compiler/rustc_middle/src/ty/error.rs

   1 use crate::traits::{ObligationCause, ObligationCauseCode};
   2 use crate::ty::diagnostics::suggest_constraining_type_param;
   3 use crate::ty::print::{FmtPrinter, Printer};
   4 use crate::ty::{self, BoundRegionKind, Region, Ty, TyCtxt};
   5 use hir::def::DefKind;
   6 use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
   7 use rustc_errors::{pluralize, Diagnostic, MultiSpan};
   8 use rustc_hir as hir;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_span::symbol::{sym, Symbol};
  11 use rustc_span::{BytePos, Span};
  12 use rustc_target::spec::abi;
  13
  14 use std::borrow::Cow;
  15 use std::fmt;
  16
  17 #[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable, TypeVisitable, Lift)]
  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(Copy, Clone, Debug, TypeFoldable, TypeVisitable, Lift)]
  35 #[rustc_pass_by_value]
  36 pub enum TypeError<'tcx> {
  37     Mismatch,
  38     ConstnessMismatch(ExpectedFound<ty::BoundConstness>),
  39     PolarityMismatch(ExpectedFound<ty::ImplPolarity>),
  40     UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
  41     AbiMismatch(ExpectedFound<abi::Abi>),
  42     Mutability,
  43     ArgumentMutability(usize),
  44     TupleSize(ExpectedFound<usize>),
  45     FixedArraySize(ExpectedFound<u64>),
  46     ArgCount,
  47     FieldMisMatch(Symbol, Symbol),
  48
  49     RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
  50     RegionsInsufficientlyPolymorphic(BoundRegionKind, Region<'tcx>),
  51     RegionsOverlyPolymorphic(BoundRegionKind, Region<'tcx>),
  52     RegionsPlaceholderMismatch,
  53
  54     Sorts(ExpectedFound<Ty<'tcx>>),
  55     ArgumentSorts(ExpectedFound<Ty<'tcx>>, usize),
  56     IntMismatch(ExpectedFound<ty::IntVarValue>),
  57     FloatMismatch(ExpectedFound<ty::FloatTy>),
  58     Traits(ExpectedFound<DefId>),
  59     VariadicMismatch(ExpectedFound<bool>),
  60
  61     /// Instantiating a type variable with the given type would have
  62     /// created a cycle (because it appears somewhere within that
  63     /// type).
  64     CyclicTy(Ty<'tcx>),
  65     CyclicConst(ty::Const<'tcx>),
  66     ProjectionMismatched(ExpectedFound<DefId>),
  67     ExistentialMismatch(
  68         ExpectedFound<&'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>>,
  69     ),
  70     ObjectUnsafeCoercion(DefId),
  71     ConstMismatch(ExpectedFound<ty::Const<'tcx>>),
  72
  73     IntrinsicCast,
  74     /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
  75     TargetFeatureCast(DefId),
  76 }
  77
  78 impl TypeError<'_> {
  79     pub fn involves_regions(self) -> bool {
  80         match self {
  81             TypeError::RegionsDoesNotOutlive(_, _)
  82             | TypeError::RegionsInsufficientlyPolymorphic(_, _)
  83             | TypeError::RegionsOverlyPolymorphic(_, _)
  84             | TypeError::RegionsPlaceholderMismatch => true,
  85             _ => false,
  86         }
  87     }
  88 }
  89
  90 /// Explains the source of a type err in a short, human readable way. This is meant to be placed
  91 /// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
  92 /// afterwards to present additional details, particularly when it comes to lifetime-related
  93 /// errors.
  94 impl<'tcx> fmt::Display for TypeError<'tcx> {
  95     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  96         use self::TypeError::*;
  97         fn report_maybe_different(
  98             f: &mut fmt::Formatter<'_>,
  99             expected: &str,
 100             found: &str,
 101         ) -> fmt::Result {
 102             // A naive approach to making sure that we're not reporting silly errors such as:
 103             // (expected closure, found closure).
 104             if expected == found {
 105                 write!(f, "expected {}, found a different {}", expected, found)
 106             } else {
 107                 write!(f, "expected {}, found {}", expected, found)
 108             }
 109         }
 110
 111         let br_string = |br: ty::BoundRegionKind| match br {
 112             ty::BrNamed(_, name) => format!(" {}", name),
 113             _ => String::new(),
 114         };
 115
 116         match *self {
 117             CyclicTy(_) => write!(f, "cyclic type of infinite size"),
 118             CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
 119             Mismatch => write!(f, "types differ"),
 120             ConstnessMismatch(values) => {
 121                 write!(f, "expected {} bound, found {} bound", values.expected, values.found)
 122             }
 123             PolarityMismatch(values) => {
 124                 write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
 125             }
 126             UnsafetyMismatch(values) => {
 127                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 128             }
 129             AbiMismatch(values) => {
 130                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 131             }
 132             ArgumentMutability(_) | Mutability => write!(f, "types differ in mutability"),
 133             TupleSize(values) => write!(
 134                 f,
 135                 "expected a tuple with {} element{}, found one with {} element{}",
 136                 values.expected,
 137                 pluralize!(values.expected),
 138                 values.found,
 139                 pluralize!(values.found)
 140             ),
 141             FixedArraySize(values) => write!(
 142                 f,
 143                 "expected an array with a fixed size of {} element{}, found one with {} element{}",
 144                 values.expected,
 145                 pluralize!(values.expected),
 146                 values.found,
 147                 pluralize!(values.found)
 148             ),
 149             ArgCount => write!(f, "incorrect number of function parameters"),
 150             FieldMisMatch(adt, field) => write!(f, "field type mismatch: {}.{}", adt, field),
 151             RegionsDoesNotOutlive(..) => write!(f, "lifetime mismatch"),
 152             // Actually naming the region here is a bit confusing because context is lacking
 153             RegionsInsufficientlyPolymorphic(..) => {
 154                 write!(f, "one type is more general than the other")
 155             }
 156             RegionsOverlyPolymorphic(br, _) => write!(
 157                 f,
 158                 "expected concrete lifetime, found bound lifetime parameter{}",
 159                 br_string(br)
 160             ),
 161             RegionsPlaceholderMismatch => write!(f, "one type is more general than the other"),
 162             ArgumentSorts(values, _) | Sorts(values) => ty::tls::with(|tcx| {
 163                 report_maybe_different(
 164                     f,
 165                     &values.expected.sort_string(tcx),
 166                     &values.found.sort_string(tcx),
 167                 )
 168             }),
 169             Traits(values) => ty::tls::with(|tcx| {
 170                 report_maybe_different(
 171                     f,
 172                     &format!("trait `{}`", tcx.def_path_str(values.expected)),
 173                     &format!("trait `{}`", tcx.def_path_str(values.found)),
 174                 )
 175             }),
 176             IntMismatch(ref values) => {
 177                 let expected = match values.expected {
 178                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 179                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 180                 };
 181                 let found = match values.found {
 182                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 183                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 184                 };
 185                 write!(f, "expected `{}`, found `{}`", expected, found)
 186             }
 187             FloatMismatch(ref values) => {
 188                 write!(
 189                     f,
 190                     "expected `{}`, found `{}`",
 191                     values.expected.name_str(),
 192                     values.found.name_str()
 193                 )
 194             }
 195             VariadicMismatch(ref values) => write!(
 196                 f,
 197                 "expected {} fn, found {} function",
 198                 if values.expected { "variadic" } else { "non-variadic" },
 199                 if values.found { "variadic" } else { "non-variadic" }
 200             ),
 201             ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
 202                 write!(
 203                     f,
 204                     "expected {}, found {}",
 205                     tcx.def_path_str(values.expected),
 206                     tcx.def_path_str(values.found)
 207                 )
 208             }),
 209             ExistentialMismatch(ref values) => report_maybe_different(
 210                 f,
 211                 &format!("trait `{}`", values.expected),
 212                 &format!("trait `{}`", values.found),
 213             ),
 214             ConstMismatch(ref values) => {
 215                 write!(f, "expected `{}`, found `{}`", values.expected, values.found)
 216             }
 217             IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
 218             TargetFeatureCast(_) => write!(
 219                 f,
 220                 "cannot coerce functions with `#[target_feature]` to safe function pointers"
 221             ),
 222             ObjectUnsafeCoercion(_) => write!(f, "coercion to object-unsafe trait object"),
 223         }
 224     }
 225 }
 226
 227 impl<'tcx> TypeError<'tcx> {
 228     pub fn must_include_note(self) -> bool {
 229         use self::TypeError::*;
 230         match self {
 231             CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
 232             | PolarityMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
 233             | ArgumentSorts(..) | Sorts(_) | IntMismatch(_) | FloatMismatch(_)
 234             | VariadicMismatch(_) | TargetFeatureCast(_) => false,
 235
 236             Mutability
 237             | ArgumentMutability(_)
 238             | TupleSize(_)
 239             | ArgCount
 240             | FieldMisMatch(..)
 241             | RegionsDoesNotOutlive(..)
 242             | RegionsInsufficientlyPolymorphic(..)
 243             | RegionsOverlyPolymorphic(..)
 244             | RegionsPlaceholderMismatch
 245             | Traits(_)
 246             | ProjectionMismatched(_)
 247             | ExistentialMismatch(_)
 248             | ConstMismatch(_)
 249             | IntrinsicCast
 250             | ObjectUnsafeCoercion(_) => true,
 251         }
 252     }
 253 }
 254
 255 impl<'tcx> Ty<'tcx> {
 256     pub fn sort_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 257         match *self.kind() {
 258             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
 259                 format!("`{}`", self).into()
 260             }
 261             ty::Tuple(ref tys) if tys.is_empty() => format!("`{}`", self).into(),
 262
 263             ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did())).into(),
 264             ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
 265             ty::Array(t, n) => {
 266                 if t.is_simple_ty() {
 267                     return format!("array `{}`", self).into();
 268                 }
 269
 270                 let n = tcx.lift(n).unwrap();
 271                 if let ty::ConstKind::Value(v) = n.kind() {
 272                     if let Some(n) = v.try_to_machine_usize(tcx) {
 273                         return format!("array of {} element{}", n, pluralize!(n)).into();
 274                     }
 275                 }
 276                 "array".into()
 277             }
 278             ty::Slice(ty) if ty.is_simple_ty() => format!("slice `{}`", self).into(),
 279             ty::Slice(_) => "slice".into(),
 280             ty::RawPtr(tymut) => {
 281                 let tymut_string = match tymut.mutbl {
 282                     hir::Mutability::Mut => tymut.to_string(),
 283                     hir::Mutability::Not => format!("const {}", tymut.ty),
 284                 };
 285
 286                 if tymut_string != "_" && (tymut.ty.is_simple_text() || tymut_string.len() < "const raw pointer".len()) {
 287                     format!("`*{}`", tymut_string).into()
 288                 } else {
 289                     // Unknown type name, it's long or has type arguments
 290                     "raw pointer".into()
 291                 }
 292             },
 293             ty::Ref(_, ty, mutbl) => {
 294                 let tymut = ty::TypeAndMut { ty, mutbl };
 295                 let tymut_string = tymut.to_string();
 296
 297                 if tymut_string != "_"
 298                     && (ty.is_simple_text() || tymut_string.len() < "mutable reference".len())
 299                 {
 300                     format!("`&{}`", tymut_string).into()
 301                 } else {
 302                     // Unknown type name, it's long or has type arguments
 303                     match mutbl {
 304                         hir::Mutability::Mut => "mutable reference",
 305                         _ => "reference",
 306                     }
 307                     .into()
 308                 }
 309             }
 310             ty::FnDef(..) => "fn item".into(),
 311             ty::FnPtr(_) => "fn pointer".into(),
 312             ty::Dynamic(ref inner, ..) if let Some(principal) = inner.principal() => {
 313                 format!("trait object `dyn {}`", tcx.def_path_str(principal.def_id())).into()
 314             }
 315             ty::Dynamic(..) => "trait object".into(),
 316             ty::Closure(..) => "closure".into(),
 317             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 318             ty::GeneratorWitness(..) => "generator witness".into(),
 319             ty::Tuple(..) => "tuple".into(),
 320             ty::Infer(ty::TyVar(_)) => "inferred type".into(),
 321             ty::Infer(ty::IntVar(_)) => "integer".into(),
 322             ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
 323             ty::Placeholder(..) => "placeholder type".into(),
 324             ty::Bound(..) => "bound type".into(),
 325             ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
 326             ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
 327             ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
 328             ty::Projection(_) => "associated type".into(),
 329             ty::Param(p) => format!("type parameter `{}`", p).into(),
 330             ty::Opaque(..) => "opaque type".into(),
 331             ty::Error(_) => "type error".into(),
 332         }
 333     }
 334
 335     pub fn prefix_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 336         match *self.kind() {
 337             ty::Infer(_)
 338             | ty::Error(_)
 339             | ty::Bool
 340             | ty::Char
 341             | ty::Int(_)
 342             | ty::Uint(_)
 343             | ty::Float(_)
 344             | ty::Str
 345             | ty::Never => "type".into(),
 346             ty::Tuple(ref tys) if tys.is_empty() => "unit type".into(),
 347             ty::Adt(def, _) => def.descr().into(),
 348             ty::Foreign(_) => "extern type".into(),
 349             ty::Array(..) => "array".into(),
 350             ty::Slice(_) => "slice".into(),
 351             ty::RawPtr(_) => "raw pointer".into(),
 352             ty::Ref(.., mutbl) => match mutbl {
 353                 hir::Mutability::Mut => "mutable reference",
 354                 _ => "reference",
 355             }
 356             .into(),
 357             ty::FnDef(..) => "fn item".into(),
 358             ty::FnPtr(_) => "fn pointer".into(),
 359             ty::Dynamic(..) => "trait object".into(),
 360             ty::Closure(..) => "closure".into(),
 361             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 362             ty::GeneratorWitness(..) => "generator witness".into(),
 363             ty::Tuple(..) => "tuple".into(),
 364             ty::Placeholder(..) => "higher-ranked type".into(),
 365             ty::Bound(..) => "bound type variable".into(),
 366             ty::Projection(_) => "associated type".into(),
 367             ty::Param(_) => "type parameter".into(),
 368             ty::Opaque(..) => "opaque type".into(),
 369         }
 370     }
 371 }
 372
 373 impl<'tcx> TyCtxt<'tcx> {
 374     pub fn note_and_explain_type_err(
 375         self,
 376         diag: &mut Diagnostic,
 377         err: TypeError<'tcx>,
 378         cause: &ObligationCause<'tcx>,
 379         sp: Span,
 380         body_owner_def_id: DefId,
 381     ) {
 382         use self::TypeError::*;
 383         debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
 384         match err {
 385             ArgumentSorts(values, _) | Sorts(values) => {
 386                 match (values.expected.kind(), values.found.kind()) {
 387                     (ty::Closure(..), ty::Closure(..)) => {
 388                         diag.note("no two closures, even if identical, have the same type");
 389                         diag.help("consider boxing your closure and/or using it as a trait object");
 390                     }
 391                     (ty::Opaque(..), ty::Opaque(..)) => {
 392                         // Issue #63167
 393                         diag.note("distinct uses of `impl Trait` result in different opaque types");
 394                     }
 395                     (ty::Float(_), ty::Infer(ty::IntVar(_)))
 396                         if let Ok(
 397                             // Issue #53280
 398                             snippet,
 399                         ) = self.sess.source_map().span_to_snippet(sp) =>
 400                     {
 401                         if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
 402                             diag.span_suggestion(
 403                                 sp,
 404                                 "use a float literal",
 405                                 format!("{}.0", snippet),
 406                                 MachineApplicable,
 407                             );
 408                         }
 409                     }
 410                     (ty::Param(expected), ty::Param(found)) => {
 411                         let generics = self.generics_of(body_owner_def_id);
 412                         let e_span = self.def_span(generics.type_param(expected, self).def_id);
 413                         if !sp.contains(e_span) {
 414                             diag.span_label(e_span, "expected type parameter");
 415                         }
 416                         let f_span = self.def_span(generics.type_param(found, self).def_id);
 417                         if !sp.contains(f_span) {
 418                             diag.span_label(f_span, "found type parameter");
 419                         }
 420                         diag.note(
 421                             "a type parameter was expected, but a different one was found; \
 422                              you might be missing a type parameter or trait bound",
 423                         );
 424                         diag.note(
 425                             "for more information, visit \
 426                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 427                              #traits-as-parameters",
 428                         );
 429                     }
 430                     (ty::Projection(_), ty::Projection(_)) => {
 431                         diag.note("an associated type was expected, but a different one was found");
 432                     }
 433                     (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p)) => {
 434                         let generics = self.generics_of(body_owner_def_id);
 435                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 436                         if !sp.contains(p_span) {
 437                             diag.span_label(p_span, "this type parameter");
 438                         }
 439                         let hir = self.hir();
 440                         let mut note = true;
 441                         if let Some(generics) = generics
 442                             .type_param(p, self)
 443                             .def_id
 444                             .as_local()
 445                             .map(|id| hir.local_def_id_to_hir_id(id))
 446                             .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
 447                             .as_ref()
 448                             .and_then(|node| node.generics())
 449                         {
 450                             // Synthesize the associated type restriction `Add<Output = Expected>`.
 451                             // FIXME: extract this logic for use in other diagnostics.
 452                             let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
 453                             let path =
 454                                 self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
 455                             let item_name = self.item_name(proj.item_def_id);
 456                             let item_args = self.format_generic_args(assoc_substs);
 457
 458                             let path = if path.ends_with('>') {
 459                                 format!(
 460                                     "{}, {}{} = {}>",
 461                                     &path[..path.len() - 1],
 462                                     item_name,
 463                                     item_args,
 464                                     p
 465                                 )
 466                             } else {
 467                                 format!("{}<{}{} = {}>", path, item_name, item_args, p)
 468                             };
 469                             note = !suggest_constraining_type_param(
 470                                 self,
 471                                 generics,
 472                                 diag,
 473                                 &format!("{}", proj.self_ty()),
 474                                 &path,
 475                                 None,
 476                             );
 477                         }
 478                         if note {
 479                             diag.note("you might be missing a type parameter or trait bound");
 480                         }
 481                     }
 482                     (ty::Param(p), ty::Dynamic(..) | ty::Opaque(..))
 483                     | (ty::Dynamic(..) | ty::Opaque(..), ty::Param(p)) => {
 484                         let generics = self.generics_of(body_owner_def_id);
 485                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 486                         if !sp.contains(p_span) {
 487                             diag.span_label(p_span, "this type parameter");
 488                         }
 489                         diag.help("type parameters must be constrained to match other types");
 490                         if self.sess.teach(&diag.get_code().unwrap()) {
 491                             diag.help(
 492                                 "given a type parameter `T` and a method `foo`:
 493 ```
 494 trait Trait<T> { fn foo(&self) -> T; }
 495 ```
 496 the only ways to implement method `foo` are:
 497 - constrain `T` with an explicit type:
 498 ```
 499 impl Trait<String> for X {
 500     fn foo(&self) -> String { String::new() }
 501 }
 502 ```
 503 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 504 ```
 505 impl<T: std::default::Default> Trait<T> for X {
 506     fn foo(&self) -> T { <T as std::default::Default>::default() }
 507 }
 508 ```
 509 - change `foo` to return an argument of type `T`:
 510 ```
 511 impl<T> Trait<T> for X {
 512     fn foo(&self, x: T) -> T { x }
 513 }
 514 ```",
 515                             );
 516                         }
 517                         diag.note(
 518                             "for more information, visit \
 519                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 520                              #traits-as-parameters",
 521                         );
 522                     }
 523                     (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
 524                         let generics = self.generics_of(body_owner_def_id);
 525                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 526                         if !sp.contains(p_span) {
 527                             diag.span_label(p_span, "this type parameter");
 528                         }
 529                         diag.help(&format!(
 530                             "every closure has a distinct type and so could not always match the \
 531                              caller-chosen type of parameter `{}`",
 532                             p
 533                         ));
 534                     }
 535                     (ty::Param(p), _) | (_, ty::Param(p)) => {
 536                         let generics = self.generics_of(body_owner_def_id);
 537                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 538                         if !sp.contains(p_span) {
 539                             diag.span_label(p_span, "this type parameter");
 540                         }
 541                     }
 542                     (ty::Projection(proj_ty), _) if self.def_kind(proj_ty.item_def_id) != DefKind::ImplTraitPlaceholder => {
 543                         self.expected_projection(
 544                             diag,
 545                             proj_ty,
 546                             values,
 547                             body_owner_def_id,
 548                             cause.code(),
 549                         );
 550                     }
 551                     (_, ty::Projection(proj_ty)) if self.def_kind(proj_ty.item_def_id) != DefKind::ImplTraitPlaceholder => {
 552                         let msg = format!(
 553                             "consider constraining the associated type `{}` to `{}`",
 554                             values.found, values.expected,
 555                         );
 556                         if !(self.suggest_constraining_opaque_associated_type(
 557                             diag,
 558                             &msg,
 559                             proj_ty,
 560                             values.expected,
 561                         ) || self.suggest_constraint(
 562                             diag,
 563                             &msg,
 564                             body_owner_def_id,
 565                             proj_ty,
 566                             values.expected,
 567                         )) {
 568                             diag.help(&msg);
 569                             diag.note(
 570                                 "for more information, visit \
 571                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 572                             );
 573                         }
 574                     }
 575                     _ => {}
 576                 }
 577                 debug!(
 578                     "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
 579                     values.expected,
 580                     values.expected.kind(),
 581                     values.found,
 582                     values.found.kind(),
 583                 );
 584             }
 585             CyclicTy(ty) => {
 586                 // Watch out for various cases of cyclic types and try to explain.
 587                 if ty.is_closure() || ty.is_generator() {
 588                     diag.note(
 589                         "closures cannot capture themselves or take themselves as argument;\n\
 590                          this error may be the result of a recent compiler bug-fix,\n\
 591                          see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
 592                          for more information",
 593                     );
 594                 }
 595             }
 596             TargetFeatureCast(def_id) => {
 597                 let target_spans =
 598                     self.get_attrs(def_id, sym::target_feature).map(|attr| attr.span);
 599                 diag.note(
 600                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 601                 );
 602                 diag.span_labels(target_spans, "`#[target_feature]` added here");
 603             }
 604             _ => {}
 605         }
 606     }
 607
 608     fn suggest_constraint(
 609         self,
 610         diag: &mut Diagnostic,
 611         msg: &str,
 612         body_owner_def_id: DefId,
 613         proj_ty: &ty::ProjectionTy<'tcx>,
 614         ty: Ty<'tcx>,
 615     ) -> bool {
 616         let assoc = self.associated_item(proj_ty.item_def_id);
 617         let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 618         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
 619             if let Some(hir_generics) = item.generics() {
 620                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 621                 // This will also work for `impl Trait`.
 622                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
 623                     let generics = self.generics_of(body_owner_def_id);
 624                     generics.type_param(param_ty, self).def_id
 625                 } else {
 626                     return false;
 627                 };
 628                 let Some(def_id) = def_id.as_local() else {
 629                     return false;
 630                 };
 631
 632                 // First look in the `where` clause, as this might be
 633                 // `fn foo<T>(x: T) where T: Trait`.
 634                 for pred in hir_generics.bounds_for_param(def_id) {
 635                     if self.constrain_generic_bound_associated_type_structured_suggestion(
 636                         diag,
 637                         &trait_ref,
 638                         pred.bounds,
 639                         &assoc,
 640                         assoc_substs,
 641                         ty,
 642                         msg,
 643                         false,
 644                     ) {
 645                         return true;
 646                     }
 647                 }
 648             }
 649         }
 650         false
 651     }
 652
 653     /// An associated type was expected and a different type was found.
 654     ///
 655     /// We perform a few different checks to see what we can suggest:
 656     ///
 657     ///  - In the current item, look for associated functions that return the expected type and
 658     ///    suggest calling them. (Not a structured suggestion.)
 659     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 660     ///    associated type to the found type.
 661     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 662     ///    mention that this is disallowed.
 663     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 664     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 665     ///    fn that returns the type.
 666     fn expected_projection(
 667         self,
 668         diag: &mut Diagnostic,
 669         proj_ty: &ty::ProjectionTy<'tcx>,
 670         values: ExpectedFound<Ty<'tcx>>,
 671         body_owner_def_id: DefId,
 672         cause_code: &ObligationCauseCode<'_>,
 673     ) {
 674         let msg = format!(
 675             "consider constraining the associated type `{}` to `{}`",
 676             values.expected, values.found
 677         );
 678         let body_owner = self.hir().get_if_local(body_owner_def_id);
 679         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 680
 681         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 682         let callable_scope = matches!(
 683             body_owner,
 684             Some(
 685                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 686                     | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 687                     | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 688             )
 689         );
 690         let impl_comparison =
 691             matches!(cause_code, ObligationCauseCode::CompareImplItemObligation { .. });
 692         let assoc = self.associated_item(proj_ty.item_def_id);
 693         if !callable_scope || impl_comparison {
 694             // We do not want to suggest calling functions when the reason of the
 695             // type error is a comparison of an `impl` with its `trait` or when the
 696             // scope is outside of a `Body`.
 697         } else {
 698             // If we find a suitable associated function that returns the expected type, we don't
 699             // want the more general suggestion later in this method about "consider constraining
 700             // the associated type or calling a method that returns the associated type".
 701             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 702                 diag,
 703                 assoc.container_id(self),
 704                 current_method_ident,
 705                 proj_ty.item_def_id,
 706                 values.expected,
 707             );
 708             // Possibly suggest constraining the associated type to conform to the
 709             // found type.
 710             if self.suggest_constraint(diag, &msg, body_owner_def_id, proj_ty, values.found)
 711                 || point_at_assoc_fn
 712             {
 713                 return;
 714             }
 715         }
 716
 717         self.suggest_constraining_opaque_associated_type(diag, &msg, proj_ty, values.found);
 718
 719         if self.point_at_associated_type(diag, body_owner_def_id, values.found) {
 720             return;
 721         }
 722
 723         if !impl_comparison {
 724             // Generic suggestion when we can't be more specific.
 725             if callable_scope {
 726                 diag.help(&format!(
 727                     "{} or calling a method that returns `{}`",
 728                     msg, values.expected
 729                 ));
 730             } else {
 731                 diag.help(&msg);
 732             }
 733             diag.note(
 734                 "for more information, visit \
 735                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 736             );
 737         }
 738         if self.sess.teach(&diag.get_code().unwrap()) {
 739             diag.help(
 740                 "given an associated type `T` and a method `foo`:
 741 ```
 742 trait Trait {
 743 type T;
 744 fn foo(&self) -> Self::T;
 745 }
 746 ```
 747 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 748 ```
 749 impl Trait for X {
 750 type T = String;
 751 fn foo(&self) -> Self::T { String::new() }
 752 }
 753 ```",
 754             );
 755         }
 756     }
 757
 758     /// When the expected `impl Trait` is not defined in the current item, it will come from
 759     /// a return type. This can occur when dealing with `TryStream` (#71035).
 760     fn suggest_constraining_opaque_associated_type(
 761         self,
 762         diag: &mut Diagnostic,
 763         msg: &str,
 764         proj_ty: &ty::ProjectionTy<'tcx>,
 765         ty: Ty<'tcx>,
 766     ) -> bool {
 767         let assoc = self.associated_item(proj_ty.item_def_id);
 768         if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
 769             let opaque_local_def_id = def_id.as_local();
 770             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
 771                 match &self.hir().expect_item(opaque_local_def_id).kind {
 772                     hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
 773                     _ => bug!("The HirId comes from a `ty::Opaque`"),
 774                 }
 775             } else {
 776                 return false;
 777             };
 778
 779             let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 780
 781             self.constrain_generic_bound_associated_type_structured_suggestion(
 782                 diag,
 783                 &trait_ref,
 784                 opaque_hir_ty.bounds,
 785                 assoc,
 786                 assoc_substs,
 787                 ty,
 788                 msg,
 789                 true,
 790             )
 791         } else {
 792             false
 793         }
 794     }
 795
 796     fn point_at_methods_that_satisfy_associated_type(
 797         self,
 798         diag: &mut Diagnostic,
 799         assoc_container_id: DefId,
 800         current_method_ident: Option<Symbol>,
 801         proj_ty_item_def_id: DefId,
 802         expected: Ty<'tcx>,
 803     ) -> bool {
 804         let items = self.associated_items(assoc_container_id);
 805         // Find all the methods in the trait that could be called to construct the
 806         // expected associated type.
 807         // FIXME: consider suggesting the use of associated `const`s.
 808         let methods: Vec<(Span, String)> = items
 809             .items
 810             .iter()
 811             .filter(|(name, item)| {
 812                 ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
 813             })
 814             .filter_map(|(_, item)| {
 815                 let method = self.fn_sig(item.def_id);
 816                 match *method.output().skip_binder().kind() {
 817                     ty::Projection(ty::ProjectionTy { item_def_id, .. })
 818                         if item_def_id == proj_ty_item_def_id =>
 819                     {
 820                         Some((
 821                             self.def_span(item.def_id),
 822                             format!("consider calling `{}`", self.def_path_str(item.def_id)),
 823                         ))
 824                     }
 825                     _ => None,
 826                 }
 827             })
 828             .collect();
 829         if !methods.is_empty() {
 830             // Use a single `help:` to show all the methods in the trait that can
 831             // be used to construct the expected associated type.
 832             let mut span: MultiSpan =
 833                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 834             let msg = format!(
 835                 "{some} method{s} {are} available that return{r} `{ty}`",
 836                 some = if methods.len() == 1 { "a" } else { "some" },
 837                 s = pluralize!(methods.len()),
 838                 are = pluralize!("is", methods.len()),
 839                 r = if methods.len() == 1 { "s" } else { "" },
 840                 ty = expected
 841             );
 842             for (sp, label) in methods.into_iter() {
 843                 span.push_span_label(sp, label);
 844             }
 845             diag.span_help(span, &msg);
 846             return true;
 847         }
 848         false
 849     }
 850
 851     fn point_at_associated_type(
 852         self,
 853         diag: &mut Diagnostic,
 854         body_owner_def_id: DefId,
 855         found: Ty<'tcx>,
 856     ) -> bool {
 857         let Some(hir_id) = body_owner_def_id.as_local() else {
 858             return false;
 859         };
 860         let hir_id = self.hir().local_def_id_to_hir_id(hir_id);
 861         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 862         // `expected` and point at it.
 863         let parent_id = self.hir().get_parent_item(hir_id);
 864         let item = self.hir().find_by_def_id(parent_id);
 865         debug!("expected_projection parent item {:?}", item);
 866         match item {
 867             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 868                 // FIXME: account for `#![feature(specialization)]`
 869                 for item in &items[..] {
 870                     match item.kind {
 871                         hir::AssocItemKind::Type => {
 872                             // FIXME: account for returning some type in a trait fn impl that has
 873                             // an assoc type as a return type (#72076).
 874                             if let hir::Defaultness::Default { has_value: true } =
 875                                 self.impl_defaultness(item.id.def_id)
 876                             {
 877                                 if self.type_of(item.id.def_id) == found {
 878                                     diag.span_label(
 879                                         item.span,
 880                                         "associated type defaults can't be assumed inside the \
 881                                             trait defining them",
 882                                     );
 883                                     return true;
 884                                 }
 885                             }
 886                         }
 887                         _ => {}
 888                     }
 889                 }
 890             }
 891             Some(hir::Node::Item(hir::Item {
 892                 kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
 893                 ..
 894             })) => {
 895                 for item in &items[..] {
 896                     if let hir::AssocItemKind::Type = item.kind {
 897                         if self.type_of(item.id.def_id) == found {
 898                             diag.span_label(item.span, "expected this associated type");
 899                             return true;
 900                         }
 901                     }
 902                 }
 903             }
 904             _ => {}
 905         }
 906         false
 907     }
 908
 909     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 910     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
 911     ///
 912     /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
 913     /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
 914     /// trait bound as the one we're looking for. This can help in cases where the associated
 915     /// type is defined on a supertrait of the one present in the bounds.
 916     fn constrain_generic_bound_associated_type_structured_suggestion(
 917         self,
 918         diag: &mut Diagnostic,
 919         trait_ref: &ty::TraitRef<'tcx>,
 920         bounds: hir::GenericBounds<'_>,
 921         assoc: &ty::AssocItem,
 922         assoc_substs: &[ty::GenericArg<'tcx>],
 923         ty: Ty<'tcx>,
 924         msg: &str,
 925         is_bound_surely_present: bool,
 926     ) -> bool {
 927         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 928
 929         let trait_bounds = bounds.iter().filter_map(|bound| match bound {
 930             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
 931             _ => None,
 932         });
 933
 934         let matching_trait_bounds = trait_bounds
 935             .clone()
 936             .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
 937             .collect::<Vec<_>>();
 938
 939         let span = match &matching_trait_bounds[..] {
 940             &[ptr] => ptr.span,
 941             &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
 942                 &[ptr] => ptr.span,
 943                 _ => return false,
 944             },
 945             _ => return false,
 946         };
 947
 948         self.constrain_associated_type_structured_suggestion(
 949             diag,
 950             span,
 951             assoc,
 952             assoc_substs,
 953             ty,
 954             msg,
 955         )
 956     }
 957
 958     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 959     /// associated type to a given type `ty`.
 960     fn constrain_associated_type_structured_suggestion(
 961         self,
 962         diag: &mut Diagnostic,
 963         span: Span,
 964         assoc: &ty::AssocItem,
 965         assoc_substs: &[ty::GenericArg<'tcx>],
 966         ty: Ty<'tcx>,
 967         msg: &str,
 968     ) -> bool {
 969         if let Ok(has_params) =
 970             self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 971         {
 972             let (span, sugg) = if has_params {
 973                 let pos = span.hi() - BytePos(1);
 974                 let span = Span::new(pos, pos, span.ctxt(), span.parent());
 975                 (span, format!(", {} = {}", assoc.ident(self), ty))
 976             } else {
 977                 let item_args = self.format_generic_args(assoc_substs);
 978                 (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(self), item_args, ty))
 979             };
 980             diag.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 981             return true;
 982         }
 983         false
 984     }
 985
 986     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
 987         FmtPrinter::new(self, hir::def::Namespace::TypeNS)
 988             .path_generic_args(Ok, args)
 989             .expect("could not write to `String`.")
 990             .into_buffer()
 991     }
 992 }