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