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::hir::{self, ExprKind, Node, QPath};
  10 use rustc::hir::def::{Res, DefKind};
  11 use rustc::hir::def_id::{CRATE_DEF_INDEX, LOCAL_CRATE, DefId};
  12 use rustc::hir::map as hir_map;
  13 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  14 use rustc::traits::Obligation;
  15 use rustc::ty::{self, Ty, TyCtxt, ToPolyTraitRef, ToPredicate, TypeFoldable};
  16 use rustc::ty::print::with_crate_prefix;
  17 use syntax_pos::{Span, FileName};
  18 use syntax::ast;
  19 use syntax::util::lev_distance;
  20
  21 use std::cmp::Ordering;
  22
  23 use super::{MethodError, NoMatchData, CandidateSource};
  24 use super::probe::Mode;
  25
  26 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  27     fn is_fn_ty(&self, ty: Ty<'tcx>, span: Span) -> bool {
  28         let tcx = self.tcx;
  29         match ty.sty {
  30             // Not all of these (e.g., unsafe fns) implement `FnOnce`,
  31             // so we look for these beforehand.
  32             ty::Closure(..) |
  33             ty::FnDef(..) |
  34             ty::FnPtr(_) => true,
  35             // If it's not a simple function, look for things which implement `FnOnce`.
  36             _ => {
  37                 let fn_once = match tcx.lang_items().require(FnOnceTraitLangItem) {
  38                     Ok(fn_once) => fn_once,
  39                     Err(..) => return false,
  40                 };
  41
  42                 self.autoderef(span, ty).any(|(ty, _)| {
  43                     self.probe(|_| {
  44                         let fn_once_substs = tcx.mk_substs_trait(ty, &[
  45                             self.next_ty_var(TypeVariableOrigin {
  46                                 kind: TypeVariableOriginKind::MiscVariable,
  47                                 span,
  48                             }).into()
  49                         ]);
  50                         let trait_ref = ty::TraitRef::new(fn_once, fn_once_substs);
  51                         let poly_trait_ref = trait_ref.to_poly_trait_ref();
  52                         let obligation =
  53                             Obligation::misc(span,
  54                                              self.body_id,
  55                                              self.param_env,
  56                                              poly_trait_ref.to_predicate());
  57                         self.predicate_may_hold(&obligation)
  58                     })
  59                 })
  60             }
  61         }
  62     }
  63
  64     pub fn report_method_error<'b>(
  65         &self,
  66         span: Span,
  67         rcvr_ty: Ty<'tcx>,
  68         item_name: ast::Ident,
  69         source: SelfSource<'b>,
  70         error: MethodError<'tcx>,
  71         args: Option<&'tcx [hir::Expr]>,
  72     ) -> Option<DiagnosticBuilder<'_>> {
  73         let orig_span = span;
  74         let mut span = span;
  75         // Avoid suggestions when we don't know what's going on.
  76         if rcvr_ty.references_error() {
  77             return None;
  78         }
  79
  80         let print_disambiguation_help = |
  81             err: &mut DiagnosticBuilder<'_>,
  82             trait_name: String,
  83         | {
  84             err.help(&format!(
  85                 "to disambiguate the method call, write `{}::{}({}{})` instead",
  86                 trait_name,
  87                 item_name,
  88                 if rcvr_ty.is_region_ptr() && args.is_some() {
  89                     if rcvr_ty.is_mutable_ptr() {
  90                         "&mut "
  91                     } else {
  92                         "&"
  93                     }
  94                 } else {
  95                     ""
  96                 },
  97                 args.map(|arg| arg
  98                     .iter()
  99                     .map(|arg| self.tcx.sess.source_map().span_to_snippet(arg.span)
 100                         .unwrap_or_else(|_| "...".to_owned()))
 101                     .collect::<Vec<_>>()
 102                     .join(", ")
 103                 ).unwrap_or_else(|| "...".to_owned())
 104             ));
 105         };
 106
 107         let report_candidates = |
 108             span: Span,
 109             err: &mut DiagnosticBuilder<'_>,
 110             mut sources: Vec<CandidateSource>,
 111         | {
 112             sources.sort();
 113             sources.dedup();
 114             // Dynamic limit to avoid hiding just one candidate, which is silly.
 115             let limit = if sources.len() == 5 { 5 } else { 4 };
 116
 117             for (idx, source) in sources.iter().take(limit).enumerate() {
 118                 match *source {
 119                     CandidateSource::ImplSource(impl_did) => {
 120                         // Provide the best span we can. Use the item, if local to crate, else
 121                         // the impl, if local to crate (item may be defaulted), else nothing.
 122                         let item = match self.associated_item(
 123                             impl_did,
 124                             item_name,
 125                             Namespace::Value,
 126                         ).or_else(|| {
 127                             let impl_trait_ref = self.tcx.impl_trait_ref(impl_did)?;
 128                             self.associated_item(
 129                                 impl_trait_ref.def_id,
 130                                 item_name,
 131                                 Namespace::Value,
 132                             )
 133                         }) {
 134                             Some(item) => item,
 135                             None => continue,
 136                         };
 137                         let note_span = self.tcx.hir().span_if_local(item.def_id).or_else(|| {
 138                             self.tcx.hir().span_if_local(impl_did)
 139                         });
 140
 141                         let impl_ty = self.impl_self_ty(span, impl_did).ty;
 142
 143                         let insertion = match self.tcx.impl_trait_ref(impl_did) {
 144                             None => String::new(),
 145                             Some(trait_ref) => {
 146                                 format!(" of the trait `{}`",
 147                                         self.tcx.def_path_str(trait_ref.def_id))
 148                             }
 149                         };
 150
 151                         let note_str = if sources.len() > 1 {
 152                             format!("candidate #{} is defined in an impl{} for the type `{}`",
 153                                     idx + 1,
 154                                     insertion,
 155                                     impl_ty)
 156                         } else {
 157                             format!("the candidate is defined in an impl{} for the type `{}`",
 158                                     insertion,
 159                                     impl_ty)
 160                         };
 161                         if let Some(note_span) = note_span {
 162                             // We have a span pointing to the method. Show note with snippet.
 163                             err.span_note(self.tcx.sess.source_map().def_span(note_span),
 164                                           &note_str);
 165                         } else {
 166                             err.note(&note_str);
 167                         }
 168                         if let Some(trait_ref) = self.tcx.impl_trait_ref(impl_did) {
 169                             print_disambiguation_help(err, self.tcx.def_path_str(trait_ref.def_id));
 170                         }
 171                     }
 172                     CandidateSource::TraitSource(trait_did) => {
 173                         let item = match self.associated_item(
 174                             trait_did,
 175                             item_name,
 176                             Namespace::Value)
 177                         {
 178                             Some(item) => item,
 179                             None => continue,
 180                         };
 181                         let item_span = self.tcx.sess.source_map()
 182                             .def_span(self.tcx.def_span(item.def_id));
 183                         if sources.len() > 1 {
 184                             span_note!(err,
 185                                        item_span,
 186                                        "candidate #{} is defined in the trait `{}`",
 187                                        idx + 1,
 188                                        self.tcx.def_path_str(trait_did));
 189                         } else {
 190                             span_note!(err,
 191                                        item_span,
 192                                        "the candidate is defined in the trait `{}`",
 193                                        self.tcx.def_path_str(trait_did));
 194                         }
 195                         print_disambiguation_help(err, self.tcx.def_path_str(trait_did));
 196                     }
 197                 }
 198             }
 199             if sources.len() > limit {
 200                 err.note(&format!("and {} others", sources.len() - limit));
 201             }
 202         };
 203
 204         match error {
 205             MethodError::NoMatch(NoMatchData {
 206                 static_candidates: static_sources,
 207                 unsatisfied_predicates,
 208                 out_of_scope_traits,
 209                 lev_candidate,
 210                 mode,
 211             }) => {
 212                 let tcx = self.tcx;
 213
 214                 let actual = self.resolve_vars_if_possible(&rcvr_ty);
 215                 let ty_str = self.ty_to_string(actual);
 216                 let is_method = mode == Mode::MethodCall;
 217                 let item_kind = if is_method {
 218                     "method"
 219                 } else if actual.is_enum() {
 220                     "variant or associated item"
 221                 } else {
 222                     match (item_name.as_str().chars().next(), actual.is_fresh_ty()) {
 223                         (Some(name), false) if name.is_lowercase() => {
 224                             "function or associated item"
 225                         }
 226                         (Some(_), false) => "associated item",
 227                         (Some(_), true) | (None, false) => {
 228                             "variant or associated item"
 229                         }
 230                         (None, true) => "variant",
 231                     }
 232                 };
 233                 let mut err = if !actual.references_error() {
 234                     // Suggest clamping down the type if the method that is being attempted to
 235                     // be used exists at all, and the type is an ambiuous numeric type
 236                     // ({integer}/{float}).
 237                     let mut candidates = all_traits(self.tcx)
 238                         .into_iter()
 239                         .filter_map(|info|
 240                             self.associated_item(info.def_id, item_name, Namespace::Value)
 241                         );
 242                     if let (true, false, SelfSource::MethodCall(expr), Some(_)) =
 243                            (actual.is_numeric(),
 244                             actual.has_concrete_skeleton(),
 245                             source,
 246                             candidates.next()) {
 247                         let mut err = struct_span_err!(
 248                             tcx.sess,
 249                             span,
 250                             E0689,
 251                             "can't call {} `{}` on ambiguous numeric type `{}`",
 252                             item_kind,
 253                             item_name,
 254                             ty_str
 255                         );
 256                         let concrete_type = if actual.is_integral() {
 257                             "i32"
 258                         } else {
 259                             "f32"
 260                         };
 261                         match expr.node {
 262                             ExprKind::Lit(ref lit) => {
 263                                 // numeric literal
 264                                 let snippet = tcx.sess.source_map().span_to_snippet(lit.span)
 265                                     .unwrap_or_else(|_| "<numeric literal>".to_owned());
 266
 267                                 err.span_suggestion(
 268                                     lit.span,
 269                                     &format!("you must specify a concrete type for \
 270                                               this numeric value, like `{}`", concrete_type),
 271                                     format!("{}_{}", snippet, concrete_type),
 272                                     Applicability::MaybeIncorrect,
 273                                 );
 274                             }
 275                             ExprKind::Path(ref qpath) => {
 276                                 // local binding
 277                                 if let &QPath::Resolved(_, ref path) = &qpath {
 278                                     if let hir::def::Res::Local(hir_id) = path.res {
 279                                         let span = tcx.hir().span(hir_id);
 280                                         let snippet = tcx.sess.source_map().span_to_snippet(span);
 281                                         let filename = tcx.sess.source_map().span_to_filename(span);
 282
 283                                         let parent_node = self.tcx.hir().get(
 284                                             self.tcx.hir().get_parent_node(hir_id),
 285                                         );
 286                                         let msg = format!(
 287                                             "you must specify a type for this binding, like `{}`",
 288                                             concrete_type,
 289                                         );
 290
 291                                         match (filename, parent_node, snippet) {
 292                                             (FileName::Real(_), Node::Local(hir::Local {
 293                                                 source: hir::LocalSource::Normal,
 294                                                 ty,
 295                                                 ..
 296                                             }), Ok(ref snippet)) => {
 297                                                 err.span_suggestion(
 298                                                     // account for `let x: _ = 42;`
 299                                                     //                  ^^^^
 300                                                     span.to(ty.as_ref().map(|ty| ty.span)
 301                                                         .unwrap_or(span)),
 302                                                     &msg,
 303                                                     format!("{}: {}", snippet, concrete_type),
 304                                                     Applicability::MaybeIncorrect,
 305                                                 );
 306                                             }
 307                                             _ => {
 308                                                 err.span_label(span, msg);
 309                                             }
 310                                         }
 311                                     }
 312                                 }
 313                             }
 314                             _ => {}
 315                         }
 316                         err.emit();
 317                         return None;
 318                     } else {
 319                         span = item_name.span;
 320                         let mut err = struct_span_err!(
 321                             tcx.sess,
 322                             span,
 323                             E0599,
 324                             "no {} named `{}` found for type `{}` in the current scope",
 325                             item_kind,
 326                             item_name,
 327                             ty_str
 328                         );
 329                         if let Some(span) = tcx.sess.confused_type_with_std_module.borrow()
 330                             .get(&span)
 331                         {
 332                             if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(*span) {
 333                                 err.span_suggestion(
 334                                     *span,
 335                                     "you are looking for the module in `std`, \
 336                                      not the primitive type",
 337                                     format!("std::{}", snippet),
 338                                     Applicability::MachineApplicable,
 339                                 );
 340                             }
 341                         }
 342                         if let ty::RawPtr(_) = &actual.sty {
 343                             err.note("try using `<*const T>::as_ref()` to get a reference to the \
 344                                       type behind the pointer: https://doc.rust-lang.org/std/\
 345                                       primitive.pointer.html#method.as_ref");
 346                             err.note("using `<*const T>::as_ref()` on a pointer \
 347                                       which is unaligned or points to invalid \
 348                                       or uninitialized memory is undefined behavior");
 349                         }
 350                         err
 351                     }
 352                 } else {
 353                     tcx.sess.diagnostic().struct_dummy()
 354                 };
 355
 356                 if let Some(def) = actual.ty_adt_def() {
 357                     if let Some(full_sp) = tcx.hir().span_if_local(def.did) {
 358                         let def_sp = tcx.sess.source_map().def_span(full_sp);
 359                         err.span_label(def_sp, format!("{} `{}` not found {}",
 360                                                        item_kind,
 361                                                        item_name,
 362                                                        if def.is_enum() && !is_method {
 363                                                            "here"
 364                                                        } else {
 365                                                            "for this"
 366                                                        }));
 367                     }
 368                 }
 369
 370                 // If the method name is the name of a field with a function or closure type,
 371                 // give a helping note that it has to be called as `(x.f)(...)`.
 372                 if let SelfSource::MethodCall(expr) = source {
 373                     let field_receiver = self
 374                         .autoderef(span, rcvr_ty)
 375                         .find_map(|(ty, _)| match ty.sty {
 376                             ty::Adt(def, substs) if !def.is_enum() => {
 377                                 let variant = &def.non_enum_variant();
 378                                 self.tcx.find_field_index(item_name, variant).map(|index| {
 379                                     let field = &variant.fields[index];
 380                                     let field_ty = field.ty(tcx, substs);
 381                                     (field, field_ty)
 382                                 })
 383                             }
 384                             _ => None,
 385                         });
 386
 387                     if let Some((field, field_ty)) = field_receiver {
 388                         let scope = self.tcx.hir().get_module_parent(self.body_id);
 389                         let is_accessible = field.vis.is_accessible_from(scope, self.tcx);
 390
 391                         if is_accessible {
 392                             if self.is_fn_ty(&field_ty, span) {
 393                                 let expr_span = expr.span.to(item_name.span);
 394                                 err.multipart_suggestion(
 395                                     &format!(
 396                                         "to call the function stored in `{}`, \
 397                                          surround the field access with parentheses",
 398                                         item_name,
 399                                     ),
 400                                     vec![
 401                                         (expr_span.shrink_to_lo(), '('.to_string()),
 402                                         (expr_span.shrink_to_hi(), ')'.to_string()),
 403                                     ],
 404                                     Applicability::MachineApplicable,
 405                                 );
 406                             } else {
 407                                 let call_expr = self.tcx.hir().expect_expr(
 408                                     self.tcx.hir().get_parent_node(expr.hir_id),
 409                                 );
 410
 411                                 if let Some(span) = call_expr.span.trim_start(item_name.span) {
 412                                     err.span_suggestion(
 413                                         span,
 414                                         "remove the arguments",
 415                                         String::new(),
 416                                         Applicability::MaybeIncorrect,
 417                                     );
 418                                 }
 419                             }
 420                         }
 421
 422                         let field_kind = if is_accessible {
 423                             "field"
 424                         } else {
 425                             "private field"
 426                         };
 427                         err.span_label(item_name.span, format!("{}, not a method", field_kind));
 428                     }
 429                 } else {
 430                     err.span_label(span, format!("{} not found in `{}`", item_kind, ty_str));
 431                     self.tcx.sess.trait_methods_not_found.borrow_mut().insert(orig_span);
 432                 }
 433
 434                 if self.is_fn_ty(&rcvr_ty, span) {
 435                     macro_rules! report_function {
 436                         ($span:expr, $name:expr) => {
 437                             err.note(&format!("{} is a function, perhaps you wish to call it",
 438                                               $name));
 439                         }
 440                     }
 441
 442                     if let SelfSource::MethodCall(expr) = source {
 443                         if let Ok(expr_string) = tcx.sess.source_map().span_to_snippet(expr.span) {
 444                             report_function!(expr.span, expr_string);
 445                         } else if let ExprKind::Path(QPath::Resolved(_, ref path)) =
 446                             expr.node
 447                         {
 448                             if let Some(segment) = path.segments.last() {
 449                                 report_function!(expr.span, segment.ident);
 450                             }
 451                         }
 452                     }
 453                 }
 454
 455                 if !static_sources.is_empty() {
 456                     err.note("found the following associated functions; to be used as methods, \
 457                               functions must have a `self` parameter");
 458                     err.span_label(span, "this is an associated function, not a method");
 459                 }
 460                 if static_sources.len() == 1 {
 461                     if let SelfSource::MethodCall(expr) = source {
 462                         err.span_suggestion(expr.span.to(span),
 463                                             "use associated function syntax instead",
 464                                             format!("{}::{}",
 465                                                     self.ty_to_string(actual),
 466                                                     item_name),
 467                                             Applicability::MachineApplicable);
 468                     } else {
 469                         err.help(&format!("try with `{}::{}`",
 470                                           self.ty_to_string(actual), item_name));
 471                     }
 472
 473                     report_candidates(span, &mut err, static_sources);
 474                 } else if static_sources.len() > 1 {
 475                     report_candidates(span, &mut err, static_sources);
 476                 }
 477
 478                 if !unsatisfied_predicates.is_empty() {
 479                     let mut bound_list = unsatisfied_predicates.iter()
 480                         .map(|p| format!("`{} : {}`", p.self_ty(), p))
 481                         .collect::<Vec<_>>();
 482                     bound_list.sort();
 483                     bound_list.dedup();  // #35677
 484                     let bound_list = bound_list.join("\n");
 485                     err.note(&format!("the method `{}` exists but the following trait bounds \
 486                                        were not satisfied:\n{}",
 487                                       item_name,
 488                                       bound_list));
 489                 }
 490
 491                 if actual.is_numeric() && actual.is_fresh() {
 492
 493                 } else {
 494                     self.suggest_traits_to_import(&mut err,
 495                                                   span,
 496                                                   rcvr_ty,
 497                                                   item_name,
 498                                                   source,
 499                                                   out_of_scope_traits);
 500                 }
 501
 502                 if actual.is_enum() {
 503                     let adt_def = actual.ty_adt_def().expect("enum is not an ADT");
 504                     if let Some(suggestion) = lev_distance::find_best_match_for_name(
 505                         adt_def.variants.iter().map(|s| &s.ident.name),
 506                         &item_name.as_str(),
 507                         None,
 508                     ) {
 509                         err.span_suggestion(
 510                             span,
 511                             "there is a variant with a similar name",
 512                             suggestion.to_string(),
 513                             Applicability::MaybeIncorrect,
 514                         );
 515                     }
 516                 }
 517
 518                 if let Some(lev_candidate) = lev_candidate {
 519                     let def_kind = lev_candidate.def_kind();
 520                     err.span_suggestion(
 521                         span,
 522                         &format!(
 523                             "there is {} {} with a similar name",
 524                             def_kind.article(),
 525                             def_kind.descr(lev_candidate.def_id),
 526                         ),
 527                         lev_candidate.ident.to_string(),
 528                         Applicability::MaybeIncorrect,
 529                     );
 530                 }
 531
 532                 return Some(err);
 533             }
 534
 535             MethodError::Ambiguity(sources) => {
 536                 let mut err = struct_span_err!(self.sess(),
 537                                                span,
 538                                                E0034,
 539                                                "multiple applicable items in scope");
 540                 err.span_label(span, format!("multiple `{}` found", item_name));
 541
 542                 report_candidates(span, &mut err, sources);
 543                 err.emit();
 544             }
 545
 546             MethodError::PrivateMatch(kind, def_id, out_of_scope_traits) => {
 547                 let mut err = struct_span_err!(self.tcx.sess, span, E0624,
 548                                                "{} `{}` is private", kind.descr(def_id), item_name);
 549                 self.suggest_valid_traits(&mut err, out_of_scope_traits);
 550                 err.emit();
 551             }
 552
 553             MethodError::IllegalSizedBound(candidates) => {
 554                 let msg = format!("the `{}` method cannot be invoked on a trait object", item_name);
 555                 let mut err = self.sess().struct_span_err(span, &msg);
 556                 if !candidates.is_empty() {
 557                     let help = format!("{an}other candidate{s} {were} found in the following \
 558                                         trait{s}, perhaps add a `use` for {one_of_them}:",
 559                                     an = if candidates.len() == 1 {"an" } else { "" },
 560                                     s = if candidates.len() == 1 { "" } else { "s" },
 561                                     were = if candidates.len() == 1 { "was" } else { "were" },
 562                                     one_of_them = if candidates.len() == 1 {
 563                                         "it"
 564                                     } else {
 565                                         "one_of_them"
 566                                     });
 567                     self.suggest_use_candidates(&mut err, help, candidates);
 568                 }
 569                 err.emit();
 570             }
 571
 572             MethodError::BadReturnType => {
 573                 bug!("no return type expectations but got BadReturnType")
 574             }
 575         }
 576         None
 577     }
 578
 579     fn suggest_use_candidates(&self,
 580                               err: &mut DiagnosticBuilder<'_>,
 581                               mut msg: String,
 582                               candidates: Vec<DefId>) {
 583         let module_did = self.tcx.hir().get_module_parent(self.body_id);
 584         let module_id = self.tcx.hir().as_local_hir_id(module_did).unwrap();
 585         let krate = self.tcx.hir().krate();
 586         let (span, found_use) = UsePlacementFinder::check(self.tcx, krate, module_id);
 587         if let Some(span) = span {
 588             let path_strings = candidates.iter().map(|did| {
 589                 // Produce an additional newline to separate the new use statement
 590                 // from the directly following item.
 591                 let additional_newline = if found_use {
 592                     ""
 593                 } else {
 594                     "\n"
 595                 };
 596                 format!(
 597                     "use {};\n{}",
 598                     with_crate_prefix(|| self.tcx.def_path_str(*did)),
 599                     additional_newline
 600                 )
 601             });
 602
 603             err.span_suggestions(span, &msg, path_strings, Applicability::MaybeIncorrect);
 604         } else {
 605             let limit = if candidates.len() == 5 { 5 } else { 4 };
 606             for (i, trait_did) in candidates.iter().take(limit).enumerate() {
 607                 if candidates.len() > 1 {
 608                     msg.push_str(
 609                         &format!(
 610                             "\ncandidate #{}: `use {};`",
 611                             i + 1,
 612                             with_crate_prefix(|| self.tcx.def_path_str(*trait_did))
 613                         )
 614                     );
 615                 } else {
 616                     msg.push_str(
 617                         &format!(
 618                             "\n`use {};`",
 619                             with_crate_prefix(|| self.tcx.def_path_str(*trait_did))
 620                         )
 621                     );
 622                 }
 623             }
 624             if candidates.len() > limit {
 625                 msg.push_str(&format!("\nand {} others", candidates.len() - limit));
 626             }
 627             err.note(&msg[..]);
 628         }
 629     }
 630
 631     fn suggest_valid_traits(&self,
 632                             err: &mut DiagnosticBuilder<'_>,
 633                             valid_out_of_scope_traits: Vec<DefId>) -> bool {
 634         if !valid_out_of_scope_traits.is_empty() {
 635             let mut candidates = valid_out_of_scope_traits;
 636             candidates.sort();
 637             candidates.dedup();
 638             err.help("items from traits can only be used if the trait is in scope");
 639             let msg = format!("the following {traits_are} implemented but not in scope, \
 640                                perhaps add a `use` for {one_of_them}:",
 641                             traits_are = if candidates.len() == 1 {
 642                                 "trait is"
 643                             } else {
 644                                 "traits are"
 645                             },
 646                             one_of_them = if candidates.len() == 1 {
 647                                 "it"
 648                             } else {
 649                                 "one of them"
 650                             });
 651
 652             self.suggest_use_candidates(err, msg, candidates);
 653             true
 654         } else {
 655             false
 656         }
 657     }
 658
 659     fn suggest_traits_to_import<'b>(
 660         &self,
 661         err: &mut DiagnosticBuilder<'_>,
 662         span: Span,
 663         rcvr_ty: Ty<'tcx>,
 664         item_name: ast::Ident,
 665         source: SelfSource<'b>,
 666         valid_out_of_scope_traits: Vec<DefId>,
 667     ) {
 668         if self.suggest_valid_traits(err, valid_out_of_scope_traits) {
 669             return;
 670         }
 671
 672         let type_is_local = self.type_derefs_to_local(span, rcvr_ty, source);
 673
 674         // There are no traits implemented, so lets suggest some traits to
 675         // implement, by finding ones that have the item name, and are
 676         // legal to implement.
 677         let mut candidates = all_traits(self.tcx)
 678             .into_iter()
 679             .filter(|info| {
 680                 // We approximate the coherence rules to only suggest
 681                 // traits that are legal to implement by requiring that
 682                 // either the type or trait is local. Multi-dispatch means
 683                 // this isn't perfect (that is, there are cases when
 684                 // implementing a trait would be legal but is rejected
 685                 // here).
 686                 (type_is_local || info.def_id.is_local()) &&
 687                     self.associated_item(info.def_id, item_name, Namespace::Value)
 688                         .filter(|item| {
 689                             // We only want to suggest public or local traits (#45781).
 690                             item.vis == ty::Visibility::Public || info.def_id.is_local()
 691                         })
 692                         .is_some()
 693             })
 694             .collect::<Vec<_>>();
 695
 696         if !candidates.is_empty() {
 697             // Sort from most relevant to least relevant.
 698             candidates.sort_by(|a, b| a.cmp(b).reverse());
 699             candidates.dedup();
 700
 701             let param_type = match rcvr_ty.sty {
 702                 ty::Param(param) => Some(param),
 703                 ty::Ref(_, ty, _) => match ty.sty {
 704                     ty::Param(param) => Some(param),
 705                     _ => None,
 706                 }
 707                 _ => None,
 708             };
 709             err.help(if param_type.is_some() {
 710                 "items from traits can only be used if the type parameter is bounded by the trait"
 711             } else {
 712                 "items from traits can only be used if the trait is implemented and in scope"
 713             });
 714             let mut msg = format!(
 715                 "the following {traits_define} an item `{name}`, perhaps you need to {action} \
 716                  {one_of_them}:",
 717                 traits_define = if candidates.len() == 1 {
 718                     "trait defines"
 719                 } else {
 720                     "traits define"
 721                 },
 722                 action = if let Some(param) = param_type {
 723                     format!("restrict type parameter `{}` with", param)
 724                 } else {
 725                     "implement".to_string()
 726                 },
 727                 one_of_them = if candidates.len() == 1 {
 728                     "it"
 729                 } else {
 730                     "one of them"
 731                 },
 732                 name = item_name,
 733             );
 734             // Obtain the span for `param` and use it for a structured suggestion.
 735             let mut suggested = false;
 736             if let (Some(ref param), Some(ref table)) = (param_type, self.in_progress_tables) {
 737                 let table = table.borrow();
 738                 if let Some(did) = table.local_id_root {
 739                     let generics = self.tcx.generics_of(did);
 740                     let type_param = generics.type_param(param, self.tcx);
 741                     let hir = &self.tcx.hir();
 742                     if let Some(id) = hir.as_local_hir_id(type_param.def_id) {
 743                         // Get the `hir::Param` to verify whether it already has any bounds.
 744                         // We do this to avoid suggesting code that ends up as `T: FooBar`,
 745                         // instead we suggest `T: Foo + Bar` in that case.
 746                         let mut has_bounds = false;
 747                         let mut impl_trait = false;
 748                         if let Node::GenericParam(ref param) = hir.get(id) {
 749                             match param.kind {
 750                                 hir::GenericParamKind::Type { synthetic: Some(_), .. } => {
 751                                     // We've found `fn foo(x: impl Trait)` instead of
 752                                     // `fn foo<T>(x: T)`. We want to suggest the correct
 753                                     // `fn foo(x: impl Trait + TraitBound)` instead of
 754                                     // `fn foo<T: TraitBound>(x: T)`. (#63706)
 755                                     impl_trait = true;
 756                                     has_bounds = param.bounds.len() > 1;
 757                                 }
 758                                 _ => {
 759                                     has_bounds = !param.bounds.is_empty();
 760                                 }
 761                             }
 762                         }
 763                         let sp = hir.span(id);
 764                         // `sp` only covers `T`, change it so that it covers
 765                         // `T:` when appropriate
 766                         let sp = if has_bounds {
 767                             sp.to(self.tcx
 768                                 .sess
 769                                 .source_map()
 770                                 .next_point(self.tcx.sess.source_map().next_point(sp)))
 771                         } else {
 772                             sp
 773                         };
 774
 775                         // FIXME: contrast `t.def_id` against `param.bounds` to not suggest traits
 776                         // already there. That can happen when the cause is that we're in a const
 777                         // scope or associated function used as a method.
 778                         err.span_suggestions(
 779                             sp,
 780                             &msg[..],
 781                             candidates.iter().map(|t| format!(
 782                                 "{}{} {}{}",
 783                                 param,
 784                                 if impl_trait { " +" } else { ":" },
 785                                 self.tcx.def_path_str(t.def_id),
 786                                 if has_bounds { " +"} else { "" },
 787                             )),
 788                             Applicability::MaybeIncorrect,
 789                         );
 790                         suggested = true;
 791                     }
 792                 };
 793             }
 794
 795             if !suggested {
 796                 for (i, trait_info) in candidates.iter().enumerate() {
 797                     msg.push_str(&format!(
 798                         "\ncandidate #{}: `{}`",
 799                         i + 1,
 800                         self.tcx.def_path_str(trait_info.def_id),
 801                     ));
 802                 }
 803                 err.note(&msg[..]);
 804             }
 805         }
 806     }
 807
 808     /// Checks whether there is a local type somewhere in the chain of
 809     /// autoderefs of `rcvr_ty`.
 810     fn type_derefs_to_local(&self,
 811                             span: Span,
 812                             rcvr_ty: Ty<'tcx>,
 813                             source: SelfSource<'_>) -> bool {
 814         fn is_local(ty: Ty<'_>) -> bool {
 815             match ty.sty {
 816                 ty::Adt(def, _) => def.did.is_local(),
 817                 ty::Foreign(did) => did.is_local(),
 818
 819                 ty::Dynamic(ref tr, ..) =>
 820                     tr.principal().map(|d| d.def_id().is_local()).unwrap_or(false),
 821
 822                 ty::Param(_) => true,
 823
 824                 // Everything else (primitive types, etc.) is effectively
 825                 // non-local (there are "edge" cases, e.g., `(LocalType,)`, but
 826                 // the noise from these sort of types is usually just really
 827                 // annoying, rather than any sort of help).
 828                 _ => false,
 829             }
 830         }
 831
 832         // This occurs for UFCS desugaring of `T::method`, where there is no
 833         // receiver expression for the method call, and thus no autoderef.
 834         if let SelfSource::QPath(_) = source {
 835             return is_local(self.resolve_type_vars_with_obligations(rcvr_ty));
 836         }
 837
 838         self.autoderef(span, rcvr_ty).any(|(ty, _)| is_local(ty))
 839     }
 840 }
 841
 842 #[derive(Copy, Clone)]
 843 pub enum SelfSource<'a> {
 844     QPath(&'a hir::Ty),
 845     MethodCall(&'a hir::Expr /* rcvr */),
 846 }
 847
 848 #[derive(Copy, Clone)]
 849 pub struct TraitInfo {
 850     pub def_id: DefId,
 851 }
 852
 853 impl PartialEq for TraitInfo {
 854     fn eq(&self, other: &TraitInfo) -> bool {
 855         self.cmp(other) == Ordering::Equal
 856     }
 857 }
 858 impl Eq for TraitInfo {}
 859 impl PartialOrd for TraitInfo {
 860     fn partial_cmp(&self, other: &TraitInfo) -> Option<Ordering> {
 861         Some(self.cmp(other))
 862     }
 863 }
 864 impl Ord for TraitInfo {
 865     fn cmp(&self, other: &TraitInfo) -> Ordering {
 866         // Local crates are more important than remote ones (local:
 867         // `cnum == 0`), and otherwise we throw in the defid for totality.
 868
 869         let lhs = (other.def_id.krate, other.def_id);
 870         let rhs = (self.def_id.krate, self.def_id);
 871         lhs.cmp(&rhs)
 872     }
 873 }
 874
 875 /// Retrieves all traits in this crate and any dependent crates.
 876 pub fn all_traits(tcx: TyCtxt<'_>) -> Vec<TraitInfo> {
 877     tcx.all_traits(LOCAL_CRATE).iter().map(|&def_id| TraitInfo { def_id }).collect()
 878 }
 879
 880 /// Computes all traits in this crate and any dependent crates.
 881 fn compute_all_traits(tcx: TyCtxt<'_>) -> Vec<DefId> {
 882     use hir::itemlikevisit;
 883
 884     let mut traits = vec![];
 885
 886     // Crate-local:
 887
 888     struct Visitor<'a, 'tcx> {
 889         map: &'a hir_map::Map<'tcx>,
 890         traits: &'a mut Vec<DefId>,
 891     }
 892
 893     impl<'v, 'a, 'tcx> itemlikevisit::ItemLikeVisitor<'v> for Visitor<'a, 'tcx> {
 894         fn visit_item(&mut self, i: &'v hir::Item) {
 895             match i.node {
 896                 hir::ItemKind::Trait(..) |
 897                 hir::ItemKind::TraitAlias(..) => {
 898                     let def_id = self.map.local_def_id(i.hir_id);
 899                     self.traits.push(def_id);
 900                 }
 901                 _ => ()
 902             }
 903         }
 904
 905         fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {}
 906
 907         fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {}
 908     }
 909
 910     tcx.hir().krate().visit_all_item_likes(&mut Visitor {
 911         map: &tcx.hir(),
 912         traits: &mut traits,
 913     });
 914
 915     // Cross-crate:
 916
 917     let mut external_mods = FxHashSet::default();
 918     fn handle_external_res(
 919         tcx: TyCtxt<'_>,
 920         traits: &mut Vec<DefId>,
 921         external_mods: &mut FxHashSet<DefId>,
 922         res: Res,
 923     ) {
 924         match res {
 925             Res::Def(DefKind::Trait, def_id) |
 926             Res::Def(DefKind::TraitAlias, def_id) => {
 927                 traits.push(def_id);
 928             }
 929             Res::Def(DefKind::Mod, def_id) => {
 930                 if !external_mods.insert(def_id) {
 931                     return;
 932                 }
 933                 for child in tcx.item_children(def_id).iter() {
 934                     handle_external_res(tcx, traits, external_mods, child.res)
 935                 }
 936             }
 937             _ => {}
 938         }
 939     }
 940     for &cnum in tcx.crates().iter() {
 941         let def_id = DefId {
 942             krate: cnum,
 943             index: CRATE_DEF_INDEX,
 944         };
 945         handle_external_res(tcx, &mut traits, &mut external_mods, Res::Def(DefKind::Mod, def_id));
 946     }
 947
 948     traits
 949 }
 950
 951 pub fn provide(providers: &mut ty::query::Providers<'_>) {
 952     providers.all_traits = |tcx, cnum| {
 953         assert_eq!(cnum, LOCAL_CRATE);
 954         &tcx.arena.alloc(compute_all_traits(tcx))[..]
 955     }
 956 }
 957
 958 struct UsePlacementFinder<'tcx> {
 959     target_module: hir::HirId,
 960     span: Option<Span>,
 961     found_use: bool,
 962     tcx: TyCtxt<'tcx>,
 963 }
 964
 965 impl UsePlacementFinder<'tcx> {
 966     fn check(
 967         tcx: TyCtxt<'tcx>,
 968         krate: &'tcx hir::Crate,
 969         target_module: hir::HirId,
 970     ) -> (Option<Span>, bool) {
 971         let mut finder = UsePlacementFinder {
 972             target_module,
 973             span: None,
 974             found_use: false,
 975             tcx,
 976         };
 977         hir::intravisit::walk_crate(&mut finder, krate);
 978         (finder.span, finder.found_use)
 979     }
 980 }
 981
 982 impl hir::intravisit::Visitor<'tcx> for UsePlacementFinder<'tcx> {
 983     fn visit_mod(
 984         &mut self,
 985         module: &'tcx hir::Mod,
 986         _: Span,
 987         hir_id: hir::HirId,
 988     ) {
 989         if self.span.is_some() {
 990             return;
 991         }
 992         if hir_id != self.target_module {
 993             hir::intravisit::walk_mod(self, module, hir_id);
 994             return;
 995         }
 996         // Find a `use` statement.
 997         for item_id in &module.item_ids {
 998             let item = self.tcx.hir().expect_item(item_id.id);
 999             match item.node {
1000                 hir::ItemKind::Use(..) => {
1001                     // Don't suggest placing a `use` before the prelude
1002                     // import or other generated ones.
1003                     if !item.span.from_expansion() {
1004                         self.span = Some(item.span.shrink_to_lo());
1005                         self.found_use = true;
1006                         return;
1007                     }
1008                 },
1009                 // Don't place `use` before `extern crate`...
1010                 hir::ItemKind::ExternCrate(_) => {}
1011                 // ...but do place them before the first other item.
1012                 _ => if self.span.map_or(true, |span| item.span < span ) {
1013                     if !item.span.from_expansion() {
1014                         // Don't insert between attributes and an item.
1015                         if item.attrs.is_empty() {
1016                             self.span = Some(item.span.shrink_to_lo());
1017                         } else {
1018                             // Find the first attribute on the item.
1019                             for attr in &item.attrs {
1020                                 if self.span.map_or(true, |span| attr.span < span) {
1021                                     self.span = Some(attr.span.shrink_to_lo());
1022                                 }
1023                             }
1024                         }
1025                     }
1026                 },
1027             }
1028         }
1029     }
1030
1031     fn nested_visit_map<'this>(
1032         &'this mut self
1033     ) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
1034         hir::intravisit::NestedVisitorMap::None
1035     }
1036 }