src/librustc_typeck/check/method/suggest.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Give useful errors and suggestions to users when an item can't be
  12 //! found or is otherwise invalid.
  13
  14 use check::FnCtxt;
  15 use rustc::hir::map as hir_map;
  16 use rustc::ty::{self, Ty, TyCtxt, ToPolyTraitRef, ToPredicate, TypeFoldable};
  17 use hir::def::Def;
  18 use hir::def_id::{CRATE_DEF_INDEX, DefId};
  19 use middle::lang_items::FnOnceTraitLangItem;
  20 use rustc::traits::{Obligation, SelectionContext};
  21 use util::nodemap::FxHashSet;
  22
  23 use syntax::ast;
  24 use errors::DiagnosticBuilder;
  25 use syntax_pos::Span;
  26
  27 use rustc::hir;
  28 use rustc::hir::print;
  29 use rustc::infer::type_variable::TypeVariableOrigin;
  30
  31 use std::cell;
  32 use std::cmp::Ordering;
  33
  34 use super::{MethodError, NoMatchData, CandidateSource};
  35 use super::probe::Mode;
  36
  37 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  38     fn is_fn_ty(&self, ty: &Ty<'tcx>, span: Span) -> bool {
  39         let tcx = self.tcx;
  40         match ty.sty {
  41             // Not all of these (e.g. unsafe fns) implement FnOnce
  42             // so we look for these beforehand
  43             ty::TyClosure(..) |
  44             ty::TyFnDef(..) |
  45             ty::TyFnPtr(_) => true,
  46             // If it's not a simple function, look for things which implement FnOnce
  47             _ => {
  48                 let fn_once = match tcx.lang_items.require(FnOnceTraitLangItem) {
  49                     Ok(fn_once) => fn_once,
  50                     Err(..) => return false,
  51                 };
  52
  53                 self.autoderef(span, ty).any(|(ty, _)| {
  54                     self.probe(|_| {
  55                         let fn_once_substs = tcx.mk_substs_trait(ty,
  56                             &[self.next_ty_var(TypeVariableOrigin::MiscVariable(span))]);
  57                         let trait_ref = ty::TraitRef::new(fn_once, fn_once_substs);
  58                         let poly_trait_ref = trait_ref.to_poly_trait_ref();
  59                         let obligation =
  60                             Obligation::misc(span,
  61                                              self.body_id,
  62                                              self.param_env,
  63                                              poly_trait_ref.to_predicate());
  64                         SelectionContext::new(self).evaluate_obligation(&obligation)
  65                     })
  66                 })
  67             }
  68         }
  69     }
  70
  71     pub fn report_method_error(&self,
  72                                span: Span,
  73                                rcvr_ty: Ty<'tcx>,
  74                                item_name: ast::Name,
  75                                rcvr_expr: Option<&hir::Expr>,
  76                                error: MethodError<'tcx>,
  77                                args: Option<&'gcx [hir::Expr]>) {
  78         // avoid suggestions when we don't know what's going on.
  79         if rcvr_ty.references_error() {
  80             return;
  81         }
  82
  83         let report_candidates = |err: &mut DiagnosticBuilder, mut sources: Vec<CandidateSource>| {
  84
  85             sources.sort();
  86             sources.dedup();
  87             // Dynamic limit to avoid hiding just one candidate, which is silly.
  88             let limit = if sources.len() == 5 { 5 } else { 4 };
  89
  90             for (idx, source) in sources.iter().take(limit).enumerate() {
  91                 match *source {
  92                     CandidateSource::ImplSource(impl_did) => {
  93                         // Provide the best span we can. Use the item, if local to crate, else
  94                         // the impl, if local to crate (item may be defaulted), else nothing.
  95                         let item = self.associated_item(impl_did, item_name)
  96                             .or_else(|| {
  97                                 self.associated_item(
  98                                     self.tcx.impl_trait_ref(impl_did).unwrap().def_id,
  99
 100                                     item_name
 101                                 )
 102                             }).unwrap();
 103                         let note_span = self.tcx.hir.span_if_local(item.def_id).or_else(|| {
 104                             self.tcx.hir.span_if_local(impl_did)
 105                         });
 106
 107                         let impl_ty = self.impl_self_ty(span, impl_did).ty;
 108
 109                         let insertion = match self.tcx.impl_trait_ref(impl_did) {
 110                             None => format!(""),
 111                             Some(trait_ref) => {
 112                                 format!(" of the trait `{}`",
 113                                         self.tcx.item_path_str(trait_ref.def_id))
 114                             }
 115                         };
 116
 117                         let note_str = format!("candidate #{} is defined in an impl{} \
 118                                                 for the type `{}`",
 119                                                idx + 1,
 120                                                insertion,
 121                                                impl_ty);
 122                         if let Some(note_span) = note_span {
 123                             // We have a span pointing to the method. Show note with snippet.
 124                             err.span_note(note_span, &note_str);
 125                         } else {
 126                             err.note(&note_str);
 127                         }
 128                     }
 129                     CandidateSource::TraitSource(trait_did) => {
 130                         let item = self.associated_item(trait_did, item_name).unwrap();
 131                         let item_span = self.tcx.def_span(item.def_id);
 132                         span_note!(err,
 133                                    item_span,
 134                                    "candidate #{} is defined in the trait `{}`",
 135                                    idx + 1,
 136                                    self.tcx.item_path_str(trait_did));
 137                         err.help(&format!("to disambiguate the method call, write `{}::{}({}{})` \
 138                                           instead",
 139                                           self.tcx.item_path_str(trait_did),
 140                                           item_name,
 141                                           if rcvr_ty.is_region_ptr() && args.is_some() {
 142                                               if rcvr_ty.is_mutable_pointer() {
 143                                                   "&mut "
 144                                               } else {
 145                                                   "&"
 146                                               }
 147                                           } else {
 148                                               ""
 149                                           },
 150                                           args.map(|arg| arg.iter()
 151                                               .map(|arg| print::to_string(print::NO_ANN,
 152                                                                           |s| s.print_expr(arg)))
 153                                               .collect::<Vec<_>>()
 154                                               .join(", ")).unwrap_or("...".to_owned())));
 155                     }
 156                 }
 157             }
 158             if sources.len() > limit {
 159                 err.note(&format!("and {} others", sources.len() - limit));
 160             }
 161         };
 162
 163         match error {
 164             MethodError::NoMatch(NoMatchData { static_candidates: static_sources,
 165                                                unsatisfied_predicates,
 166                                                out_of_scope_traits,
 167                                                mode,
 168                                                .. }) => {
 169                 let tcx = self.tcx;
 170
 171                 let actual = self.resolve_type_vars_if_possible(&rcvr_ty);
 172                 let mut err = if !actual.references_error() {
 173                     struct_span_err!(tcx.sess, span, E0599,
 174                                      "no {} named `{}` found for type `{}` in the \
 175                                       current scope",
 176                                      if mode == Mode::MethodCall {
 177                                          "method"
 178                                      } else {
 179                                          match item_name.as_str().chars().next() {
 180                                              Some(name) => {
 181                                                  if name.is_lowercase() {
 182                                                      "function or associated item"
 183                                                  } else {
 184                                                      "associated item"
 185                                                  }
 186                                              },
 187                                              None => {
 188                                                  ""
 189                                              },
 190                                          }
 191                                      },
 192                                      item_name,
 193                                      self.ty_to_string(actual))
 194                 } else {
 195                     self.tcx.sess.diagnostic().struct_dummy()
 196                 };
 197
 198                 // If the method name is the name of a field with a function or closure type,
 199                 // give a helping note that it has to be called as (x.f)(...).
 200                 if let Some(expr) = rcvr_expr {
 201                     for (ty, _) in self.autoderef(span, rcvr_ty) {
 202                         match ty.sty {
 203                             ty::TyAdt(def, substs) if !def.is_enum() => {
 204                                 if let Some(field) = def.struct_variant()
 205                                     .find_field_named(item_name) {
 206                                     let snippet = tcx.sess.codemap().span_to_snippet(expr.span);
 207                                     let expr_string = match snippet {
 208                                         Ok(expr_string) => expr_string,
 209                                         _ => "s".into(), // Default to a generic placeholder for the
 210                                         // expression when we can't generate a
 211                                         // string snippet
 212                                     };
 213
 214                                     let field_ty = field.ty(tcx, substs);
 215                                     let scope = self.tcx.hir.get_module_parent(self.body_id);
 216                                     if field.vis.is_accessible_from(scope, self.tcx) {
 217                                         if self.is_fn_ty(&field_ty, span) {
 218                                             err.help(&format!("use `({0}.{1})(...)` if you \
 219                                                                meant to call the function \
 220                                                                stored in the `{1}` field",
 221                                                               expr_string,
 222                                                               item_name));
 223                                         } else {
 224                                             err.help(&format!("did you mean to write `{0}.{1}` \
 225                                                                instead of `{0}.{1}(...)`?",
 226                                                               expr_string,
 227                                                               item_name));
 228                                         }
 229                                         err.span_label(span, "field, not a method");
 230                                     } else {
 231                                         err.span_label(span, "private field, not a method");
 232                                     }
 233                                     break;
 234                                 }
 235                             }
 236                             _ => {}
 237                         }
 238                     }
 239                 }
 240
 241                 if self.is_fn_ty(&rcvr_ty, span) {
 242                     macro_rules! report_function {
 243                         ($span:expr, $name:expr) => {
 244                             err.note(&format!("{} is a function, perhaps you wish to call it",
 245                                               $name));
 246                         }
 247                     }
 248
 249                     if let Some(expr) = rcvr_expr {
 250                         if let Ok(expr_string) = tcx.sess.codemap().span_to_snippet(expr.span) {
 251                             report_function!(expr.span, expr_string);
 252                         } else if let hir::ExprPath(hir::QPath::Resolved(_, ref path)) = expr.node {
 253                             if let Some(segment) = path.segments.last() {
 254                                 report_function!(expr.span, segment.name);
 255                             }
 256                         }
 257                     }
 258                 }
 259
 260                 if !static_sources.is_empty() {
 261                     err.note("found the following associated functions; to be used as methods, \
 262                               functions must have a `self` parameter");
 263                     err.help(&format!("try with `{}::{}`", self.ty_to_string(actual), item_name));
 264
 265                     report_candidates(&mut err, static_sources);
 266                 }
 267
 268                 if !unsatisfied_predicates.is_empty() {
 269                     let bound_list = unsatisfied_predicates.iter()
 270                         .map(|p| format!("`{} : {}`", p.self_ty(), p))
 271                         .collect::<Vec<_>>()
 272                         .join("\n");
 273                     err.note(&format!("the method `{}` exists but the following trait bounds \
 274                                        were not satisfied:\n{}",
 275                                       item_name,
 276                                       bound_list));
 277                 }
 278
 279                 self.suggest_traits_to_import(&mut err,
 280                                               span,
 281                                               rcvr_ty,
 282                                               item_name,
 283                                               rcvr_expr,
 284                                               out_of_scope_traits);
 285                 err.emit();
 286             }
 287
 288             MethodError::Ambiguity(sources) => {
 289                 let mut err = struct_span_err!(self.sess(),
 290                                                span,
 291                                                E0034,
 292                                                "multiple applicable items in scope");
 293                 err.span_label(span, format!("multiple `{}` found", item_name));
 294
 295                 report_candidates(&mut err, sources);
 296                 err.emit();
 297             }
 298
 299             MethodError::ClosureAmbiguity(trait_def_id) => {
 300                 let msg = format!("the `{}` method from the `{}` trait cannot be explicitly \
 301                                    invoked on this closure as we have not yet inferred what \
 302                                    kind of closure it is",
 303                                   item_name,
 304                                   self.tcx.item_path_str(trait_def_id));
 305                 let msg = if let Some(callee) = rcvr_expr {
 306                     format!("{}; use overloaded call notation instead (e.g., `{}()`)",
 307                             msg,
 308                             self.tcx.hir.node_to_pretty_string(callee.id))
 309                 } else {
 310                     msg
 311                 };
 312                 self.sess().span_err(span, &msg);
 313             }
 314
 315             MethodError::PrivateMatch(def, out_of_scope_traits) => {
 316                 let mut err = struct_span_err!(self.tcx.sess, span, E0624,
 317                                                "{} `{}` is private", def.kind_name(), item_name);
 318                 self.suggest_valid_traits(&mut err, out_of_scope_traits);
 319                 err.emit();
 320             }
 321
 322             MethodError::IllegalSizedBound(candidates) => {
 323                 let msg = format!("the `{}` method cannot be invoked on a trait object", item_name);
 324                 let mut err = self.sess().struct_span_err(span, &msg);
 325                 if !candidates.is_empty() {
 326                     let help = format!("{an}other candidate{s} {were} found in the following \
 327                                         trait{s}, perhaps add a `use` for {one_of_them}:",
 328                                     an = if candidates.len() == 1 {"an" } else { "" },
 329                                     s = if candidates.len() == 1 { "" } else { "s" },
 330                                     were = if candidates.len() == 1 { "was" } else { "were" },
 331                                     one_of_them = if candidates.len() == 1 {
 332                                         "it"
 333                                     } else {
 334                                         "one_of_them"
 335                                     });
 336                     self.suggest_use_candidates(&mut err, help, candidates);
 337                 }
 338                 err.emit();
 339             }
 340         }
 341     }
 342
 343     fn suggest_use_candidates(&self,
 344                               err: &mut DiagnosticBuilder,
 345                               mut msg: String,
 346                               candidates: Vec<DefId>) {
 347         let limit = if candidates.len() == 5 { 5 } else { 4 };
 348         for (i, trait_did) in candidates.iter().take(limit).enumerate() {
 349             msg.push_str(&format!("\ncandidate #{}: `use {};`",
 350                                     i + 1,
 351                                     self.tcx.item_path_str(*trait_did)));
 352         }
 353         if candidates.len() > limit {
 354             msg.push_str(&format!("\nand {} others", candidates.len() - limit));
 355         }
 356         err.note(&msg[..]);
 357     }
 358
 359     fn suggest_valid_traits(&self,
 360                             err: &mut DiagnosticBuilder,
 361                             valid_out_of_scope_traits: Vec<DefId>) -> bool {
 362         if !valid_out_of_scope_traits.is_empty() {
 363             let mut candidates = valid_out_of_scope_traits;
 364             candidates.sort();
 365             candidates.dedup();
 366             err.help("items from traits can only be used if the trait is in scope");
 367             let msg = format!("the following {traits_are} implemented but not in scope, \
 368                                perhaps add a `use` for {one_of_them}:",
 369                             traits_are = if candidates.len() == 1 {
 370                                 "trait is"
 371                             } else {
 372                                 "traits are"
 373                             },
 374                             one_of_them = if candidates.len() == 1 {
 375                                 "it"
 376                             } else {
 377                                 "one of them"
 378                             });
 379
 380             self.suggest_use_candidates(err, msg, candidates);
 381             true
 382         } else {
 383             false
 384         }
 385     }
 386
 387     fn suggest_traits_to_import(&self,
 388                                 err: &mut DiagnosticBuilder,
 389                                 span: Span,
 390                                 rcvr_ty: Ty<'tcx>,
 391                                 item_name: ast::Name,
 392                                 rcvr_expr: Option<&hir::Expr>,
 393                                 valid_out_of_scope_traits: Vec<DefId>) {
 394         if self.suggest_valid_traits(err, valid_out_of_scope_traits) {
 395             return;
 396         }
 397
 398         let type_is_local = self.type_derefs_to_local(span, rcvr_ty, rcvr_expr);
 399
 400         // there's no implemented traits, so lets suggest some traits to
 401         // implement, by finding ones that have the item name, and are
 402         // legal to implement.
 403         let mut candidates = all_traits(self.tcx)
 404             .filter(|info| {
 405                 // we approximate the coherence rules to only suggest
 406                 // traits that are legal to implement by requiring that
 407                 // either the type or trait is local. Multidispatch means
 408                 // this isn't perfect (that is, there are cases when
 409                 // implementing a trait would be legal but is rejected
 410                 // here).
 411                 (type_is_local || info.def_id.is_local())
 412                     && self.associated_item(info.def_id, item_name).is_some()
 413             })
 414             .collect::<Vec<_>>();
 415
 416         if !candidates.is_empty() {
 417             // sort from most relevant to least relevant
 418             candidates.sort_by(|a, b| a.cmp(b).reverse());
 419             candidates.dedup();
 420
 421             // FIXME #21673 this help message could be tuned to the case
 422             // of a type parameter: suggest adding a trait bound rather
 423             // than implementing.
 424             err.help("items from traits can only be used if the trait is implemented and in scope");
 425             let mut msg = format!("the following {traits_define} an item `{name}`, \
 426                                    perhaps you need to implement {one_of_them}:",
 427                                   traits_define = if candidates.len() == 1 {
 428                                       "trait defines"
 429                                   } else {
 430                                       "traits define"
 431                                   },
 432                                   one_of_them = if candidates.len() == 1 {
 433                                       "it"
 434                                   } else {
 435                                       "one of them"
 436                                   },
 437                                   name = item_name);
 438
 439             for (i, trait_info) in candidates.iter().enumerate() {
 440                 msg.push_str(&format!("\ncandidate #{}: `{}`",
 441                                       i + 1,
 442                                       self.tcx.item_path_str(trait_info.def_id)));
 443             }
 444             err.note(&msg[..]);
 445         }
 446     }
 447
 448     /// Checks whether there is a local type somewhere in the chain of
 449     /// autoderefs of `rcvr_ty`.
 450     fn type_derefs_to_local(&self,
 451                             span: Span,
 452                             rcvr_ty: Ty<'tcx>,
 453                             rcvr_expr: Option<&hir::Expr>)
 454                             -> bool {
 455         fn is_local(ty: Ty) -> bool {
 456             match ty.sty {
 457                 ty::TyAdt(def, _) => def.did.is_local(),
 458
 459                 ty::TyDynamic(ref tr, ..) => tr.principal()
 460                     .map_or(false, |p| p.def_id().is_local()),
 461
 462                 ty::TyParam(_) => true,
 463
 464                 // everything else (primitive types etc.) is effectively
 465                 // non-local (there are "edge" cases, e.g. (LocalType,), but
 466                 // the noise from these sort of types is usually just really
 467                 // annoying, rather than any sort of help).
 468                 _ => false,
 469             }
 470         }
 471
 472         // This occurs for UFCS desugaring of `T::method`, where there is no
 473         // receiver expression for the method call, and thus no autoderef.
 474         if rcvr_expr.is_none() {
 475             return is_local(self.resolve_type_vars_with_obligations(rcvr_ty));
 476         }
 477
 478         self.autoderef(span, rcvr_ty).any(|(ty, _)| is_local(ty))
 479     }
 480 }
 481
 482 pub type AllTraitsVec = Vec<DefId>;
 483
 484 #[derive(Copy, Clone)]
 485 pub struct TraitInfo {
 486     pub def_id: DefId,
 487 }
 488
 489 impl TraitInfo {
 490     fn new(def_id: DefId) -> TraitInfo {
 491         TraitInfo { def_id: def_id }
 492     }
 493 }
 494 impl PartialEq for TraitInfo {
 495     fn eq(&self, other: &TraitInfo) -> bool {
 496         self.cmp(other) == Ordering::Equal
 497     }
 498 }
 499 impl Eq for TraitInfo {}
 500 impl PartialOrd for TraitInfo {
 501     fn partial_cmp(&self, other: &TraitInfo) -> Option<Ordering> {
 502         Some(self.cmp(other))
 503     }
 504 }
 505 impl Ord for TraitInfo {
 506     fn cmp(&self, other: &TraitInfo) -> Ordering {
 507         // local crates are more important than remote ones (local:
 508         // cnum == 0), and otherwise we throw in the defid for totality
 509
 510         let lhs = (other.def_id.krate, other.def_id);
 511         let rhs = (self.def_id.krate, self.def_id);
 512         lhs.cmp(&rhs)
 513     }
 514 }
 515
 516 /// Retrieve all traits in this crate and any dependent crates.
 517 pub fn all_traits<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> AllTraits<'a> {
 518     if tcx.all_traits.borrow().is_none() {
 519         use rustc::hir::itemlikevisit;
 520
 521         let mut traits = vec![];
 522
 523         // Crate-local:
 524         //
 525         // meh.
 526         struct Visitor<'a, 'tcx: 'a> {
 527             map: &'a hir_map::Map<'tcx>,
 528             traits: &'a mut AllTraitsVec,
 529         }
 530         impl<'v, 'a, 'tcx> itemlikevisit::ItemLikeVisitor<'v> for Visitor<'a, 'tcx> {
 531             fn visit_item(&mut self, i: &'v hir::Item) {
 532                 match i.node {
 533                     hir::ItemTrait(..) => {
 534                         let def_id = self.map.local_def_id(i.id);
 535                         self.traits.push(def_id);
 536                     }
 537                     _ => {}
 538                 }
 539             }
 540
 541             fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {
 542             }
 543
 544             fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {
 545             }
 546         }
 547         tcx.hir.krate().visit_all_item_likes(&mut Visitor {
 548             map: &tcx.hir,
 549             traits: &mut traits,
 550         });
 551
 552         // Cross-crate:
 553         let mut external_mods = FxHashSet();
 554         fn handle_external_def(tcx: TyCtxt,
 555                                traits: &mut AllTraitsVec,
 556                                external_mods: &mut FxHashSet<DefId>,
 557                                def: Def) {
 558             let def_id = def.def_id();
 559             match def {
 560                 Def::Trait(..) => {
 561                     traits.push(def_id);
 562                 }
 563                 Def::Mod(..) => {
 564                     if !external_mods.insert(def_id) {
 565                         return;
 566                     }
 567                     for child in tcx.sess.cstore.item_children(def_id, tcx.sess) {
 568                         handle_external_def(tcx, traits, external_mods, child.def)
 569                     }
 570                 }
 571                 _ => {}
 572             }
 573         }
 574         for cnum in tcx.sess.cstore.crates() {
 575             let def_id = DefId {
 576                 krate: cnum,
 577                 index: CRATE_DEF_INDEX,
 578             };
 579             handle_external_def(tcx, &mut traits, &mut external_mods, Def::Mod(def_id));
 580         }
 581
 582         *tcx.all_traits.borrow_mut() = Some(traits);
 583     }
 584
 585     let borrow = tcx.all_traits.borrow();
 586     assert!(borrow.is_some());
 587     AllTraits {
 588         borrow: borrow,
 589         idx: 0,
 590     }
 591 }
 592
 593 pub struct AllTraits<'a> {
 594     borrow: cell::Ref<'a, Option<AllTraitsVec>>,
 595     idx: usize,
 596 }
 597
 598 impl<'a> Iterator for AllTraits<'a> {
 599     type Item = TraitInfo;
 600
 601     fn next(&mut self) -> Option<TraitInfo> {
 602         let AllTraits { ref borrow, ref mut idx } = *self;
 603         // ugh.
 604         borrow.as_ref().unwrap().get(*idx).map(|info| {
 605             *idx += 1;
 606             TraitInfo::new(*info)
 607         })
 608     }
 609 }