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(self.body_id);
 350                                     if field.vis.is_accessible_from(scope, self.tcx) {
 351                                         if self.is_fn_ty(&field_ty, span) {
 352                                             err.help(&format!("use `({0}.{1})(...)` if you \
 353                                                                meant to call the function \
 354                                                                stored in the `{1}` field",
 355                                                               expr_string,
 356                                                               item_name));
 357                                         } else {
 358                                             err.help(&format!("did you mean to write `{0}.{1}` \
 359                                                                instead of `{0}.{1}(...)`?",
 360                                                               expr_string,
 361                                                               item_name));
 362                                         }
 363                                         err.span_label(span, "field, not a method");
 364                                     } else {
 365                                         err.span_label(span, "private field, not a method");
 366                                     }
 367                                     break;
 368                                 }
 369                             }
 370                         }
 371                     }
 372                 } else {
 373                     err.span_label(span, format!("{} not found in `{}`", item_kind, ty_str));
 374                 }
 375
 376                 if self.is_fn_ty(&rcvr_ty, span) {
 377                     macro_rules! report_function {
 378                         ($span:expr, $name:expr) => {
 379                             err.note(&format!("{} is a function, perhaps you wish to call it",
 380                                               $name));
 381                         }
 382                     }
 383
 384                     if let SelfSource::MethodCall(expr) = source {
 385                         if let Ok(expr_string) = tcx.sess.source_map().span_to_snippet(expr.span) {
 386                             report_function!(expr.span, expr_string);
 387                         } else if let ExprKind::Path(QPath::Resolved(_, ref path)) =
 388                             expr.node
 389                         {
 390                             if let Some(segment) = path.segments.last() {
 391                                 report_function!(expr.span, segment.ident);
 392                             }
 393                         }
 394                     }
 395                 }
 396
 397                 if !static_sources.is_empty() {
 398                     err.note("found the following associated functions; to be used as methods, \
 399                               functions must have a `self` parameter");
 400                     err.span_label(span, "this is an associated function, not a method");
 401                 }
 402                 if static_sources.len() == 1 {
 403                     if let SelfSource::MethodCall(expr) = source {
 404                         err.span_suggestion(expr.span.to(span),
 405                                             "use associated function syntax instead",
 406                                             format!("{}::{}",
 407                                                     self.ty_to_string(actual),
 408                                                     item_name),
 409                                             Applicability::MachineApplicable);
 410                     } else {
 411                         err.help(&format!("try with `{}::{}`",
 412                                           self.ty_to_string(actual), item_name));
 413                     }
 414
 415                     report_candidates(&mut err, static_sources);
 416                 } else if static_sources.len() > 1 {
 417                     report_candidates(&mut err, static_sources);
 418                 }
 419
 420                 if !unsatisfied_predicates.is_empty() {
 421                     let mut bound_list = unsatisfied_predicates.iter()
 422                         .map(|p| format!("`{} : {}`", p.self_ty(), p))
 423                         .collect::<Vec<_>>();
 424                     bound_list.sort();
 425                     bound_list.dedup();  // #35677
 426                     let bound_list = bound_list.join("\n");
 427                     err.note(&format!("the method `{}` exists but the following trait bounds \
 428                                        were not satisfied:\n{}",
 429                                       item_name,
 430                                       bound_list));
 431                 }
 432
 433                 if actual.is_numeric() && actual.is_fresh() {
 434
 435                 } else {
 436                     self.suggest_traits_to_import(&mut err,
 437                                                   span,
 438                                                   rcvr_ty,
 439                                                   item_name,
 440                                                   source,
 441                                                   out_of_scope_traits);
 442                 }
 443
 444                 if let Some(lev_candidate) = lev_candidate {
 445                     err.span_suggestion(
 446                         span,
 447                         "did you mean",
 448                         lev_candidate.ident.to_string(),
 449                         Applicability::MaybeIncorrect,
 450                     );
 451                 }
 452                 err.emit();
 453             }
 454
 455             MethodError::Ambiguity(sources) => {
 456                 let mut err = struct_span_err!(self.sess(),
 457                                                span,
 458                                                E0034,
 459                                                "multiple applicable items in scope");
 460                 err.span_label(span, format!("multiple `{}` found", item_name));
 461
 462                 report_candidates(&mut err, sources);
 463                 err.emit();
 464             }
 465
 466             MethodError::PrivateMatch(def, out_of_scope_traits) => {
 467                 let mut err = struct_span_err!(self.tcx.sess, span, E0624,
 468                                                "{} `{}` is private", def.kind_name(), item_name);
 469                 self.suggest_valid_traits(&mut err, out_of_scope_traits);
 470                 err.emit();
 471             }
 472
 473             MethodError::IllegalSizedBound(candidates) => {
 474                 let msg = format!("the `{}` method cannot be invoked on a trait object", item_name);
 475                 let mut err = self.sess().struct_span_err(span, &msg);
 476                 if !candidates.is_empty() {
 477                     let help = format!("{an}other candidate{s} {were} found in the following \
 478                                         trait{s}, perhaps add a `use` for {one_of_them}:",
 479                                     an = if candidates.len() == 1 {"an" } else { "" },
 480                                     s = if candidates.len() == 1 { "" } else { "s" },
 481                                     were = if candidates.len() == 1 { "was" } else { "were" },
 482                                     one_of_them = if candidates.len() == 1 {
 483                                         "it"
 484                                     } else {
 485                                         "one_of_them"
 486                                     });
 487                     self.suggest_use_candidates(&mut err, help, candidates);
 488                 }
 489                 err.emit();
 490             }
 491
 492             MethodError::BadReturnType => {
 493                 bug!("no return type expectations but got BadReturnType")
 494             }
 495         }
 496     }
 497
 498     fn suggest_use_candidates(&self,
 499                               err: &mut DiagnosticBuilder,
 500                               mut msg: String,
 501                               candidates: Vec<DefId>) {
 502         let module_did = self.tcx.hir().get_module_parent(self.body_id);
 503         let module_id = self.tcx.hir().as_local_node_id(module_did).unwrap();
 504         let krate = self.tcx.hir().krate();
 505         let (span, found_use) = UsePlacementFinder::check(self.tcx, krate, module_id);
 506         if let Some(span) = span {
 507             let path_strings = candidates.iter().map(|did| {
 508                 // Produce an additional newline to separate the new use statement
 509                 // from the directly following item.
 510                 let additional_newline = if found_use {
 511                     ""
 512                 } else {
 513                     "\n"
 514                 };
 515                 format!(
 516                     "use {};\n{}",
 517                     with_crate_prefix(|| self.tcx.item_path_str(*did)),
 518                     additional_newline
 519                 )
 520             });
 521
 522             err.span_suggestions(span, &msg, path_strings, Applicability::MaybeIncorrect);
 523         } else {
 524             let limit = if candidates.len() == 5 { 5 } else { 4 };
 525             for (i, trait_did) in candidates.iter().take(limit).enumerate() {
 526                 if candidates.len() > 1 {
 527                     msg.push_str(
 528                         &format!(
 529                             "\ncandidate #{}: `use {};`",
 530                             i + 1,
 531                             with_crate_prefix(|| self.tcx.item_path_str(*trait_did))
 532                         )
 533                     );
 534                 } else {
 535                     msg.push_str(
 536                         &format!(
 537                             "\n`use {};`",
 538                             with_crate_prefix(|| self.tcx.item_path_str(*trait_did))
 539                         )
 540                     );
 541                 }
 542             }
 543             if candidates.len() > limit {
 544                 msg.push_str(&format!("\nand {} others", candidates.len() - limit));
 545             }
 546             err.note(&msg[..]);
 547         }
 548     }
 549
 550     fn suggest_valid_traits(&self,
 551                             err: &mut DiagnosticBuilder,
 552                             valid_out_of_scope_traits: Vec<DefId>) -> bool {
 553         if !valid_out_of_scope_traits.is_empty() {
 554             let mut candidates = valid_out_of_scope_traits;
 555             candidates.sort();
 556             candidates.dedup();
 557             err.help("items from traits can only be used if the trait is in scope");
 558             let msg = format!("the following {traits_are} implemented but not in scope, \
 559                                perhaps add a `use` for {one_of_them}:",
 560                             traits_are = if candidates.len() == 1 {
 561                                 "trait is"
 562                             } else {
 563                                 "traits are"
 564                             },
 565                             one_of_them = if candidates.len() == 1 {
 566                                 "it"
 567                             } else {
 568                                 "one of them"
 569                             });
 570
 571             self.suggest_use_candidates(err, msg, candidates);
 572             true
 573         } else {
 574             false
 575         }
 576     }
 577
 578     fn suggest_traits_to_import<'b>(&self,
 579                                     err: &mut DiagnosticBuilder,
 580                                     span: Span,
 581                                     rcvr_ty: Ty<'tcx>,
 582                                     item_name: ast::Ident,
 583                                     source: SelfSource<'b>,
 584                                     valid_out_of_scope_traits: Vec<DefId>) {
 585         if self.suggest_valid_traits(err, valid_out_of_scope_traits) {
 586             return;
 587         }
 588
 589         let type_is_local = self.type_derefs_to_local(span, rcvr_ty, source);
 590
 591         // There are no traits implemented, so lets suggest some traits to
 592         // implement, by finding ones that have the item name, and are
 593         // legal to implement.
 594         let mut candidates = all_traits(self.tcx)
 595             .into_iter()
 596             .filter(|info| {
 597                 // We approximate the coherence rules to only suggest
 598                 // traits that are legal to implement by requiring that
 599                 // either the type or trait is local. Multi-dispatch means
 600                 // this isn't perfect (that is, there are cases when
 601                 // implementing a trait would be legal but is rejected
 602                 // here).
 603                 (type_is_local || info.def_id.is_local()) &&
 604                     self.associated_item(info.def_id, item_name, Namespace::Value)
 605                         .filter(|item| {
 606                             // We only want to suggest public or local traits (#45781).
 607                             item.vis == ty::Visibility::Public || info.def_id.is_local()
 608                         })
 609                         .is_some()
 610             })
 611             .collect::<Vec<_>>();
 612
 613         if !candidates.is_empty() {
 614             // Sort from most relevant to least relevant.
 615             candidates.sort_by(|a, b| a.cmp(b).reverse());
 616             candidates.dedup();
 617
 618             // FIXME #21673: this help message could be tuned to the case
 619             // of a type parameter: suggest adding a trait bound rather
 620             // than implementing.
 621             err.help("items from traits can only be used if the trait is implemented and in scope");
 622             let mut msg = format!("the following {traits_define} an item `{name}`, \
 623                                    perhaps you need to implement {one_of_them}:",
 624                                   traits_define = if candidates.len() == 1 {
 625                                       "trait defines"
 626                                   } else {
 627                                       "traits define"
 628                                   },
 629                                   one_of_them = if candidates.len() == 1 {
 630                                       "it"
 631                                   } else {
 632                                       "one of them"
 633                                   },
 634                                   name = item_name);
 635
 636             for (i, trait_info) in candidates.iter().enumerate() {
 637                 msg.push_str(&format!("\ncandidate #{}: `{}`",
 638                                       i + 1,
 639                                       self.tcx.item_path_str(trait_info.def_id)));
 640             }
 641             err.note(&msg[..]);
 642         }
 643     }
 644
 645     /// Checks whether there is a local type somewhere in the chain of
 646     /// autoderefs of `rcvr_ty`.
 647     fn type_derefs_to_local(&self,
 648                             span: Span,
 649                             rcvr_ty: Ty<'tcx>,
 650                             source: SelfSource) -> bool {
 651         fn is_local(ty: Ty) -> bool {
 652             match ty.sty {
 653                 ty::Adt(def, _) => def.did.is_local(),
 654                 ty::Foreign(did) => did.is_local(),
 655
 656                 ty::Dynamic(ref tr, ..) =>
 657                     tr.principal().map(|d| d.def_id().is_local()).unwrap_or(false),
 658
 659                 ty::Param(_) => true,
 660
 661                 // Everything else (primitive types, etc.) is effectively
 662                 // non-local (there are "edge" cases, e.g., `(LocalType,)`, but
 663                 // the noise from these sort of types is usually just really
 664                 // annoying, rather than any sort of help).
 665                 _ => false,
 666             }
 667         }
 668
 669         // This occurs for UFCS desugaring of `T::method`, where there is no
 670         // receiver expression for the method call, and thus no autoderef.
 671         if let SelfSource::QPath(_) = source {
 672             return is_local(self.resolve_type_vars_with_obligations(rcvr_ty));
 673         }
 674
 675         self.autoderef(span, rcvr_ty).any(|(ty, _)| is_local(ty))
 676     }
 677 }
 678
 679 #[derive(Copy, Clone)]
 680 pub enum SelfSource<'a> {
 681     QPath(&'a hir::Ty),
 682     MethodCall(&'a hir::Expr /* rcvr */),
 683 }
 684
 685 #[derive(Copy, Clone)]
 686 pub struct TraitInfo {
 687     pub def_id: DefId,
 688 }
 689
 690 impl PartialEq for TraitInfo {
 691     fn eq(&self, other: &TraitInfo) -> bool {
 692         self.cmp(other) == Ordering::Equal
 693     }
 694 }
 695 impl Eq for TraitInfo {}
 696 impl PartialOrd for TraitInfo {
 697     fn partial_cmp(&self, other: &TraitInfo) -> Option<Ordering> {
 698         Some(self.cmp(other))
 699     }
 700 }
 701 impl Ord for TraitInfo {
 702     fn cmp(&self, other: &TraitInfo) -> Ordering {
 703         // Local crates are more important than remote ones (local:
 704         // `cnum == 0`), and otherwise we throw in the defid for totality.
 705
 706         let lhs = (other.def_id.krate, other.def_id);
 707         let rhs = (self.def_id.krate, self.def_id);
 708         lhs.cmp(&rhs)
 709     }
 710 }
 711
 712 /// Retrieves all traits in this crate and any dependent crates.
 713 pub fn all_traits<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> Vec<TraitInfo> {
 714     tcx.all_traits(LOCAL_CRATE).iter().map(|&def_id| TraitInfo { def_id }).collect()
 715 }
 716
 717 /// Computes all traits in this crate and any dependent crates.
 718 fn compute_all_traits<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> Vec<DefId> {
 719     use hir::itemlikevisit;
 720
 721     let mut traits = vec![];
 722
 723     // Crate-local:
 724
 725     struct Visitor<'a, 'tcx: 'a> {
 726         map: &'a hir_map::Map<'tcx>,
 727         traits: &'a mut Vec<DefId>,
 728     }
 729
 730     impl<'v, 'a, 'tcx> itemlikevisit::ItemLikeVisitor<'v> for Visitor<'a, 'tcx> {
 731         fn visit_item(&mut self, i: &'v hir::Item) {
 732             if let hir::ItemKind::Trait(..) = i.node {
 733                 let def_id = self.map.local_def_id(i.id);
 734                 self.traits.push(def_id);
 735             }
 736         }
 737
 738         fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {}
 739
 740         fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {}
 741     }
 742
 743     tcx.hir().krate().visit_all_item_likes(&mut Visitor {
 744         map: &tcx.hir(),
 745         traits: &mut traits,
 746     });
 747
 748     // Cross-crate:
 749
 750     let mut external_mods = FxHashSet::default();
 751     fn handle_external_def(tcx: TyCtxt,
 752                            traits: &mut Vec<DefId>,
 753                            external_mods: &mut FxHashSet<DefId>,
 754                            def: Def) {
 755         match def {
 756             Def::Trait(def_id) => {
 757                 traits.push(def_id);
 758             }
 759             Def::Mod(def_id) => {
 760                 if !external_mods.insert(def_id) {
 761                     return;
 762                 }
 763                 for child in tcx.item_children(def_id).iter() {
 764                     handle_external_def(tcx, traits, external_mods, child.def)
 765                 }
 766             }
 767             _ => {}
 768         }
 769     }
 770     for &cnum in tcx.crates().iter() {
 771         let def_id = DefId {
 772             krate: cnum,
 773             index: CRATE_DEF_INDEX,
 774         };
 775         handle_external_def(tcx, &mut traits, &mut external_mods, Def::Mod(def_id));
 776     }
 777
 778     traits
 779 }
 780
 781 pub fn provide(providers: &mut ty::query::Providers) {
 782     providers.all_traits = |tcx, cnum| {
 783         assert_eq!(cnum, LOCAL_CRATE);
 784         Lrc::new(compute_all_traits(tcx))
 785     }
 786 }
 787
 788 struct UsePlacementFinder<'a, 'tcx: 'a, 'gcx: 'tcx> {
 789     target_module: ast::NodeId,
 790     span: Option<Span>,
 791     found_use: bool,
 792     tcx: TyCtxt<'a, 'gcx, 'tcx>
 793 }
 794
 795 impl<'a, 'tcx, 'gcx> UsePlacementFinder<'a, 'tcx, 'gcx> {
 796     fn check(
 797         tcx: TyCtxt<'a, 'gcx, 'tcx>,
 798         krate: &'tcx hir::Crate,
 799         target_module: ast::NodeId,
 800     ) -> (Option<Span>, bool) {
 801         let mut finder = UsePlacementFinder {
 802             target_module,
 803             span: None,
 804             found_use: false,
 805             tcx,
 806         };
 807         hir::intravisit::walk_crate(&mut finder, krate);
 808         (finder.span, finder.found_use)
 809     }
 810 }
 811
 812 impl<'a, 'tcx, 'gcx> hir::intravisit::Visitor<'tcx> for UsePlacementFinder<'a, 'tcx, 'gcx> {
 813     fn visit_mod(
 814         &mut self,
 815         module: &'tcx hir::Mod,
 816         _: Span,
 817         node_id: ast::NodeId,
 818     ) {
 819         if self.span.is_some() {
 820             return;
 821         }
 822         if node_id != self.target_module {
 823             hir::intravisit::walk_mod(self, module, node_id);
 824             return;
 825         }
 826         // Find a `use` statement.
 827         for item_id in &module.item_ids {
 828             let item = self.tcx.hir().expect_item(item_id.id);
 829             match item.node {
 830                 hir::ItemKind::Use(..) => {
 831                     // Don't suggest placing a `use` before the prelude
 832                     // import or other generated ones.
 833                     if item.span.ctxt().outer().expn_info().is_none() {
 834                         self.span = Some(item.span.shrink_to_lo());
 835                         self.found_use = true;
 836                         return;
 837                     }
 838                 },
 839                 // Don't place `use` before `extern crate`...
 840                 hir::ItemKind::ExternCrate(_) => {}
 841                 // ...but do place them before the first other item.
 842                 _ => if self.span.map_or(true, |span| item.span < span ) {
 843                     if item.span.ctxt().outer().expn_info().is_none() {
 844                         // Don't insert between attributes and an item.
 845                         if item.attrs.is_empty() {
 846                             self.span = Some(item.span.shrink_to_lo());
 847                         } else {
 848                             // Find the first attribute on the item.
 849                             for attr in &item.attrs {
 850                                 if self.span.map_or(true, |span| attr.span < span) {
 851                                     self.span = Some(attr.span.shrink_to_lo());
 852                                 }
 853                             }
 854                         }
 855                     }
 856                 },
 857             }
 858         }
 859     }
 860
 861     fn nested_visit_map<'this>(
 862         &'this mut self
 863     ) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 864         hir::intravisit::NestedVisitorMap::None
 865     }
 866 }