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::{with_forced_trimmed_paths, 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::collections::hash_map::DefaultHasher;
  16 use std::fmt;
  17 use std::hash::{Hash, Hasher};
  18 use std::path::PathBuf;
  19
  20 use super::print::PrettyPrinter;
  21
  22 #[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable, TypeVisitable, Lift)]
  23 pub struct ExpectedFound<T> {
  24     pub expected: T,
  25     pub found: T,
  26 }
  27
  28 impl<T> ExpectedFound<T> {
  29     pub fn new(a_is_expected: bool, a: T, b: T) -> Self {
  30         if a_is_expected {
  31             ExpectedFound { expected: a, found: b }
  32         } else {
  33             ExpectedFound { expected: b, found: a }
  34         }
  35     }
  36 }
  37
  38 // Data structures used in type unification
  39 #[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable, Lift)]
  40 #[rustc_pass_by_value]
  41 pub enum TypeError<'tcx> {
  42     Mismatch,
  43     ConstnessMismatch(ExpectedFound<ty::BoundConstness>),
  44     PolarityMismatch(ExpectedFound<ty::ImplPolarity>),
  45     UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
  46     AbiMismatch(ExpectedFound<abi::Abi>),
  47     Mutability,
  48     ArgumentMutability(usize),
  49     TupleSize(ExpectedFound<usize>),
  50     FixedArraySize(ExpectedFound<u64>),
  51     ArgCount,
  52     FieldMisMatch(Symbol, Symbol),
  53
  54     RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
  55     RegionsInsufficientlyPolymorphic(BoundRegionKind, Region<'tcx>),
  56     RegionsOverlyPolymorphic(BoundRegionKind, Region<'tcx>),
  57     RegionsPlaceholderMismatch,
  58
  59     Sorts(ExpectedFound<Ty<'tcx>>),
  60     ArgumentSorts(ExpectedFound<Ty<'tcx>>, usize),
  61     IntMismatch(ExpectedFound<ty::IntVarValue>),
  62     FloatMismatch(ExpectedFound<ty::FloatTy>),
  63     Traits(ExpectedFound<DefId>),
  64     VariadicMismatch(ExpectedFound<bool>),
  65
  66     /// Instantiating a type variable with the given type would have
  67     /// created a cycle (because it appears somewhere within that
  68     /// type).
  69     CyclicTy(Ty<'tcx>),
  70     CyclicConst(ty::Const<'tcx>),
  71     ProjectionMismatched(ExpectedFound<DefId>),
  72     ExistentialMismatch(ExpectedFound<&'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>>),
  73     ConstMismatch(ExpectedFound<ty::Const<'tcx>>),
  74
  75     IntrinsicCast,
  76     /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
  77     TargetFeatureCast(DefId),
  78 }
  79
  80 impl TypeError<'_> {
  81     pub fn involves_regions(self) -> bool {
  82         match self {
  83             TypeError::RegionsDoesNotOutlive(_, _)
  84             | TypeError::RegionsInsufficientlyPolymorphic(_, _)
  85             | TypeError::RegionsOverlyPolymorphic(_, _)
  86             | TypeError::RegionsPlaceholderMismatch => true,
  87             _ => false,
  88         }
  89     }
  90 }
  91
  92 /// Explains the source of a type err in a short, human readable way. This is meant to be placed
  93 /// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
  94 /// afterwards to present additional details, particularly when it comes to lifetime-related
  95 /// errors.
  96 impl<'tcx> fmt::Display for TypeError<'tcx> {
  97     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  98         use self::TypeError::*;
  99         fn report_maybe_different(
 100             f: &mut fmt::Formatter<'_>,
 101             expected: &str,
 102             found: &str,
 103         ) -> fmt::Result {
 104             // A naive approach to making sure that we're not reporting silly errors such as:
 105             // (expected closure, found closure).
 106             if expected == found {
 107                 write!(f, "expected {}, found a different {}", expected, found)
 108             } else {
 109                 write!(f, "expected {}, found {}", expected, found)
 110             }
 111         }
 112
 113         let br_string = |br: ty::BoundRegionKind| match br {
 114             ty::BrNamed(_, name) => format!(" {}", name),
 115             _ => String::new(),
 116         };
 117
 118         match *self {
 119             CyclicTy(_) => write!(f, "cyclic type of infinite size"),
 120             CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
 121             Mismatch => write!(f, "types differ"),
 122             ConstnessMismatch(values) => {
 123                 write!(f, "expected {} bound, found {} bound", values.expected, values.found)
 124             }
 125             PolarityMismatch(values) => {
 126                 write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
 127             }
 128             UnsafetyMismatch(values) => {
 129                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 130             }
 131             AbiMismatch(values) => {
 132                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 133             }
 134             ArgumentMutability(_) | Mutability => write!(f, "types differ in mutability"),
 135             TupleSize(values) => write!(
 136                 f,
 137                 "expected a tuple with {} element{}, found one with {} element{}",
 138                 values.expected,
 139                 pluralize!(values.expected),
 140                 values.found,
 141                 pluralize!(values.found)
 142             ),
 143             FixedArraySize(values) => write!(
 144                 f,
 145                 "expected an array with a fixed size of {} element{}, found one with {} element{}",
 146                 values.expected,
 147                 pluralize!(values.expected),
 148                 values.found,
 149                 pluralize!(values.found)
 150             ),
 151             ArgCount => write!(f, "incorrect number of function parameters"),
 152             FieldMisMatch(adt, field) => write!(f, "field type mismatch: {}.{}", adt, field),
 153             RegionsDoesNotOutlive(..) => write!(f, "lifetime mismatch"),
 154             // Actually naming the region here is a bit confusing because context is lacking
 155             RegionsInsufficientlyPolymorphic(..) => {
 156                 write!(f, "one type is more general than the other")
 157             }
 158             RegionsOverlyPolymorphic(br, _) => write!(
 159                 f,
 160                 "expected concrete lifetime, found bound lifetime parameter{}",
 161                 br_string(br)
 162             ),
 163             RegionsPlaceholderMismatch => write!(f, "one type is more general than the other"),
 164             ArgumentSorts(values, _) | Sorts(values) => ty::tls::with(|tcx| {
 165                 let (mut expected, mut found) = with_forced_trimmed_paths!((
 166                     values.expected.sort_string(tcx),
 167                     values.found.sort_string(tcx),
 168                 ));
 169                 if expected == found {
 170                     expected = values.expected.sort_string(tcx);
 171                     found = values.found.sort_string(tcx);
 172                 }
 173                 report_maybe_different(f, &expected, &found)
 174             }),
 175             Traits(values) => ty::tls::with(|tcx| {
 176                 let (mut expected, mut found) = with_forced_trimmed_paths!((
 177                     tcx.def_path_str(values.expected),
 178                     tcx.def_path_str(values.found),
 179                 ));
 180                 if expected == found {
 181                     expected = tcx.def_path_str(values.expected);
 182                     found = tcx.def_path_str(values.found);
 183                 }
 184                 report_maybe_different(
 185                     f,
 186                     &format!("trait `{expected}`"),
 187                     &format!("trait `{found}`"),
 188                 )
 189             }),
 190             IntMismatch(ref values) => {
 191                 let expected = match values.expected {
 192                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 193                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 194                 };
 195                 let found = match values.found {
 196                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 197                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 198                 };
 199                 write!(f, "expected `{}`, found `{}`", expected, found)
 200             }
 201             FloatMismatch(ref values) => {
 202                 write!(
 203                     f,
 204                     "expected `{}`, found `{}`",
 205                     values.expected.name_str(),
 206                     values.found.name_str()
 207                 )
 208             }
 209             VariadicMismatch(ref values) => write!(
 210                 f,
 211                 "expected {} fn, found {} function",
 212                 if values.expected { "variadic" } else { "non-variadic" },
 213                 if values.found { "variadic" } else { "non-variadic" }
 214             ),
 215             ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
 216                 write!(
 217                     f,
 218                     "expected {}, found {}",
 219                     tcx.def_path_str(values.expected),
 220                     tcx.def_path_str(values.found)
 221                 )
 222             }),
 223             ExistentialMismatch(ref values) => report_maybe_different(
 224                 f,
 225                 &format!("trait `{}`", values.expected),
 226                 &format!("trait `{}`", values.found),
 227             ),
 228             ConstMismatch(ref values) => {
 229                 write!(f, "expected `{}`, found `{}`", values.expected, values.found)
 230             }
 231             IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
 232             TargetFeatureCast(_) => write!(
 233                 f,
 234                 "cannot coerce functions with `#[target_feature]` to safe function pointers"
 235             ),
 236         }
 237     }
 238 }
 239
 240 impl<'tcx> TypeError<'tcx> {
 241     pub fn must_include_note(self) -> bool {
 242         use self::TypeError::*;
 243         match self {
 244             CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
 245             | PolarityMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
 246             | ArgumentSorts(..) | Sorts(_) | IntMismatch(_) | FloatMismatch(_)
 247             | VariadicMismatch(_) | TargetFeatureCast(_) => false,
 248
 249             Mutability
 250             | ArgumentMutability(_)
 251             | TupleSize(_)
 252             | ArgCount
 253             | FieldMisMatch(..)
 254             | RegionsDoesNotOutlive(..)
 255             | RegionsInsufficientlyPolymorphic(..)
 256             | RegionsOverlyPolymorphic(..)
 257             | RegionsPlaceholderMismatch
 258             | Traits(_)
 259             | ProjectionMismatched(_)
 260             | ExistentialMismatch(_)
 261             | ConstMismatch(_)
 262             | IntrinsicCast => true,
 263         }
 264     }
 265 }
 266
 267 impl<'tcx> Ty<'tcx> {
 268     pub fn sort_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 269         match *self.kind() {
 270             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
 271                 format!("`{}`", self).into()
 272             }
 273             ty::Tuple(ref tys) if tys.is_empty() => format!("`{}`", self).into(),
 274
 275             ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did())).into(),
 276             ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
 277             ty::Array(t, n) => {
 278                 if t.is_simple_ty() {
 279                     return format!("array `{}`", self).into();
 280                 }
 281
 282                 let n = tcx.lift(n).unwrap();
 283                 if let ty::ConstKind::Value(v) = n.kind() {
 284                     if let Some(n) = v.try_to_machine_usize(tcx) {
 285                         return format!("array of {} element{}", n, pluralize!(n)).into();
 286                     }
 287                 }
 288                 "array".into()
 289             }
 290             ty::Slice(ty) if ty.is_simple_ty() => format!("slice `{}`", self).into(),
 291             ty::Slice(_) => "slice".into(),
 292             ty::RawPtr(tymut) => {
 293                 let tymut_string = match tymut.mutbl {
 294                     hir::Mutability::Mut => tymut.to_string(),
 295                     hir::Mutability::Not => format!("const {}", tymut.ty),
 296                 };
 297
 298                 if tymut_string != "_" && (tymut.ty.is_simple_text() || tymut_string.len() < "const raw pointer".len()) {
 299                     format!("`*{}`", tymut_string).into()
 300                 } else {
 301                     // Unknown type name, it's long or has type arguments
 302                     "raw pointer".into()
 303                 }
 304             },
 305             ty::Ref(_, ty, mutbl) => {
 306                 let tymut = ty::TypeAndMut { ty, mutbl };
 307                 let tymut_string = tymut.to_string();
 308
 309                 if tymut_string != "_"
 310                     && (ty.is_simple_text() || tymut_string.len() < "mutable reference".len())
 311                 {
 312                     format!("`&{}`", tymut_string).into()
 313                 } else {
 314                     // Unknown type name, it's long or has type arguments
 315                     match mutbl {
 316                         hir::Mutability::Mut => "mutable reference",
 317                         _ => "reference",
 318                     }
 319                     .into()
 320                 }
 321             }
 322             ty::FnDef(..) => "fn item".into(),
 323             ty::FnPtr(_) => "fn pointer".into(),
 324             ty::Dynamic(ref inner, ..) if let Some(principal) = inner.principal() => {
 325                 format!("trait object `dyn {}`", tcx.def_path_str(principal.def_id())).into()
 326             }
 327             ty::Dynamic(..) => "trait object".into(),
 328             ty::Closure(..) => "closure".into(),
 329             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 330             ty::GeneratorWitness(..) => "generator witness".into(),
 331             ty::Tuple(..) => "tuple".into(),
 332             ty::Infer(ty::TyVar(_)) => "inferred type".into(),
 333             ty::Infer(ty::IntVar(_)) => "integer".into(),
 334             ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
 335             ty::Placeholder(..) => "placeholder type".into(),
 336             ty::Bound(..) => "bound type".into(),
 337             ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
 338             ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
 339             ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
 340             ty::Alias(ty::Projection, _) => "associated type".into(),
 341             ty::Param(p) => format!("type parameter `{}`", p).into(),
 342             ty::Alias(ty::Opaque, ..) => "opaque type".into(),
 343             ty::Error(_) => "type error".into(),
 344         }
 345     }
 346
 347     pub fn prefix_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 348         match *self.kind() {
 349             ty::Infer(_)
 350             | ty::Error(_)
 351             | ty::Bool
 352             | ty::Char
 353             | ty::Int(_)
 354             | ty::Uint(_)
 355             | ty::Float(_)
 356             | ty::Str
 357             | ty::Never => "type".into(),
 358             ty::Tuple(ref tys) if tys.is_empty() => "unit type".into(),
 359             ty::Adt(def, _) => def.descr().into(),
 360             ty::Foreign(_) => "extern type".into(),
 361             ty::Array(..) => "array".into(),
 362             ty::Slice(_) => "slice".into(),
 363             ty::RawPtr(_) => "raw pointer".into(),
 364             ty::Ref(.., mutbl) => match mutbl {
 365                 hir::Mutability::Mut => "mutable reference",
 366                 _ => "reference",
 367             }
 368             .into(),
 369             ty::FnDef(..) => "fn item".into(),
 370             ty::FnPtr(_) => "fn pointer".into(),
 371             ty::Dynamic(..) => "trait object".into(),
 372             ty::Closure(..) => "closure".into(),
 373             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 374             ty::GeneratorWitness(..) => "generator witness".into(),
 375             ty::Tuple(..) => "tuple".into(),
 376             ty::Placeholder(..) => "higher-ranked type".into(),
 377             ty::Bound(..) => "bound type variable".into(),
 378             ty::Alias(ty::Projection, _) => "associated type".into(),
 379             ty::Param(_) => "type parameter".into(),
 380             ty::Alias(ty::Opaque, ..) => "opaque type".into(),
 381         }
 382     }
 383 }
 384
 385 impl<'tcx> TyCtxt<'tcx> {
 386     pub fn note_and_explain_type_err(
 387         self,
 388         diag: &mut Diagnostic,
 389         err: TypeError<'tcx>,
 390         cause: &ObligationCause<'tcx>,
 391         sp: Span,
 392         body_owner_def_id: DefId,
 393     ) {
 394         use self::TypeError::*;
 395         debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
 396         match err {
 397             ArgumentSorts(values, _) | Sorts(values) => {
 398                 match (values.expected.kind(), values.found.kind()) {
 399                     (ty::Closure(..), ty::Closure(..)) => {
 400                         diag.note("no two closures, even if identical, have the same type");
 401                         diag.help("consider boxing your closure and/or using it as a trait object");
 402                     }
 403                     (ty::Alias(ty::Opaque, ..), ty::Alias(ty::Opaque, ..)) => {
 404                         // Issue #63167
 405                         diag.note("distinct uses of `impl Trait` result in different opaque types");
 406                     }
 407                     (ty::Float(_), ty::Infer(ty::IntVar(_)))
 408                         if let Ok(
 409                             // Issue #53280
 410                             snippet,
 411                         ) = self.sess.source_map().span_to_snippet(sp) =>
 412                     {
 413                         if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
 414                             diag.span_suggestion(
 415                                 sp,
 416                                 "use a float literal",
 417                                 format!("{}.0", snippet),
 418                                 MachineApplicable,
 419                             );
 420                         }
 421                     }
 422                     (ty::Param(expected), ty::Param(found)) => {
 423                         let generics = self.generics_of(body_owner_def_id);
 424                         let e_span = self.def_span(generics.type_param(expected, self).def_id);
 425                         if !sp.contains(e_span) {
 426                             diag.span_label(e_span, "expected type parameter");
 427                         }
 428                         let f_span = self.def_span(generics.type_param(found, self).def_id);
 429                         if !sp.contains(f_span) {
 430                             diag.span_label(f_span, "found type parameter");
 431                         }
 432                         diag.note(
 433                             "a type parameter was expected, but a different one was found; \
 434                              you might be missing a type parameter or trait bound",
 435                         );
 436                         diag.note(
 437                             "for more information, visit \
 438                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 439                              #traits-as-parameters",
 440                         );
 441                     }
 442                     (ty::Alias(ty::Projection, _), ty::Alias(ty::Projection, _)) => {
 443                         diag.note("an associated type was expected, but a different one was found");
 444                     }
 445                     (ty::Param(p), ty::Alias(ty::Projection, proj)) | (ty::Alias(ty::Projection, proj), ty::Param(p))
 446                         if self.def_kind(proj.def_id) != DefKind::ImplTraitPlaceholder =>
 447                     {
 448                         let generics = self.generics_of(body_owner_def_id);
 449                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 450                         if !sp.contains(p_span) {
 451                             diag.span_label(p_span, "this type parameter");
 452                         }
 453                         let hir = self.hir();
 454                         let mut note = true;
 455                         if let Some(generics) = generics
 456                             .type_param(p, self)
 457                             .def_id
 458                             .as_local()
 459                             .map(|id| hir.local_def_id_to_hir_id(id))
 460                             .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
 461                             .as_ref()
 462                             .and_then(|node| node.generics())
 463                         {
 464                             // Synthesize the associated type restriction `Add<Output = Expected>`.
 465                             // FIXME: extract this logic for use in other diagnostics.
 466                             let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
 467                             let path =
 468                                 self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
 469                             let item_name = self.item_name(proj.def_id);
 470                             let item_args = self.format_generic_args(assoc_substs);
 471
 472                             let path = if path.ends_with('>') {
 473                                 format!(
 474                                     "{}, {}{} = {}>",
 475                                     &path[..path.len() - 1],
 476                                     item_name,
 477                                     item_args,
 478                                     p
 479                                 )
 480                             } else {
 481                                 format!("{}<{}{} = {}>", path, item_name, item_args, p)
 482                             };
 483                             note = !suggest_constraining_type_param(
 484                                 self,
 485                                 generics,
 486                                 diag,
 487                                 &format!("{}", proj.self_ty()),
 488                                 &path,
 489                                 None,
 490                             );
 491                         }
 492                         if note {
 493                             diag.note("you might be missing a type parameter or trait bound");
 494                         }
 495                     }
 496                     (ty::Param(p), ty::Dynamic(..) | ty::Alias(ty::Opaque, ..))
 497                     | (ty::Dynamic(..) | ty::Alias(ty::Opaque, ..), ty::Param(p)) => {
 498                         let generics = self.generics_of(body_owner_def_id);
 499                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 500                         if !sp.contains(p_span) {
 501                             diag.span_label(p_span, "this type parameter");
 502                         }
 503                         diag.help("type parameters must be constrained to match other types");
 504                         if self.sess.teach(&diag.get_code().unwrap()) {
 505                             diag.help(
 506                                 "given a type parameter `T` and a method `foo`:
 507 ```
 508 trait Trait<T> { fn foo(&self) -> T; }
 509 ```
 510 the only ways to implement method `foo` are:
 511 - constrain `T` with an explicit type:
 512 ```
 513 impl Trait<String> for X {
 514     fn foo(&self) -> String { String::new() }
 515 }
 516 ```
 517 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 518 ```
 519 impl<T: std::default::Default> Trait<T> for X {
 520     fn foo(&self) -> T { <T as std::default::Default>::default() }
 521 }
 522 ```
 523 - change `foo` to return an argument of type `T`:
 524 ```
 525 impl<T> Trait<T> for X {
 526     fn foo(&self, x: T) -> T { x }
 527 }
 528 ```",
 529                             );
 530                         }
 531                         diag.note(
 532                             "for more information, visit \
 533                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 534                              #traits-as-parameters",
 535                         );
 536                     }
 537                     (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
 538                         let generics = self.generics_of(body_owner_def_id);
 539                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 540                         if !sp.contains(p_span) {
 541                             diag.span_label(p_span, "this type parameter");
 542                         }
 543                         diag.help(&format!(
 544                             "every closure has a distinct type and so could not always match the \
 545                              caller-chosen type of parameter `{}`",
 546                             p
 547                         ));
 548                     }
 549                     (ty::Param(p), _) | (_, ty::Param(p)) => {
 550                         let generics = self.generics_of(body_owner_def_id);
 551                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 552                         if !sp.contains(p_span) {
 553                             diag.span_label(p_span, "this type parameter");
 554                         }
 555                     }
 556                     (ty::Alias(ty::Projection, proj_ty), _) if self.def_kind(proj_ty.def_id) != DefKind::ImplTraitPlaceholder => {
 557                         self.expected_projection(
 558                             diag,
 559                             proj_ty,
 560                             values,
 561                             body_owner_def_id,
 562                             cause.code(),
 563                         );
 564                     }
 565                     (_, ty::Alias(ty::Projection, proj_ty)) if self.def_kind(proj_ty.def_id) != DefKind::ImplTraitPlaceholder => {
 566                         let msg = format!(
 567                             "consider constraining the associated type `{}` to `{}`",
 568                             values.found, values.expected,
 569                         );
 570                         if !(self.suggest_constraining_opaque_associated_type(
 571                             diag,
 572                             &msg,
 573                             proj_ty,
 574                             values.expected,
 575                         ) || self.suggest_constraint(
 576                             diag,
 577                             &msg,
 578                             body_owner_def_id,
 579                             proj_ty,
 580                             values.expected,
 581                         )) {
 582                             diag.help(&msg);
 583                             diag.note(
 584                                 "for more information, visit \
 585                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 586                             );
 587                         }
 588                     }
 589                     _ => {}
 590                 }
 591                 debug!(
 592                     "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
 593                     values.expected,
 594                     values.expected.kind(),
 595                     values.found,
 596                     values.found.kind(),
 597                 );
 598             }
 599             CyclicTy(ty) => {
 600                 // Watch out for various cases of cyclic types and try to explain.
 601                 if ty.is_closure() || ty.is_generator() {
 602                     diag.note(
 603                         "closures cannot capture themselves or take themselves as argument;\n\
 604                          this error may be the result of a recent compiler bug-fix,\n\
 605                          see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
 606                          for more information",
 607                     );
 608                 }
 609             }
 610             TargetFeatureCast(def_id) => {
 611                 let target_spans =
 612                     self.get_attrs(def_id, sym::target_feature).map(|attr| attr.span);
 613                 diag.note(
 614                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 615                 );
 616                 diag.span_labels(target_spans, "`#[target_feature]` added here");
 617             }
 618             _ => {}
 619         }
 620     }
 621
 622     fn suggest_constraint(
 623         self,
 624         diag: &mut Diagnostic,
 625         msg: &str,
 626         body_owner_def_id: DefId,
 627         proj_ty: &ty::AliasTy<'tcx>,
 628         ty: Ty<'tcx>,
 629     ) -> bool {
 630         let assoc = self.associated_item(proj_ty.def_id);
 631         let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 632         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
 633             if let Some(hir_generics) = item.generics() {
 634                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 635                 // This will also work for `impl Trait`.
 636                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
 637                     let generics = self.generics_of(body_owner_def_id);
 638                     generics.type_param(param_ty, self).def_id
 639                 } else {
 640                     return false;
 641                 };
 642                 let Some(def_id) = def_id.as_local() else {
 643                     return false;
 644                 };
 645
 646                 // First look in the `where` clause, as this might be
 647                 // `fn foo<T>(x: T) where T: Trait`.
 648                 for pred in hir_generics.bounds_for_param(def_id) {
 649                     if self.constrain_generic_bound_associated_type_structured_suggestion(
 650                         diag,
 651                         &trait_ref,
 652                         pred.bounds,
 653                         &assoc,
 654                         assoc_substs,
 655                         ty,
 656                         msg,
 657                         false,
 658                     ) {
 659                         return true;
 660                     }
 661                 }
 662             }
 663         }
 664         false
 665     }
 666
 667     /// An associated type was expected and a different type was found.
 668     ///
 669     /// We perform a few different checks to see what we can suggest:
 670     ///
 671     ///  - In the current item, look for associated functions that return the expected type and
 672     ///    suggest calling them. (Not a structured suggestion.)
 673     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 674     ///    associated type to the found type.
 675     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 676     ///    mention that this is disallowed.
 677     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 678     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 679     ///    fn that returns the type.
 680     fn expected_projection(
 681         self,
 682         diag: &mut Diagnostic,
 683         proj_ty: &ty::AliasTy<'tcx>,
 684         values: ExpectedFound<Ty<'tcx>>,
 685         body_owner_def_id: DefId,
 686         cause_code: &ObligationCauseCode<'_>,
 687     ) {
 688         let msg = format!(
 689             "consider constraining the associated type `{}` to `{}`",
 690             values.expected, values.found
 691         );
 692         let body_owner = self.hir().get_if_local(body_owner_def_id);
 693         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 694
 695         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 696         let callable_scope = matches!(
 697             body_owner,
 698             Some(
 699                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 700                     | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 701                     | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 702             )
 703         );
 704         let impl_comparison =
 705             matches!(cause_code, ObligationCauseCode::CompareImplItemObligation { .. });
 706         let assoc = self.associated_item(proj_ty.def_id);
 707         if !callable_scope || impl_comparison {
 708             // We do not want to suggest calling functions when the reason of the
 709             // type error is a comparison of an `impl` with its `trait` or when the
 710             // scope is outside of a `Body`.
 711         } else {
 712             // If we find a suitable associated function that returns the expected type, we don't
 713             // want the more general suggestion later in this method about "consider constraining
 714             // the associated type or calling a method that returns the associated type".
 715             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 716                 diag,
 717                 assoc.container_id(self),
 718                 current_method_ident,
 719                 proj_ty.def_id,
 720                 values.expected,
 721             );
 722             // Possibly suggest constraining the associated type to conform to the
 723             // found type.
 724             if self.suggest_constraint(diag, &msg, body_owner_def_id, proj_ty, values.found)
 725                 || point_at_assoc_fn
 726             {
 727                 return;
 728             }
 729         }
 730
 731         self.suggest_constraining_opaque_associated_type(diag, &msg, proj_ty, values.found);
 732
 733         if self.point_at_associated_type(diag, body_owner_def_id, values.found) {
 734             return;
 735         }
 736
 737         if !impl_comparison {
 738             // Generic suggestion when we can't be more specific.
 739             if callable_scope {
 740                 diag.help(&format!(
 741                     "{} or calling a method that returns `{}`",
 742                     msg, values.expected
 743                 ));
 744             } else {
 745                 diag.help(&msg);
 746             }
 747             diag.note(
 748                 "for more information, visit \
 749                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 750             );
 751         }
 752         if self.sess.teach(&diag.get_code().unwrap()) {
 753             diag.help(
 754                 "given an associated type `T` and a method `foo`:
 755 ```
 756 trait Trait {
 757 type T;
 758 fn foo(&self) -> Self::T;
 759 }
 760 ```
 761 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 762 ```
 763 impl Trait for X {
 764 type T = String;
 765 fn foo(&self) -> Self::T { String::new() }
 766 }
 767 ```",
 768             );
 769         }
 770     }
 771
 772     /// When the expected `impl Trait` is not defined in the current item, it will come from
 773     /// a return type. This can occur when dealing with `TryStream` (#71035).
 774     fn suggest_constraining_opaque_associated_type(
 775         self,
 776         diag: &mut Diagnostic,
 777         msg: &str,
 778         proj_ty: &ty::AliasTy<'tcx>,
 779         ty: Ty<'tcx>,
 780     ) -> bool {
 781         let assoc = self.associated_item(proj_ty.def_id);
 782         if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = *proj_ty.self_ty().kind() {
 783             let opaque_local_def_id = def_id.as_local();
 784             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
 785                 match &self.hir().expect_item(opaque_local_def_id).kind {
 786                     hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
 787                     _ => bug!("The HirId comes from a `ty::Opaque`"),
 788                 }
 789             } else {
 790                 return false;
 791             };
 792
 793             let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 794
 795             self.constrain_generic_bound_associated_type_structured_suggestion(
 796                 diag,
 797                 &trait_ref,
 798                 opaque_hir_ty.bounds,
 799                 assoc,
 800                 assoc_substs,
 801                 ty,
 802                 msg,
 803                 true,
 804             )
 805         } else {
 806             false
 807         }
 808     }
 809
 810     fn point_at_methods_that_satisfy_associated_type(
 811         self,
 812         diag: &mut Diagnostic,
 813         assoc_container_id: DefId,
 814         current_method_ident: Option<Symbol>,
 815         proj_ty_item_def_id: DefId,
 816         expected: Ty<'tcx>,
 817     ) -> bool {
 818         let items = self.associated_items(assoc_container_id);
 819         // Find all the methods in the trait that could be called to construct the
 820         // expected associated type.
 821         // FIXME: consider suggesting the use of associated `const`s.
 822         let methods: Vec<(Span, String)> = items
 823             .items
 824             .iter()
 825             .filter(|(name, item)| {
 826                 ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
 827             })
 828             .filter_map(|(_, item)| {
 829                 let method = self.fn_sig(item.def_id);
 830                 match *method.output().skip_binder().kind() {
 831                     ty::Alias(ty::Projection, ty::AliasTy { def_id: item_def_id, .. })
 832                         if item_def_id == proj_ty_item_def_id =>
 833                     {
 834                         Some((
 835                             self.def_span(item.def_id),
 836                             format!("consider calling `{}`", self.def_path_str(item.def_id)),
 837                         ))
 838                     }
 839                     _ => None,
 840                 }
 841             })
 842             .collect();
 843         if !methods.is_empty() {
 844             // Use a single `help:` to show all the methods in the trait that can
 845             // be used to construct the expected associated type.
 846             let mut span: MultiSpan =
 847                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 848             let msg = format!(
 849                 "{some} method{s} {are} available that return{r} `{ty}`",
 850                 some = if methods.len() == 1 { "a" } else { "some" },
 851                 s = pluralize!(methods.len()),
 852                 are = pluralize!("is", methods.len()),
 853                 r = if methods.len() == 1 { "s" } else { "" },
 854                 ty = expected
 855             );
 856             for (sp, label) in methods.into_iter() {
 857                 span.push_span_label(sp, label);
 858             }
 859             diag.span_help(span, &msg);
 860             return true;
 861         }
 862         false
 863     }
 864
 865     fn point_at_associated_type(
 866         self,
 867         diag: &mut Diagnostic,
 868         body_owner_def_id: DefId,
 869         found: Ty<'tcx>,
 870     ) -> bool {
 871         let Some(hir_id) = body_owner_def_id.as_local() else {
 872             return false;
 873         };
 874         let hir_id = self.hir().local_def_id_to_hir_id(hir_id);
 875         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 876         // `expected` and point at it.
 877         let parent_id = self.hir().get_parent_item(hir_id);
 878         let item = self.hir().find_by_def_id(parent_id.def_id);
 879         debug!("expected_projection parent item {:?}", item);
 880         match item {
 881             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 882                 // FIXME: account for `#![feature(specialization)]`
 883                 for item in &items[..] {
 884                     match item.kind {
 885                         hir::AssocItemKind::Type => {
 886                             // FIXME: account for returning some type in a trait fn impl that has
 887                             // an assoc type as a return type (#72076).
 888                             if let hir::Defaultness::Default { has_value: true } =
 889                                 self.impl_defaultness(item.id.owner_id)
 890                             {
 891                                 if self.type_of(item.id.owner_id) == found {
 892                                     diag.span_label(
 893                                         item.span,
 894                                         "associated type defaults can't be assumed inside the \
 895                                             trait defining them",
 896                                     );
 897                                     return true;
 898                                 }
 899                             }
 900                         }
 901                         _ => {}
 902                     }
 903                 }
 904             }
 905             Some(hir::Node::Item(hir::Item {
 906                 kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
 907                 ..
 908             })) => {
 909                 for item in &items[..] {
 910                     if let hir::AssocItemKind::Type = item.kind {
 911                         if self.type_of(item.id.owner_id) == found {
 912                             diag.span_label(item.span, "expected this associated type");
 913                             return true;
 914                         }
 915                     }
 916                 }
 917             }
 918             _ => {}
 919         }
 920         false
 921     }
 922
 923     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 924     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
 925     ///
 926     /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
 927     /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
 928     /// trait bound as the one we're looking for. This can help in cases where the associated
 929     /// type is defined on a supertrait of the one present in the bounds.
 930     fn constrain_generic_bound_associated_type_structured_suggestion(
 931         self,
 932         diag: &mut Diagnostic,
 933         trait_ref: &ty::TraitRef<'tcx>,
 934         bounds: hir::GenericBounds<'_>,
 935         assoc: &ty::AssocItem,
 936         assoc_substs: &[ty::GenericArg<'tcx>],
 937         ty: Ty<'tcx>,
 938         msg: &str,
 939         is_bound_surely_present: bool,
 940     ) -> bool {
 941         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 942
 943         let trait_bounds = bounds.iter().filter_map(|bound| match bound {
 944             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
 945             _ => None,
 946         });
 947
 948         let matching_trait_bounds = trait_bounds
 949             .clone()
 950             .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
 951             .collect::<Vec<_>>();
 952
 953         let span = match &matching_trait_bounds[..] {
 954             &[ptr] => ptr.span,
 955             &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
 956                 &[ptr] => ptr.span,
 957                 _ => return false,
 958             },
 959             _ => return false,
 960         };
 961
 962         self.constrain_associated_type_structured_suggestion(
 963             diag,
 964             span,
 965             assoc,
 966             assoc_substs,
 967             ty,
 968             msg,
 969         )
 970     }
 971
 972     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 973     /// associated type to a given type `ty`.
 974     fn constrain_associated_type_structured_suggestion(
 975         self,
 976         diag: &mut Diagnostic,
 977         span: Span,
 978         assoc: &ty::AssocItem,
 979         assoc_substs: &[ty::GenericArg<'tcx>],
 980         ty: Ty<'tcx>,
 981         msg: &str,
 982     ) -> bool {
 983         if let Ok(has_params) =
 984             self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 985         {
 986             let (span, sugg) = if has_params {
 987                 let pos = span.hi() - BytePos(1);
 988                 let span = Span::new(pos, pos, span.ctxt(), span.parent());
 989                 (span, format!(", {} = {}", assoc.ident(self), ty))
 990             } else {
 991                 let item_args = self.format_generic_args(assoc_substs);
 992                 (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(self), item_args, ty))
 993             };
 994             diag.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 995             return true;
 996         }
 997         false
 998     }
 999
1000     pub fn short_ty_string(self, ty: Ty<'tcx>) -> (String, Option<PathBuf>) {
1001         let width = self.sess.diagnostic_width();
1002         let length_limit = width.saturating_sub(30);
1003         let mut type_limit = 50;
1004         let regular = FmtPrinter::new(self, hir::def::Namespace::TypeNS)
1005             .pretty_print_type(ty)
1006             .expect("could not write to `String`")
1007             .into_buffer();
1008         if regular.len() <= width {
1009             return (regular, None);
1010         }
1011         let mut short;
1012         loop {
1013             // Look for the longest properly trimmed path that still fits in length_limit.
1014             short = with_forced_trimmed_paths!(
1015                 FmtPrinter::new_with_limit(
1016                     self,
1017                     hir::def::Namespace::TypeNS,
1018                     rustc_session::Limit(type_limit),
1019                 )
1020                 .pretty_print_type(ty)
1021                 .expect("could not write to `String`")
1022                 .into_buffer()
1023             );
1024             if short.len() <= length_limit || type_limit == 0 {
1025                 break;
1026             }
1027             type_limit -= 1;
1028         }
1029         if regular == short {
1030             return (regular, None);
1031         }
1032         // Multiple types might be shortened in a single error, ensure we create a file for each.
1033         let mut s = DefaultHasher::new();
1034         ty.hash(&mut s);
1035         let hash = s.finish();
1036         let path = self.output_filenames(()).temp_path_ext(&format!("long-type-{hash}.txt"), None);
1037         match std::fs::write(&path, &regular) {
1038             Ok(_) => (short, Some(path)),
1039             Err(_) => (regular, None),
1040         }
1041     }
1042
1043     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
1044         FmtPrinter::new(self, hir::def::Namespace::TypeNS)
1045             .path_generic_args(Ok, args)
1046             .expect("could not write to `String`.")
1047             .into_buffer()
1048     }
1049 }