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