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, DiagnosticBuilder};
   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, MultiSpan, 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(&'tcx 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<&'tcx 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::TyS<'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         db: &mut DiagnosticBuilder<'_>,
 351         err: &TypeError<'tcx>,
 352         cause: &ObligationCause<'tcx>,
 353         sp: Span,
 354         body_owner_def_id: DefId,
 355     ) {
 356         use self::TypeError::*;
 357         debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
 358         match err {
 359             ArgumentSorts(values, _) | Sorts(values) => {
 360                 match (values.expected.kind(), values.found.kind()) {
 361                     (ty::Closure(..), ty::Closure(..)) => {
 362                         db.note("no two closures, even if identical, have the same type");
 363                         db.help("consider boxing your closure and/or using it as a trait object");
 364                     }
 365                     (ty::Opaque(..), ty::Opaque(..)) => {
 366                         // Issue #63167
 367                         db.note("distinct uses of `impl Trait` result in different opaque types");
 368                     }
 369                     (ty::Float(_), ty::Infer(ty::IntVar(_)))
 370                         if let Ok(
 371                             // Issue #53280
 372                             snippet,
 373                         ) = self.sess.source_map().span_to_snippet(sp) =>
 374                     {
 375                         if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
 376                             db.span_suggestion(
 377                                 sp,
 378                                 "use a float literal",
 379                                 format!("{}.0", snippet),
 380                                 MachineApplicable,
 381                             );
 382                         }
 383                     }
 384                     (ty::Param(expected), ty::Param(found)) => {
 385                         let generics = self.generics_of(body_owner_def_id);
 386                         let e_span = self.def_span(generics.type_param(expected, self).def_id);
 387                         if !sp.contains(e_span) {
 388                             db.span_label(e_span, "expected type parameter");
 389                         }
 390                         let f_span = self.def_span(generics.type_param(found, self).def_id);
 391                         if !sp.contains(f_span) {
 392                             db.span_label(f_span, "found type parameter");
 393                         }
 394                         db.note(
 395                             "a type parameter was expected, but a different one was found; \
 396                              you might be missing a type parameter or trait bound",
 397                         );
 398                         db.note(
 399                             "for more information, visit \
 400                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 401                              #traits-as-parameters",
 402                         );
 403                     }
 404                     (ty::Projection(_), ty::Projection(_)) => {
 405                         db.note("an associated type was expected, but a different one was found");
 406                     }
 407                     (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p)) => {
 408                         let generics = self.generics_of(body_owner_def_id);
 409                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 410                         if !sp.contains(p_span) {
 411                             db.span_label(p_span, "this type parameter");
 412                         }
 413                         let hir = self.hir();
 414                         let mut note = true;
 415                         if let Some(generics) = generics
 416                             .type_param(p, self)
 417                             .def_id
 418                             .as_local()
 419                             .map(|id| hir.local_def_id_to_hir_id(id))
 420                             .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
 421                             .as_ref()
 422                             .and_then(|node| node.generics())
 423                         {
 424                             // Synthesize the associated type restriction `Add<Output = Expected>`.
 425                             // FIXME: extract this logic for use in other diagnostics.
 426                             let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
 427                             let path =
 428                                 self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
 429                             let item_name = self.item_name(proj.item_def_id);
 430                             let item_args = self.format_generic_args(assoc_substs);
 431
 432                             let path = if path.ends_with('>') {
 433                                 format!(
 434                                     "{}, {}{} = {}>",
 435                                     &path[..path.len() - 1],
 436                                     item_name,
 437                                     item_args,
 438                                     p
 439                                 )
 440                             } else {
 441                                 format!("{}<{}{} = {}>", path, item_name, item_args, p)
 442                             };
 443                             note = !suggest_constraining_type_param(
 444                                 self,
 445                                 generics,
 446                                 db,
 447                                 &format!("{}", proj.self_ty()),
 448                                 &path,
 449                                 None,
 450                             );
 451                         }
 452                         if note {
 453                             db.note("you might be missing a type parameter or trait bound");
 454                         }
 455                     }
 456                     (ty::Param(p), ty::Dynamic(..) | ty::Opaque(..))
 457                     | (ty::Dynamic(..) | ty::Opaque(..), ty::Param(p)) => {
 458                         let generics = self.generics_of(body_owner_def_id);
 459                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 460                         if !sp.contains(p_span) {
 461                             db.span_label(p_span, "this type parameter");
 462                         }
 463                         db.help("type parameters must be constrained to match other types");
 464                         if self.sess.teach(&db.get_code().unwrap()) {
 465                             db.help(
 466                                 "given a type parameter `T` and a method `foo`:
 467 ```
 468 trait Trait<T> { fn foo(&self) -> T; }
 469 ```
 470 the only ways to implement method `foo` are:
 471 - constrain `T` with an explicit type:
 472 ```
 473 impl Trait<String> for X {
 474     fn foo(&self) -> String { String::new() }
 475 }
 476 ```
 477 - add a trait bound to `T` and call a method on that trait that returns `Self`:
 478 ```
 479 impl<T: std::default::Default> Trait<T> for X {
 480     fn foo(&self) -> T { <T as std::default::Default>::default() }
 481 }
 482 ```
 483 - change `foo` to return an argument of type `T`:
 484 ```
 485 impl<T> Trait<T> for X {
 486     fn foo(&self, x: T) -> T { x }
 487 }
 488 ```",
 489                             );
 490                         }
 491                         db.note(
 492                             "for more information, visit \
 493                              https://doc.rust-lang.org/book/ch10-02-traits.html\
 494                              #traits-as-parameters",
 495                         );
 496                     }
 497                     (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
 498                         let generics = self.generics_of(body_owner_def_id);
 499                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 500                         if !sp.contains(p_span) {
 501                             db.span_label(p_span, "this type parameter");
 502                         }
 503                         db.help(&format!(
 504                             "every closure has a distinct type and so could not always match the \
 505                              caller-chosen type of parameter `{}`",
 506                             p
 507                         ));
 508                     }
 509                     (ty::Param(p), _) | (_, ty::Param(p)) => {
 510                         let generics = self.generics_of(body_owner_def_id);
 511                         let p_span = self.def_span(generics.type_param(p, self).def_id);
 512                         if !sp.contains(p_span) {
 513                             db.span_label(p_span, "this type parameter");
 514                         }
 515                     }
 516                     (ty::Projection(proj_ty), _) => {
 517                         self.expected_projection(
 518                             db,
 519                             proj_ty,
 520                             values,
 521                             body_owner_def_id,
 522                             cause.code(),
 523                         );
 524                     }
 525                     (_, ty::Projection(proj_ty)) => {
 526                         let msg = format!(
 527                             "consider constraining the associated type `{}` to `{}`",
 528                             values.found, values.expected,
 529                         );
 530                         if !(self.suggest_constraining_opaque_associated_type(
 531                             db,
 532                             &msg,
 533                             proj_ty,
 534                             values.expected,
 535                         ) || self.suggest_constraint(
 536                             db,
 537                             &msg,
 538                             body_owner_def_id,
 539                             proj_ty,
 540                             values.expected,
 541                         )) {
 542                             db.help(&msg);
 543                             db.note(
 544                                 "for more information, visit \
 545                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 546                             );
 547                         }
 548                     }
 549                     _ => {}
 550                 }
 551                 debug!(
 552                     "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
 553                     values.expected,
 554                     values.expected.kind(),
 555                     values.found,
 556                     values.found.kind(),
 557                 );
 558             }
 559             CyclicTy(ty) => {
 560                 // Watch out for various cases of cyclic types and try to explain.
 561                 if ty.is_closure() || ty.is_generator() {
 562                     db.note(
 563                         "closures cannot capture themselves or take themselves as argument;\n\
 564                          this error may be the result of a recent compiler bug-fix,\n\
 565                          see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
 566                          for more information",
 567                     );
 568                 }
 569             }
 570             TargetFeatureCast(def_id) => {
 571                 let attrs = self.get_attrs(*def_id);
 572                 let target_spans = attrs
 573                     .iter()
 574                     .filter(|attr| attr.has_name(sym::target_feature))
 575                     .map(|attr| attr.span);
 576                 db.note(
 577                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 578                 );
 579                 db.span_labels(target_spans, "`#[target_feature]` added here");
 580             }
 581             _ => {}
 582         }
 583     }
 584
 585     fn suggest_constraint(
 586         self,
 587         db: &mut DiagnosticBuilder<'_>,
 588         msg: &str,
 589         body_owner_def_id: DefId,
 590         proj_ty: &ty::ProjectionTy<'tcx>,
 591         ty: Ty<'tcx>,
 592     ) -> bool {
 593         let assoc = self.associated_item(proj_ty.item_def_id);
 594         let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 595         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
 596             if let Some(hir_generics) = item.generics() {
 597                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 598                 // This will also work for `impl Trait`.
 599                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
 600                     let generics = self.generics_of(body_owner_def_id);
 601                     generics.type_param(param_ty, self).def_id
 602                 } else {
 603                     return false;
 604                 };
 605
 606                 // First look in the `where` clause, as this might be
 607                 // `fn foo<T>(x: T) where T: Trait`.
 608                 for predicate in hir_generics.where_clause.predicates {
 609                     if let hir::WherePredicate::BoundPredicate(pred) = predicate {
 610                         if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) =
 611                             pred.bounded_ty.kind
 612                         {
 613                             if path.res.opt_def_id() == Some(def_id) {
 614                                 // This predicate is binding type param `A` in `<A as T>::Foo` to
 615                                 // something, potentially `T`.
 616                             } else {
 617                                 continue;
 618                             }
 619                         } else {
 620                             continue;
 621                         }
 622
 623                         if self.constrain_generic_bound_associated_type_structured_suggestion(
 624                             db,
 625                             &trait_ref,
 626                             pred.bounds,
 627                             &assoc,
 628                             assoc_substs,
 629                             ty,
 630                             msg,
 631                             false,
 632                         ) {
 633                             return true;
 634                         }
 635                     }
 636                 }
 637                 for param in hir_generics.params {
 638                     if self.hir().opt_local_def_id(param.hir_id).map(|id| id.to_def_id())
 639                         == Some(def_id)
 640                     {
 641                         // This is type param `A` in `<A as T>::Foo`.
 642                         return self.constrain_generic_bound_associated_type_structured_suggestion(
 643                             db,
 644                             &trait_ref,
 645                             param.bounds,
 646                             &assoc,
 647                             assoc_substs,
 648                             ty,
 649                             msg,
 650                             false,
 651                         );
 652                     }
 653                 }
 654             }
 655         }
 656         false
 657     }
 658
 659     /// An associated type was expected and a different type was found.
 660     ///
 661     /// We perform a few different checks to see what we can suggest:
 662     ///
 663     ///  - In the current item, look for associated functions that return the expected type and
 664     ///    suggest calling them. (Not a structured suggestion.)
 665     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 666     ///    associated type to the found type.
 667     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 668     ///    mention that this is disallowed.
 669     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 670     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 671     ///    fn that returns the type.
 672     fn expected_projection(
 673         self,
 674         db: &mut DiagnosticBuilder<'_>,
 675         proj_ty: &ty::ProjectionTy<'tcx>,
 676         values: &ExpectedFound<Ty<'tcx>>,
 677         body_owner_def_id: DefId,
 678         cause_code: &ObligationCauseCode<'_>,
 679     ) {
 680         let msg = format!(
 681             "consider constraining the associated type `{}` to `{}`",
 682             values.expected, values.found
 683         );
 684         let body_owner = self.hir().get_if_local(body_owner_def_id);
 685         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 686
 687         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 688         let callable_scope = matches!(
 689             body_owner,
 690             Some(
 691                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 692                     | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 693                     | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 694             )
 695         );
 696         let impl_comparison = matches!(
 697             cause_code,
 698             ObligationCauseCode::CompareImplMethodObligation { .. }
 699                 | ObligationCauseCode::CompareImplTypeObligation { .. }
 700                 | ObligationCauseCode::CompareImplConstObligation
 701         );
 702         let assoc = self.associated_item(proj_ty.item_def_id);
 703         if !callable_scope || impl_comparison {
 704             // We do not want to suggest calling functions when the reason of the
 705             // type error is a comparison of an `impl` with its `trait` or when the
 706             // scope is outside of a `Body`.
 707         } else {
 708             // If we find a suitable associated function that returns the expected type, we don't
 709             // want the more general suggestion later in this method about "consider constraining
 710             // the associated type or calling a method that returns the associated type".
 711             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 712                 db,
 713                 assoc.container.id(),
 714                 current_method_ident,
 715                 proj_ty.item_def_id,
 716                 values.expected,
 717             );
 718             // Possibly suggest constraining the associated type to conform to the
 719             // found type.
 720             if self.suggest_constraint(db, &msg, body_owner_def_id, proj_ty, values.found)
 721                 || point_at_assoc_fn
 722             {
 723                 return;
 724             }
 725         }
 726
 727         self.suggest_constraining_opaque_associated_type(db, &msg, proj_ty, values.found);
 728
 729         if self.point_at_associated_type(db, body_owner_def_id, values.found) {
 730             return;
 731         }
 732
 733         if !impl_comparison {
 734             // Generic suggestion when we can't be more specific.
 735             if callable_scope {
 736                 db.help(&format!("{} or calling a method that returns `{}`", msg, values.expected));
 737             } else {
 738                 db.help(&msg);
 739             }
 740             db.note(
 741                 "for more information, visit \
 742                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 743             );
 744         }
 745         if self.sess.teach(&db.get_code().unwrap()) {
 746             db.help(
 747                 "given an associated type `T` and a method `foo`:
 748 ```
 749 trait Trait {
 750 type T;
 751 fn foo(&self) -> Self::T;
 752 }
 753 ```
 754 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 755 ```
 756 impl Trait for X {
 757 type T = String;
 758 fn foo(&self) -> Self::T { String::new() }
 759 }
 760 ```",
 761             );
 762         }
 763     }
 764
 765     /// When the expected `impl Trait` is not defined in the current item, it will come from
 766     /// a return type. This can occur when dealing with `TryStream` (#71035).
 767     fn suggest_constraining_opaque_associated_type(
 768         self,
 769         db: &mut DiagnosticBuilder<'_>,
 770         msg: &str,
 771         proj_ty: &ty::ProjectionTy<'tcx>,
 772         ty: Ty<'tcx>,
 773     ) -> bool {
 774         let assoc = self.associated_item(proj_ty.item_def_id);
 775         if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
 776             let opaque_local_def_id = def_id.as_local();
 777             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
 778                 match &self.hir().expect_item(opaque_local_def_id).kind {
 779                     hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
 780                     _ => bug!("The HirId comes from a `ty::Opaque`"),
 781                 }
 782             } else {
 783                 return false;
 784             };
 785
 786             let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 787
 788             self.constrain_generic_bound_associated_type_structured_suggestion(
 789                 db,
 790                 &trait_ref,
 791                 opaque_hir_ty.bounds,
 792                 assoc,
 793                 assoc_substs,
 794                 ty,
 795                 msg,
 796                 true,
 797             )
 798         } else {
 799             false
 800         }
 801     }
 802
 803     fn point_at_methods_that_satisfy_associated_type(
 804         self,
 805         db: &mut DiagnosticBuilder<'_>,
 806         assoc_container_id: DefId,
 807         current_method_ident: Option<Symbol>,
 808         proj_ty_item_def_id: DefId,
 809         expected: Ty<'tcx>,
 810     ) -> bool {
 811         let items = self.associated_items(assoc_container_id);
 812         // Find all the methods in the trait that could be called to construct the
 813         // expected associated type.
 814         // FIXME: consider suggesting the use of associated `const`s.
 815         let methods: Vec<(Span, String)> = items
 816             .items
 817             .iter()
 818             .filter(|(name, item)| {
 819                 ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
 820             })
 821             .filter_map(|(_, item)| {
 822                 let method = self.fn_sig(item.def_id);
 823                 match *method.output().skip_binder().kind() {
 824                     ty::Projection(ty::ProjectionTy { item_def_id, .. })
 825                         if item_def_id == proj_ty_item_def_id =>
 826                     {
 827                         Some((
 828                             self.sess.source_map().guess_head_span(self.def_span(item.def_id)),
 829                             format!("consider calling `{}`", self.def_path_str(item.def_id)),
 830                         ))
 831                     }
 832                     _ => None,
 833                 }
 834             })
 835             .collect();
 836         if !methods.is_empty() {
 837             // Use a single `help:` to show all the methods in the trait that can
 838             // be used to construct the expected associated type.
 839             let mut span: MultiSpan =
 840                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 841             let msg = format!(
 842                 "{some} method{s} {are} available that return{r} `{ty}`",
 843                 some = if methods.len() == 1 { "a" } else { "some" },
 844                 s = pluralize!(methods.len()),
 845                 are = if methods.len() == 1 { "is" } else { "are" },
 846                 r = if methods.len() == 1 { "s" } else { "" },
 847                 ty = expected
 848             );
 849             for (sp, label) in methods.into_iter() {
 850                 span.push_span_label(sp, label);
 851             }
 852             db.span_help(span, &msg);
 853             return true;
 854         }
 855         false
 856     }
 857
 858     fn point_at_associated_type(
 859         self,
 860         db: &mut DiagnosticBuilder<'_>,
 861         body_owner_def_id: DefId,
 862         found: Ty<'tcx>,
 863     ) -> bool {
 864         let hir_id =
 865             match body_owner_def_id.as_local().map(|id| self.hir().local_def_id_to_hir_id(id)) {
 866                 Some(hir_id) => hir_id,
 867                 None => return false,
 868             };
 869         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 870         // `expected` and point at it.
 871         let parent_id = self.hir().get_parent_item(hir_id);
 872         let item = self.hir().find_by_def_id(parent_id);
 873         debug!("expected_projection parent item {:?}", item);
 874         match item {
 875             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 876                 // FIXME: account for `#![feature(specialization)]`
 877                 for item in &items[..] {
 878                     match item.kind {
 879                         hir::AssocItemKind::Type => {
 880                             // FIXME: account for returning some type in a trait fn impl that has
 881                             // an assoc type as a return type (#72076).
 882                             if let hir::Defaultness::Default { has_value: true } = item.defaultness
 883                             {
 884                                 if self.type_of(item.id.def_id) == found {
 885                                     db.span_label(
 886                                         item.span,
 887                                         "associated type defaults can't be assumed inside the \
 888                                             trait defining them",
 889                                     );
 890                                     return true;
 891                                 }
 892                             }
 893                         }
 894                         _ => {}
 895                     }
 896                 }
 897             }
 898             Some(hir::Node::Item(hir::Item {
 899                 kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
 900                 ..
 901             })) => {
 902                 for item in &items[..] {
 903                     if let hir::AssocItemKind::Type = item.kind {
 904                         if self.type_of(item.id.def_id) == found {
 905                             db.span_label(item.span, "expected this associated type");
 906                             return true;
 907                         }
 908                     }
 909                 }
 910             }
 911             _ => {}
 912         }
 913         false
 914     }
 915
 916     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 917     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
 918     ///
 919     /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
 920     /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
 921     /// trait bound as the one we're looking for. This can help in cases where the associated
 922     /// type is defined on a supertrait of the one present in the bounds.
 923     fn constrain_generic_bound_associated_type_structured_suggestion(
 924         self,
 925         db: &mut DiagnosticBuilder<'_>,
 926         trait_ref: &ty::TraitRef<'tcx>,
 927         bounds: hir::GenericBounds<'_>,
 928         assoc: &ty::AssocItem,
 929         assoc_substs: &[ty::GenericArg<'tcx>],
 930         ty: Ty<'tcx>,
 931         msg: &str,
 932         is_bound_surely_present: bool,
 933     ) -> bool {
 934         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 935
 936         let trait_bounds = bounds.iter().filter_map(|bound| match bound {
 937             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
 938             _ => None,
 939         });
 940
 941         let matching_trait_bounds = trait_bounds
 942             .clone()
 943             .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
 944             .collect::<Vec<_>>();
 945
 946         let span = match &matching_trait_bounds[..] {
 947             &[ptr] => ptr.span,
 948             &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
 949                 &[ptr] => ptr.span,
 950                 _ => return false,
 951             },
 952             _ => return false,
 953         };
 954
 955         self.constrain_associated_type_structured_suggestion(db, span, assoc, assoc_substs, ty, msg)
 956     }
 957
 958     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 959     /// associated type to a given type `ty`.
 960     fn constrain_associated_type_structured_suggestion(
 961         self,
 962         db: &mut DiagnosticBuilder<'_>,
 963         span: Span,
 964         assoc: &ty::AssocItem,
 965         assoc_substs: &[ty::GenericArg<'tcx>],
 966         ty: Ty<'tcx>,
 967         msg: &str,
 968     ) -> bool {
 969         if let Ok(has_params) =
 970             self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 971         {
 972             let (span, sugg) = if has_params {
 973                 let pos = span.hi() - BytePos(1);
 974                 let span = Span::new(pos, pos, span.ctxt(), span.parent());
 975                 (span, format!(", {} = {}", assoc.ident(self), ty))
 976             } else {
 977                 let item_args = self.format_generic_args(assoc_substs);
 978                 (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(self), item_args, ty))
 979             };
 980             db.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 981             return true;
 982         }
 983         false
 984     }
 985
 986     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
 987         let mut item_args = String::new();
 988         FmtPrinter::new(self, &mut item_args, hir::def::Namespace::TypeNS)
 989             .path_generic_args(Ok, args)
 990             .expect("could not write to `String`.");
 991         item_args
 992     }
 993 }