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