src/librustc_middle/ty/error.rs

   1 use crate::traits::{ObligationCause, ObligationCauseCode};
   2 use crate::ty::diagnostics::suggest_constraining_type_param;
   3 use crate::ty::{self, BoundRegion, Region, Ty, TyCtxt};
   4 use rustc_ast::ast;
   5 use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
   6 use rustc_errors::{pluralize, DiagnosticBuilder};
   7 use rustc_hir as hir;
   8 use rustc_hir::def_id::DefId;
   9 use rustc_span::symbol::{sym, Symbol};
  10 use rustc_span::{BytePos, MultiSpan, Span};
  11 use rustc_target::spec::abi;
  12
  13 use std::borrow::Cow;
  14 use std::fmt;
  15 use std::ops::Deref;
  16
  17 #[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable)]
  18 pub struct ExpectedFound<T> {
  19     pub expected: T,
  20     pub found: T,
  21 }
  22
  23 impl<T> ExpectedFound<T> {
  24     pub fn new(a_is_expected: bool, a: T, b: T) -> Self {
  25         if a_is_expected {
  26             ExpectedFound { expected: a, found: b }
  27         } else {
  28             ExpectedFound { expected: b, found: a }
  29         }
  30     }
  31 }
  32
  33 // Data structures used in type unification
  34 #[derive(Clone, Debug, TypeFoldable)]
  35 pub enum TypeError<'tcx> {
  36     Mismatch,
  37     UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
  38     AbiMismatch(ExpectedFound<abi::Abi>),
  39     Mutability,
  40     TupleSize(ExpectedFound<usize>),
  41     FixedArraySize(ExpectedFound<u64>),
  42     ArgCount,
  43
  44     RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
  45     RegionsInsufficientlyPolymorphic(BoundRegion, Region<'tcx>),
  46     RegionsOverlyPolymorphic(BoundRegion, Region<'tcx>),
  47     RegionsPlaceholderMismatch,
  48
  49     Sorts(ExpectedFound<Ty<'tcx>>),
  50     IntMismatch(ExpectedFound<ty::IntVarValue>),
  51     FloatMismatch(ExpectedFound<ast::FloatTy>),
  52     Traits(ExpectedFound<DefId>),
  53     VariadicMismatch(ExpectedFound<bool>),
  54
  55     /// Instantiating a type variable with the given type would have
  56     /// created a cycle (because it appears somewhere within that
  57     /// type).
  58     CyclicTy(Ty<'tcx>),
  59     ProjectionMismatched(ExpectedFound<DefId>),
  60     ExistentialMismatch(ExpectedFound<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>),
  61     ObjectUnsafeCoercion(DefId),
  62     ConstMismatch(ExpectedFound<&'tcx ty::Const<'tcx>>),
  63
  64     IntrinsicCast,
  65     /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
  66     TargetFeatureCast(DefId),
  67 }
  68
  69 pub enum UnconstrainedNumeric {
  70     UnconstrainedFloat,
  71     UnconstrainedInt,
  72     Neither,
  73 }
  74
  75 /// Explains the source of a type err in a short, human readable way. This is meant to be placed
  76 /// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
  77 /// afterwards to present additional details, particularly when it comes to lifetime-related
  78 /// errors.
  79 impl<'tcx> fmt::Display for TypeError<'tcx> {
  80     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  81         use self::TypeError::*;
  82         fn report_maybe_different(
  83             f: &mut fmt::Formatter<'_>,
  84             expected: &str,
  85             found: &str,
  86         ) -> fmt::Result {
  87             // A naive approach to making sure that we're not reporting silly errors such as:
  88             // (expected closure, found closure).
  89             if expected == found {
  90                 write!(f, "expected {}, found a different {}", expected, found)
  91             } else {
  92                 write!(f, "expected {}, found {}", expected, found)
  93             }
  94         }
  95
  96         let br_string = |br: ty::BoundRegion| match br {
  97             ty::BrNamed(_, name) => format!(" {}", name),
  98             _ => String::new(),
  99         };
 100
 101         match *self {
 102             CyclicTy(_) => write!(f, "cyclic type of infinite size"),
 103             Mismatch => write!(f, "types differ"),
 104             UnsafetyMismatch(values) => {
 105                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 106             }
 107             AbiMismatch(values) => {
 108                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 109             }
 110             Mutability => write!(f, "types differ in mutability"),
 111             TupleSize(values) => write!(
 112                 f,
 113                 "expected a tuple with {} element{}, \
 114                            found one with {} element{}",
 115                 values.expected,
 116                 pluralize!(values.expected),
 117                 values.found,
 118                 pluralize!(values.found)
 119             ),
 120             FixedArraySize(values) => write!(
 121                 f,
 122                 "expected an array with a fixed size of {} element{}, \
 123                            found one with {} element{}",
 124                 values.expected,
 125                 pluralize!(values.expected),
 126                 values.found,
 127                 pluralize!(values.found)
 128             ),
 129             ArgCount => write!(f, "incorrect number of function parameters"),
 130             RegionsDoesNotOutlive(..) => write!(f, "lifetime mismatch"),
 131             RegionsInsufficientlyPolymorphic(br, _) => write!(
 132                 f,
 133                 "expected bound lifetime parameter{}, found concrete lifetime",
 134                 br_string(br)
 135             ),
 136             RegionsOverlyPolymorphic(br, _) => write!(
 137                 f,
 138                 "expected concrete lifetime, found bound lifetime parameter{}",
 139                 br_string(br)
 140             ),
 141             RegionsPlaceholderMismatch => write!(f, "one type is more general than the other"),
 142             Sorts(values) => ty::tls::with(|tcx| {
 143                 report_maybe_different(
 144                     f,
 145                     &values.expected.sort_string(tcx),
 146                     &values.found.sort_string(tcx),
 147                 )
 148             }),
 149             Traits(values) => ty::tls::with(|tcx| {
 150                 report_maybe_different(
 151                     f,
 152                     &format!("trait `{}`", tcx.def_path_str(values.expected)),
 153                     &format!("trait `{}`", tcx.def_path_str(values.found)),
 154                 )
 155             }),
 156             IntMismatch(ref values) => {
 157                 write!(f, "expected `{:?}`, found `{:?}`", values.expected, values.found)
 158             }
 159             FloatMismatch(ref values) => {
 160                 write!(f, "expected `{:?}`, found `{:?}`", values.expected, values.found)
 161             }
 162             VariadicMismatch(ref values) => write!(
 163                 f,
 164                 "expected {} fn, found {} function",
 165                 if values.expected { "variadic" } else { "non-variadic" },
 166                 if values.found { "variadic" } else { "non-variadic" }
 167             ),
 168             ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
 169                 write!(
 170                     f,
 171                     "expected {}, found {}",
 172                     tcx.def_path_str(values.expected),
 173                     tcx.def_path_str(values.found)
 174                 )
 175             }),
 176             ExistentialMismatch(ref values) => report_maybe_different(
 177                 f,
 178                 &format!("trait `{}`", values.expected),
 179                 &format!("trait `{}`", values.found),
 180             ),
 181             ConstMismatch(ref values) => {
 182                 write!(f, "expected `{}`, found `{}`", values.expected, values.found)
 183             }
 184             IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
 185             TargetFeatureCast(_) => write!(
 186                 f,
 187                 "cannot coerce functions with `#[target_feature]` to safe function pointers"
 188             ),
 189             ObjectUnsafeCoercion(_) => write!(f, "coercion to object-unsafe trait object"),
 190         }
 191     }
 192 }
 193
 194 impl<'tcx> TypeError<'tcx> {
 195     pub fn must_include_note(&self) -> bool {
 196         use self::TypeError::*;
 197         match self {
 198             CyclicTy(_) | UnsafetyMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
 199             | Sorts(_) | IntMismatch(_) | FloatMismatch(_) | VariadicMismatch(_)
 200             | TargetFeatureCast(_) => false,
 201
 202             Mutability
 203             | TupleSize(_)
 204             | ArgCount
 205             | RegionsDoesNotOutlive(..)
 206             | RegionsInsufficientlyPolymorphic(..)
 207             | RegionsOverlyPolymorphic(..)
 208             | RegionsPlaceholderMismatch
 209             | Traits(_)
 210             | ProjectionMismatched(_)
 211             | ExistentialMismatch(_)
 212             | ConstMismatch(_)
 213             | IntrinsicCast
 214             | ObjectUnsafeCoercion(_) => true,
 215         }
 216     }
 217 }
 218
 219 impl<'tcx> ty::TyS<'tcx> {
 220     pub fn sort_string(&self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 221         match self.kind {
 222             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
 223                 format!("`{}`", self).into()
 224             }
 225             ty::Tuple(ref tys) if tys.is_empty() => format!("`{}`", self).into(),
 226
 227             ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did)).into(),
 228             ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
 229             ty::Array(t, n) => {
 230                 let n = tcx.lift(&n).unwrap();
 231                 match n.try_eval_usize(tcx, ty::ParamEnv::empty()) {
 232                     _ if t.is_simple_ty() => format!("array `{}`", self).into(),
 233                     Some(n) => format!("array of {} element{} ", n, pluralize!(n)).into(),
 234                     None => "array".into(),
 235                 }
 236             }
 237             ty::Slice(ty) if ty.is_simple_ty() => format!("slice `{}`", self).into(),
 238             ty::Slice(_) => "slice".into(),
 239             ty::RawPtr(_) => "*-ptr".into(),
 240             ty::Ref(_, ty, mutbl) => {
 241                 let tymut = ty::TypeAndMut { ty, mutbl };
 242                 let tymut_string = tymut.to_string();
 243                 if tymut_string != "_"
 244                     && (ty.is_simple_text() || tymut_string.len() < "mutable reference".len())
 245                 {
 246                     format!("`&{}`", tymut_string).into()
 247                 } else {
 248                     // Unknown type name, it's long or has type arguments
 249                     match mutbl {
 250                         hir::Mutability::Mut => "mutable reference",
 251                         _ => "reference",
 252                     }
 253                     .into()
 254                 }
 255             }
 256             ty::FnDef(..) => "fn item".into(),
 257             ty::FnPtr(_) => "fn pointer".into(),
 258             ty::Dynamic(ref inner, ..) => {
 259                 if let Some(principal) = inner.principal() {
 260                     format!("trait object `dyn {}`", tcx.def_path_str(principal.def_id())).into()
 261                 } else {
 262                     "trait object".into()
 263                 }
 264             }
 265             ty::Closure(..) => "closure".into(),
 266             ty::Generator(..) => "generator".into(),
 267             ty::GeneratorWitness(..) => "generator witness".into(),
 268             ty::Tuple(..) => "tuple".into(),
 269             ty::Infer(ty::TyVar(_)) => "inferred type".into(),
 270             ty::Infer(ty::IntVar(_)) => "integer".into(),
 271             ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
 272             ty::Placeholder(..) => "placeholder type".into(),
 273             ty::Bound(..) => "bound type".into(),
 274             ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
 275             ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
 276             ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
 277             ty::Projection(_) => "associated type".into(),
 278             ty::Param(p) => format!("type parameter `{}`", p).into(),
 279             ty::Opaque(..) => "opaque type".into(),
 280             ty::Error(_) => "type error".into(),
 281         }
 282     }
 283
 284     pub fn prefix_string(&self) -> Cow<'static, str> {
 285         match self.kind {
 286             ty::Infer(_)
 287             | ty::Error(_)
 288             | ty::Bool
 289             | ty::Char
 290             | ty::Int(_)
 291             | ty::Uint(_)
 292             | ty::Float(_)
 293             | ty::Str
 294             | ty::Never => "type".into(),
 295             ty::Tuple(ref tys) if tys.is_empty() => "unit type".into(),
 296             ty::Adt(def, _) => def.descr().into(),
 297             ty::Foreign(_) => "extern type".into(),
 298             ty::Array(..) => "array".into(),
 299             ty::Slice(_) => "slice".into(),
 300             ty::RawPtr(_) => "raw pointer".into(),
 301             ty::Ref(.., mutbl) => match mutbl {
 302                 hir::Mutability::Mut => "mutable reference",
 303                 _ => "reference",
 304             }
 305             .into(),
 306             ty::FnDef(..) => "fn item".into(),
 307             ty::FnPtr(_) => "fn pointer".into(),
 308             ty::Dynamic(..) => "trait object".into(),
 309             ty::Closure(..) => "closure".into(),
 310             ty::Generator(..) => "generator".into(),
 311             ty::GeneratorWitness(..) => "generator witness".into(),
 312             ty::Tuple(..) => "tuple".into(),
 313             ty::Placeholder(..) => "higher-ranked type".into(),
 314             ty::Bound(..) => "bound type variable".into(),
 315             ty::Projection(_) => "associated type".into(),
 316             ty::Param(_) => "type parameter".into(),
 317             ty::Opaque(..) => "opaque type".into(),
 318         }
 319     }
 320 }
 321
 322 impl<'tcx> TyCtxt<'tcx> {
 323     pub fn note_and_explain_type_err(
 324         self,
 325         db: &mut DiagnosticBuilder<'_>,
 326         err: &TypeError<'tcx>,
 327         cause: &ObligationCause<'tcx>,
 328         sp: Span,
 329         body_owner_def_id: DefId,
 330     ) {
 331         use self::TypeError::*;
 332         debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
 333         match err {
 334             Sorts(values) => {
 335                 let expected_str = values.expected.sort_string(self);
 336                 let found_str = values.found.sort_string(self);
 337                 if expected_str == found_str && expected_str == "closure" {
 338                     db.note("no two closures, even if identical, have the same type");
 339                     db.help("consider boxing your closure and/or using it as a trait object");
 340                 }
 341                 if expected_str == found_str && expected_str == "opaque type" {
 342                     // Issue #63167
 343                     db.note("distinct uses of `impl Trait` result in different opaque types");
 344                     let e_str = values.expected.to_string();
 345                     let f_str = values.found.to_string();
 346                     if e_str == f_str && &e_str == "impl std::future::Future" {
 347                         // FIXME: use non-string based check.
 348                         db.help(
 349                             "if both `Future`s have the same `Output` type, consider \
 350                                  `.await`ing on both of them",
 351                         );
 352                     }
 353                 }
 354                 match (&values.expected.kind, &values.found.kind) {
 355                     (ty::Float(_), ty::Infer(ty::IntVar(_))) => {
 356                         if let Ok(
 357                             // Issue #53280
 358                             snippet,
 359                         ) = self.sess.source_map().span_to_snippet(sp)
 360                         {
 361                             if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
 362                                 db.span_suggestion(
 363                                     sp,
 364                                     "use a float literal",
 365                                     format!("{}.0", snippet),
 366                                     MachineApplicable,
 367                                 );
 368                             }
 369                         }
 370                     }
 371                     (ty::Param(expected), ty::Param(found)) => {
 372                         let generics = self.generics_of(body_owner_def_id);
 373                         let e_span = self.def_span(generics.type_param(expected, self).def_id);
 374                         if !sp.contains(e_span) {
 375                             db.span_label(e_span, "expected type parameter");
 376                         }
 377                         let f_span = self.def_span(generics.type_param(found, self).def_id);
 378                         if !sp.contains(f_span) {
 379                             db.span_label(f_span, "found type parameter");
 380                         }
 381                         db.note(
 382                             "a type parameter was expected, but a different one was found; \
 383                                  you might be missing a type parameter or trait bound",
 384                         );
 385                         db.note(
 386                             "for more information, visit \
 387                                  https://doc.rust-lang.org/book/ch10-02-traits.html\
 388                                  #traits-as-parameters",
 389                         );
 390                     }
 391                     (ty::Projection(_), ty::Projection(_)) => {
 392                         db.note("an associated type was expected, but a different one was found");
 393                     }
 394                     (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p)) => {
 395                         let generics = self.generics_of(body_owner_def_id);
 396                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 397                         if !sp.contains(p_span) {
 398                             db.span_label(p_span, "this type parameter");
 399                         }
 400                         let hir = self.hir();
 401                         let mut note = true;
 402                         if let Some(generics) = generics
 403                             .type_param(p, self)
 404                             .def_id
 405                             .as_local()
 406                             .map(|id| hir.as_local_hir_id(id))
 407                             .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
 408                             .as_ref()
 409                             .and_then(|node| node.generics())
 410                         {
 411                             // Synthesize the associated type restriction `Add<Output = Expected>`.
 412                             // FIXME: extract this logic for use in other diagnostics.
 413                             let trait_ref = proj.trait_ref(self);
 414                             let path =
 415                                 self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
 416                             let item_name = self.item_name(proj.item_def_id);
 417                             let path = if path.ends_with('>') {
 418                                 format!("{}, {} = {}>", &path[..path.len() - 1], item_name, p)
 419                             } else {
 420                                 format!("{}<{} = {}>", path, item_name, p)
 421                             };
 422                             note = !suggest_constraining_type_param(
 423                                 self,
 424                                 generics,
 425                                 db,
 426                                 &format!("{}", proj.self_ty()),
 427                                 &path,
 428                                 None,
 429                             );
 430                         }
 431                         if note {
 432                             db.note("you might be missing a type parameter or trait bound");
 433                         }
 434                     }
 435                     (ty::Param(p), ty::Dynamic(..) | ty::Opaque(..))
 436                     | (ty::Dynamic(..) | ty::Opaque(..), ty::Param(p)) => {
 437                         let generics = self.generics_of(body_owner_def_id);
 438                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 439                         if !sp.contains(p_span) {
 440                             db.span_label(p_span, "this type parameter");
 441                         }
 442                         db.help("type parameters must be constrained to match other types");
 443                         if self.sess.teach(&db.get_code().unwrap()) {
 444                             db.help(
 445                                 "given a type parameter `T` and a method `foo`:
 446 ```
 447 trait Trait<T> { fn foo(&self) -> T; }
 448 ```
 449 the only ways to implement method `foo` are:
 450 - constrain `T` with an explicit type:
 451 ```
 452 impl Trait<String> for X {
 453     fn foo(&self) -> String { String::new() }
 454 }
 455 ```
 456 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 457 ```
 458 impl<T: std::default::Default> Trait<T> for X {
 459     fn foo(&self) -> T { <T as std::default::Default>::default() }
 460 }
 461 ```
 462 - change `foo` to return an argument of type `T`:
 463 ```
 464 impl<T> Trait<T> for X {
 465     fn foo(&self, x: T) -> T { x }
 466 }
 467 ```",
 468                             );
 469                         }
 470                         db.note(
 471                             "for more information, visit \
 472                                  https://doc.rust-lang.org/book/ch10-02-traits.html\
 473                                  #traits-as-parameters",
 474                         );
 475                     }
 476                     (ty::Param(p), _) | (_, ty::Param(p)) => {
 477                         let generics = self.generics_of(body_owner_def_id);
 478                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 479                         if !sp.contains(p_span) {
 480                             db.span_label(p_span, "this type parameter");
 481                         }
 482                     }
 483                     (ty::Projection(proj_ty), _) => {
 484                         self.expected_projection(
 485                             db,
 486                             proj_ty,
 487                             values,
 488                             body_owner_def_id,
 489                             &cause.code,
 490                         );
 491                     }
 492                     (_, ty::Projection(proj_ty)) => {
 493                         let msg = format!(
 494                             "consider constraining the associated type `{}` to `{}`",
 495                             values.found, values.expected,
 496                         );
 497                         if !self.suggest_constraint(
 498                             db,
 499                             &msg,
 500                             body_owner_def_id,
 501                             proj_ty,
 502                             values.expected,
 503                         ) {
 504                             db.help(&msg);
 505                             db.note(
 506                                 "for more information, visit \
 507                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 508                             );
 509                         }
 510                     }
 511                     _ => {}
 512                 }
 513                 debug!(
 514                     "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
 515                     values.expected, values.expected.kind, values.found, values.found.kind,
 516                 );
 517             }
 518             CyclicTy(ty) => {
 519                 // Watch out for various cases of cyclic types and try to explain.
 520                 if ty.is_closure() || ty.is_generator() {
 521                     db.note(
 522                         "closures cannot capture themselves or take themselves as argument;\n\
 523                          this error may be the result of a recent compiler bug-fix,\n\
 524                          see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
 525                          for more information",
 526                     );
 527                 }
 528             }
 529             TargetFeatureCast(def_id) => {
 530                 let attrs = self.get_attrs(*def_id);
 531                 let target_spans = attrs
 532                     .deref()
 533                     .iter()
 534                     .filter(|attr| attr.has_name(sym::target_feature))
 535                     .map(|attr| attr.span);
 536                 db.note(
 537                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 538                 );
 539                 db.span_labels(target_spans, "`#[target_feature]` added here");
 540             }
 541             _ => {}
 542         }
 543     }
 544
 545     fn suggest_constraint(
 546         &self,
 547         db: &mut DiagnosticBuilder<'_>,
 548         msg: &str,
 549         body_owner_def_id: DefId,
 550         proj_ty: &ty::ProjectionTy<'tcx>,
 551         ty: Ty<'tcx>,
 552     ) -> bool {
 553         let assoc = self.associated_item(proj_ty.item_def_id);
 554         let trait_ref = proj_ty.trait_ref(*self);
 555         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
 556             if let Some(hir_generics) = item.generics() {
 557                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 558                 // This will also work for `impl Trait`.
 559                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind {
 560                     let generics = self.generics_of(body_owner_def_id);
 561                     generics.type_param(&param_ty, *self).def_id
 562                 } else {
 563                     return false;
 564                 };
 565
 566                 // First look in the `where` clause, as this might be
 567                 // `fn foo<T>(x: T) where T: Trait`.
 568                 for predicate in hir_generics.where_clause.predicates {
 569                     if let hir::WherePredicate::BoundPredicate(pred) = predicate {
 570                         if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) =
 571                             pred.bounded_ty.kind
 572                         {
 573                             if path.res.opt_def_id() == Some(def_id) {
 574                                 // This predicate is binding type param `A` in `<A as T>::Foo` to
 575                                 // something, potentially `T`.
 576                             } else {
 577                                 continue;
 578                             }
 579                         } else {
 580                             continue;
 581                         }
 582
 583                         if self.constrain_generic_bound_associated_type_structured_suggestion(
 584                             db,
 585                             &trait_ref,
 586                             pred.bounds,
 587                             &assoc,
 588                             ty,
 589                             msg,
 590                         ) {
 591                             return true;
 592                         }
 593                     }
 594                 }
 595                 for param in hir_generics.params {
 596                     if self.hir().opt_local_def_id(param.hir_id).map(|id| id.to_def_id())
 597                         == Some(def_id)
 598                     {
 599                         // This is type param `A` in `<A as T>::Foo`.
 600                         return self.constrain_generic_bound_associated_type_structured_suggestion(
 601                             db,
 602                             &trait_ref,
 603                             param.bounds,
 604                             &assoc,
 605                             ty,
 606                             msg,
 607                         );
 608                     }
 609                 }
 610             }
 611         }
 612         false
 613     }
 614
 615     /// An associated type was expected and a different type was found.
 616     ///
 617     /// We perform a few different checks to see what we can suggest:
 618     ///
 619     ///  - In the current item, look for associated functions that return the expected type and
 620     ///    suggest calling them. (Not a structured suggestion.)
 621     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 622     ///    associated type to the found type.
 623     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 624     ///    mention that this is disallowed.
 625     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 626     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 627     ///    fn that returns the type.
 628     fn expected_projection(
 629         &self,
 630         db: &mut DiagnosticBuilder<'_>,
 631         proj_ty: &ty::ProjectionTy<'tcx>,
 632         values: &ExpectedFound<Ty<'tcx>>,
 633         body_owner_def_id: DefId,
 634         cause_code: &ObligationCauseCode<'_>,
 635     ) {
 636         let msg = format!(
 637             "consider constraining the associated type `{}` to `{}`",
 638             values.expected, values.found
 639         );
 640         let body_owner = self.hir().get_if_local(body_owner_def_id);
 641         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 642
 643         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 644         let callable_scope = match body_owner {
 645             Some(
 646                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 647                 | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 648                 | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 649             ) => true,
 650             _ => false,
 651         };
 652         let impl_comparison = matches!(
 653             cause_code,
 654             ObligationCauseCode::CompareImplMethodObligation { .. }
 655                 | ObligationCauseCode::CompareImplTypeObligation { .. }
 656                 | ObligationCauseCode::CompareImplConstObligation
 657         );
 658         let assoc = self.associated_item(proj_ty.item_def_id);
 659         if !callable_scope || impl_comparison {
 660             // We do not want to suggest calling functions when the reason of the
 661             // type error is a comparison of an `impl` with its `trait` or when the
 662             // scope is outside of a `Body`.
 663         } else {
 664             // If we find a suitable associated function that returns the expected type, we don't
 665             // want the more general suggestion later in this method about "consider constraining
 666             // the associated type or calling a method that returns the associated type".
 667             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 668                 db,
 669                 assoc.container.id(),
 670                 current_method_ident,
 671                 proj_ty.item_def_id,
 672                 values.expected,
 673             );
 674             // Possibly suggest constraining the associated type to conform to the
 675             // found type.
 676             if self.suggest_constraint(db, &msg, body_owner_def_id, proj_ty, values.found)
 677                 || point_at_assoc_fn
 678             {
 679                 return;
 680             }
 681         }
 682
 683         if let ty::Opaque(def_id, _) = proj_ty.self_ty().kind {
 684             // When the expected `impl Trait` is not defined in the current item, it will come from
 685             // a return type. This can occur when dealing with `TryStream` (#71035).
 686             if self.constrain_associated_type_structured_suggestion(
 687                 db,
 688                 self.def_span(def_id),
 689                 &assoc,
 690                 values.found,
 691                 &msg,
 692             ) {
 693                 return;
 694             }
 695         }
 696
 697         if self.point_at_associated_type(db, body_owner_def_id, values.found) {
 698             return;
 699         }
 700
 701         if !impl_comparison {
 702             // Generic suggestion when we can't be more specific.
 703             if callable_scope {
 704                 db.help(&format!("{} or calling a method that returns `{}`", msg, values.expected));
 705             } else {
 706                 db.help(&msg);
 707             }
 708             db.note(
 709                 "for more information, visit \
 710                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 711             );
 712         }
 713         if self.sess.teach(&db.get_code().unwrap()) {
 714             db.help(
 715                 "given an associated type `T` and a method `foo`:
 716 ```
 717 trait Trait {
 718 type T;
 719 fn foo(&self) -> Self::T;
 720 }
 721 ```
 722 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 723 ```
 724 impl Trait for X {
 725 type T = String;
 726 fn foo(&self) -> Self::T { String::new() }
 727 }
 728 ```",
 729             );
 730         }
 731     }
 732
 733     fn point_at_methods_that_satisfy_associated_type(
 734         &self,
 735         db: &mut DiagnosticBuilder<'_>,
 736         assoc_container_id: DefId,
 737         current_method_ident: Option<Symbol>,
 738         proj_ty_item_def_id: DefId,
 739         expected: Ty<'tcx>,
 740     ) -> bool {
 741         let items = self.associated_items(assoc_container_id);
 742         // Find all the methods in the trait that could be called to construct the
 743         // expected associated type.
 744         // FIXME: consider suggesting the use of associated `const`s.
 745         let methods: Vec<(Span, String)> = items
 746             .items
 747             .iter()
 748             .filter(|(name, item)| {
 749                 ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
 750             })
 751             .filter_map(|(_, item)| {
 752                 let method = self.fn_sig(item.def_id);
 753                 match method.output().skip_binder().kind {
 754                     ty::Projection(ty::ProjectionTy { item_def_id, .. })
 755                         if item_def_id == proj_ty_item_def_id =>
 756                     {
 757                         Some((
 758                             self.sess.source_map().guess_head_span(self.def_span(item.def_id)),
 759                             format!("consider calling `{}`", self.def_path_str(item.def_id)),
 760                         ))
 761                     }
 762                     _ => None,
 763                 }
 764             })
 765             .collect();
 766         if !methods.is_empty() {
 767             // Use a single `help:` to show all the methods in the trait that can
 768             // be used to construct the expected associated type.
 769             let mut span: MultiSpan =
 770                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 771             let msg = format!(
 772                 "{some} method{s} {are} available that return{r} `{ty}`",
 773                 some = if methods.len() == 1 { "a" } else { "some" },
 774                 s = pluralize!(methods.len()),
 775                 are = if methods.len() == 1 { "is" } else { "are" },
 776                 r = if methods.len() == 1 { "s" } else { "" },
 777                 ty = expected
 778             );
 779             for (sp, label) in methods.into_iter() {
 780                 span.push_span_label(sp, label);
 781             }
 782             db.span_help(span, &msg);
 783             return true;
 784         }
 785         false
 786     }
 787
 788     fn point_at_associated_type(
 789         &self,
 790         db: &mut DiagnosticBuilder<'_>,
 791         body_owner_def_id: DefId,
 792         found: Ty<'tcx>,
 793     ) -> bool {
 794         let hir_id = match body_owner_def_id.as_local().map(|id| self.hir().as_local_hir_id(id)) {
 795             Some(hir_id) => hir_id,
 796             None => return false,
 797         };
 798         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 799         // `expected` and point at it.
 800         let parent_id = self.hir().get_parent_item(hir_id);
 801         let item = self.hir().find(parent_id);
 802         debug!("expected_projection parent item {:?}", item);
 803         match item {
 804             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 805                 // FIXME: account for `#![feature(specialization)]`
 806                 for item in &items[..] {
 807                     match item.kind {
 808                         hir::AssocItemKind::Type => {
 809                             // FIXME: account for returning some type in a trait fn impl that has
 810                             // an assoc type as a return type (#72076).
 811                             if let hir::Defaultness::Default { has_value: true } = item.defaultness
 812                             {
 813                                 if self.type_of(self.hir().local_def_id(item.id.hir_id)) == found {
 814                                     db.span_label(
 815                                         item.span,
 816                                         "associated type defaults can't be assumed inside the \
 817                                             trait defining them",
 818                                     );
 819                                     return true;
 820                                 }
 821                             }
 822                         }
 823                         _ => {}
 824                     }
 825                 }
 826             }
 827             Some(hir::Node::Item(hir::Item {
 828                 kind: hir::ItemKind::Impl { items, .. }, ..
 829             })) => {
 830                 for item in &items[..] {
 831                     match item.kind {
 832                         hir::AssocItemKind::Type => {
 833                             if self.type_of(self.hir().local_def_id(item.id.hir_id)) == found {
 834                                 db.span_label(item.span, "expected this associated type");
 835                                 return true;
 836                             }
 837                         }
 838                         _ => {}
 839                     }
 840                 }
 841             }
 842             _ => {}
 843         }
 844         false
 845     }
 846
 847     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 848     /// requirement, provide a strucuted suggestion to constrain it to a given type `ty`.
 849     fn constrain_generic_bound_associated_type_structured_suggestion(
 850         &self,
 851         db: &mut DiagnosticBuilder<'_>,
 852         trait_ref: &ty::TraitRef<'tcx>,
 853         bounds: hir::GenericBounds<'_>,
 854         assoc: &ty::AssocItem,
 855         ty: Ty<'tcx>,
 856         msg: &str,
 857     ) -> bool {
 858         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 859         bounds.iter().any(|bound| match bound {
 860             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => {
 861                 // Relate the type param against `T` in `<A as T>::Foo`.
 862                 ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id)
 863                     && self.constrain_associated_type_structured_suggestion(
 864                         db, ptr.span, assoc, ty, msg,
 865                     )
 866             }
 867             _ => false,
 868         })
 869     }
 870
 871     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 872     /// associated type to a given type `ty`.
 873     fn constrain_associated_type_structured_suggestion(
 874         &self,
 875         db: &mut DiagnosticBuilder<'_>,
 876         span: Span,
 877         assoc: &ty::AssocItem,
 878         ty: Ty<'tcx>,
 879         msg: &str,
 880     ) -> bool {
 881         if let Ok(has_params) =
 882             self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 883         {
 884             let (span, sugg) = if has_params {
 885                 let pos = span.hi() - BytePos(1);
 886                 let span = Span::new(pos, pos, span.ctxt());
 887                 (span, format!(", {} = {}", assoc.ident, ty))
 888             } else {
 889                 (span.shrink_to_hi(), format!("<{} = {}>", assoc.ident, ty))
 890             };
 891             db.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 892             return true;
 893         }
 894         false
 895     }
 896 }