compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

   1 use super::TypeErrCtxt;
   2 use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
   3 use rustc_errors::{pluralize, Diagnostic, MultiSpan};
   4 use rustc_hir::{self as hir, def::DefKind};
   5 use rustc_middle::traits::ObligationCauseCode;
   6 use rustc_middle::ty::error::ExpectedFound;
   7 use rustc_middle::ty::print::Printer;
   8 use rustc_middle::{
   9     traits::ObligationCause,
  10     ty::{self, error::TypeError, print::FmtPrinter, suggest_constraining_type_param, Ty},
  11 };
  12 use rustc_span::{def_id::DefId, sym, BytePos, Span, Symbol};
  13
  14 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
  15     pub fn note_and_explain_type_err(
  16         &self,
  17         diag: &mut Diagnostic,
  18         err: TypeError<'tcx>,
  19         cause: &ObligationCause<'tcx>,
  20         sp: Span,
  21         body_owner_def_id: DefId,
  22     ) {
  23         use ty::error::TypeError::*;
  24         debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
  25
  26         let tcx = self.tcx;
  27
  28         match err {
  29             ArgumentSorts(values, _) | Sorts(values) => {
  30                 match (values.expected.kind(), values.found.kind()) {
  31                     (ty::Closure(..), ty::Closure(..)) => {
  32                         diag.note("no two closures, even if identical, have the same type");
  33                         diag.help("consider boxing your closure and/or using it as a trait object");
  34                     }
  35                     (ty::Alias(ty::Opaque, ..), ty::Alias(ty::Opaque, ..)) => {
  36                         // Issue #63167
  37                         diag.note("distinct uses of `impl Trait` result in different opaque types");
  38                     }
  39                     (ty::Float(_), ty::Infer(ty::IntVar(_)))
  40                         if let Ok(
  41                             // Issue #53280
  42                             snippet,
  43                         ) = tcx.sess.source_map().span_to_snippet(sp) =>
  44                     {
  45                         if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
  46                             diag.span_suggestion(
  47                                 sp,
  48                                 "use a float literal",
  49                                 format!("{}.0", snippet),
  50                                 MachineApplicable,
  51                             );
  52                         }
  53                     }
  54                     (ty::Param(expected), ty::Param(found)) => {
  55                         let generics = tcx.generics_of(body_owner_def_id);
  56                         let e_span = tcx.def_span(generics.type_param(expected, tcx).def_id);
  57                         if !sp.contains(e_span) {
  58                             diag.span_label(e_span, "expected type parameter");
  59                         }
  60                         let f_span = tcx.def_span(generics.type_param(found, tcx).def_id);
  61                         if !sp.contains(f_span) {
  62                             diag.span_label(f_span, "found type parameter");
  63                         }
  64                         diag.note(
  65                             "a type parameter was expected, but a different one was found; \
  66                              you might be missing a type parameter or trait bound",
  67                         );
  68                         diag.note(
  69                             "for more information, visit \
  70                              https://doc.rust-lang.org/book/ch10-02-traits.html\
  71                              #traits-as-parameters",
  72                         );
  73                     }
  74                     (ty::Alias(ty::Projection, _), ty::Alias(ty::Projection, _)) => {
  75                         diag.note("an associated type was expected, but a different one was found");
  76                     }
  77                     (ty::Param(p), ty::Alias(ty::Projection, proj)) | (ty::Alias(ty::Projection, proj), ty::Param(p))
  78                         if tcx.def_kind(proj.def_id) != DefKind::ImplTraitPlaceholder =>
  79                     {
  80                         let p_def_id = tcx
  81                             .generics_of(body_owner_def_id)
  82                             .type_param(p, tcx)
  83                             .def_id;
  84                         let p_span = tcx.def_span(p_def_id);
  85                         if !sp.contains(p_span) {
  86                             diag.span_label(p_span, "this type parameter");
  87                         }
  88                         let hir = tcx.hir();
  89                         let mut note = true;
  90                         let parent = p_def_id
  91                             .as_local()
  92                             .and_then(|id| {
  93                                 let local_id = hir.local_def_id_to_hir_id(id);
  94                                 let generics = tcx.hir().find_parent(local_id)?.generics()?;
  95                                 Some((id, generics))
  96                             });
  97                         if let Some((local_id, generics)) = parent
  98                         {
  99                             // Synthesize the associated type restriction `Add<Output = Expected>`.
 100                             // FIXME: extract this logic for use in other diagnostics.
 101                             let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(tcx);
 102                             let item_name = tcx.item_name(proj.def_id);
 103                             let item_args = self.format_generic_args(assoc_substs);
 104
 105                             // Here, we try to see if there's an existing
 106                             // trait implementation that matches the one that
 107                             // we're suggesting to restrict. If so, find the
 108                             // "end", whether it be at the end of the trait
 109                             // or the end of the generic arguments.
 110                             let mut matching_span = None;
 111                             let mut matched_end_of_args = false;
 112                             for bound in generics.bounds_for_param(local_id) {
 113                                 let potential_spans = bound
 114                                     .bounds
 115                                     .iter()
 116                                     .find_map(|bound| {
 117                                         let bound_trait_path = bound.trait_ref()?.path;
 118                                         let def_id = bound_trait_path.res.opt_def_id()?;
 119                                         let generic_args = bound_trait_path.segments.iter().last().map(|path| path.args());
 120                                         (def_id == trait_ref.def_id).then_some((bound_trait_path.span, generic_args))
 121                                     });
 122
 123                                 if let Some((end_of_trait, end_of_args)) = potential_spans {
 124                                     let args_span = end_of_args.and_then(|args| args.span());
 125                                     matched_end_of_args = args_span.is_some();
 126                                     matching_span = args_span
 127                                         .or_else(|| Some(end_of_trait))
 128                                         .map(|span| span.shrink_to_hi());
 129                                     break;
 130                                 }
 131                             }
 132
 133                             if matched_end_of_args {
 134                                 // Append suggestion to the end of our args
 135                                 let path = format!(", {}{} = {}",item_name, item_args, p);
 136                                 note = !suggest_constraining_type_param(
 137                                     tcx,
 138                                     generics,
 139                                     diag,
 140                                     &format!("{}", proj.self_ty()),
 141                                     &path,
 142                                     None,
 143                                     matching_span,
 144                                 );
 145                             } else {
 146                                 // Suggest adding a bound to an existing trait
 147                                 // or if the trait doesn't exist, add the trait
 148                                 // and the suggested bounds.
 149                                 let path = format!("<{}{} = {}>", item_name, item_args, p);
 150                                 note = !suggest_constraining_type_param(
 151                                     tcx,
 152                                     generics,
 153                                     diag,
 154                                     &format!("{}", proj.self_ty()),
 155                                     &path,
 156                                     None,
 157                                     matching_span,
 158                                 );
 159                             }
 160                         }
 161                         if note {
 162                             diag.note("you might be missing a type parameter or trait bound");
 163                         }
 164                     }
 165                     (ty::Param(p), ty::Dynamic(..) | ty::Alias(ty::Opaque, ..))
 166                     | (ty::Dynamic(..) | ty::Alias(ty::Opaque, ..), ty::Param(p)) => {
 167                         let generics = tcx.generics_of(body_owner_def_id);
 168                         let p_span = tcx.def_span(generics.type_param(p, tcx).def_id);
 169                         if !sp.contains(p_span) {
 170                             diag.span_label(p_span, "this type parameter");
 171                         }
 172                         diag.help("type parameters must be constrained to match other types");
 173                         if tcx.sess.teach(&diag.get_code().unwrap()) {
 174                             diag.help(
 175                                 "given a type parameter `T` and a method `foo`:
 176 ```
 177 trait Trait<T> { fn foo(&self) -> T; }
 178 ```
 179 the only ways to implement method `foo` are:
 180 - constrain `T` with an explicit type:
 181 ```
 182 impl Trait<String> for X {
 183     fn foo(&self) -> String { String::new() }
 184 }
 185 ```
 186 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 187 ```
 188 impl<T: std::default::Default> Trait<T> for X {
 189     fn foo(&self) -> T { <T as std::default::Default>::default() }
 190 }
 191 ```
 192 - change `foo` to return an argument of type `T`:
 193 ```
 194 impl<T> Trait<T> for X {
 195     fn foo(&self, x: T) -> T { x }
 196 }
 197 ```",
 198                             );
 199                         }
 200                         diag.note(
 201                             "for more information, visit \
 202                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 203                              #traits-as-parameters",
 204                         );
 205                     }
 206                     (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
 207                         let generics = tcx.generics_of(body_owner_def_id);
 208                         let p_span = tcx.def_span(generics.type_param(p, tcx).def_id);
 209                         if !sp.contains(p_span) {
 210                             diag.span_label(p_span, "this type parameter");
 211                         }
 212                         diag.help(&format!(
 213                             "every closure has a distinct type and so could not always match the \
 214                              caller-chosen type of parameter `{}`",
 215                             p
 216                         ));
 217                     }
 218                     (ty::Param(p), _) | (_, ty::Param(p)) => {
 219                         let generics = tcx.generics_of(body_owner_def_id);
 220                         let p_span = tcx.def_span(generics.type_param(p, tcx).def_id);
 221                         if !sp.contains(p_span) {
 222                             diag.span_label(p_span, "this type parameter");
 223                         }
 224                     }
 225                     (ty::Alias(ty::Projection, proj_ty), _) if tcx.def_kind(proj_ty.def_id) != DefKind::ImplTraitPlaceholder => {
 226                         self.expected_projection(
 227                             diag,
 228                             proj_ty,
 229                             values,
 230                             body_owner_def_id,
 231                             cause.code(),
 232                         );
 233                     }
 234                     (_, ty::Alias(ty::Projection, proj_ty)) if tcx.def_kind(proj_ty.def_id) != DefKind::ImplTraitPlaceholder => {
 235                         let msg = format!(
 236                             "consider constraining the associated type `{}` to `{}`",
 237                             values.found, values.expected,
 238                         );
 239                         if !(self.suggest_constraining_opaque_associated_type(
 240                             diag,
 241                             &msg,
 242                             proj_ty,
 243                             values.expected,
 244                         ) || self.suggest_constraint(
 245                             diag,
 246                             &msg,
 247                             body_owner_def_id,
 248                             proj_ty,
 249                             values.expected,
 250                         )) {
 251                             diag.help(&msg);
 252                             diag.note(
 253                                 "for more information, visit \
 254                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 255                             );
 256                         }
 257                     }
 258                     (ty::FnPtr(_), ty::FnDef(def, _))
 259                     if let hir::def::DefKind::Fn = tcx.def_kind(def) => {
 260                         diag.note(
 261                             "when the arguments and return types match, functions can be coerced \
 262                              to function pointers",
 263                         );
 264                     }
 265                     _ => {}
 266                 }
 267                 debug!(
 268                     "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
 269                     values.expected,
 270                     values.expected.kind(),
 271                     values.found,
 272                     values.found.kind(),
 273                 );
 274             }
 275             CyclicTy(ty) => {
 276                 // Watch out for various cases of cyclic types and try to explain.
 277                 if ty.is_closure() || ty.is_generator() {
 278                     diag.note(
 279                         "closures cannot capture themselves or take themselves as argument;\n\
 280                          this error may be the result of a recent compiler bug-fix,\n\
 281                          see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
 282                          for more information",
 283                     );
 284                 }
 285             }
 286             TargetFeatureCast(def_id) => {
 287                 let target_spans = tcx.get_attrs(def_id, sym::target_feature).map(|attr| attr.span);
 288                 diag.note(
 289                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 290                 );
 291                 diag.span_labels(target_spans, "`#[target_feature]` added here");
 292             }
 293             _ => {}
 294         }
 295     }
 296
 297     fn suggest_constraint(
 298         &self,
 299         diag: &mut Diagnostic,
 300         msg: &str,
 301         body_owner_def_id: DefId,
 302         proj_ty: &ty::AliasTy<'tcx>,
 303         ty: Ty<'tcx>,
 304     ) -> bool {
 305         let tcx = self.tcx;
 306         let assoc = tcx.associated_item(proj_ty.def_id);
 307         let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(tcx);
 308         if let Some(item) = tcx.hir().get_if_local(body_owner_def_id) {
 309             if let Some(hir_generics) = item.generics() {
 310                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 311                 // This will also work for `impl Trait`.
 312                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
 313                     let generics = tcx.generics_of(body_owner_def_id);
 314                     generics.type_param(param_ty, tcx).def_id
 315                 } else {
 316                     return false;
 317                 };
 318                 let Some(def_id) = def_id.as_local() else {
 319                     return false;
 320                 };
 321
 322                 // First look in the `where` clause, as this might be
 323                 // `fn foo<T>(x: T) where T: Trait`.
 324                 for pred in hir_generics.bounds_for_param(def_id) {
 325                     if self.constrain_generic_bound_associated_type_structured_suggestion(
 326                         diag,
 327                         &trait_ref,
 328                         pred.bounds,
 329                         &assoc,
 330                         assoc_substs,
 331                         ty,
 332                         msg,
 333                         false,
 334                     ) {
 335                         return true;
 336                     }
 337                 }
 338             }
 339         }
 340         false
 341     }
 342
 343     /// An associated type was expected and a different type was found.
 344     ///
 345     /// We perform a few different checks to see what we can suggest:
 346     ///
 347     ///  - In the current item, look for associated functions that return the expected type and
 348     ///    suggest calling them. (Not a structured suggestion.)
 349     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 350     ///    associated type to the found type.
 351     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 352     ///    mention that this is disallowed.
 353     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 354     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 355     ///    fn that returns the type.
 356     fn expected_projection(
 357         &self,
 358         diag: &mut Diagnostic,
 359         proj_ty: &ty::AliasTy<'tcx>,
 360         values: ExpectedFound<Ty<'tcx>>,
 361         body_owner_def_id: DefId,
 362         cause_code: &ObligationCauseCode<'_>,
 363     ) {
 364         let tcx = self.tcx;
 365
 366         let msg = format!(
 367             "consider constraining the associated type `{}` to `{}`",
 368             values.expected, values.found
 369         );
 370         let body_owner = tcx.hir().get_if_local(body_owner_def_id);
 371         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 372
 373         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 374         let callable_scope = matches!(
 375             body_owner,
 376             Some(
 377                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 378                     | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 379                     | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 380             )
 381         );
 382         let impl_comparison =
 383             matches!(cause_code, ObligationCauseCode::CompareImplItemObligation { .. });
 384         let assoc = tcx.associated_item(proj_ty.def_id);
 385         if !callable_scope || impl_comparison {
 386             // We do not want to suggest calling functions when the reason of the
 387             // type error is a comparison of an `impl` with its `trait` or when the
 388             // scope is outside of a `Body`.
 389         } else {
 390             // If we find a suitable associated function that returns the expected type, we don't
 391             // want the more general suggestion later in this method about "consider constraining
 392             // the associated type or calling a method that returns the associated type".
 393             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 394                 diag,
 395                 assoc.container_id(tcx),
 396                 current_method_ident,
 397                 proj_ty.def_id,
 398                 values.expected,
 399             );
 400             // Possibly suggest constraining the associated type to conform to the
 401             // found type.
 402             if self.suggest_constraint(diag, &msg, body_owner_def_id, proj_ty, values.found)
 403                 || point_at_assoc_fn
 404             {
 405                 return;
 406             }
 407         }
 408
 409         self.suggest_constraining_opaque_associated_type(diag, &msg, proj_ty, values.found);
 410
 411         if self.point_at_associated_type(diag, body_owner_def_id, values.found) {
 412             return;
 413         }
 414
 415         if !impl_comparison {
 416             // Generic suggestion when we can't be more specific.
 417             if callable_scope {
 418                 diag.help(&format!(
 419                     "{} or calling a method that returns `{}`",
 420                     msg, values.expected
 421                 ));
 422             } else {
 423                 diag.help(&msg);
 424             }
 425             diag.note(
 426                 "for more information, visit \
 427                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 428             );
 429         }
 430         if tcx.sess.teach(&diag.get_code().unwrap()) {
 431             diag.help(
 432                 "given an associated type `T` and a method `foo`:
 433 ```
 434 trait Trait {
 435 type T;
 436 fn foo(&self) -> Self::T;
 437 }
 438 ```
 439 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 440 ```
 441 impl Trait for X {
 442 type T = String;
 443 fn foo(&self) -> Self::T { String::new() }
 444 }
 445 ```",
 446             );
 447         }
 448     }
 449
 450     /// When the expected `impl Trait` is not defined in the current item, it will come from
 451     /// a return type. This can occur when dealing with `TryStream` (#71035).
 452     fn suggest_constraining_opaque_associated_type(
 453         &self,
 454         diag: &mut Diagnostic,
 455         msg: &str,
 456         proj_ty: &ty::AliasTy<'tcx>,
 457         ty: Ty<'tcx>,
 458     ) -> bool {
 459         let tcx = self.tcx;
 460
 461         let assoc = tcx.associated_item(proj_ty.def_id);
 462         if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = *proj_ty.self_ty().kind() {
 463             let opaque_local_def_id = def_id.as_local();
 464             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
 465                 match &tcx.hir().expect_item(opaque_local_def_id).kind {
 466                     hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
 467                     _ => bug!("The HirId comes from a `ty::Opaque`"),
 468                 }
 469             } else {
 470                 return false;
 471             };
 472
 473             let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(tcx);
 474
 475             self.constrain_generic_bound_associated_type_structured_suggestion(
 476                 diag,
 477                 &trait_ref,
 478                 opaque_hir_ty.bounds,
 479                 assoc,
 480                 assoc_substs,
 481                 ty,
 482                 msg,
 483                 true,
 484             )
 485         } else {
 486             false
 487         }
 488     }
 489
 490     fn point_at_methods_that_satisfy_associated_type(
 491         &self,
 492         diag: &mut Diagnostic,
 493         assoc_container_id: DefId,
 494         current_method_ident: Option<Symbol>,
 495         proj_ty_item_def_id: DefId,
 496         expected: Ty<'tcx>,
 497     ) -> bool {
 498         let tcx = self.tcx;
 499
 500         let items = tcx.associated_items(assoc_container_id);
 501         // Find all the methods in the trait that could be called to construct the
 502         // expected associated type.
 503         // FIXME: consider suggesting the use of associated `const`s.
 504         let methods: Vec<(Span, String)> = items
 505             .in_definition_order()
 506             .filter(|item| {
 507                 ty::AssocKind::Fn == item.kind && Some(item.name) != current_method_ident
 508             })
 509             .filter_map(|item| {
 510                 let method = tcx.fn_sig(item.def_id).subst_identity();
 511                 match *method.output().skip_binder().kind() {
 512                     ty::Alias(ty::Projection, ty::AliasTy { def_id: item_def_id, .. })
 513                         if item_def_id == proj_ty_item_def_id =>
 514                     {
 515                         Some((
 516                             tcx.def_span(item.def_id),
 517                             format!("consider calling `{}`", tcx.def_path_str(item.def_id)),
 518                         ))
 519                     }
 520                     _ => None,
 521                 }
 522             })
 523             .collect();
 524         if !methods.is_empty() {
 525             // Use a single `help:` to show all the methods in the trait that can
 526             // be used to construct the expected associated type.
 527             let mut span: MultiSpan =
 528                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 529             let msg = format!(
 530                 "{some} method{s} {are} available that return{r} `{ty}`",
 531                 some = if methods.len() == 1 { "a" } else { "some" },
 532                 s = pluralize!(methods.len()),
 533                 are = pluralize!("is", methods.len()),
 534                 r = if methods.len() == 1 { "s" } else { "" },
 535                 ty = expected
 536             );
 537             for (sp, label) in methods.into_iter() {
 538                 span.push_span_label(sp, label);
 539             }
 540             diag.span_help(span, &msg);
 541             return true;
 542         }
 543         false
 544     }
 545
 546     fn point_at_associated_type(
 547         &self,
 548         diag: &mut Diagnostic,
 549         body_owner_def_id: DefId,
 550         found: Ty<'tcx>,
 551     ) -> bool {
 552         let tcx = self.tcx;
 553
 554         let Some(hir_id) = body_owner_def_id.as_local() else {
 555             return false;
 556         };
 557         let hir_id = tcx.hir().local_def_id_to_hir_id(hir_id);
 558         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 559         // `expected` and point at it.
 560         let parent_id = tcx.hir().get_parent_item(hir_id);
 561         let item = tcx.hir().find_by_def_id(parent_id.def_id);
 562
 563         debug!("expected_projection parent item {:?}", item);
 564
 565         let param_env = tcx.param_env(body_owner_def_id);
 566
 567         match item {
 568             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 569                 // FIXME: account for `#![feature(specialization)]`
 570                 for item in &items[..] {
 571                     match item.kind {
 572                         hir::AssocItemKind::Type => {
 573                             // FIXME: account for returning some type in a trait fn impl that has
 574                             // an assoc type as a return type (#72076).
 575                             if let hir::Defaultness::Default { has_value: true } =
 576                                 tcx.impl_defaultness(item.id.owner_id)
 577                             {
 578                                 let assoc_ty = tcx.bound_type_of(item.id.owner_id).subst_identity();
 579                                 if self.infcx.can_eq(param_env, assoc_ty, found).is_ok() {
 580                                     diag.span_label(
 581                                         item.span,
 582                                         "associated type defaults can't be assumed inside the \
 583                                             trait defining them",
 584                                     );
 585                                     return true;
 586                                 }
 587                             }
 588                         }
 589                         _ => {}
 590                     }
 591                 }
 592             }
 593             Some(hir::Node::Item(hir::Item {
 594                 kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
 595                 ..
 596             })) => {
 597                 for item in &items[..] {
 598                     if let hir::AssocItemKind::Type = item.kind {
 599                         let assoc_ty = tcx.bound_type_of(item.id.owner_id).subst_identity();
 600
 601                         if self.infcx.can_eq(param_env, assoc_ty, found).is_ok() {
 602                             diag.span_label(item.span, "expected this associated type");
 603                             return true;
 604                         }
 605                     }
 606                 }
 607             }
 608             _ => {}
 609         }
 610         false
 611     }
 612
 613     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 614     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
 615     ///
 616     /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
 617     /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
 618     /// trait bound as the one we're looking for. This can help in cases where the associated
 619     /// type is defined on a supertrait of the one present in the bounds.
 620     fn constrain_generic_bound_associated_type_structured_suggestion(
 621         &self,
 622         diag: &mut Diagnostic,
 623         trait_ref: &ty::TraitRef<'tcx>,
 624         bounds: hir::GenericBounds<'_>,
 625         assoc: &ty::AssocItem,
 626         assoc_substs: &[ty::GenericArg<'tcx>],
 627         ty: Ty<'tcx>,
 628         msg: &str,
 629         is_bound_surely_present: bool,
 630     ) -> bool {
 631         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 632
 633         let trait_bounds = bounds.iter().filter_map(|bound| match bound {
 634             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
 635             _ => None,
 636         });
 637
 638         let matching_trait_bounds = trait_bounds
 639             .clone()
 640             .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
 641             .collect::<Vec<_>>();
 642
 643         let span = match &matching_trait_bounds[..] {
 644             &[ptr] => ptr.span,
 645             &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
 646                 &[ptr] => ptr.span,
 647                 _ => return false,
 648             },
 649             _ => return false,
 650         };
 651
 652         self.constrain_associated_type_structured_suggestion(
 653             diag,
 654             span,
 655             assoc,
 656             assoc_substs,
 657             ty,
 658             msg,
 659         )
 660     }
 661
 662     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 663     /// associated type to a given type `ty`.
 664     fn constrain_associated_type_structured_suggestion(
 665         &self,
 666         diag: &mut Diagnostic,
 667         span: Span,
 668         assoc: &ty::AssocItem,
 669         assoc_substs: &[ty::GenericArg<'tcx>],
 670         ty: Ty<'tcx>,
 671         msg: &str,
 672     ) -> bool {
 673         let tcx = self.tcx;
 674
 675         if let Ok(has_params) =
 676             tcx.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 677         {
 678             let (span, sugg) = if has_params {
 679                 let pos = span.hi() - BytePos(1);
 680                 let span = Span::new(pos, pos, span.ctxt(), span.parent());
 681                 (span, format!(", {} = {}", assoc.ident(tcx), ty))
 682             } else {
 683                 let item_args = self.format_generic_args(assoc_substs);
 684                 (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(tcx), item_args, ty))
 685             };
 686             diag.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 687             return true;
 688         }
 689         false
 690     }
 691
 692     pub fn format_generic_args(&self, args: &[ty::GenericArg<'tcx>]) -> String {
 693         FmtPrinter::new(self.tcx, hir::def::Namespace::TypeNS)
 694             .path_generic_args(Ok, args)
 695             .expect("could not write to `String`.")
 696             .into_buffer()
 697     }
 698 }