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 CrateCtxt;
  15
  16 use astconv::AstConv;
  17 use check::{self, FnCtxt};
  18 use middle::ty::{self, Ty, ToPolyTraitRef, ToPredicate, HasTypeFlags};
  19 use middle::def;
  20 use middle::lang_items::FnOnceTraitLangItem;
  21 use middle::subst::Substs;
  22 use middle::traits::{Obligation, SelectionContext};
  23 use metadata::{csearch, cstore, decoder};
  24
  25 use syntax::{ast, ast_util};
  26 use syntax::codemap::Span;
  27 use syntax::print::pprust;
  28
  29 use std::cell;
  30 use std::cmp::Ordering;
  31
  32 use super::{MethodError, NoMatchData, CandidateSource, impl_item, trait_item};
  33 use super::probe::Mode;
  34
  35 pub fn report_error<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
  36                               span: Span,
  37                               rcvr_ty: Ty<'tcx>,
  38                               item_name: ast::Name,
  39                               rcvr_expr: Option<&ast::Expr>,
  40                               error: MethodError<'tcx>)
  41 {
  42     // avoid suggestions when we don't know what's going on.
  43     if rcvr_ty.references_error() {
  44         return
  45     }
  46
  47     match error {
  48         MethodError::NoMatch(NoMatchData { static_candidates: static_sources,
  49                                            unsatisfied_predicates,
  50                                            out_of_scope_traits,
  51                                            mode }) => {
  52             let cx = fcx.tcx();
  53
  54             fcx.type_error_message(
  55                 span,
  56                 |actual| {
  57                     format!("no {} named `{}` found for type `{}` \
  58                              in the current scope",
  59                             if mode == Mode::MethodCall { "method" }
  60                             else { "associated item" },
  61                             item_name,
  62                             actual)
  63                 },
  64                 rcvr_ty,
  65                 None);
  66
  67             // If the item has the name of a field, give a help note
  68             if let (&ty::TyStruct(did, substs), Some(expr)) = (&rcvr_ty.sty, rcvr_expr) {
  69                 let fields = cx.lookup_struct_fields(did);
  70
  71                 if let Some(field) = fields.iter().find(|f| f.name == item_name) {
  72                     let expr_string = match cx.sess.codemap().span_to_snippet(expr.span) {
  73                         Ok(expr_string) => expr_string,
  74                         _ => "s".into() // Default to a generic placeholder for the
  75                                         // expression when we can't generate a string
  76                                         // snippet
  77                     };
  78
  79                     let span_stored_function = || {
  80                         cx.sess.span_note(span,
  81                                           &format!("use `({0}.{1})(...)` if you meant to call \
  82                                                     the function stored in the `{1}` field",
  83                                                    expr_string, item_name));
  84                     };
  85
  86                     let span_did_you_mean = || {
  87                         cx.sess.span_note(span, &format!("did you mean to write `{0}.{1}`?",
  88                                                          expr_string, item_name));
  89                     };
  90
  91                     // Determine if the field can be used as a function in some way
  92                     let field_ty = cx.lookup_field_type(did, field.id, substs);
  93                     if let Ok(fn_once_trait_did) = cx.lang_items.require(FnOnceTraitLangItem) {
  94                         let infcx = fcx.infcx();
  95                         infcx.probe(|_| {
  96                             let fn_once_substs = Substs::new_trait(vec![infcx.next_ty_var()],
  97                                                                    Vec::new(),
  98                                                                    field_ty);
  99                             let trait_ref = ty::TraitRef::new(fn_once_trait_did,
 100                                                               cx.mk_substs(fn_once_substs));
 101                             let poly_trait_ref = trait_ref.to_poly_trait_ref();
 102                             let obligation = Obligation::misc(span,
 103                                                               fcx.body_id,
 104                                                               poly_trait_ref.to_predicate());
 105                             let mut selcx = SelectionContext::new(infcx);
 106
 107                             if selcx.evaluate_obligation(&obligation) {
 108                                 span_stored_function();
 109                             } else {
 110                                 span_did_you_mean();
 111                             }
 112                         });
 113                     } else {
 114                         match field_ty.sty {
 115                             // fallback to matching a closure or function pointer
 116                             ty::TyClosure(..) | ty::TyBareFn(..) => span_stored_function(),
 117                             _ => span_did_you_mean(),
 118                         }
 119                     }
 120                 }
 121             }
 122
 123             if !static_sources.is_empty() {
 124                 cx.sess.fileline_note(
 125                     span,
 126                     "found defined static methods, maybe a `self` is missing?");
 127
 128                 report_candidates(fcx, span, item_name, static_sources);
 129             }
 130
 131             if !unsatisfied_predicates.is_empty() {
 132                 let bound_list = unsatisfied_predicates.iter()
 133                     .map(|p| format!("`{} : {}`",
 134                                      p.self_ty(),
 135                                      p))
 136                     .collect::<Vec<_>>()
 137                     .join(", ");
 138                 cx.sess.fileline_note(
 139                     span,
 140                     &format!("the method `{}` exists but the \
 141                              following trait bounds were not satisfied: {}",
 142                              item_name,
 143                              bound_list));
 144             }
 145
 146             suggest_traits_to_import(fcx, span, rcvr_ty, item_name,
 147                                      rcvr_expr, out_of_scope_traits)
 148         }
 149
 150         MethodError::Ambiguity(sources) => {
 151             span_err!(fcx.sess(), span, E0034,
 152                       "multiple applicable items in scope");
 153
 154             report_candidates(fcx, span, item_name, sources);
 155         }
 156
 157         MethodError::ClosureAmbiguity(trait_def_id) => {
 158             let msg = format!("the `{}` method from the `{}` trait cannot be explicitly \
 159                                invoked on this closure as we have not yet inferred what \
 160                                kind of closure it is",
 161                                item_name,
 162                                fcx.tcx().item_path_str(trait_def_id));
 163             let msg = if let Some(callee) = rcvr_expr {
 164                 format!("{}; use overloaded call notation instead (e.g., `{}()`)",
 165                         msg, pprust::expr_to_string(callee))
 166             } else {
 167                 msg
 168             };
 169             fcx.sess().span_err(span, &msg);
 170         }
 171     }
 172
 173     fn report_candidates(fcx: &FnCtxt,
 174                          span: Span,
 175                          item_name: ast::Name,
 176                          mut sources: Vec<CandidateSource>) {
 177         sources.sort();
 178         sources.dedup();
 179
 180         for (idx, source) in sources.iter().enumerate() {
 181             match *source {
 182                 CandidateSource::ImplSource(impl_did) => {
 183                     // Provide the best span we can. Use the item, if local to crate, else
 184                     // the impl, if local to crate (item may be defaulted), else the call site.
 185                     let item = impl_item(fcx.tcx(), impl_did, item_name).unwrap();
 186                     let impl_span = fcx.tcx().map.def_id_span(impl_did, span);
 187                     let item_span = fcx.tcx().map.def_id_span(item.def_id(), impl_span);
 188
 189                     let impl_ty = check::impl_self_ty(fcx, span, impl_did).ty;
 190
 191                     let insertion = match fcx.tcx().impl_trait_ref(impl_did) {
 192                         None => format!(""),
 193                         Some(trait_ref) => {
 194                             format!(" of the trait `{}`",
 195                                     fcx.tcx().item_path_str(trait_ref.def_id))
 196                         }
 197                     };
 198
 199                     span_note!(fcx.sess(), item_span,
 200                                "candidate #{} is defined in an impl{} for the type `{}`",
 201                                idx + 1,
 202                                insertion,
 203                                impl_ty);
 204                 }
 205                 CandidateSource::TraitSource(trait_did) => {
 206                     let item = trait_item(fcx.tcx(), trait_did, item_name).unwrap();
 207                     let item_span = fcx.tcx().map.def_id_span(item.def_id(), span);
 208                     span_note!(fcx.sess(), item_span,
 209                                "candidate #{} is defined in the trait `{}`",
 210                                idx + 1,
 211                                fcx.tcx().item_path_str(trait_did));
 212                 }
 213             }
 214         }
 215     }
 216 }
 217
 218
 219 pub type AllTraitsVec = Vec<TraitInfo>;
 220
 221 fn suggest_traits_to_import<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 222                                       span: Span,
 223                                       rcvr_ty: Ty<'tcx>,
 224                                       item_name: ast::Name,
 225                                       rcvr_expr: Option<&ast::Expr>,
 226                                       valid_out_of_scope_traits: Vec<ast::DefId>)
 227 {
 228     let tcx = fcx.tcx();
 229
 230     if !valid_out_of_scope_traits.is_empty() {
 231         let mut candidates = valid_out_of_scope_traits;
 232         candidates.sort();
 233         candidates.dedup();
 234         let msg = format!(
 235             "items from traits can only be used if the trait is in scope; \
 236              the following {traits_are} implemented but not in scope, \
 237              perhaps add a `use` for {one_of_them}:",
 238             traits_are = if candidates.len() == 1 {"trait is"} else {"traits are"},
 239             one_of_them = if candidates.len() == 1 {"it"} else {"one of them"});
 240
 241         fcx.sess().fileline_help(span, &msg[..]);
 242
 243         for (i, trait_did) in candidates.iter().enumerate() {
 244             fcx.sess().fileline_help(span,
 245                                      &*format!("candidate #{}: use `{}`",
 246                                                i + 1,
 247                                                fcx.tcx().item_path_str(*trait_did)))
 248
 249         }
 250         return
 251     }
 252
 253     let type_is_local = type_derefs_to_local(fcx, span, rcvr_ty, rcvr_expr);
 254
 255     // there's no implemented traits, so lets suggest some traits to
 256     // implement, by finding ones that have the item name, and are
 257     // legal to implement.
 258     let mut candidates = all_traits(fcx.ccx)
 259         .filter(|info| {
 260             // we approximate the coherence rules to only suggest
 261             // traits that are legal to implement by requiring that
 262             // either the type or trait is local. Multidispatch means
 263             // this isn't perfect (that is, there are cases when
 264             // implementing a trait would be legal but is rejected
 265             // here).
 266             (type_is_local || ast_util::is_local(info.def_id))
 267                 && trait_item(tcx, info.def_id, item_name).is_some()
 268         })
 269         .collect::<Vec<_>>();
 270
 271     if !candidates.is_empty() {
 272         // sort from most relevant to least relevant
 273         candidates.sort_by(|a, b| a.cmp(b).reverse());
 274         candidates.dedup();
 275
 276         // FIXME #21673 this help message could be tuned to the case
 277         // of a type parameter: suggest adding a trait bound rather
 278         // than implementing.
 279         let msg = format!(
 280             "items from traits can only be used if the trait is implemented and in scope; \
 281              the following {traits_define} an item `{name}`, \
 282              perhaps you need to implement {one_of_them}:",
 283             traits_define = if candidates.len() == 1 {"trait defines"} else {"traits define"},
 284             one_of_them = if candidates.len() == 1 {"it"} else {"one of them"},
 285             name = item_name);
 286
 287         fcx.sess().fileline_help(span, &msg[..]);
 288
 289         for (i, trait_info) in candidates.iter().enumerate() {
 290             fcx.sess().fileline_help(span,
 291                                      &*format!("candidate #{}: `{}`",
 292                                                i + 1,
 293                                                fcx.tcx().item_path_str(trait_info.def_id)))
 294         }
 295     }
 296 }
 297
 298 /// Checks whether there is a local type somewhere in the chain of
 299 /// autoderefs of `rcvr_ty`.
 300 fn type_derefs_to_local<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 301                                   span: Span,
 302                                   rcvr_ty: Ty<'tcx>,
 303                                   rcvr_expr: Option<&ast::Expr>) -> bool {
 304     fn is_local(ty: Ty) -> bool {
 305         match ty.sty {
 306             ty::TyEnum(did, _) | ty::TyStruct(did, _) => ast_util::is_local(did),
 307
 308             ty::TyTrait(ref tr) => ast_util::is_local(tr.principal_def_id()),
 309
 310             ty::TyParam(_) => true,
 311
 312             // everything else (primitive types etc.) is effectively
 313             // non-local (there are "edge" cases, e.g. (LocalType,), but
 314             // the noise from these sort of types is usually just really
 315             // annoying, rather than any sort of help).
 316             _ => false
 317         }
 318     }
 319
 320     // This occurs for UFCS desugaring of `T::method`, where there is no
 321     // receiver expression for the method call, and thus no autoderef.
 322     if rcvr_expr.is_none() {
 323         return is_local(fcx.resolve_type_vars_if_possible(rcvr_ty));
 324     }
 325
 326     check::autoderef(fcx, span, rcvr_ty, None,
 327                      check::UnresolvedTypeAction::Ignore, check::NoPreference,
 328                      |ty, _| {
 329         if is_local(ty) {
 330             Some(())
 331         } else {
 332             None
 333         }
 334     }).2.is_some()
 335 }
 336
 337 #[derive(Copy, Clone)]
 338 pub struct TraitInfo {
 339     pub def_id: ast::DefId,
 340 }
 341
 342 impl TraitInfo {
 343     fn new(def_id: ast::DefId) -> TraitInfo {
 344         TraitInfo {
 345             def_id: def_id,
 346         }
 347     }
 348 }
 349 impl PartialEq for TraitInfo {
 350     fn eq(&self, other: &TraitInfo) -> bool {
 351         self.cmp(other) == Ordering::Equal
 352     }
 353 }
 354 impl Eq for TraitInfo {}
 355 impl PartialOrd for TraitInfo {
 356     fn partial_cmp(&self, other: &TraitInfo) -> Option<Ordering> { Some(self.cmp(other)) }
 357 }
 358 impl Ord for TraitInfo {
 359     fn cmp(&self, other: &TraitInfo) -> Ordering {
 360         // accessible traits are more important/relevant than
 361         // inaccessible ones, local crates are more important than
 362         // remote ones (local: cnum == 0), and NodeIds just for
 363         // totality.
 364
 365         let lhs = (other.def_id.krate, other.def_id.node);
 366         let rhs = (self.def_id.krate, self.def_id.node);
 367         lhs.cmp(&rhs)
 368     }
 369 }
 370
 371 /// Retrieve all traits in this crate and any dependent crates.
 372 pub fn all_traits<'a>(ccx: &'a CrateCtxt) -> AllTraits<'a> {
 373     if ccx.all_traits.borrow().is_none() {
 374         use syntax::visit;
 375
 376         let mut traits = vec![];
 377
 378         // Crate-local:
 379         //
 380         // meh.
 381         struct Visitor<'a> {
 382             traits: &'a mut AllTraitsVec,
 383         }
 384         impl<'v, 'a> visit::Visitor<'v> for Visitor<'a> {
 385             fn visit_item(&mut self, i: &'v ast::Item) {
 386                 match i.node {
 387                     ast::ItemTrait(..) => {
 388                         self.traits.push(TraitInfo::new(ast_util::local_def(i.id)));
 389                     }
 390                     _ => {}
 391                 }
 392                 visit::walk_item(self, i)
 393             }
 394         }
 395         visit::walk_crate(&mut Visitor {
 396             traits: &mut traits
 397         }, ccx.tcx.map.krate());
 398
 399         // Cross-crate:
 400         fn handle_external_def(traits: &mut AllTraitsVec,
 401                                ccx: &CrateCtxt,
 402                                cstore: &cstore::CStore,
 403                                dl: decoder::DefLike) {
 404             match dl {
 405                 decoder::DlDef(def::DefTrait(did)) => {
 406                     traits.push(TraitInfo::new(did));
 407                 }
 408                 decoder::DlDef(def::DefMod(did)) => {
 409                     csearch::each_child_of_item(cstore, did, |dl, _, _| {
 410                         handle_external_def(traits, ccx, cstore, dl)
 411                     })
 412                 }
 413                 _ => {}
 414             }
 415         }
 416         let cstore = &ccx.tcx.sess.cstore;
 417         cstore.iter_crate_data(|cnum, _| {
 418             csearch::each_top_level_item_of_crate(cstore, cnum, |dl, _, _| {
 419                 handle_external_def(&mut traits, ccx, cstore, dl)
 420             })
 421         });
 422
 423         *ccx.all_traits.borrow_mut() = Some(traits);
 424     }
 425
 426     let borrow = ccx.all_traits.borrow();
 427     assert!(borrow.is_some());
 428     AllTraits {
 429         borrow: borrow,
 430         idx: 0
 431     }
 432 }
 433
 434 pub struct AllTraits<'a> {
 435     borrow: cell::Ref<'a, Option<AllTraitsVec>>,
 436     idx: usize
 437 }
 438
 439 impl<'a> Iterator for AllTraits<'a> {
 440     type Item = TraitInfo;
 441
 442     fn next(&mut self) -> Option<TraitInfo> {
 443         let AllTraits { ref borrow, ref mut idx } = *self;
 444         // ugh.
 445         borrow.as_ref().unwrap().get(*idx).map(|info| {
 446             *idx += 1;
 447             *info
 448         })
 449     }
 450 }