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