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