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
  46     RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
  47     RegionsInsufficientlyPolymorphic(BoundRegionKind, Region<'tcx>),
  48     RegionsOverlyPolymorphic(BoundRegionKind, Region<'tcx>),
  49     RegionsPlaceholderMismatch,
  50
  51     Sorts(ExpectedFound<Ty<'tcx>>),
  52     ArgumentSorts(ExpectedFound<Ty<'tcx>>, usize),
  53     IntMismatch(ExpectedFound<ty::IntVarValue>),
  54     FloatMismatch(ExpectedFound<ty::FloatTy>),
  55     Traits(ExpectedFound<DefId>),
  56     VariadicMismatch(ExpectedFound<bool>),
  57
  58     /// Instantiating a type variable with the given type would have
  59     /// created a cycle (because it appears somewhere within that
  60     /// type).
  61     CyclicTy(Ty<'tcx>),
  62     CyclicConst(&'tcx ty::Const<'tcx>),
  63     ProjectionMismatched(ExpectedFound<DefId>),
  64     ExistentialMismatch(
  65         ExpectedFound<&'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>>,
  66     ),
  67     ObjectUnsafeCoercion(DefId),
  68     ConstMismatch(ExpectedFound<&'tcx ty::Const<'tcx>>),
  69
  70     IntrinsicCast,
  71     /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
  72     TargetFeatureCast(DefId),
  73 }
  74
  75 /// Explains the source of a type err in a short, human readable way. This is meant to be placed
  76 /// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
  77 /// afterwards to present additional details, particularly when it comes to lifetime-related
  78 /// errors.
  79 impl<'tcx> fmt::Display for TypeError<'tcx> {
  80     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  81         use self::TypeError::*;
  82         fn report_maybe_different(
  83             f: &mut fmt::Formatter<'_>,
  84             expected: &str,
  85             found: &str,
  86         ) -> fmt::Result {
  87             // A naive approach to making sure that we're not reporting silly errors such as:
  88             // (expected closure, found closure).
  89             if expected == found {
  90                 write!(f, "expected {}, found a different {}", expected, found)
  91             } else {
  92                 write!(f, "expected {}, found {}", expected, found)
  93             }
  94         }
  95
  96         let br_string = |br: ty::BoundRegionKind| match br {
  97             ty::BrNamed(_, name) => format!(" {}", name),
  98             _ => String::new(),
  99         };
 100
 101         match *self {
 102             CyclicTy(_) => write!(f, "cyclic type of infinite size"),
 103             CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
 104             Mismatch => write!(f, "types differ"),
 105             ConstnessMismatch(values) => {
 106                 write!(f, "expected {} bound, found {} bound", values.expected, values.found)
 107             }
 108             PolarityMismatch(values) => {
 109                 write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
 110             }
 111             UnsafetyMismatch(values) => {
 112                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 113             }
 114             AbiMismatch(values) => {
 115                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)
 116             }
 117             ArgumentMutability(_) | Mutability => write!(f, "types differ in mutability"),
 118             TupleSize(values) => write!(
 119                 f,
 120                 "expected a tuple with {} element{}, \
 121                            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{}, \
 130                            found one with {} element{}",
 131                 values.expected,
 132                 pluralize!(values.expected),
 133                 values.found,
 134                 pluralize!(values.found)
 135             ),
 136             ArgCount => write!(f, "incorrect number of function parameters"),
 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             | 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::TyS<'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.val {
 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         db: &mut DiagnosticBuilder<'_>,
 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                         db.note("no two closures, even if identical, have the same type");
 362                         db.help("consider boxing your closure and/or using it as a trait object");
 363                     }
 364                     (ty::Opaque(..), ty::Opaque(..)) => {
 365                         // Issue #63167
 366                         db.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                             db.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                             db.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                             db.span_label(f_span, "found type parameter");
 392                         }
 393                         db.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                         db.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                         db.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                             db.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                                 db,
 446                                 &format!("{}", proj.self_ty()),
 447                                 &path,
 448                                 None,
 449                             );
 450                         }
 451                         if note {
 452                             db.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                             db.span_label(p_span, "this type parameter");
 461                         }
 462                         db.help("type parameters must be constrained to match other types");
 463                         if self.sess.teach(&db.get_code().unwrap()) {
 464                             db.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                         db.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                             db.span_label(p_span, "this type parameter");
 501                         }
 502                         db.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                             db.span_label(p_span, "this type parameter");
 513                         }
 514                     }
 515                     (ty::Projection(proj_ty), _) => {
 516                         self.expected_projection(
 517                             db,
 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                             db,
 531                             &msg,
 532                             proj_ty,
 533                             values.expected,
 534                         ) || self.suggest_constraint(
 535                             db,
 536                             &msg,
 537                             body_owner_def_id,
 538                             proj_ty,
 539                             values.expected,
 540                         )) {
 541                             db.help(&msg);
 542                             db.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                     db.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 attrs = self.get_attrs(*def_id);
 571                 let target_spans = attrs
 572                     .iter()
 573                     .filter(|attr| attr.has_name(sym::target_feature))
 574                     .map(|attr| attr.span);
 575                 db.note(
 576                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
 577                 );
 578                 db.span_labels(target_spans, "`#[target_feature]` added here");
 579             }
 580             _ => {}
 581         }
 582     }
 583
 584     fn suggest_constraint(
 585         self,
 586         db: &mut DiagnosticBuilder<'_>,
 587         msg: &str,
 588         body_owner_def_id: DefId,
 589         proj_ty: &ty::ProjectionTy<'tcx>,
 590         ty: Ty<'tcx>,
 591     ) -> bool {
 592         let assoc = self.associated_item(proj_ty.item_def_id);
 593         let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 594         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
 595             if let Some(hir_generics) = item.generics() {
 596                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
 597                 // This will also work for `impl Trait`.
 598                 let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
 599                     let generics = self.generics_of(body_owner_def_id);
 600                     generics.type_param(param_ty, self).def_id
 601                 } 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 predicate in hir_generics.where_clause.predicates {
 608                     if let hir::WherePredicate::BoundPredicate(pred) = predicate {
 609                         if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) =
 610                             pred.bounded_ty.kind
 611                         {
 612                             if path.res.opt_def_id() == Some(def_id) {
 613                                 // This predicate is binding type param `A` in `<A as T>::Foo` to
 614                                 // something, potentially `T`.
 615                             } else {
 616                                 continue;
 617                             }
 618                         } else {
 619                             continue;
 620                         }
 621
 622                         if self.constrain_generic_bound_associated_type_structured_suggestion(
 623                             db,
 624                             &trait_ref,
 625                             pred.bounds,
 626                             &assoc,
 627                             assoc_substs,
 628                             ty,
 629                             msg,
 630                             false,
 631                         ) {
 632                             return true;
 633                         }
 634                     }
 635                 }
 636                 for param in hir_generics.params {
 637                     if self.hir().opt_local_def_id(param.hir_id).map(|id| id.to_def_id())
 638                         == Some(def_id)
 639                     {
 640                         // This is type param `A` in `<A as T>::Foo`.
 641                         return self.constrain_generic_bound_associated_type_structured_suggestion(
 642                             db,
 643                             &trait_ref,
 644                             param.bounds,
 645                             &assoc,
 646                             assoc_substs,
 647                             ty,
 648                             msg,
 649                             false,
 650                         );
 651                     }
 652                 }
 653             }
 654         }
 655         false
 656     }
 657
 658     /// An associated type was expected and a different type was found.
 659     ///
 660     /// We perform a few different checks to see what we can suggest:
 661     ///
 662     ///  - In the current item, look for associated functions that return the expected type and
 663     ///    suggest calling them. (Not a structured suggestion.)
 664     ///  - If any of the item's generic bounds can be constrained, we suggest constraining the
 665     ///    associated type to the found type.
 666     ///  - If the associated type has a default type and was expected inside of a `trait`, we
 667     ///    mention that this is disallowed.
 668     ///  - If all other things fail, and the error is not because of a mismatch between the `trait`
 669     ///    and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
 670     ///    fn that returns the type.
 671     fn expected_projection(
 672         self,
 673         db: &mut DiagnosticBuilder<'_>,
 674         proj_ty: &ty::ProjectionTy<'tcx>,
 675         values: &ExpectedFound<Ty<'tcx>>,
 676         body_owner_def_id: DefId,
 677         cause_code: &ObligationCauseCode<'_>,
 678     ) {
 679         let msg = format!(
 680             "consider constraining the associated type `{}` to `{}`",
 681             values.expected, values.found
 682         );
 683         let body_owner = self.hir().get_if_local(body_owner_def_id);
 684         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 685
 686         // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
 687         let callable_scope = matches!(
 688             body_owner,
 689             Some(
 690                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
 691                     | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
 692                     | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
 693             )
 694         );
 695         let impl_comparison = matches!(
 696             cause_code,
 697             ObligationCauseCode::CompareImplMethodObligation { .. }
 698                 | ObligationCauseCode::CompareImplTypeObligation { .. }
 699                 | ObligationCauseCode::CompareImplConstObligation
 700         );
 701         let assoc = self.associated_item(proj_ty.item_def_id);
 702         if !callable_scope || impl_comparison {
 703             // We do not want to suggest calling functions when the reason of the
 704             // type error is a comparison of an `impl` with its `trait` or when the
 705             // scope is outside of a `Body`.
 706         } else {
 707             // If we find a suitable associated function that returns the expected type, we don't
 708             // want the more general suggestion later in this method about "consider constraining
 709             // the associated type or calling a method that returns the associated type".
 710             let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
 711                 db,
 712                 assoc.container.id(),
 713                 current_method_ident,
 714                 proj_ty.item_def_id,
 715                 values.expected,
 716             );
 717             // Possibly suggest constraining the associated type to conform to the
 718             // found type.
 719             if self.suggest_constraint(db, &msg, body_owner_def_id, proj_ty, values.found)
 720                 || point_at_assoc_fn
 721             {
 722                 return;
 723             }
 724         }
 725
 726         self.suggest_constraining_opaque_associated_type(db, &msg, proj_ty, values.found);
 727
 728         if self.point_at_associated_type(db, body_owner_def_id, values.found) {
 729             return;
 730         }
 731
 732         if !impl_comparison {
 733             // Generic suggestion when we can't be more specific.
 734             if callable_scope {
 735                 db.help(&format!("{} or calling a method that returns `{}`", msg, values.expected));
 736             } else {
 737                 db.help(&msg);
 738             }
 739             db.note(
 740                 "for more information, visit \
 741                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
 742             );
 743         }
 744         if self.sess.teach(&db.get_code().unwrap()) {
 745             db.help(
 746                 "given an associated type `T` and a method `foo`:
 747 ```
 748 trait Trait {
 749 type T;
 750 fn foo(&self) -> Self::T;
 751 }
 752 ```
 753 the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
 754 ```
 755 impl Trait for X {
 756 type T = String;
 757 fn foo(&self) -> Self::T { String::new() }
 758 }
 759 ```",
 760             );
 761         }
 762     }
 763
 764     /// When the expected `impl Trait` is not defined in the current item, it will come from
 765     /// a return type. This can occur when dealing with `TryStream` (#71035).
 766     fn suggest_constraining_opaque_associated_type(
 767         self,
 768         db: &mut DiagnosticBuilder<'_>,
 769         msg: &str,
 770         proj_ty: &ty::ProjectionTy<'tcx>,
 771         ty: Ty<'tcx>,
 772     ) -> bool {
 773         let assoc = self.associated_item(proj_ty.item_def_id);
 774         if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
 775             let opaque_local_def_id = def_id.as_local();
 776             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
 777                 let hir = self.hir();
 778                 let opaque_hir_id = hir.local_def_id_to_hir_id(opaque_local_def_id);
 779                 match &hir.expect_item(opaque_hir_id).kind {
 780                     hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
 781                     _ => bug!("The HirId comes from a `ty::Opaque`"),
 782                 }
 783             } else {
 784                 return false;
 785             };
 786
 787             let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
 788
 789             self.constrain_generic_bound_associated_type_structured_suggestion(
 790                 db,
 791                 &trait_ref,
 792                 opaque_hir_ty.bounds,
 793                 assoc,
 794                 assoc_substs,
 795                 ty,
 796                 msg,
 797                 true,
 798             )
 799         } else {
 800             false
 801         }
 802     }
 803
 804     fn point_at_methods_that_satisfy_associated_type(
 805         self,
 806         db: &mut DiagnosticBuilder<'_>,
 807         assoc_container_id: DefId,
 808         current_method_ident: Option<Symbol>,
 809         proj_ty_item_def_id: DefId,
 810         expected: Ty<'tcx>,
 811     ) -> bool {
 812         let items = self.associated_items(assoc_container_id);
 813         // Find all the methods in the trait that could be called to construct the
 814         // expected associated type.
 815         // FIXME: consider suggesting the use of associated `const`s.
 816         let methods: Vec<(Span, String)> = items
 817             .items
 818             .iter()
 819             .filter(|(name, item)| {
 820                 ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
 821             })
 822             .filter_map(|(_, item)| {
 823                 let method = self.fn_sig(item.def_id);
 824                 match *method.output().skip_binder().kind() {
 825                     ty::Projection(ty::ProjectionTy { item_def_id, .. })
 826                         if item_def_id == proj_ty_item_def_id =>
 827                     {
 828                         Some((
 829                             self.sess.source_map().guess_head_span(self.def_span(item.def_id)),
 830                             format!("consider calling `{}`", self.def_path_str(item.def_id)),
 831                         ))
 832                     }
 833                     _ => None,
 834                 }
 835             })
 836             .collect();
 837         if !methods.is_empty() {
 838             // Use a single `help:` to show all the methods in the trait that can
 839             // be used to construct the expected associated type.
 840             let mut span: MultiSpan =
 841                 methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
 842             let msg = format!(
 843                 "{some} method{s} {are} available that return{r} `{ty}`",
 844                 some = if methods.len() == 1 { "a" } else { "some" },
 845                 s = pluralize!(methods.len()),
 846                 are = if methods.len() == 1 { "is" } else { "are" },
 847                 r = if methods.len() == 1 { "s" } else { "" },
 848                 ty = expected
 849             );
 850             for (sp, label) in methods.into_iter() {
 851                 span.push_span_label(sp, label);
 852             }
 853             db.span_help(span, &msg);
 854             return true;
 855         }
 856         false
 857     }
 858
 859     fn point_at_associated_type(
 860         self,
 861         db: &mut DiagnosticBuilder<'_>,
 862         body_owner_def_id: DefId,
 863         found: Ty<'tcx>,
 864     ) -> bool {
 865         let hir_id =
 866             match body_owner_def_id.as_local().map(|id| self.hir().local_def_id_to_hir_id(id)) {
 867                 Some(hir_id) => hir_id,
 868                 None => return false,
 869             };
 870         // When `body_owner` is an `impl` or `trait` item, look in its associated types for
 871         // `expected` and point at it.
 872         let parent_id = self.hir().get_parent_item(hir_id);
 873         let item = self.hir().find(parent_id);
 874         debug!("expected_projection parent item {:?}", item);
 875         match item {
 876             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
 877                 // FIXME: account for `#![feature(specialization)]`
 878                 for item in &items[..] {
 879                     match item.kind {
 880                         hir::AssocItemKind::Type => {
 881                             // FIXME: account for returning some type in a trait fn impl that has
 882                             // an assoc type as a return type (#72076).
 883                             if let hir::Defaultness::Default { has_value: true } = item.defaultness
 884                             {
 885                                 if self.type_of(item.id.def_id) == found {
 886                                     db.span_label(
 887                                         item.span,
 888                                         "associated type defaults can't be assumed inside the \
 889                                             trait defining them",
 890                                     );
 891                                     return true;
 892                                 }
 893                             }
 894                         }
 895                         _ => {}
 896                     }
 897                 }
 898             }
 899             Some(hir::Node::Item(hir::Item {
 900                 kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
 901                 ..
 902             })) => {
 903                 for item in &items[..] {
 904                     if let hir::AssocItemKind::Type = item.kind {
 905                         if self.type_of(item.id.def_id) == found {
 906                             db.span_label(item.span, "expected this associated type");
 907                             return true;
 908                         }
 909                     }
 910                 }
 911             }
 912             _ => {}
 913         }
 914         false
 915     }
 916
 917     /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
 918     /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
 919     ///
 920     /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
 921     /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
 922     /// trait bound as the one we're looking for. This can help in cases where the associated
 923     /// type is defined on a supertrait of the one present in the bounds.
 924     fn constrain_generic_bound_associated_type_structured_suggestion(
 925         self,
 926         db: &mut DiagnosticBuilder<'_>,
 927         trait_ref: &ty::TraitRef<'tcx>,
 928         bounds: hir::GenericBounds<'_>,
 929         assoc: &ty::AssocItem,
 930         assoc_substs: &[ty::GenericArg<'tcx>],
 931         ty: Ty<'tcx>,
 932         msg: &str,
 933         is_bound_surely_present: bool,
 934     ) -> bool {
 935         // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
 936
 937         let trait_bounds = bounds.iter().filter_map(|bound| match bound {
 938             hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
 939             _ => None,
 940         });
 941
 942         let matching_trait_bounds = trait_bounds
 943             .clone()
 944             .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
 945             .collect::<Vec<_>>();
 946
 947         let span = match &matching_trait_bounds[..] {
 948             &[ptr] => ptr.span,
 949             &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
 950                 &[ptr] => ptr.span,
 951                 _ => return false,
 952             },
 953             _ => return false,
 954         };
 955
 956         self.constrain_associated_type_structured_suggestion(db, span, assoc, assoc_substs, ty, msg)
 957     }
 958
 959     /// Given a span corresponding to a bound, provide a structured suggestion to set an
 960     /// associated type to a given type `ty`.
 961     fn constrain_associated_type_structured_suggestion(
 962         self,
 963         db: &mut DiagnosticBuilder<'_>,
 964         span: Span,
 965         assoc: &ty::AssocItem,
 966         assoc_substs: &[ty::GenericArg<'tcx>],
 967         ty: Ty<'tcx>,
 968         msg: &str,
 969     ) -> bool {
 970         if let Ok(has_params) =
 971             self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
 972         {
 973             let (span, sugg) = if has_params {
 974                 let pos = span.hi() - BytePos(1);
 975                 let span = Span::new(pos, pos, span.ctxt(), span.parent());
 976                 (span, format!(", {} = {}", assoc.ident, ty))
 977             } else {
 978                 let item_args = self.format_generic_args(assoc_substs);
 979                 (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident, item_args, ty))
 980             };
 981             db.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
 982             return true;
 983         }
 984         false
 985     }
 986
 987     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
 988         let mut item_args = String::new();
 989         FmtPrinter::new(self, &mut item_args, hir::def::Namespace::TypeNS)
 990             .path_generic_args(Ok, args)
 991             .expect("could not write to `String`.");
 992         item_args
 993     }
 994 }