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::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     ObjectUnsafeCoercion(DefId),
  74     ConstMismatch(ExpectedFound<ty::Const<'tcx>>),
  75
  76     IntrinsicCast,
  77     /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
  78     TargetFeatureCast(DefId),
  79 }
  80
  81 impl TypeError<'_> {
  82     pub fn involves_regions(self) -> bool {
  83         match self {
  84             TypeError::RegionsDoesNotOutlive(_, _)
  85             | TypeError::RegionsInsufficientlyPolymorphic(_, _)
  86             | TypeError::RegionsOverlyPolymorphic(_, _)
  87             | TypeError::RegionsPlaceholderMismatch => true,
  88             _ => false,
  89         }
  90     }
  91 }
  92
  93 /// Explains the source of a type err in a short, human readable way. This is meant to be placed
  94 /// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
  95 /// afterwards to present additional details, particularly when it comes to lifetime-related
  96 /// errors.
  97 impl<'tcx> fmt::Display for TypeError<'tcx> {
  98     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  99         use self::TypeError::*;
 100         fn report_maybe_different(
 101             f: &mut fmt::Formatter<'_>,
 102             expected: &str,
 103             found: &str,
 104         ) -> fmt::Result {
 105             // A naive approach to making sure that we're not reporting silly errors such as:
 106             // (expected closure, found closure).
 107             if expected == found {
 108                 write!(f, "expected {}, found a different {}", expected, found)
 109             } else {
 110                 write!(f, "expected {}, found {}", expected, found)
 111             }
 112         }
 113
 114         let br_string = |br: ty::BoundRegionKind| match br {
 115             ty::BrNamed(_, name) => format!(" {}", name),
 116             _ => String::new(),
 117         };
 118
 119         match *self {
 120             CyclicTy(_) => write!(f, "cyclic type of infinite size"),
 121             CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
 122             Mismatch => write!(f, "types differ"),
 123             ConstnessMismatch(values) => {
 124                 write!(f, "expected {} bound, found {} bound", values.expected, values.found)
 125             }
 126             PolarityMismatch(values) => {
 127                 write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
 128             }
 129             UnsafetyMismatch(values) => {
 130                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 131             }
 132             AbiMismatch(values) => {
 133                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 134             }
 135             ArgumentMutability(_) | Mutability => write!(f, "types differ in mutability"),
 136             TupleSize(values) => write!(
 137                 f,
 138                 "expected a tuple with {} element{}, found one with {} element{}",
 139                 values.expected,
 140                 pluralize!(values.expected),
 141                 values.found,
 142                 pluralize!(values.found)
 143             ),
 144             FixedArraySize(values) => write!(
 145                 f,
 146                 "expected an array with a fixed size of {} element{}, found one with {} element{}",
 147                 values.expected,
 148                 pluralize!(values.expected),
 149                 values.found,
 150                 pluralize!(values.found)
 151             ),
 152             ArgCount => write!(f, "incorrect number of function parameters"),
 153             FieldMisMatch(adt, field) => write!(f, "field type mismatch: {}.{}", adt, field),
 154             RegionsDoesNotOutlive(..) => write!(f, "lifetime mismatch"),
 155             // Actually naming the region here is a bit confusing because context is lacking
 156             RegionsInsufficientlyPolymorphic(..) => {
 157                 write!(f, "one type is more general than the other")
 158             }
 159             RegionsOverlyPolymorphic(br, _) => write!(
 160                 f,
 161                 "expected concrete lifetime, found bound lifetime parameter{}",
 162                 br_string(br)
 163             ),
 164             RegionsPlaceholderMismatch => write!(f, "one type is more general than the other"),
 165             ArgumentSorts(values, _) | Sorts(values) => ty::tls::with(|tcx| {
 166                 report_maybe_different(
 167                     f,
 168                     &values.expected.sort_string(tcx),
 169                     &values.found.sort_string(tcx),
 170                 )
 171             }),
 172             Traits(values) => ty::tls::with(|tcx| {
 173                 report_maybe_different(
 174                     f,
 175                     &format!("trait `{}`", tcx.def_path_str(values.expected)),
 176                     &format!("trait `{}`", tcx.def_path_str(values.found)),
 177                 )
 178             }),
 179             IntMismatch(ref values) => {
 180                 let expected = match values.expected {
 181                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 182                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 183                 };
 184                 let found = match values.found {
 185                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 186                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 187                 };
 188                 write!(f, "expected `{}`, found `{}`", expected, found)
 189             }
 190             FloatMismatch(ref values) => {
 191                 write!(
 192                     f,
 193                     "expected `{}`, found `{}`",
 194                     values.expected.name_str(),
 195                     values.found.name_str()
 196                 )
 197             }
 198             VariadicMismatch(ref values) => write!(
 199                 f,
 200                 "expected {} fn, found {} function",
 201                 if values.expected { "variadic" } else { "non-variadic" },
 202                 if values.found { "variadic" } else { "non-variadic" }
 203             ),
 204             ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
 205                 write!(
 206                     f,
 207                     "expected {}, found {}",
 208                     tcx.def_path_str(values.expected),
 209                     tcx.def_path_str(values.found)
 210                 )
 211             }),
 212             ExistentialMismatch(ref values) => report_maybe_different(
 213                 f,
 214                 &format!("trait `{}`", values.expected),
 215                 &format!("trait `{}`", values.found),
 216             ),
 217             ConstMismatch(ref values) => {
 218                 write!(f, "expected `{}`, found `{}`", values.expected, values.found)
 219             }
 220             IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
 221             TargetFeatureCast(_) => write!(
 222                 f,
 223                 "cannot coerce functions with `#[target_feature]` to safe function pointers"
 224             ),
 225             ObjectUnsafeCoercion(_) => write!(f, "coercion to object-unsafe trait object"),
 226         }
 227     }
 228 }
 229
 230 impl<'tcx> TypeError<'tcx> {
 231     pub fn must_include_note(self) -> bool {
 232         use self::TypeError::*;
 233         match self {
 234             CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
 235             | PolarityMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
 236             | ArgumentSorts(..) | Sorts(_) | IntMismatch(_) | FloatMismatch(_)
 237             | VariadicMismatch(_) | TargetFeatureCast(_) => false,
 238
 239             Mutability
 240             | ArgumentMutability(_)
 241             | TupleSize(_)
 242             | ArgCount
 243             | FieldMisMatch(..)
 244             | RegionsDoesNotOutlive(..)
 245             | RegionsInsufficientlyPolymorphic(..)
 246             | RegionsOverlyPolymorphic(..)
 247             | RegionsPlaceholderMismatch
 248             | Traits(_)
 249             | ProjectionMismatched(_)
 250             | ExistentialMismatch(_)
 251             | ConstMismatch(_)
 252             | IntrinsicCast
 253             | ObjectUnsafeCoercion(_) => true,
 254         }
 255     }
 256 }
 257
 258 impl<'tcx> Ty<'tcx> {
 259     pub fn sort_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 260         match *self.kind() {
 261             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
 262                 format!("`{}`", self).into()
 263             }
 264             ty::Tuple(ref tys) if tys.is_empty() => format!("`{}`", self).into(),
 265
 266             ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did())).into(),
 267             ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
 268             ty::Array(t, n) => {
 269                 if t.is_simple_ty() {
 270                     return format!("array `{}`", self).into();
 271                 }
 272
 273                 let n = tcx.lift(n).unwrap();
 274                 if let ty::ConstKind::Value(v) = n.kind() {
 275                     if let Some(n) = v.try_to_machine_usize(tcx) {
 276                         return format!("array of {} element{}", n, pluralize!(n)).into();
 277                     }
 278                 }
 279                 "array".into()
 280             }
 281             ty::Slice(ty) if ty.is_simple_ty() => format!("slice `{}`", self).into(),
 282             ty::Slice(_) => "slice".into(),
 283             ty::RawPtr(tymut) => {
 284                 let tymut_string = match tymut.mutbl {
 285                     hir::Mutability::Mut => tymut.to_string(),
 286                     hir::Mutability::Not => format!("const {}", tymut.ty),
 287                 };
 288
 289                 if tymut_string != "_" && (tymut.ty.is_simple_text() || tymut_string.len() < "const raw pointer".len()) {
 290                     format!("`*{}`", tymut_string).into()
 291                 } else {
 292                     // Unknown type name, it's long or has type arguments
 293                     "raw pointer".into()
 294                 }
 295             },
 296             ty::Ref(_, ty, mutbl) => {
 297                 let tymut = ty::TypeAndMut { ty, mutbl };
 298                 let tymut_string = tymut.to_string();
 299
 300                 if tymut_string != "_"
 301                     && (ty.is_simple_text() || tymut_string.len() < "mutable reference".len())
 302                 {
 303                     format!("`&{}`", tymut_string).into()
 304                 } else {
 305                     // Unknown type name, it's long or has type arguments
 306                     match mutbl {
 307                         hir::Mutability::Mut => "mutable reference",
 308                         _ => "reference",
 309                     }
 310                     .into()
 311                 }
 312             }
 313             ty::FnDef(..) => "fn item".into(),
 314             ty::FnPtr(_) => "fn pointer".into(),
 315             ty::Dynamic(ref inner, ..) if let Some(principal) = inner.principal() => {
 316                 format!("trait object `dyn {}`", tcx.def_path_str(principal.def_id())).into()
 317             }
 318             ty::Dynamic(..) => "trait object".into(),
 319             ty::Closure(..) => "closure".into(),
 320             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 321             ty::GeneratorWitness(..) => "generator witness".into(),
 322             ty::Tuple(..) => "tuple".into(),
 323             ty::Infer(ty::TyVar(_)) => "inferred type".into(),
 324             ty::Infer(ty::IntVar(_)) => "integer".into(),
 325             ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
 326             ty::Placeholder(..) => "placeholder type".into(),
 327             ty::Bound(..) => "bound type".into(),
 328             ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
 329             ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
 330             ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
 331             ty::Projection(_) => "associated type".into(),
 332             ty::Param(p) => format!("type parameter `{}`", p).into(),
 333             ty::Opaque(..) => "opaque type".into(),
 334             ty::Error(_) => "type error".into(),
 335         }
 336     }
 337
 338     pub fn prefix_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 339         match *self.kind() {
 340             ty::Infer(_)
 341             | ty::Error(_)
 342             | ty::Bool
 343             | ty::Char
 344             | ty::Int(_)
 345             | ty::Uint(_)
 346             | ty::Float(_)
 347             | ty::Str
 348             | ty::Never => "type".into(),
 349             ty::Tuple(ref tys) if tys.is_empty() => "unit type".into(),
 350             ty::Adt(def, _) => def.descr().into(),
 351             ty::Foreign(_) => "extern type".into(),
 352             ty::Array(..) => "array".into(),
 353             ty::Slice(_) => "slice".into(),
 354             ty::RawPtr(_) => "raw pointer".into(),
 355             ty::Ref(.., mutbl) => match mutbl {
 356                 hir::Mutability::Mut => "mutable reference",
 357                 _ => "reference",
 358             }
 359             .into(),
 360             ty::FnDef(..) => "fn item".into(),
 361             ty::FnPtr(_) => "fn pointer".into(),
 362             ty::Dynamic(..) => "trait object".into(),
 363             ty::Closure(..) => "closure".into(),
 364             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 365             ty::GeneratorWitness(..) => "generator witness".into(),
 366             ty::Tuple(..) => "tuple".into(),
 367             ty::Placeholder(..) => "higher-ranked type".into(),
 368             ty::Bound(..) => "bound type variable".into(),
 369             ty::Projection(_) => "associated type".into(),
 370             ty::Param(_) => "type parameter".into(),
 371             ty::Opaque(..) => "opaque type".into(),
 372         }
 373     }
 374 }
 375
 376 impl<'tcx> TyCtxt<'tcx> {
 377     pub fn note_and_explain_type_err(
 378         self,
 379         diag: &mut Diagnostic,
 380         err: TypeError<'tcx>,
 381         cause: &ObligationCause<'tcx>,
 382         sp: Span,
 383         body_owner_def_id: DefId,
 384     ) {
 385         use self::TypeError::*;
 386         debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
 387         match err {
 388             ArgumentSorts(values, _) | Sorts(values) => {
 389                 match (values.expected.kind(), values.found.kind()) {
 390                     (ty::Closure(..), ty::Closure(..)) => {
 391                         diag.note("no two closures, even if identical, have the same type");
 392                         diag.help("consider boxing your closure and/or using it as a trait object");
 393                     }
 394                     (ty::Opaque(..), ty::Opaque(..)) => {
 395                         // Issue #63167
 396                         diag.note("distinct uses of `impl Trait` result in different opaque types");
 397                     }
 398                     (ty::Float(_), ty::Infer(ty::IntVar(_)))
 399                         if let Ok(
 400                             // Issue #53280
 401                             snippet,
 402                         ) = self.sess.source_map().span_to_snippet(sp) =>
 403                     {
 404                         if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
 405                             diag.span_suggestion(
 406                                 sp,
 407                                 "use a float literal",
 408                                 format!("{}.0", snippet),
 409                                 MachineApplicable,
 410                             );
 411                         }
 412                     }
 413                     (ty::Param(expected), ty::Param(found)) => {
 414                         let generics = self.generics_of(body_owner_def_id);
 415                         let e_span = self.def_span(generics.type_param(expected, self).def_id);
 416                         if !sp.contains(e_span) {
 417                             diag.span_label(e_span, "expected type parameter");
 418                         }
 419                         let f_span = self.def_span(generics.type_param(found, self).def_id);
 420                         if !sp.contains(f_span) {
 421                             diag.span_label(f_span, "found type parameter");
 422                         }
 423                         diag.note(
 424                             "a type parameter was expected, but a different one was found; \
 425                              you might be missing a type parameter or trait bound",
 426                         );
 427                         diag.note(
 428                             "for more information, visit \
 429                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 430                              #traits-as-parameters",
 431                         );
 432                     }
 433                     (ty::Projection(_), ty::Projection(_)) => {
 434                         diag.note("an associated type was expected, but a different one was found");
 435                     }
 436                     (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p))
 437                         if self.def_kind(proj.item_def_id) != DefKind::ImplTraitPlaceholder =>
 438                     {
 439                         let generics = self.generics_of(body_owner_def_id);
 440                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 441                         if !sp.contains(p_span) {
 442                             diag.span_label(p_span, "this type parameter");
 443                         }
 444                         let hir = self.hir();
 445                         let mut note = true;
 446                         if let Some(generics) = generics
 447                             .type_param(p, self)
 448                             .def_id
 449                             .as_local()
 450                             .map(|id| hir.local_def_id_to_hir_id(id))
 451                             .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
 452                             .as_ref()
 453                             .and_then(|node| node.generics())
 454                         {
 455                             // Synthesize the associated type restriction `Add<Output = Expected>`.
 456                             // FIXME: extract this logic for use in other diagnostics.
 457                             let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
 458                             let path =
 459                                 self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
 460                             let item_name = self.item_name(proj.item_def_id);
 461                             let item_args = self.format_generic_args(assoc_substs);
 462
 463                             let path = if path.ends_with('>') {
 464                                 format!(
 465                                     "{}, {}{} = {}>",
 466                                     &path[..path.len() - 1],
 467                                     item_name,
 468                                     item_args,
 469                                     p
 470                                 )
 471                             } else {
 472                                 format!("{}<{}{} = {}>", path, item_name, item_args, p)
 473                             };
 474                             note = !suggest_constraining_type_param(
 475                                 self,
 476                                 generics,
 477                                 diag,
 478                                 &format!("{}", proj.self_ty()),
 479                                 &path,
 480                                 None,
 481                             );
 482                         }
 483                         if note {
 484                             diag.note("you might be missing a type parameter or trait bound");
 485                         }
 486                     }
 487                     (ty::Param(p), ty::Dynamic(..) | ty::Opaque(..))
 488                     | (ty::Dynamic(..) | ty::Opaque(..), ty::Param(p)) => {
 489                         let generics = self.generics_of(body_owner_def_id);
 490                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 491                         if !sp.contains(p_span) {
 492                             diag.span_label(p_span, "this type parameter");
 493                         }
 494                         diag.help("type parameters must be constrained to match other types");
 495                         if self.sess.teach(&diag.get_code().unwrap()) {
 496                             diag.help(
 497                                 "given a type parameter `T` and a method `foo`:
 498 ```
 499 trait Trait<T> { fn foo(&self) -> T; }
 500 ```
 501 the only ways to implement method `foo` are:
 502 - constrain `T` with an explicit type:
 503 ```
 504 impl Trait<String> for X {
 505     fn foo(&self) -> String { String::new() }
 506 }
 507 ```
 508 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 509 ```
 510 impl<T: std::default::Default> Trait<T> for X {
 511     fn foo(&self) -> T { <T as std::default::Default>::default() }
 512 }
 513 ```
 514 - change `foo` to return an argument of type `T`:
 515 ```
 516 impl<T> Trait<T> for X {
 517     fn foo(&self, x: T) -> T { x }
 518 }
 519 ```",
 520                             );
 521                         }
 522                         diag.note(
 523                             "for more information, visit \
 524                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 525                              #traits-as-parameters",
 526                         );
 527                     }
 528                     (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
 529                         let generics = self.generics_of(body_owner_def_id);
 530                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 531                         if !sp.contains(p_span) {
 532                             diag.span_label(p_span, "this type parameter");
 533                         }
 534                         diag.help(&format!(
 535                             "every closure has a distinct type and so could not always match the \
 536                              caller-chosen type of parameter `{}`",
 537                             p
 538                         ));
 539                     }
 540                     (ty::Param(p), _) | (_, ty::Param(p)) => {
 541                         let generics = self.generics_of(body_owner_def_id);
 542                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 543                         if !sp.contains(p_span) {
 544                             diag.span_label(p_span, "this type parameter");
 545                         }
 546                     }
 547                     (ty::Projection(proj_ty), _) if self.def_kind(proj_ty.item_def_id) != DefKind::ImplTraitPlaceholder => {
 548                         self.expected_projection(
 549                             diag,
 550                             proj_ty,
 551                             values,
 552                             body_owner_def_id,
 553                             cause.code(),
 554                         );
 555                     }
 556                     (_, ty::Projection(proj_ty)) if self.def_kind(proj_ty.item_def_id) != DefKind::ImplTraitPlaceholder => {
 557                         let msg = format!(
 558                             "consider constraining the associated type `{}` to `{}`",
 559                             values.found, values.expected,
 560                         );
 561                         if !(self.suggest_constraining_opaque_associated_type(
 562                             diag,
 563                             &msg,
 564                             proj_ty,
 565                             values.expected,
 566                         ) || self.suggest_constraint(
 567                             diag,
 568                             &msg,
 569                             body_owner_def_id,
 570                             proj_ty,
 571                             values.expected,
 572                         )) {
 573                             diag.help(&msg);
 574                             diag.note(
 575                                 "for more information, visit \
 576                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 577                             );
 578                         }
 579                     }
 580                     _ => {}
 581                 }
 582                 debug!(
 583                     "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
 584                     values.expected,
 585                     values.expected.kind(),
 586                     values.found,
 587                     values.found.kind(),
 588                 );
 589             }
 590             CyclicTy(ty) => {
 591                 // Watch out for various cases of cyclic types and try to explain.
 592                 if ty.is_closure() || ty.is_generator() {
 593                     diag.note(
 594                         "closures cannot capture themselves or take themselves as argument;\n\
 595                          this error may be the result of a recent compiler bug-fix,\n\
 596                          see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
 597                          for more information",
 598                     );
 599                 }
 600             }
 601             TargetFeatureCast(def_id) => {
 602                 let target_spans =
 603                     self.get_attrs(def_id, sym::target_feature).map(|attr| attr.span);
 604                 diag.note(
 605                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 606                 );
 607                 diag.span_labels(target_spans, "`#[target_feature]` added here");
 608             }
 609             _ => {}
 610         }
 611     }
 612
 613     fn suggest_constraint(
 614         self,
 615         diag: &mut Diagnostic,
 616         msg: &str,
 617         body_owner_def_id: DefId,
 618         proj_ty: &ty::ProjectionTy<'tcx>,
 619         ty: Ty<'tcx>,
 620     ) -> bool {
 621         let assoc = self.associated_item(proj_ty.item_def_id);
 622         let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 623         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
 624             if let Some(hir_generics) = item.generics() {
 625                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 626                 // This will also work for `impl Trait`.
 627                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
 628                     let generics = self.generics_of(body_owner_def_id);
 629                     generics.type_param(param_ty, self).def_id
 630                 } else {
 631                     return false;
 632                 };
 633                 let Some(def_id) = def_id.as_local() else {
 634                     return false;
 635                 };
 636
 637                 // First look in the `where` clause, as this might be
 638                 // `fn foo<T>(x: T) where T: Trait`.
 639                 for pred in hir_generics.bounds_for_param(def_id) {
 640                     if self.constrain_generic_bound_associated_type_structured_suggestion(
 641                         diag,
 642                         &trait_ref,
 643                         pred.bounds,
 644                         &assoc,
 645                         assoc_substs,
 646                         ty,
 647                         msg,
 648                         false,
 649                     ) {
 650                         return true;
 651                     }
 652                 }
 653             }
 654         }
 655         false
 656     }
 657
 658     /// An associated type was expected and a different type was found.
 659     ///
 660     /// We perform a few different checks to see what we can suggest:
 661     ///
 662     ///  - In the current item, look for associated functions that return the expected type and
 663     ///    suggest calling them. (Not a structured suggestion.)
 664     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 665     ///    associated type to the found type.
 666     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 667     ///    mention that this is disallowed.
 668     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 669     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 670     ///    fn that returns the type.
 671     fn expected_projection(
 672         self,
 673         diag: &mut Diagnostic,
 674         proj_ty: &ty::ProjectionTy<'tcx>,
 675         values: ExpectedFound<Ty<'tcx>>,
 676         body_owner_def_id: DefId,
 677         cause_code: &ObligationCauseCode<'_>,
 678     ) {
 679         let msg = format!(
 680             "consider constraining the associated type `{}` to `{}`",
 681             values.expected, values.found
 682         );
 683         let body_owner = self.hir().get_if_local(body_owner_def_id);
 684         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 685
 686         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 687         let callable_scope = matches!(
 688             body_owner,
 689             Some(
 690                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 691                     | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 692                     | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 693             )
 694         );
 695         let impl_comparison =
 696             matches!(cause_code, ObligationCauseCode::CompareImplItemObligation { .. });
 697         let assoc = self.associated_item(proj_ty.item_def_id);
 698         if !callable_scope || impl_comparison {
 699             // We do not want to suggest calling functions when the reason of the
 700             // type error is a comparison of an `impl` with its `trait` or when the
 701             // scope is outside of a `Body`.
 702         } else {
 703             // If we find a suitable associated function that returns the expected type, we don't
 704             // want the more general suggestion later in this method about "consider constraining
 705             // the associated type or calling a method that returns the associated type".
 706             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 707                 diag,
 708                 assoc.container_id(self),
 709                 current_method_ident,
 710                 proj_ty.item_def_id,
 711                 values.expected,
 712             );
 713             // Possibly suggest constraining the associated type to conform to the
 714             // found type.
 715             if self.suggest_constraint(diag, &msg, body_owner_def_id, proj_ty, values.found)
 716                 || point_at_assoc_fn
 717             {
 718                 return;
 719             }
 720         }
 721
 722         self.suggest_constraining_opaque_associated_type(diag, &msg, proj_ty, values.found);
 723
 724         if self.point_at_associated_type(diag, body_owner_def_id, values.found) {
 725             return;
 726         }
 727
 728         if !impl_comparison {
 729             // Generic suggestion when we can't be more specific.
 730             if callable_scope {
 731                 diag.help(&format!(
 732                     "{} or calling a method that returns `{}`",
 733                     msg, values.expected
 734                 ));
 735             } else {
 736                 diag.help(&msg);
 737             }
 738             diag.note(
 739                 "for more information, visit \
 740                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 741             );
 742         }
 743         if self.sess.teach(&diag.get_code().unwrap()) {
 744             diag.help(
 745                 "given an associated type `T` and a method `foo`:
 746 ```
 747 trait Trait {
 748 type T;
 749 fn foo(&self) -> Self::T;
 750 }
 751 ```
 752 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 753 ```
 754 impl Trait for X {
 755 type T = String;
 756 fn foo(&self) -> Self::T { String::new() }
 757 }
 758 ```",
 759             );
 760         }
 761     }
 762
 763     /// When the expected `impl Trait` is not defined in the current item, it will come from
 764     /// a return type. This can occur when dealing with `TryStream` (#71035).
 765     fn suggest_constraining_opaque_associated_type(
 766         self,
 767         diag: &mut Diagnostic,
 768         msg: &str,
 769         proj_ty: &ty::ProjectionTy<'tcx>,
 770         ty: Ty<'tcx>,
 771     ) -> bool {
 772         let assoc = self.associated_item(proj_ty.item_def_id);
 773         if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
 774             let opaque_local_def_id = def_id.as_local();
 775             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
 776                 match &self.hir().expect_item(opaque_local_def_id).kind {
 777                     hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
 778                     _ => bug!("The HirId comes from a `ty::Opaque`"),
 779                 }
 780             } else {
 781                 return false;
 782             };
 783
 784             let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 785
 786             self.constrain_generic_bound_associated_type_structured_suggestion(
 787                 diag,
 788                 &trait_ref,
 789                 opaque_hir_ty.bounds,
 790                 assoc,
 791                 assoc_substs,
 792                 ty,
 793                 msg,
 794                 true,
 795             )
 796         } else {
 797             false
 798         }
 799     }
 800
 801     fn point_at_methods_that_satisfy_associated_type(
 802         self,
 803         diag: &mut Diagnostic,
 804         assoc_container_id: DefId,
 805         current_method_ident: Option<Symbol>,
 806         proj_ty_item_def_id: DefId,
 807         expected: Ty<'tcx>,
 808     ) -> bool {
 809         let items = self.associated_items(assoc_container_id);
 810         // Find all the methods in the trait that could be called to construct the
 811         // expected associated type.
 812         // FIXME: consider suggesting the use of associated `const`s.
 813         let methods: Vec<(Span, String)> = items
 814             .items
 815             .iter()
 816             .filter(|(name, item)| {
 817                 ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
 818             })
 819             .filter_map(|(_, item)| {
 820                 let method = self.fn_sig(item.def_id);
 821                 match *method.output().skip_binder().kind() {
 822                     ty::Projection(ty::ProjectionTy { item_def_id, .. })
 823                         if item_def_id == proj_ty_item_def_id =>
 824                     {
 825                         Some((
 826                             self.def_span(item.def_id),
 827                             format!("consider calling `{}`", self.def_path_str(item.def_id)),
 828                         ))
 829                     }
 830                     _ => None,
 831                 }
 832             })
 833             .collect();
 834         if !methods.is_empty() {
 835             // Use a single `help:` to show all the methods in the trait that can
 836             // be used to construct the expected associated type.
 837             let mut span: MultiSpan =
 838                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 839             let msg = format!(
 840                 "{some} method{s} {are} available that return{r} `{ty}`",
 841                 some = if methods.len() == 1 { "a" } else { "some" },
 842                 s = pluralize!(methods.len()),
 843                 are = pluralize!("is", methods.len()),
 844                 r = if methods.len() == 1 { "s" } else { "" },
 845                 ty = expected
 846             );
 847             for (sp, label) in methods.into_iter() {
 848                 span.push_span_label(sp, label);
 849             }
 850             diag.span_help(span, &msg);
 851             return true;
 852         }
 853         false
 854     }
 855
 856     fn point_at_associated_type(
 857         self,
 858         diag: &mut Diagnostic,
 859         body_owner_def_id: DefId,
 860         found: Ty<'tcx>,
 861     ) -> bool {
 862         let Some(hir_id) = body_owner_def_id.as_local() else {
 863             return false;
 864         };
 865         let hir_id = self.hir().local_def_id_to_hir_id(hir_id);
 866         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 867         // `expected` and point at it.
 868         let parent_id = self.hir().get_parent_item(hir_id);
 869         let item = self.hir().find_by_def_id(parent_id.def_id);
 870         debug!("expected_projection parent item {:?}", item);
 871         match item {
 872             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 873                 // FIXME: account for `#![feature(specialization)]`
 874                 for item in &items[..] {
 875                     match item.kind {
 876                         hir::AssocItemKind::Type => {
 877                             // FIXME: account for returning some type in a trait fn impl that has
 878                             // an assoc type as a return type (#72076).
 879                             if let hir::Defaultness::Default { has_value: true } =
 880                                 self.impl_defaultness(item.id.owner_id)
 881                             {
 882                                 if self.type_of(item.id.owner_id) == found {
 883                                     diag.span_label(
 884                                         item.span,
 885                                         "associated type defaults can't be assumed inside the \
 886                                             trait defining them",
 887                                     );
 888                                     return true;
 889                                 }
 890                             }
 891                         }
 892                         _ => {}
 893                     }
 894                 }
 895             }
 896             Some(hir::Node::Item(hir::Item {
 897                 kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
 898                 ..
 899             })) => {
 900                 for item in &items[..] {
 901                     if let hir::AssocItemKind::Type = item.kind {
 902                         if self.type_of(item.id.owner_id) == found {
 903                             diag.span_label(item.span, "expected this associated type");
 904                             return true;
 905                         }
 906                     }
 907                 }
 908             }
 909             _ => {}
 910         }
 911         false
 912     }
 913
 914     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 915     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
 916     ///
 917     /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
 918     /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
 919     /// trait bound as the one we're looking for. This can help in cases where the associated
 920     /// type is defined on a supertrait of the one present in the bounds.
 921     fn constrain_generic_bound_associated_type_structured_suggestion(
 922         self,
 923         diag: &mut Diagnostic,
 924         trait_ref: &ty::TraitRef<'tcx>,
 925         bounds: hir::GenericBounds<'_>,
 926         assoc: &ty::AssocItem,
 927         assoc_substs: &[ty::GenericArg<'tcx>],
 928         ty: Ty<'tcx>,
 929         msg: &str,
 930         is_bound_surely_present: bool,
 931     ) -> bool {
 932         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 933
 934         let trait_bounds = bounds.iter().filter_map(|bound| match bound {
 935             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
 936             _ => None,
 937         });
 938
 939         let matching_trait_bounds = trait_bounds
 940             .clone()
 941             .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
 942             .collect::<Vec<_>>();
 943
 944         let span = match &matching_trait_bounds[..] {
 945             &[ptr] => ptr.span,
 946             &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
 947                 &[ptr] => ptr.span,
 948                 _ => return false,
 949             },
 950             _ => return false,
 951         };
 952
 953         self.constrain_associated_type_structured_suggestion(
 954             diag,
 955             span,
 956             assoc,
 957             assoc_substs,
 958             ty,
 959             msg,
 960         )
 961     }
 962
 963     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 964     /// associated type to a given type `ty`.
 965     fn constrain_associated_type_structured_suggestion(
 966         self,
 967         diag: &mut Diagnostic,
 968         span: Span,
 969         assoc: &ty::AssocItem,
 970         assoc_substs: &[ty::GenericArg<'tcx>],
 971         ty: Ty<'tcx>,
 972         msg: &str,
 973     ) -> bool {
 974         if let Ok(has_params) =
 975             self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 976         {
 977             let (span, sugg) = if has_params {
 978                 let pos = span.hi() - BytePos(1);
 979                 let span = Span::new(pos, pos, span.ctxt(), span.parent());
 980                 (span, format!(", {} = {}", assoc.ident(self), ty))
 981             } else {
 982                 let item_args = self.format_generic_args(assoc_substs);
 983                 (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(self), item_args, ty))
 984             };
 985             diag.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 986             return true;
 987         }
 988         false
 989     }
 990
 991     pub fn short_ty_string(self, ty: Ty<'tcx>) -> (String, Option<PathBuf>) {
 992         let length_limit = 50;
 993         let type_limit = 4;
 994         let regular = FmtPrinter::new(self, hir::def::Namespace::TypeNS)
 995             .pretty_print_type(ty)
 996             .expect("could not write to `String`")
 997             .into_buffer();
 998         if regular.len() <= length_limit {
 999             return (regular, None);
1000         }
1001         let short = FmtPrinter::new_with_limit(
1002             self,
1003             hir::def::Namespace::TypeNS,
1004             rustc_session::Limit(type_limit),
1005         )
1006         .pretty_print_type(ty)
1007         .expect("could not write to `String`")
1008         .into_buffer();
1009         if regular == short {
1010             return (regular, None);
1011         }
1012         // Multiple types might be shortened in a single error, ensure we create a file for each.
1013         let mut s = DefaultHasher::new();
1014         ty.hash(&mut s);
1015         let hash = s.finish();
1016         let path = self.output_filenames(()).temp_path_ext(&format!("long-type-{hash}.txt"), None);
1017         match std::fs::write(&path, &regular) {
1018             Ok(_) => (short, Some(path)),
1019             Err(_) => (regular, None),
1020         }
1021     }
1022
1023     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
1024         FmtPrinter::new(self, hir::def::Namespace::TypeNS)
1025             .path_generic_args(Ok, args)
1026             .expect("could not write to `String`.")
1027             .into_buffer()
1028     }
1029 }