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