else(|| "...".to_owned())));
 182                     }
 183                 }
 184             }
 185             if sources.len() > limit {
 186                 err.note(&format!("and {} others", sources.len() - limit));
 187             }
 188         };
 189
 190         match error {
 191             MethodError::NoMatch(NoMatchData {
 192                 static_candidates: static_sources,
 193                 unsatisfied_predicates,
 194                 out_of_scope_traits,
 195                 lev_candidate,
 196                 mode,
 197             }) => {
 198                 let tcx = self.tcx;
 199
 200                 let actual = self.resolve_type_vars_if_possible(&rcvr_ty);
 201                 let ty_str = self.ty_to_string(actual);
 202                 let is_method = mode == Mode::MethodCall;
 203                 let item_kind = if is_method {
 204                     "method"
 205                 } else if actual.is_enum() {
 206                     "variant or associated item"
 207                 } else {
 208                     match (item_name.as_str().chars().next(), actual.is_fresh_ty()) {
 209                         (Some(name), false) if name.is_lowercase() => {
 210                             "function or associated item"
 211                         }
 212                         (Some(_), false) => "associated item",
 213                         (Some(_), true) | (None, false) => {
 214                             "variant or associated item"
 215                         }
 216                         (None, true) => "variant",
 217                     }
 218                 };
 219                 let mut err = if !actual.references_error() {
 220                     // Suggest clamping down the type if the method that is being attempted to
 221                     // be used exists at all, and the type is an ambiuous numeric type
 222                     // ({integer}/{float}).
 223                     let mut candidates = all_traits(self.tcx)
 224                         .into_iter()
 225                         .filter_map(|info|
 226                             self.associated_item(info.def_id, item_name, Namespace::Value)
 227                         );
 228                     if let (true, false, SelfSource::MethodCall(expr), Some(_)) =
 229                            (actual.is_numeric(),
 230                             actual.has_concrete_skeleton(),
 231                             source,
 232                             candidates.next()) {
 233                         let mut err = struct_span_err!(
 234                             tcx.sess,
 235                             span,
 236                             E0689,
 237                             "can't call {} `{}` on ambiguous numeric type `{}`",
 238                             item_kind,
 239                             item_name,
 240                             ty_str
 241                         );
 242                         let concrete_type = if actual.is_integral() {
 243                             "i32"
 244                         } else {
 245                             "f32"
 246                         };
 247                         match expr.node {
 248                             ExprKind::Lit(ref lit) => {
 249                                 // numeric literal
 250                                 let snippet = tcx.sess.source_map().span_to_snippet(lit.span)
 251                                     .unwrap_or_else(|_| "<numeric literal>".to_owned());
 252
 253                                 err.span_suggestion(
 254                                     lit.span,
 255                                     &format!("you must specify a concrete type for \
 256                                               this numeric value, like `{}`", concrete_type),
 257                                     format!("{}_{}", snippet, concrete_type),
 258                                     Applicability::MaybeIncorrect,
 259                                 );
 260                             }
 261                             ExprKind::Path(ref qpath) => {
 262                                 // local binding
 263                                 if let &QPath::Resolved(_, ref path) = &qpath {
 264                                     if let hir::def::Res::Local(hir_id) = path.res {
 265                                         let span = tcx.hir().span_by_hir_id(hir_id);
 266                                         let snippet = tcx.sess.source_map().span_to_snippet(span);
 267                                         let filename = tcx.sess.source_map().span_to_filename(span);
 268
 269                                         let parent_node = self.tcx.hir().get_by_hir_id(
 270                                             self.tcx.hir().get_parent_node_by_hir_id(hir_id),
 271                                         );
 272                                         let msg = format!(
 273                                             "you must specify a type for this binding, like `{}`",
 274                                             concrete_type,
 275                                         );
 276
 277                                         match (filename, parent_node, snippet) {
 278                                             (FileName::Real(_), Node::Local(hir::Local {
 279                                                 source: hir::LocalSource::Normal,
 280                                                 ty,
 281                                                 ..
 282                                             }), Ok(ref snippet)) => {
 283                                                 err.span_suggestion(
 284                                                     // account for `let x: _ = 42;`
 285                                                     //                  ^^^^
 286                                                     span.to(ty.as_ref().map(|ty| ty.span)
 287                                                         .unwrap_or(span)),
 288                                                     &msg,
 289                                                     format!("{}: {}", snippet, concrete_type),
 290                                                     Applicability::MaybeIncorrect,
 291                                                 );
 292                                             }
 293                                             _ => {
 294                                                 err.span_label(span, msg);
 295                                             }
 296                                         }
 297                                     }
 298                                 }
 299                             }
 300                             _ => {}
 301                         }
 302                         err.emit();
 303                         return;
 304                     } else {
 305                         span = item_name.span;
 306                         let mut err = struct_span_err!(
 307                             tcx.sess,
 308                             span,
 309                             E0599,
 310                             "no {} named `{}` found for type `{}` in the current scope",
 311                             item_kind,
 312                             item_name,
 313                             ty_str
 314                         );
 315                         if let Some(span) = tcx.sess.confused_type_with_std_module.borrow()
 316                             .get(&span)
 317                         {
 318                             if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(*span) {
 319                                 err.span_suggestion(
 320                                     *span,
 321                                     "you are looking for the module in `std`, \
 322                                      not the primitive type",
 323                                     format!("std::{}", snippet),
 324                                     Applicability::MachineApplicable,
 325                                 );
 326                             }
 327                         }
 328                         err
 329                     }
 330                 } else {
 331                     tcx.sess.diagnostic().struct_dummy()
 332                 };
 333
 334                 if let Some(def) = actual.ty_adt_def() {
 335                     if let Some(full_sp) = tcx.hir().span_if_local(def.did) {
 336                         let def_sp = tcx.sess.source_map().def_span(full_sp);
 337                         err.span_label(def_sp, format!("{} `{}` not found {}",
 338                                                        item_kind,
 339                                                        item_name,
 340                                                        if def.is_enum() && !is_method {
 341                                                            "here"
 342                                                        } else {
 343                                                            "for this"
 344                                                        }));
 345                     }
 346                 }
 347
 348                 // If the method name is the name of a field with a function or closure type,
 349                 // give a helping note that it has to be called as `(x.f)(...)`.
 350                 if let SelfSource::MethodCall(expr) = source {
 351                     let field_receiver = self
 352                         .autoderef(span, rcvr_ty)
 353                         .find_map(|(ty, _)| match ty.sty {
 354                             ty::Adt(def, substs) if !def.is_enum() => {
 355                                 let variant = &def.non_enum_variant();
 356                                 self.tcx.find_field_index(item_name, variant).map(|index| {
 357                                     let field = &variant.fields[index];
 358                                     let field_ty = field.ty(tcx, substs);
 359                                     (field, field_ty)
 360                                 })
 361                             }
 362                             _ => None,
 363                         });
 364
 365                     if let Some((field, field_ty)) = field_receiver {
 366                         let scope = self.tcx.hir().get_module_parent_by_hir_id(self.body_id);
 367                         let is_accessible = field.vis.is_accessible_from(scope, self.tcx);
 368
 369                         if is_accessible {
 370                             if self.is_fn_ty(&field_ty, span) {
 371                                 let expr_span = expr.span.to(item_name.span);
 372                                 err.multipart_suggestion(
 373                                     &format!(
 374                                         "to call the function stored in `{}`, \
 375                                          surround the field access with parentheses",
 376                                         item_name,
 377                                     ),
 378                                     vec![
 379                                         (expr_span.shrink_to_lo(), '('.to_string()),
 380                                         (expr_span.shrink_to_hi(), ')'.to_string()),
 381                                     ],
 382                                     Applicability::MachineApplicable,
 383                                 );
 384                             } else {
 385                                 let call_expr = self.tcx.hir().expect_expr_by_hir_id(
 386                                     self.tcx.hir().get_parent_node_by_hir_id(expr.hir_id),
 387                                 );
 388
 389                                 if let Some(span) = call_expr.span.trim_start(item_name.span) {
 390                                     err.span_suggestion(
 391                                         span,
 392                                         "remove the arguments",
 393                                         String::new(),
 394                                         Applicability::MaybeIncorrect,
 395                                     );
 396                                 }
 397                             }
 398                         }
 399
 400                         let field_kind = if is_accessible {
 401                             "field"
 402                         } else {
 403                             "private field"
 404                         };
 405                         err.span_label(item_name.span, format!("{}, not a method", field_kind));
 406                     }
 407                 } else {
 408                     err.span_label(span, format!("{} not found in `{}`", item_kind, ty_str));
 409                     self.tcx.sess.trait_methods_not_found.borrow_mut().insert(orig_span);
 410                 }
 411
 412                 if self.is_fn_ty(&rcvr_ty, span) {
 413                     macro_rules! report_function {
 414                         ($span:expr, $name:expr) => {
 415                             err.note(&format!("{} is a function, perhaps you wish to call it",
 416                                               $name));
 417                         }
 418                     }
 419
 420                     if let SelfSource::MethodCall(expr) = source {
 421                         if let Ok(expr_string) = tcx.sess.source_map().span_to_snippet(expr.span) {
 422                             report_function!(expr.span, expr_string);
 423                         } else if let ExprKind::Path(QPath::Resolved(_, ref path)) =
 424                             expr.node
 425                         {
 426                             if let Some(segment) = path.segments.last() {
 427                                 report_function!(expr.span, segment.ident);
 428                             }
 429                         }
 430                     }
 431                 }
 432
 433                 if !static_sources.is_empty() {
 434                     err.note("found the following associated functions; to be used as methods, \
 435                               functions must have a `self` parameter");
 436                     err.span_label(span, "this is an associated function, not a method");
 437                 }
 438                 if static_sources.len() == 1 {
 439                     if let SelfSource::MethodCall(expr) = source {
 440                         err.span_suggestion(expr.span.to(span),
 441                                             "use associated function syntax instead",
 442                                             format!("{}::{}",
 443                                                     self.ty_to_string(actual),
 444                                                     item_name),
 445                                             Applicability::MachineApplicable);
 446                     } else {
 447                         err.help(&format!("try with `{}::{}`",
 448                                           self.ty_to_string(actual), item_name));
 449                     }
 450
 451                     report_candidates(span, &mut err, static_sources);
 452                 } else if static_sources.len() > 1 {
 453                     report_candidates(span, &mut err, static_sources);
 454                 }
 455
 456                 if !unsatisfied_predicates.is_empty() {
 457                     let mut bound_list = unsatisfied_predicates.iter()
 458                         .map(|p| format!("`{} : {}`", p.self_ty(), p))
 459                         .collect::<Vec<_>>();
 460                     bound_list.sort();
 461                     bound_list.dedup();  // #35677
 462                     let bound_list = bound_list.join("\n");
 463                     err.note(&format!("the method `{}` exists but the following trait bounds \
 464                                        were not satisfied:\n{}",
 465                                       item_name,
 466                                       bound_list));
 467                 }
 468
 469                 if actual.is_numeric() && actual.is_fresh() {
 470
 471                 } else {
 472                     self.suggest_traits_to_import(&mut err,
 473                                                   span,
 474                                                   rcvr_ty,
 475                                                   item_name,
 476                                                   source,
 477                                                   out_of_scope_traits);
 478                 }
 479
 480                 if actual.is_enum() {
 481                     let adt_def = actual.ty_adt_def().expect("enum is not an ADT");
 482                     if let Some(suggestion) = lev_distance::find_best_match_for_name(
 483                         adt_def.variants.iter().map(|s| &s.ident.name),
 484                         &item_name.as_str(),
 485                         None,
 486                     ) {
 487                         err.span_suggestion(
 488                             span,
 489                             "there is a variant with a similar name",
 490                             suggestion.to_string(),
 491                             Applicability::MaybeIncorrect,
 492                         );
 493                     }
 494                 }
 495
 496                 if let Some(lev_candidate) = lev_candidate {
 497                     let def_kind = lev_candidate.def_kind();
 498                     err.span_suggestion(
 499                         span,
 500                         &format!(
 501                             "there is {} {} with a similar name",
 502                             def_kind.article(),
 503                             def_kind.descr(),
 504                         ),
 505                         lev_candidate.ident.to_string(),
 506                         Applicability::MaybeIncorrect,
 507                     );
 508                 }
 509
 510                 err.emit();
 511             }
 512
 513             MethodError::Ambiguity(sources) => {
 514                 let mut err = struct_span_err!(self.sess(),
 515                                                span,
 516                                                E0034,
 517                                                "multiple applicable items in scope");
 518                 err.span_label(span, format!("multiple `{}` found", item_name));
 519
 520                 report_candidates(span, &mut err, sources);
 521                 err.emit();
 522             }
 523
 524             MethodError::PrivateMatch(kind, _, out_of_scope_traits) => {
 525                 let mut err = struct_span_err!(self.tcx.sess, span, E0624,
 526                                                "{} `{}` is private", kind.descr(), item_name);
 527                 self.suggest_valid_traits(&mut err, out_of_scope_traits);
 528                 err.emit();
 529             }
 530
 531             MethodError::IllegalSizedBound(candidates) => {
 532                 let msg = format!("the `{}` method cannot be invoked on a trait object", item_name);
 533                 let mut err = self.sess().struct_span_err(span, &msg);
 534                 if !candidates.is_empty() {
 535                     let help = format!("{an}other candidate{s} {were} found in the following \
 536                                         trait{s}, perhaps add a `use` for {one_of_them}:",
 537                                     an = if candidates.len() == 1 {"an" } else { "" },
 538                                     s = if candidates.len() == 1 { "" } else { "s" },
 539                                     were = if candidates.len() == 1 { "was" } else { "were" },
 540                                     one_of_them = if candidates.len() == 1 {
 541                                         "it"
 542                                     } else {
 543                                         "one_of_them"
 544                                     });
 545                     self.suggest_use_candidates(&mut err, help, candidates);
 546                 }
 547                 err.emit();
 548             }
 549
 550             MethodError::BadReturnType => {
 551                 bug!("no return type expectations but got BadReturnType")
 552             }
 553         }
 554     }
 555
 556     fn suggest_use_candidates(&self,
 557                               err: &mut DiagnosticBuilder<'_>,
 558                               mut msg: String,
 559                               candidates: Vec<DefId>) {
 560         let module_did = self.tcx.hir().get_module_parent_by_hir_id(self.body_id);
 561         let module_id = self.tcx.hir().as_local_hir_id(module_did).unwrap();
 562         let krate = self.tcx.hir().krate();
 563         let (span, found_use) = UsePlacementFinder::check(self.tcx, krate, module_id);
 564         if let Some(span) = span {
 565             let path_strings = candidates.iter().map(|did| {
 566                 // Produce an additional newline to separate the new use statement
 567                 // from the directly following item.
 568                 let additional_newline = if found_use {
 569                     ""
 570                 } else {
 571                     "\n"
 572                 };
 573                 format!(
 574                     "use {};\n{}",
 575                     with_crate_prefix(|| self.tcx.def_path_str(*did)),
 576                     additional_newline
 577                 )
 578             });
 579
 580             err.span_suggestions(span, &msg, path_strings, Applicability::MaybeIncorrect);
 581         } else {
 582             let limit = if candidates.len() == 5 { 5 } else { 4 };
 583             for (i, trait_did) in candidates.iter().take(limit).enumerate() {
 584                 if candidates.len() > 1 {
 585                     msg.push_str(
 586                         &format!(
 587                             "\ncandidate #{}: `use {};`",
 588                             i + 1,
 589                             with_crate_prefix(|| self.tcx.def_path_str(*trait_did))
 590                         )
 591                     );
 592                 } else {
 593                     msg.push_str(
 594                         &format!(
 595                             "\n`use {};`",
 596                             with_crate_prefix(|| self.tcx.def_path_str(*trait_did))
 597                         )
 598                     );
 599                 }
 600             }
 601             if candidates.len() > limit {
 602                 msg.push_str(&format!("\nand {} others", candidates.len() - limit));
 603             }
 604             err.note(&msg[..]);
 605         }
 606     }
 607
 608     fn suggest_valid_traits(&self,
 609                             err: &mut DiagnosticBuilder<'_>,
 610                             valid_out_of_scope_traits: Vec<DefId>) -> bool {
 611         if !valid_out_of_scope_traits.is_empty() {
 612             let mut candidates = valid_out_of_scope_traits;
 613             candidates.sort();
 614             candidates.dedup();
 615             err.help("items from traits can only be used if the trait is in scope");
 616             let msg = format!("the following {traits_are} implemented but not in scope, \
 617                                perhaps add a `use` for {one_of_them}:",
 618                             traits_are = if candidates.len() == 1 {
 619                                 "trait is"
 620                             } else {
 621                                 "traits are"
 622                             },
 623                             one_of_them = if candidates.len() == 1 {
 624                                 "it"
 625                             } else {
 626                                 "one of them"
 627                             });
 628
 629             self.suggest_use_candidates(err, msg, candidates);
 630             true
 631         } else {
 632             false
 633         }
 634     }
 635
 636     fn suggest_traits_to_import<'b>(&self,
 637                                     err: &mut DiagnosticBuilder<'_>,
 638                                     span: Span,
 639                                     rcvr_ty: Ty<'tcx>,
 640                                     item_name: ast::Ident,
 641                                     source: SelfSource<'b>,
 642                                     valid_out_of_scope_traits: Vec<DefId>) {
 643         if self.suggest_valid_traits(err, valid_out_of_scope_traits) {
 644             return;
 645         }
 646
 647         let type_is_local = self.type_derefs_to_local(span, rcvr_ty, source);
 648
 649         // There are no traits implemented, so lets suggest some traits to
 650         // implement, by finding ones that have the item name, and are
 651         // legal to implement.
 652         let mut candidates = all_traits(self.tcx)
 653             .into_iter()
 654             .filter(|info| {
 655                 // We approximate the coherence rules to only suggest
 656                 // traits that are legal to implement by requiring that
 657                 // either the type or trait is local. Multi-dispatch means
 658                 // this isn't perfect (that is, there are cases when
 659                 // implementing a trait would be legal but is rejected
 660                 // here).
 661                 (type_is_local || info.def_id.is_local()) &&
 662                     self.associated_item(info.def_id, item_name, Namespace::Value)
 663                         .filter(|item| {
 664                             // We only want to suggest public or local traits (#45781).
 665                             item.vis == ty::Visibility::Public || info.def_id.is_local()
 666                         })
 667                         .is_some()
 668             })
 669             .collect::<Vec<_>>();
 670
 671         if !candidates.is_empty() {
 672             // Sort from most relevant to least relevant.
 673             candidates.sort_by(|a, b| a.cmp(b).reverse());
 674             candidates.dedup();
 675
 676             // FIXME #21673: this help message could be tuned to the case
 677             // of a type parameter: suggest adding a trait bound rather
 678             // than implementing.
 679             err.help("items from traits can only be used if the trait is implemented and in scope");
 680             let mut msg = format!("the following {traits_define} an item `{name}`, \
 681                                    perhaps you need to implement {one_of_them}:",
 682                                   traits_define = if candidates.len() == 1 {
 683                                       "trait defines"
 684                                   } else {
 685                                       "traits define"
 686                                   },
 687                                   one_of_them = if candidates.len() == 1 {
 688                                       "it"
 689                                   } else {
 690                                       "one of them"
 691                                   },
 692                                   name = item_name);
 693
 694             for (i, trait_info) in candidates.iter().enumerate() {
 695                 msg.push_str(&format!("\ncandidate #{}: `{}`",
 696                                       i + 1,
 697                                       self.tcx.def_path_str(trait_info.def_id)));
 698             }
 699             err.note(&msg[..]);
 700         }
 701     }
 702
 703     /// Checks whether there is a local type somewhere in the chain of
 704     /// autoderefs of `rcvr_ty`.
 705     fn type_derefs_to_local(&self,
 706                             span: Span,
 707                             rcvr_ty: Ty<'tcx>,
 708                             source: SelfSource<'_>) -> bool {
 709         fn is_local(ty: Ty<'_>) -> bool {
 710             match ty.sty {
 711                 ty::Adt(def, _) => def.did.is_local(),
 712                 ty::Foreign(did) => did.is_local(),
 713
 714                 ty::Dynamic(ref tr, ..) =>
 715                     tr.principal().map(|d| d.def_id().is_local()).unwrap_or(false),
 716
 717                 ty::Param(_) => true,
 718
 719                 // Everything else (primitive types, etc.) is effectively
 720                 // non-local (there are "edge" cases, e.g., `(LocalType,)`, but
 721                 // the noise from these sort of types is usually just really
 722                 // annoying, rather than any sort of help).
 723                 _ => false,
 724             }
 725         }
 726
 727         // This occurs for UFCS desugaring of `T::method`, where there is no
 728         // receiver expression for the method call, and thus no autoderef.
 729         if let SelfSource::QPath(_) = source {
 730             return is_local(self.resolve_type_vars_with_obligations(rcvr_ty));
 731         }
 732
 733         self.autoderef(span, rcvr_ty).any(|(ty, _)| is_local(ty))
 734     }
 735 }
 736
 737 #[derive(Copy, Clone)]
 738 pub enum SelfSource<'a> {
 739     QPath(&'a hir::Ty),
 740     MethodCall(&'a hir::Expr /* rcvr */),
 741 }
 742
 743 #[derive(Copy, Clone)]
 744 pub struct TraitInfo {
 745     pub def_id: DefId,
 746 }
 747
 748 impl PartialEq for TraitInfo {
 749     fn eq(&self, other: &TraitInfo) -> bool {
 750         self.cmp(other) == Ordering::Equal
 751     }
 752 }
 753 impl Eq for TraitInfo {}
 754 impl PartialOrd for TraitInfo {
 755     fn partial_cmp(&self, other: &TraitInfo) -> Option<Ordering> {
 756         Some(self.cmp(other))
 757     }
 758 }
 759 impl Ord for TraitInfo {
 760     fn cmp(&self, other: &TraitInfo) -> Ordering {
 761         // Local crates are more important than remote ones (local:
 762         // `cnum == 0`), and otherwise we throw in the defid for totality.
 763
 764         let lhs = (other.def_id.krate, other.def_id);
 765         let rhs = (self.def_id.krate, self.def_id);
 766         lhs.cmp(&rhs)
 767     }
 768 }
 769
 770 /// Retrieves all traits in this crate and any dependent crates.
 771 pub fn all_traits<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> Vec<TraitInfo> {
 772     tcx.all_traits(LOCAL_CRATE).iter().map(|&def_id| TraitInfo { def_id }).collect()
 773 }
 774
 775 /// Computes all traits in this crate and any dependent crates.
 776 fn compute_all_traits<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> Vec<DefId> {
 777     use hir::itemlikevisit;
 778
 779     let mut traits = vec![];
 780
 781     // Crate-local:
 782
 783     struct Visitor<'a, 'tcx: 'a> {
 784         map: &'a hir_map::Map<'tcx>,
 785         traits: &'a mut Vec<DefId>,
 786     }
 787
 788     impl<'v, 'a, 'tcx> itemlikevisit::ItemLikeVisitor<'v> for Visitor<'a, 'tcx> {
 789         fn visit_item(&mut self, i: &'v hir::Item) {
 790             match i.node {
 791                 hir::ItemKind::Trait(..) |
 792                 hir::ItemKind::TraitAlias(..) => {
 793                     let def_id = self.map.local_def_id_from_hir_id(i.hir_id);
 794                     self.traits.push(def_id);
 795                 }
 796                 _ => ()
 797             }
 798         }
 799
 800         fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {}
 801
 802         fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {}
 803     }
 804
 805     tcx.hir().krate().visit_all_item_likes(&mut Visitor {
 806         map: &tcx.hir(),
 807         traits: &mut traits,
 808     });
 809
 810     // Cross-crate:
 811
 812     let mut external_mods = FxHashSet::default();
 813     fn handle_external_res(tcx: TyCtxt<'_, '_, '_>,
 814                            traits: &mut Vec<DefId>,
 815                            external_mods: &mut FxHashSet<DefId>,
 816                            res: Res) {
 817         match res {
 818             Res::Def(DefKind::Trait, def_id) |
 819             Res::Def(DefKind::TraitAlias, def_id) => {
 820                 traits.push(def_id);
 821             }
 822             Res::Def(DefKind::Mod, def_id) => {
 823                 if !external_mods.insert(def_id) {
 824                     return;
 825                 }
 826                 for child in tcx.item_children(def_id).iter() {
 827                     handle_external_res(tcx, traits, external_mods, child.res)
 828                 }
 829             }
 830             _ => {}
 831         }
 832     }
 833     for &cnum in tcx.crates().iter() {
 834         let def_id = DefId {
 835             krate: cnum,
 836             index: CRATE_DEF_INDEX,
 837         };
 838         handle_external_res(tcx, &mut traits, &mut external_mods, Res::Def(DefKind::Mod, def_id));
 839     }
 840
 841     traits
 842 }
 843
 844 pub fn provide(providers: &mut ty::query::Providers<'_>) {
 845     providers.all_traits = |tcx, cnum| {
 846         assert_eq!(cnum, LOCAL_CRATE);
 847         Lrc::new(compute_all_traits(tcx))
 848     }
 849 }
 850
 851 struct UsePlacementFinder<'a, 'tcx: 'a, 'gcx: 'tcx> {
 852     target_module: hir::HirId,
 853     span: Option<Span>,
 854     found_use: bool,
 855     tcx: TyCtxt<'a, 'gcx, 'tcx>
 856 }
 857
 858 impl<'a, 'tcx, 'gcx> UsePlacementFinder<'a, 'tcx, 'gcx> {
 859     fn check(
 860         tcx: TyCtxt<'a, 'gcx, 'tcx>,
 861         krate: &'tcx hir::Crate,
 862         target_module: hir::HirId,
 863     ) -> (Option<Span>, bool) {
 864         let mut finder = UsePlacementFinder {
 865             target_module,
 866             span: None,
 867             found_use: false,
 868             tcx,
 869         };
 870         hir::intravisit::walk_crate(&mut finder, krate);
 871         (finder.span, finder.found_use)
 872     }
 873 }
 874
 875 impl<'a, 'tcx, 'gcx> hir::intravisit::Visitor<'tcx> for UsePlacementFinder<'a, 'tcx, 'gcx> {
 876     fn visit_mod(
 877         &mut self,
 878         module: &'tcx hir::Mod,
 879         _: Span,
 880         hir_id: hir::HirId,
 881     ) {
 882         if self.span.is_some() {
 883             return;
 884         }
 885         if hir_id != self.target_module {
 886             hir::intravisit::walk_mod(self, module, hir_id);
 887             return;
 888         }
 889         // Find a `use` statement.
 890         for item_id in &module.item_ids {
 891             let item = self.tcx.hir().expect_item_by_hir_id(item_id.id);
 892             match item.node {
 893                 hir::ItemKind::Use(..) => {
 894                     // Don't suggest placing a `use` before the prelude
 895                     // import or other generated ones.
 896                     if item.span.ctxt().outer().expn_info().is_none() {
 897                         self.span = Some(item.span.shrink_to_lo());
 898                         self.found_use = true;
 899                         return;
 900                     }
 901                 },
 902                 // Don't place `use` before `extern crate`...
 903                 hir::ItemKind::ExternCrate(_) => {}
 904                 // ...but do place them before the first other item.
 905                 _ => if self.span.map_or(true, |span| item.span < span ) {
 906                     if item.span.ctxt().outer().expn_info().is_none() {
 907                         // Don't insert between attributes and an item.
 908                         if item.attrs.is_empty() {
 909                             self.span = Some(item.span.shrink_to_lo());
 910                         } else {
 911                             // Find the first attribute on the item.
 912                             for attr in &item.attrs {
 913                                 if self.span.map_or(true, |span| attr.span < span) {
 914                                     self.span = Some(attr.span.shrink_to_lo());
 915                                 }
 916                             }
 917                         }
 918                     }
 919                 },
 920             }
 921         }
 922     }
 923
 924     fn nested_visit_map<'this>(
 925         &'this mut self
 926     ) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 927         hir::intravisit::NestedVisitorMap::None
 928     }
 929 }