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