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