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