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 rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
   6 use rustc_errors::{pluralize, Diagnostic, MultiSpan};
   7 use rustc_hir as hir;
   8 use rustc_hir::def_id::DefId;
   9 use rustc_span::symbol::{sym, Symbol};
  10 use rustc_span::{BytePos, Span};
  11 use rustc_target::spec::abi;
  12
  13 use std::borrow::Cow;
  14 use std::fmt;
  15
  16 #[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable)]
  17 pub struct ExpectedFound<T> {
  18     pub expected: T,
  19     pub found: T,
  20 }
  21
  22 impl<T> ExpectedFound<T> {
  23     pub fn new(a_is_expected: bool, a: T, b: T) -> Self {
  24         if a_is_expected {
  25             ExpectedFound { expected: a, found: b }
  26         } else {
  27             ExpectedFound { expected: b, found: a }
  28         }
  29     }
  30 }
  31
  32 // Data structures used in type unification
  33 #[derive(Clone, Debug, TypeFoldable)]
  34 pub enum TypeError<'tcx> {
  35     Mismatch,
  36     ConstnessMismatch(ExpectedFound<ty::BoundConstness>),
  37     PolarityMismatch(ExpectedFound<ty::ImplPolarity>),
  38     UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
  39     AbiMismatch(ExpectedFound<abi::Abi>),
  40     Mutability,
  41     ArgumentMutability(usize),
  42     TupleSize(ExpectedFound<usize>),
  43     FixedArraySize(ExpectedFound<u64>),
  44     ArgCount,
  45     FieldMisMatch(Symbol, Symbol),
  46
  47     RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
  48     RegionsInsufficientlyPolymorphic(BoundRegionKind, Region<'tcx>),
  49     RegionsOverlyPolymorphic(BoundRegionKind, Region<'tcx>),
  50     RegionsPlaceholderMismatch,
  51
  52     Sorts(ExpectedFound<Ty<'tcx>>),
  53     ArgumentSorts(ExpectedFound<Ty<'tcx>>, usize),
  54     IntMismatch(ExpectedFound<ty::IntVarValue>),
  55     FloatMismatch(ExpectedFound<ty::FloatTy>),
  56     Traits(ExpectedFound<DefId>),
  57     VariadicMismatch(ExpectedFound<bool>),
  58
  59     /// Instantiating a type variable with the given type would have
  60     /// created a cycle (because it appears somewhere within that
  61     /// type).
  62     CyclicTy(Ty<'tcx>),
  63     CyclicConst(ty::Const<'tcx>),
  64     ProjectionMismatched(ExpectedFound<DefId>),
  65     ExistentialMismatch(
  66         ExpectedFound<&'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>>,
  67     ),
  68     ObjectUnsafeCoercion(DefId),
  69     ConstMismatch(ExpectedFound<ty::Const<'tcx>>),
  70
  71     IntrinsicCast,
  72     /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
  73     TargetFeatureCast(DefId),
  74 }
  75
  76 /// Explains the source of a type err in a short, human readable way. This is meant to be placed
  77 /// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
  78 /// afterwards to present additional details, particularly when it comes to lifetime-related
  79 /// errors.
  80 impl<'tcx> fmt::Display for TypeError<'tcx> {
  81     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  82         use self::TypeError::*;
  83         fn report_maybe_different(
  84             f: &mut fmt::Formatter<'_>,
  85             expected: &str,
  86             found: &str,
  87         ) -> fmt::Result {
  88             // A naive approach to making sure that we're not reporting silly errors such as:
  89             // (expected closure, found closure).
  90             if expected == found {
  91                 write!(f, "expected {}, found a different {}", expected, found)
  92             } else {
  93                 write!(f, "expected {}, found {}", expected, found)
  94             }
  95         }
  96
  97         let br_string = |br: ty::BoundRegionKind| match br {
  98             ty::BrNamed(_, name) => format!(" {}", name),
  99             _ => String::new(),
 100         };
 101
 102         match *self {
 103             CyclicTy(_) => write!(f, "cyclic type of infinite size"),
 104             CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
 105             Mismatch => write!(f, "types differ"),
 106             ConstnessMismatch(values) => {
 107                 write!(f, "expected {} bound, found {} bound", values.expected, values.found)
 108             }
 109             PolarityMismatch(values) => {
 110                 write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
 111             }
 112             UnsafetyMismatch(values) => {
 113                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 114             }
 115             AbiMismatch(values) => {
 116                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 117             }
 118             ArgumentMutability(_) | Mutability => write!(f, "types differ in mutability"),
 119             TupleSize(values) => write!(
 120                 f,
 121                 "expected a tuple with {} element{}, found one with {} element{}",
 122                 values.expected,
 123                 pluralize!(values.expected),
 124                 values.found,
 125                 pluralize!(values.found)
 126             ),
 127             FixedArraySize(values) => write!(
 128                 f,
 129                 "expected an array with a fixed size of {} element{}, found one with {} element{}",
 130                 values.expected,
 131                 pluralize!(values.expected),
 132                 values.found,
 133                 pluralize!(values.found)
 134             ),
 135             ArgCount => write!(f, "incorrect number of function parameters"),
 136             FieldMisMatch(adt, field) => write!(f, "field type mismatch: {}.{}", adt, field),
 137             RegionsDoesNotOutlive(..) => write!(f, "lifetime mismatch"),
 138             RegionsInsufficientlyPolymorphic(br, _) => write!(
 139                 f,
 140                 "expected bound lifetime parameter{}, found concrete lifetime",
 141                 br_string(br)
 142             ),
 143             RegionsOverlyPolymorphic(br, _) => write!(
 144                 f,
 145                 "expected concrete lifetime, found bound lifetime parameter{}",
 146                 br_string(br)
 147             ),
 148             RegionsPlaceholderMismatch => write!(f, "one type is more general than the other"),
 149             ArgumentSorts(values, _) | Sorts(values) => ty::tls::with(|tcx| {
 150                 report_maybe_different(
 151                     f,
 152                     &values.expected.sort_string(tcx),
 153                     &values.found.sort_string(tcx),
 154                 )
 155             }),
 156             Traits(values) => ty::tls::with(|tcx| {
 157                 report_maybe_different(
 158                     f,
 159                     &format!("trait `{}`", tcx.def_path_str(values.expected)),
 160                     &format!("trait `{}`", tcx.def_path_str(values.found)),
 161                 )
 162             }),
 163             IntMismatch(ref values) => {
 164                 let expected = match values.expected {
 165                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 166                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 167                 };
 168                 let found = match values.found {
 169                     ty::IntVarValue::IntType(ty) => ty.name_str(),
 170                     ty::IntVarValue::UintType(ty) => ty.name_str(),
 171                 };
 172                 write!(f, "expected `{}`, found `{}`", expected, found)
 173             }
 174             FloatMismatch(ref values) => {
 175                 write!(
 176                     f,
 177                     "expected `{}`, found `{}`",
 178                     values.expected.name_str(),
 179                     values.found.name_str()
 180                 )
 181             }
 182             VariadicMismatch(ref values) => write!(
 183                 f,
 184                 "expected {} fn, found {} function",
 185                 if values.expected { "variadic" } else { "non-variadic" },
 186                 if values.found { "variadic" } else { "non-variadic" }
 187             ),
 188             ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
 189                 write!(
 190                     f,
 191                     "expected {}, found {}",
 192                     tcx.def_path_str(values.expected),
 193                     tcx.def_path_str(values.found)
 194                 )
 195             }),
 196             ExistentialMismatch(ref values) => report_maybe_different(
 197                 f,
 198                 &format!("trait `{}`", values.expected),
 199                 &format!("trait `{}`", values.found),
 200             ),
 201             ConstMismatch(ref values) => {
 202                 write!(f, "expected `{}`, found `{}`", values.expected, values.found)
 203             }
 204             IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
 205             TargetFeatureCast(_) => write!(
 206                 f,
 207                 "cannot coerce functions with `#[target_feature]` to safe function pointers"
 208             ),
 209             ObjectUnsafeCoercion(_) => write!(f, "coercion to object-unsafe trait object"),
 210         }
 211     }
 212 }
 213
 214 impl<'tcx> TypeError<'tcx> {
 215     pub fn must_include_note(&self) -> bool {
 216         use self::TypeError::*;
 217         match self {
 218             CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
 219             | PolarityMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
 220             | ArgumentSorts(..) | Sorts(_) | IntMismatch(_) | FloatMismatch(_)
 221             | VariadicMismatch(_) | TargetFeatureCast(_) => false,
 222
 223             Mutability
 224             | ArgumentMutability(_)
 225             | TupleSize(_)
 226             | ArgCount
 227             | FieldMisMatch(..)
 228             | RegionsDoesNotOutlive(..)
 229             | RegionsInsufficientlyPolymorphic(..)
 230             | RegionsOverlyPolymorphic(..)
 231             | RegionsPlaceholderMismatch
 232             | Traits(_)
 233             | ProjectionMismatched(_)
 234             | ExistentialMismatch(_)
 235             | ConstMismatch(_)
 236             | IntrinsicCast
 237             | ObjectUnsafeCoercion(_) => true,
 238         }
 239     }
 240 }
 241
 242 impl<'tcx> Ty<'tcx> {
 243     pub fn sort_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 244         match *self.kind() {
 245             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
 246                 format!("`{}`", self).into()
 247             }
 248             ty::Tuple(ref tys) if tys.is_empty() => format!("`{}`", self).into(),
 249
 250             ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did())).into(),
 251             ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
 252             ty::Array(t, n) => {
 253                 if t.is_simple_ty() {
 254                     return format!("array `{}`", self).into();
 255                 }
 256
 257                 let n = tcx.lift(n).unwrap();
 258                 if let ty::ConstKind::Value(v) = n.val() {
 259                     if let Some(n) = v.try_to_machine_usize(tcx) {
 260                         return format!("array of {} element{}", n, pluralize!(n)).into();
 261                     }
 262                 }
 263                 "array".into()
 264             }
 265             ty::Slice(ty) if ty.is_simple_ty() => format!("slice `{}`", self).into(),
 266             ty::Slice(_) => "slice".into(),
 267             ty::RawPtr(_) => "*-ptr".into(),
 268             ty::Ref(_, ty, mutbl) => {
 269                 let tymut = ty::TypeAndMut { ty, mutbl };
 270                 let tymut_string = tymut.to_string();
 271                 if tymut_string != "_"
 272                     && (ty.is_simple_text() || tymut_string.len() < "mutable reference".len())
 273                 {
 274                     format!("`&{}`", tymut_string).into()
 275                 } else {
 276                     // Unknown type name, it's long or has type arguments
 277                     match mutbl {
 278                         hir::Mutability::Mut => "mutable reference",
 279                         _ => "reference",
 280                     }
 281                     .into()
 282                 }
 283             }
 284             ty::FnDef(..) => "fn item".into(),
 285             ty::FnPtr(_) => "fn pointer".into(),
 286             ty::Dynamic(ref inner, ..) if let Some(principal) = inner.principal() => {
 287                 format!("trait object `dyn {}`", tcx.def_path_str(principal.def_id())).into()
 288             }
 289             ty::Dynamic(..) => "trait object".into(),
 290             ty::Closure(..) => "closure".into(),
 291             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 292             ty::GeneratorWitness(..) => "generator witness".into(),
 293             ty::Tuple(..) => "tuple".into(),
 294             ty::Infer(ty::TyVar(_)) => "inferred type".into(),
 295             ty::Infer(ty::IntVar(_)) => "integer".into(),
 296             ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
 297             ty::Placeholder(..) => "placeholder type".into(),
 298             ty::Bound(..) => "bound type".into(),
 299             ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
 300             ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
 301             ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
 302             ty::Projection(_) => "associated type".into(),
 303             ty::Param(p) => format!("type parameter `{}`", p).into(),
 304             ty::Opaque(..) => "opaque type".into(),
 305             ty::Error(_) => "type error".into(),
 306         }
 307     }
 308
 309     pub fn prefix_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
 310         match *self.kind() {
 311             ty::Infer(_)
 312             | ty::Error(_)
 313             | ty::Bool
 314             | ty::Char
 315             | ty::Int(_)
 316             | ty::Uint(_)
 317             | ty::Float(_)
 318             | ty::Str
 319             | ty::Never => "type".into(),
 320             ty::Tuple(ref tys) if tys.is_empty() => "unit type".into(),
 321             ty::Adt(def, _) => def.descr().into(),
 322             ty::Foreign(_) => "extern type".into(),
 323             ty::Array(..) => "array".into(),
 324             ty::Slice(_) => "slice".into(),
 325             ty::RawPtr(_) => "raw pointer".into(),
 326             ty::Ref(.., mutbl) => match mutbl {
 327                 hir::Mutability::Mut => "mutable reference",
 328                 _ => "reference",
 329             }
 330             .into(),
 331             ty::FnDef(..) => "fn item".into(),
 332             ty::FnPtr(_) => "fn pointer".into(),
 333             ty::Dynamic(..) => "trait object".into(),
 334             ty::Closure(..) => "closure".into(),
 335             ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
 336             ty::GeneratorWitness(..) => "generator witness".into(),
 337             ty::Tuple(..) => "tuple".into(),
 338             ty::Placeholder(..) => "higher-ranked type".into(),
 339             ty::Bound(..) => "bound type variable".into(),
 340             ty::Projection(_) => "associated type".into(),
 341             ty::Param(_) => "type parameter".into(),
 342             ty::Opaque(..) => "opaque type".into(),
 343         }
 344     }
 345 }
 346
 347 impl<'tcx> TyCtxt<'tcx> {
 348     pub fn note_and_explain_type_err(
 349         self,
 350         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 351         db: &mut Diagnostic,
 352         err: &TypeError<'tcx>,
 353         cause: &ObligationCause<'tcx>,
 354         sp: Span,
 355         body_owner_def_id: DefId,
 356     ) {
 357         use self::TypeError::*;
 358         debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
 359         match err {
 360             ArgumentSorts(values, _) | Sorts(values) => {
 361                 match (values.expected.kind(), values.found.kind()) {
 362                     (ty::Closure(..), ty::Closure(..)) => {
 363                         db.note("no two closures, even if identical, have the same type");
 364                         db.help("consider boxing your closure and/or using it as a trait object");
 365                     }
 366                     (ty::Opaque(..), ty::Opaque(..)) => {
 367                         // Issue #63167
 368                         db.note("distinct uses of `impl Trait` result in different opaque types");
 369                     }
 370                     (ty::Float(_), ty::Infer(ty::IntVar(_)))
 371                         if let Ok(
 372                             // Issue #53280
 373                             snippet,
 374                         ) = self.sess.source_map().span_to_snippet(sp) =>
 375                     {
 376                         if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
 377                             db.span_suggestion(
 378                                 sp,
 379                                 "use a float literal",
 380                                 format!("{}.0", snippet),
 381                                 MachineApplicable,
 382                             );
 383                         }
 384                     }
 385                     (ty::Param(expected), ty::Param(found)) => {
 386                         let generics = self.generics_of(body_owner_def_id);
 387                         let e_span = self.def_span(generics.type_param(expected, self).def_id);
 388                         if !sp.contains(e_span) {
 389                             db.span_label(e_span, "expected type parameter");
 390                         }
 391                         let f_span = self.def_span(generics.type_param(found, self).def_id);
 392                         if !sp.contains(f_span) {
 393                             db.span_label(f_span, "found type parameter");
 394                         }
 395                         db.note(
 396                             "a type parameter was expected, but a different one was found; \
 397                              you might be missing a type parameter or trait bound",
 398                         );
 399                         db.note(
 400                             "for more information, visit \
 401                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 402                              #traits-as-parameters",
 403                         );
 404                     }
 405                     (ty::Projection(_), ty::Projection(_)) => {
 406                         db.note("an associated type was expected, but a different one was found");
 407                     }
 408                     (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p)) => {
 409                         let generics = self.generics_of(body_owner_def_id);
 410                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 411                         if !sp.contains(p_span) {
 412                             db.span_label(p_span, "this type parameter");
 413                         }
 414                         let hir = self.hir();
 415                         let mut note = true;
 416                         if let Some(generics) = generics
 417                             .type_param(p, self)
 418                             .def_id
 419                             .as_local()
 420                             .map(|id| hir.local_def_id_to_hir_id(id))
 421                             .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
 422                             .as_ref()
 423                             .and_then(|node| node.generics())
 424                         {
 425                             // Synthesize the associated type restriction `Add<Output = Expected>`.
 426                             // FIXME: extract this logic for use in other diagnostics.
 427                             let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
 428                             let path =
 429                                 self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
 430                             let item_name = self.item_name(proj.item_def_id);
 431                             let item_args = self.format_generic_args(assoc_substs);
 432
 433                             let path = if path.ends_with('>') {
 434                                 format!(
 435                                     "{}, {}{} = {}>",
 436                                     &path[..path.len() - 1],
 437                                     item_name,
 438                                     item_args,
 439                                     p
 440                                 )
 441                             } else {
 442                                 format!("{}<{}{} = {}>", path, item_name, item_args, p)
 443                             };
 444                             note = !suggest_constraining_type_param(
 445                                 self,
 446                                 generics,
 447                                 db,
 448                                 &format!("{}", proj.self_ty()),
 449                                 &path,
 450                                 None,
 451                             );
 452                         }
 453                         if note {
 454                             db.note("you might be missing a type parameter or trait bound");
 455                         }
 456                     }
 457                     (ty::Param(p), ty::Dynamic(..) | ty::Opaque(..))
 458                     | (ty::Dynamic(..) | ty::Opaque(..), ty::Param(p)) => {
 459                         let generics = self.generics_of(body_owner_def_id);
 460                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 461                         if !sp.contains(p_span) {
 462                             db.span_label(p_span, "this type parameter");
 463                         }
 464                         db.help("type parameters must be constrained to match other types");
 465                         if self.sess.teach(&db.get_code().unwrap()) {
 466                             db.help(
 467                                 "given a type parameter `T` and a method `foo`:
 468 ```
 469 trait Trait<T> { fn foo(&self) -> T; }
 470 ```
 471 the only ways to implement method `foo` are:
 472 - constrain `T` with an explicit type:
 473 ```
 474 impl Trait<String> for X {
 475     fn foo(&self) -> String { String::new() }
 476 }
 477 ```
 478 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 479 ```
 480 impl<T: std::default::Default> Trait<T> for X {
 481     fn foo(&self) -> T { <T as std::default::Default>::default() }
 482 }
 483 ```
 484 - change `foo` to return an argument of type `T`:
 485 ```
 486 impl<T> Trait<T> for X {
 487     fn foo(&self, x: T) -> T { x }
 488 }
 489 ```",
 490                             );
 491                         }
 492                         db.note(
 493                             "for more information, visit \
 494                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 495                              #traits-as-parameters",
 496                         );
 497                     }
 498                     (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
 499                         let generics = self.generics_of(body_owner_def_id);
 500                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 501                         if !sp.contains(p_span) {
 502                             db.span_label(p_span, "this type parameter");
 503                         }
 504                         db.help(&format!(
 505                             "every closure has a distinct type and so could not always match the \
 506                              caller-chosen type of parameter `{}`",
 507                             p
 508                         ));
 509                     }
 510                     (ty::Param(p), _) | (_, ty::Param(p)) => {
 511                         let generics = self.generics_of(body_owner_def_id);
 512                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 513                         if !sp.contains(p_span) {
 514                             db.span_label(p_span, "this type parameter");
 515                         }
 516                     }
 517                     (ty::Projection(proj_ty), _) => {
 518                         self.expected_projection(
 519                             db,
 520                             proj_ty,
 521                             values,
 522                             body_owner_def_id,
 523                             cause.code(),
 524                         );
 525                     }
 526                     (_, ty::Projection(proj_ty)) => {
 527                         let msg = format!(
 528                             "consider constraining the associated type `{}` to `{}`",
 529                             values.found, values.expected,
 530                         );
 531                         if !(self.suggest_constraining_opaque_associated_type(
 532                             db,
 533                             &msg,
 534                             proj_ty,
 535                             values.expected,
 536                         ) || self.suggest_constraint(
 537                             db,
 538                             &msg,
 539                             body_owner_def_id,
 540                             proj_ty,
 541                             values.expected,
 542                         )) {
 543                             db.help(&msg);
 544                             db.note(
 545                                 "for more information, visit \
 546                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 547                             );
 548                         }
 549                     }
 550                     _ => {}
 551                 }
 552                 debug!(
 553                     "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
 554                     values.expected,
 555                     values.expected.kind(),
 556                     values.found,
 557                     values.found.kind(),
 558                 );
 559             }
 560             CyclicTy(ty) => {
 561                 // Watch out for various cases of cyclic types and try to explain.
 562                 if ty.is_closure() || ty.is_generator() {
 563                     db.note(
 564                         "closures cannot capture themselves or take themselves as argument;\n\
 565                          this error may be the result of a recent compiler bug-fix,\n\
 566                          see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
 567                          for more information",
 568                     );
 569                 }
 570             }
 571             TargetFeatureCast(def_id) => {
 572                 let attrs = self.get_attrs(*def_id);
 573                 let target_spans = attrs
 574                     .iter()
 575                     .filter(|attr| attr.has_name(sym::target_feature))
 576                     .map(|attr| attr.span);
 577                 db.note(
 578                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 579                 );
 580                 db.span_labels(target_spans, "`#[target_feature]` added here");
 581             }
 582             _ => {}
 583         }
 584     }
 585
 586     fn suggest_constraint(
 587         self,
 588         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 589         db: &mut Diagnostic,
 590         msg: &str,
 591         body_owner_def_id: DefId,
 592         proj_ty: &ty::ProjectionTy<'tcx>,
 593         ty: Ty<'tcx>,
 594     ) -> bool {
 595         let assoc = self.associated_item(proj_ty.item_def_id);
 596         let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 597         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
 598             if let Some(hir_generics) = item.generics() {
 599                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 600                 // This will also work for `impl Trait`.
 601                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
 602                     let generics = self.generics_of(body_owner_def_id);
 603                     generics.type_param(param_ty, self).def_id
 604                 } else {
 605                     return false;
 606                 };
 607
 608                 // First look in the `where` clause, as this might be
 609                 // `fn foo<T>(x: T) where T: Trait`.
 610                 for predicate in hir_generics.where_clause.predicates {
 611                     if let hir::WherePredicate::BoundPredicate(pred) = predicate {
 612                         if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) =
 613                             pred.bounded_ty.kind
 614                         {
 615                             if path.res.opt_def_id() == Some(def_id) {
 616                                 // This predicate is binding type param `A` in `<A as T>::Foo` to
 617                                 // something, potentially `T`.
 618                             } else {
 619                                 continue;
 620                             }
 621                         } else {
 622                             continue;
 623                         }
 624
 625                         if self.constrain_generic_bound_associated_type_structured_suggestion(
 626                             db,
 627                             &trait_ref,
 628                             pred.bounds,
 629                             &assoc,
 630                             assoc_substs,
 631                             ty,
 632                             msg,
 633                             false,
 634                         ) {
 635                             return true;
 636                         }
 637                     }
 638                 }
 639                 for param in hir_generics.params {
 640                     if self.hir().opt_local_def_id(param.hir_id).map(|id| id.to_def_id())
 641                         == Some(def_id)
 642                     {
 643                         // This is type param `A` in `<A as T>::Foo`.
 644                         return self.constrain_generic_bound_associated_type_structured_suggestion(
 645                             db,
 646                             &trait_ref,
 647                             param.bounds,
 648                             &assoc,
 649                             assoc_substs,
 650                             ty,
 651                             msg,
 652                             false,
 653                         );
 654                     }
 655                 }
 656             }
 657         }
 658         false
 659     }
 660
 661     /// An associated type was expected and a different type was found.
 662     ///
 663     /// We perform a few different checks to see what we can suggest:
 664     ///
 665     ///  - In the current item, look for associated functions that return the expected type and
 666     ///    suggest calling them. (Not a structured suggestion.)
 667     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 668     ///    associated type to the found type.
 669     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 670     ///    mention that this is disallowed.
 671     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 672     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 673     ///    fn that returns the type.
 674     fn expected_projection(
 675         self,
 676         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 677         db: &mut Diagnostic,
 678         proj_ty: &ty::ProjectionTy<'tcx>,
 679         values: &ExpectedFound<Ty<'tcx>>,
 680         body_owner_def_id: DefId,
 681         cause_code: &ObligationCauseCode<'_>,
 682     ) {
 683         let msg = format!(
 684             "consider constraining the associated type `{}` to `{}`",
 685             values.expected, values.found
 686         );
 687         let body_owner = self.hir().get_if_local(body_owner_def_id);
 688         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 689
 690         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 691         let callable_scope = matches!(
 692             body_owner,
 693             Some(
 694                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 695                     | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 696                     | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 697             )
 698         );
 699         let impl_comparison = matches!(
 700             cause_code,
 701             ObligationCauseCode::CompareImplMethodObligation { .. }
 702                 | ObligationCauseCode::CompareImplTypeObligation { .. }
 703                 | ObligationCauseCode::CompareImplConstObligation
 704         );
 705         let assoc = self.associated_item(proj_ty.item_def_id);
 706         if !callable_scope || impl_comparison {
 707             // We do not want to suggest calling functions when the reason of the
 708             // type error is a comparison of an `impl` with its `trait` or when the
 709             // scope is outside of a `Body`.
 710         } else {
 711             // If we find a suitable associated function that returns the expected type, we don't
 712             // want the more general suggestion later in this method about "consider constraining
 713             // the associated type or calling a method that returns the associated type".
 714             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 715                 db,
 716                 assoc.container.id(),
 717                 current_method_ident,
 718                 proj_ty.item_def_id,
 719                 values.expected,
 720             );
 721             // Possibly suggest constraining the associated type to conform to the
 722             // found type.
 723             if self.suggest_constraint(db, &msg, body_owner_def_id, proj_ty, values.found)
 724                 || point_at_assoc_fn
 725             {
 726                 return;
 727             }
 728         }
 729
 730         self.suggest_constraining_opaque_associated_type(db, &msg, proj_ty, values.found);
 731
 732         if self.point_at_associated_type(db, body_owner_def_id, values.found) {
 733             return;
 734         }
 735
 736         if !impl_comparison {
 737             // Generic suggestion when we can't be more specific.
 738             if callable_scope {
 739                 db.help(&format!("{} or calling a method that returns `{}`", msg, values.expected));
 740             } else {
 741                 db.help(&msg);
 742             }
 743             db.note(
 744                 "for more information, visit \
 745                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 746             );
 747         }
 748         if self.sess.teach(&db.get_code().unwrap()) {
 749             db.help(
 750                 "given an associated type `T` and a method `foo`:
 751 ```
 752 trait Trait {
 753 type T;
 754 fn foo(&self) -> Self::T;
 755 }
 756 ```
 757 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 758 ```
 759 impl Trait for X {
 760 type T = String;
 761 fn foo(&self) -> Self::T { String::new() }
 762 }
 763 ```",
 764             );
 765         }
 766     }
 767
 768     /// When the expected `impl Trait` is not defined in the current item, it will come from
 769     /// a return type. This can occur when dealing with `TryStream` (#71035).
 770     fn suggest_constraining_opaque_associated_type(
 771         self,
 772         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 773         db: &mut Diagnostic,
 774         msg: &str,
 775         proj_ty: &ty::ProjectionTy<'tcx>,
 776         ty: Ty<'tcx>,
 777     ) -> bool {
 778         let assoc = self.associated_item(proj_ty.item_def_id);
 779         if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
 780             let opaque_local_def_id = def_id.as_local();
 781             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
 782                 match &self.hir().expect_item(opaque_local_def_id).kind {
 783                     hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
 784                     _ => bug!("The HirId comes from a `ty::Opaque`"),
 785                 }
 786             } else {
 787                 return false;
 788             };
 789
 790             let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 791
 792             self.constrain_generic_bound_associated_type_structured_suggestion(
 793                 db,
 794                 &trait_ref,
 795                 opaque_hir_ty.bounds,
 796                 assoc,
 797                 assoc_substs,
 798                 ty,
 799                 msg,
 800                 true,
 801             )
 802         } else {
 803             false
 804         }
 805     }
 806
 807     fn point_at_methods_that_satisfy_associated_type(
 808         self,
 809         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 810         db: &mut Diagnostic,
 811         assoc_container_id: DefId,
 812         current_method_ident: Option<Symbol>,
 813         proj_ty_item_def_id: DefId,
 814         expected: Ty<'tcx>,
 815     ) -> bool {
 816         let items = self.associated_items(assoc_container_id);
 817         // Find all the methods in the trait that could be called to construct the
 818         // expected associated type.
 819         // FIXME: consider suggesting the use of associated `const`s.
 820         let methods: Vec<(Span, String)> = items
 821             .items
 822             .iter()
 823             .filter(|(name, item)| {
 824                 ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
 825             })
 826             .filter_map(|(_, item)| {
 827                 let method = self.fn_sig(item.def_id);
 828                 match *method.output().skip_binder().kind() {
 829                     ty::Projection(ty::ProjectionTy { item_def_id, .. })
 830                         if item_def_id == proj_ty_item_def_id =>
 831                     {
 832                         Some((
 833                             self.sess.source_map().guess_head_span(self.def_span(item.def_id)),
 834                             format!("consider calling `{}`", self.def_path_str(item.def_id)),
 835                         ))
 836                     }
 837                     _ => None,
 838                 }
 839             })
 840             .collect();
 841         if !methods.is_empty() {
 842             // Use a single `help:` to show all the methods in the trait that can
 843             // be used to construct the expected associated type.
 844             let mut span: MultiSpan =
 845                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 846             let msg = format!(
 847                 "{some} method{s} {are} available that return{r} `{ty}`",
 848                 some = if methods.len() == 1 { "a" } else { "some" },
 849                 s = pluralize!(methods.len()),
 850                 are = pluralize!("is", methods.len()),
 851                 r = if methods.len() == 1 { "s" } else { "" },
 852                 ty = expected
 853             );
 854             for (sp, label) in methods.into_iter() {
 855                 span.push_span_label(sp, label);
 856             }
 857             db.span_help(span, &msg);
 858             return true;
 859         }
 860         false
 861     }
 862
 863     fn point_at_associated_type(
 864         self,
 865         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 866         db: &mut Diagnostic,
 867         body_owner_def_id: DefId,
 868         found: Ty<'tcx>,
 869     ) -> bool {
 870         let Some(hir_id) = body_owner_def_id.as_local() else {
 871             return false;
 872         };
 873         let hir_id = self.hir().local_def_id_to_hir_id(hir_id);
 874         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 875         // `expected` and point at it.
 876         let parent_id = self.hir().get_parent_item(hir_id);
 877         let item = self.hir().find_by_def_id(parent_id);
 878         debug!("expected_projection parent item {:?}", item);
 879         match item {
 880             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 881                 // FIXME: account for `#![feature(specialization)]`
 882                 for item in &items[..] {
 883                     match item.kind {
 884                         hir::AssocItemKind::Type => {
 885                             // FIXME: account for returning some type in a trait fn impl that has
 886                             // an assoc type as a return type (#72076).
 887                             if let hir::Defaultness::Default { has_value: true } = item.defaultness
 888                             {
 889                                 if self.type_of(item.id.def_id) == found {
 890                                     db.span_label(
 891                                         item.span,
 892                                         "associated type defaults can't be assumed inside the \
 893                                             trait defining them",
 894                                     );
 895                                     return true;
 896                                 }
 897                             }
 898                         }
 899                         _ => {}
 900                     }
 901                 }
 902             }
 903             Some(hir::Node::Item(hir::Item {
 904                 kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
 905                 ..
 906             })) => {
 907                 for item in &items[..] {
 908                     if let hir::AssocItemKind::Type = item.kind {
 909                         if self.type_of(item.id.def_id) == found {
 910                             db.span_label(item.span, "expected this associated type");
 911                             return true;
 912                         }
 913                     }
 914                 }
 915             }
 916             _ => {}
 917         }
 918         false
 919     }
 920
 921     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 922     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
 923     ///
 924     /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
 925     /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
 926     /// trait bound as the one we're looking for. This can help in cases where the associated
 927     /// type is defined on a supertrait of the one present in the bounds.
 928     fn constrain_generic_bound_associated_type_structured_suggestion(
 929         self,
 930         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 931         db: &mut Diagnostic,
 932         trait_ref: &ty::TraitRef<'tcx>,
 933         bounds: hir::GenericBounds<'_>,
 934         assoc: &ty::AssocItem,
 935         assoc_substs: &[ty::GenericArg<'tcx>],
 936         ty: Ty<'tcx>,
 937         msg: &str,
 938         is_bound_surely_present: bool,
 939     ) -> bool {
 940         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 941
 942         let trait_bounds = bounds.iter().filter_map(|bound| match bound {
 943             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
 944             _ => None,
 945         });
 946
 947         let matching_trait_bounds = trait_bounds
 948             .clone()
 949             .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
 950             .collect::<Vec<_>>();
 951
 952         let span = match &matching_trait_bounds[..] {
 953             &[ptr] => ptr.span,
 954             &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
 955                 &[ptr] => ptr.span,
 956                 _ => return false,
 957             },
 958             _ => return false,
 959         };
 960
 961         self.constrain_associated_type_structured_suggestion(db, span, assoc, assoc_substs, ty, msg)
 962     }
 963
 964     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 965     /// associated type to a given type `ty`.
 966     fn constrain_associated_type_structured_suggestion(
 967         self,
 968         // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
 969         db: &mut Diagnostic,
 970         span: Span,
 971         assoc: &ty::AssocItem,
 972         assoc_substs: &[ty::GenericArg<'tcx>],
 973         ty: Ty<'tcx>,
 974         msg: &str,
 975     ) -> bool {
 976         if let Ok(has_params) =
 977             self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 978         {
 979             let (span, sugg) = if has_params {
 980                 let pos = span.hi() - BytePos(1);
 981                 let span = Span::new(pos, pos, span.ctxt(), span.parent());
 982                 (span, format!(", {} = {}", assoc.ident(self), ty))
 983             } else {
 984                 let item_args = self.format_generic_args(assoc_substs);
 985                 (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(self), item_args, ty))
 986             };
 987             db.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 988             return true;
 989         }
 990         false
 991     }
 992
 993     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
 994         FmtPrinter::new(self, hir::def::Namespace::TypeNS)
 995             .path_generic_args(Ok, args)
 996             .expect("could not write to `String`.")
 997             .into_buffer()
 998     }
 999 }