compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

   1 use crate::infer::type_variable::TypeVariableOriginKind;
   2 use crate::infer::InferCtxt;
   3 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder};
   4 use rustc_hir as hir;
   5 use rustc_hir::def::{DefKind, Namespace};
   6 use rustc_hir::def_id::DefId;
   7 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
   8 use rustc_hir::{Body, Expr, ExprKind, FnRetTy, HirId, Local, Pat};
   9 use rustc_middle::hir::map::Map;
  10 use rustc_middle::infer::unify_key::ConstVariableOriginKind;
  11 use rustc_middle::ty::print::Print;
  12 use rustc_middle::ty::subst::{GenericArg, GenericArgKind};
  13 use rustc_middle::ty::{self, DefIdTree, InferConst, Ty, TyCtxt};
  14 use rustc_span::source_map::DesugaringKind;
  15 use rustc_span::symbol::kw;
  16 use rustc_span::Span;
  17 use std::borrow::Cow;
  18
  19 struct FindHirNodeVisitor<'a, 'tcx> {
  20     infcx: &'a InferCtxt<'a, 'tcx>,
  21     target: GenericArg<'tcx>,
  22     target_span: Span,
  23     found_node_ty: Option<Ty<'tcx>>,
  24     found_local_pattern: Option<&'tcx Pat<'tcx>>,
  25     found_arg_pattern: Option<&'tcx Pat<'tcx>>,
  26     found_closure: Option<&'tcx Expr<'tcx>>,
  27     found_method_call: Option<&'tcx Expr<'tcx>>,
  28     found_exact_method_call: Option<&'tcx Expr<'tcx>>,
  29     found_use_diagnostic: Option<UseDiagnostic<'tcx>>,
  30 }
  31
  32 impl<'a, 'tcx> FindHirNodeVisitor<'a, 'tcx> {
  33     fn new(infcx: &'a InferCtxt<'a, 'tcx>, target: GenericArg<'tcx>, target_span: Span) -> Self {
  34         Self {
  35             infcx,
  36             target,
  37             target_span,
  38             found_node_ty: None,
  39             found_local_pattern: None,
  40             found_arg_pattern: None,
  41             found_closure: None,
  42             found_method_call: None,
  43             found_exact_method_call: None,
  44             found_use_diagnostic: None,
  45         }
  46     }
  47
  48     fn node_type_opt(&self, hir_id: HirId) -> Option<Ty<'tcx>> {
  49         self.infcx.in_progress_typeck_results?.borrow().node_type_opt(hir_id)
  50     }
  51
  52     fn node_ty_contains_target(&self, hir_id: HirId) -> Option<Ty<'tcx>> {
  53         self.node_type_opt(hir_id).map(|ty| self.infcx.resolve_vars_if_possible(ty)).filter(|ty| {
  54             ty.walk(self.infcx.tcx).any(|inner| {
  55                 inner == self.target
  56                     || match (inner.unpack(), self.target.unpack()) {
  57                         (GenericArgKind::Type(inner_ty), GenericArgKind::Type(target_ty)) => {
  58                             use ty::{Infer, TyVar};
  59                             match (inner_ty.kind(), target_ty.kind()) {
  60                                 (&Infer(TyVar(a_vid)), &Infer(TyVar(b_vid))) => self
  61                                     .infcx
  62                                     .inner
  63                                     .borrow_mut()
  64                                     .type_variables()
  65                                     .sub_unified(a_vid, b_vid),
  66                                 _ => false,
  67                             }
  68                         }
  69                         _ => false,
  70                     }
  71             })
  72         })
  73     }
  74
  75     /// Determine whether the expression, assumed to be the callee within a `Call`,
  76     /// corresponds to the `From::from` emitted in desugaring of the `?` operator.
  77     fn is_try_conversion(&self, callee: &Expr<'tcx>) -> bool {
  78         self.infcx
  79             .trait_def_from_hir_fn(callee.hir_id)
  80             .map_or(false, |def_id| self.infcx.is_try_conversion(callee.span, def_id))
  81     }
  82 }
  83
  84 impl<'a, 'tcx> Visitor<'tcx> for FindHirNodeVisitor<'a, 'tcx> {
  85     type Map = Map<'tcx>;
  86
  87     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
  88         NestedVisitorMap::OnlyBodies(self.infcx.tcx.hir())
  89     }
  90
  91     fn visit_local(&mut self, local: &'tcx Local<'tcx>) {
  92         if let (None, Some(ty)) =
  93             (self.found_local_pattern, self.node_ty_contains_target(local.hir_id))
  94         {
  95             self.found_local_pattern = Some(&*local.pat);
  96             self.found_node_ty = Some(ty);
  97         }
  98         intravisit::walk_local(self, local);
  99     }
 100
 101     fn visit_body(&mut self, body: &'tcx Body<'tcx>) {
 102         for param in body.params {
 103             if let (None, Some(ty)) =
 104                 (self.found_arg_pattern, self.node_ty_contains_target(param.hir_id))
 105             {
 106                 self.found_arg_pattern = Some(&*param.pat);
 107                 self.found_node_ty = Some(ty);
 108             }
 109         }
 110         intravisit::walk_body(self, body);
 111     }
 112
 113     fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
 114         if let ExprKind::MethodCall(_, call_span, exprs, _) = expr.kind {
 115             if call_span == self.target_span
 116                 && Some(self.target)
 117                     == self.infcx.in_progress_typeck_results.and_then(|typeck_results| {
 118                         typeck_results
 119                             .borrow()
 120                             .node_type_opt(exprs.first().unwrap().hir_id)
 121                             .map(Into::into)
 122                     })
 123             {
 124                 self.found_exact_method_call = Some(&expr);
 125                 return;
 126             }
 127         }
 128
 129         // FIXME(const_generics): Currently, any uninferred `const` generics arguments
 130         // are handled specially, but instead they should be handled in `annotate_method_call`,
 131         // which currently doesn't work because this evaluates to `false` for const arguments.
 132         // See https://github.com/rust-lang/rust/pull/77758 for more details.
 133         if let Some(ty) = self.node_ty_contains_target(expr.hir_id) {
 134             match expr.kind {
 135                 ExprKind::Closure(..) => self.found_closure = Some(&expr),
 136                 ExprKind::MethodCall(..) => self.found_method_call = Some(&expr),
 137
 138                 // If the given expression falls within the target span and is a
 139                 // `From::from(e)` call emitted during desugaring of the `?` operator,
 140                 // extract the types inferred before and after the call
 141                 ExprKind::Call(callee, [arg])
 142                     if self.target_span.contains(expr.span)
 143                         && self.found_use_diagnostic.is_none()
 144                         && self.is_try_conversion(callee) =>
 145                 {
 146                     self.found_use_diagnostic = self.node_type_opt(arg.hir_id).map(|pre_ty| {
 147                         UseDiagnostic::TryConversion { pre_ty, post_ty: ty, span: callee.span }
 148                     });
 149                 }
 150                 _ => {}
 151             }
 152         }
 153         intravisit::walk_expr(self, expr);
 154     }
 155 }
 156
 157 /// An observation about the use site of a type to be emitted as an additional
 158 /// note in an inference failure error.
 159 enum UseDiagnostic<'tcx> {
 160     /// Records the types inferred before and after `From::from` is called on the
 161     /// error value within the desugaring of the `?` operator.
 162     TryConversion { pre_ty: Ty<'tcx>, post_ty: Ty<'tcx>, span: Span },
 163 }
 164
 165 impl UseDiagnostic<'_> {
 166     /// Return a descriptor of the value at the use site
 167     fn descr(&self) -> &'static str {
 168         match self {
 169             Self::TryConversion { .. } => "error for `?` operator",
 170         }
 171     }
 172
 173     /// Return a descriptor of the type at the use site
 174     fn type_descr(&self) -> &'static str {
 175         match self {
 176             Self::TryConversion { .. } => "error type for `?` operator",
 177         }
 178     }
 179
 180     fn applies_to(&self, span: Span) -> bool {
 181         match *self {
 182             // In some cases the span for an inference failure due to try
 183             // conversion contains the antecedent expression as well as the `?`
 184             Self::TryConversion { span: s, .. } => span.contains(s) && span.hi() == s.hi(),
 185         }
 186     }
 187
 188     fn attach_note(&self, err: &mut DiagnosticBuilder<'_>) {
 189         match *self {
 190             Self::TryConversion { pre_ty, post_ty, .. } => {
 191                 let intro = "`?` implicitly converts the error value";
 192
 193                 let msg = match (pre_ty.is_ty_infer(), post_ty.is_ty_infer()) {
 194                     (true, true) => format!("{} using the `From` trait", intro),
 195                     (false, true) => {
 196                         format!("{} into a type implementing `From<{}>`", intro, pre_ty)
 197                     }
 198                     (true, false) => {
 199                         format!("{} into `{}` using the `From` trait", intro, post_ty)
 200                     }
 201                     (false, false) => {
 202                         format!(
 203                             "{} into `{}` using its implementation of `From<{}>`",
 204                             intro, post_ty, pre_ty
 205                         )
 206                     }
 207                 };
 208
 209                 err.note(&msg);
 210             }
 211         }
 212     }
 213 }
 214
 215 /// Suggest giving an appropriate return type to a closure expression.
 216 fn closure_return_type_suggestion(
 217     err: &mut DiagnosticBuilder<'_>,
 218     output: &FnRetTy<'_>,
 219     body: &Body<'_>,
 220     ret: &str,
 221 ) {
 222     let (arrow, post) = match output {
 223         FnRetTy::DefaultReturn(_) => ("-> ", " "),
 224         _ => ("", ""),
 225     };
 226     let suggestion = match body.value.kind {
 227         ExprKind::Block(..) => vec![(output.span(), format!("{}{}{}", arrow, ret, post))],
 228         _ => vec![
 229             (output.span(), format!("{}{}{}{{ ", arrow, ret, post)),
 230             (body.value.span.shrink_to_hi(), " }".to_string()),
 231         ],
 232     };
 233     err.multipart_suggestion(
 234         "give this closure an explicit return type without `_` placeholders",
 235         suggestion,
 236         Applicability::HasPlaceholders,
 237     );
 238 }
 239
 240 /// Given a closure signature, return a `String` containing a list of all its argument types.
 241 fn closure_args(fn_sig: &ty::PolyFnSig<'_>) -> String {
 242     fn_sig
 243         .inputs()
 244         .skip_binder()
 245         .iter()
 246         .next()
 247         .map(|args| args.tuple_fields().map(|arg| arg.to_string()).collect::<Vec<_>>().join(", "))
 248         .unwrap_or_default()
 249 }
 250
 251 pub enum TypeAnnotationNeeded {
 252     /// ```compile_fail,E0282
 253     /// let x = "hello".chars().rev().collect();
 254     /// ```
 255     E0282,
 256     /// An implementation cannot be chosen unambiguously because of lack of information.
 257     /// ```compile_fail,E0283
 258     /// let _ = Default::default();
 259     /// ```
 260     E0283,
 261     /// ```compile_fail,E0284
 262     /// let mut d: u64 = 2;
 263     /// d = d % 1u32.into();
 264     /// ```
 265     E0284,
 266 }
 267
 268 impl Into<rustc_errors::DiagnosticId> for TypeAnnotationNeeded {
 269     fn into(self) -> rustc_errors::DiagnosticId {
 270         match self {
 271             Self::E0282 => rustc_errors::error_code!(E0282),
 272             Self::E0283 => rustc_errors::error_code!(E0283),
 273             Self::E0284 => rustc_errors::error_code!(E0284),
 274         }
 275     }
 276 }
 277
 278 /// Information about a constant or a type containing inference variables.
 279 pub struct InferenceDiagnosticsData {
 280     pub name: String,
 281     pub span: Option<Span>,
 282     pub kind: UnderspecifiedArgKind,
 283     pub parent: Option<InferenceDiagnosticsParentData>,
 284 }
 285
 286 /// Data on the parent definition where a generic argument was declared.
 287 pub struct InferenceDiagnosticsParentData {
 288     pub prefix: &'static str,
 289     pub name: String,
 290     pub def_id: DefId,
 291 }
 292
 293 pub enum UnderspecifiedArgKind {
 294     Type { prefix: Cow<'static, str> },
 295     Const { is_parameter: bool },
 296 }
 297
 298 impl InferenceDiagnosticsData {
 299     /// Generate a label for a generic argument which can't be inferred. When not
 300     /// much is known about the argument, `use_diag` may be used to describe the
 301     /// labeled value.
 302     fn cannot_infer_msg(&self, use_diag: Option<&UseDiagnostic<'_>>) -> String {
 303         if self.name == "_" && matches!(self.kind, UnderspecifiedArgKind::Type { .. }) {
 304             if let Some(use_diag) = use_diag {
 305                 return format!("cannot infer type of {}", use_diag.descr());
 306             }
 307
 308             return "cannot infer type".to_string();
 309         }
 310
 311         let suffix = match (&self.parent, use_diag) {
 312             (Some(parent), _) => format!(" declared on the {} `{}`", parent.prefix, parent.name),
 313             (None, Some(use_diag)) => format!(" in {}", use_diag.type_descr()),
 314             (None, None) => String::new(),
 315         };
 316
 317         // For example: "cannot infer type for type parameter `T`"
 318         format!("cannot infer {} `{}`{}", self.kind.prefix_string(), self.name, suffix)
 319     }
 320 }
 321
 322 impl InferenceDiagnosticsParentData {
 323     fn for_def_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<InferenceDiagnosticsParentData> {
 324         let parent_def_id = tcx.parent(def_id)?;
 325
 326         let parent_name =
 327             tcx.def_key(parent_def_id).disambiguated_data.data.get_opt_name()?.to_string();
 328
 329         Some(InferenceDiagnosticsParentData {
 330             prefix: tcx.def_kind(parent_def_id).descr(parent_def_id),
 331             name: parent_name,
 332             def_id: parent_def_id,
 333         })
 334     }
 335 }
 336
 337 impl UnderspecifiedArgKind {
 338     fn prefix_string(&self) -> Cow<'static, str> {
 339         match self {
 340             Self::Type { prefix } => format!("type for {}", prefix).into(),
 341             Self::Const { is_parameter: true } => "the value of const parameter".into(),
 342             Self::Const { is_parameter: false } => "the value of the constant".into(),
 343         }
 344     }
 345 }
 346
 347 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 348     /// Extracts data used by diagnostic for either types or constants
 349     /// which were stuck during inference.
 350     pub fn extract_inference_diagnostics_data(
 351         &self,
 352         arg: GenericArg<'tcx>,
 353         highlight: Option<ty::print::RegionHighlightMode>,
 354     ) -> InferenceDiagnosticsData {
 355         match arg.unpack() {
 356             GenericArgKind::Type(ty) => {
 357                 if let ty::Infer(ty::TyVar(ty_vid)) = *ty.kind() {
 358                     let mut inner = self.inner.borrow_mut();
 359                     let ty_vars = &inner.type_variables();
 360                     let var_origin = ty_vars.var_origin(ty_vid);
 361                     if let TypeVariableOriginKind::TypeParameterDefinition(name, def_id) =
 362                         var_origin.kind
 363                     {
 364                         if name != kw::SelfUpper {
 365                             return InferenceDiagnosticsData {
 366                                 name: name.to_string(),
 367                                 span: Some(var_origin.span),
 368                                 kind: UnderspecifiedArgKind::Type {
 369                                     prefix: "type parameter".into(),
 370                                 },
 371                                 parent: def_id.and_then(|def_id| {
 372                                     InferenceDiagnosticsParentData::for_def_id(self.tcx, def_id)
 373                                 }),
 374                             };
 375                         }
 376                     }
 377                 }
 378
 379                 let mut s = String::new();
 380                 let mut printer = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS);
 381                 if let Some(highlight) = highlight {
 382                     printer.region_highlight_mode = highlight;
 383                 }
 384                 let _ = ty.print(printer);
 385                 InferenceDiagnosticsData {
 386                     name: s,
 387                     span: None,
 388                     kind: UnderspecifiedArgKind::Type { prefix: ty.prefix_string(self.tcx) },
 389                     parent: None,
 390                 }
 391             }
 392             GenericArgKind::Const(ct) => {
 393                 if let ty::ConstKind::Infer(InferConst::Var(vid)) = ct.val {
 394                     let origin =
 395                         self.inner.borrow_mut().const_unification_table().probe_value(vid).origin;
 396                     if let ConstVariableOriginKind::ConstParameterDefinition(name, def_id) =
 397                         origin.kind
 398                     {
 399                         return InferenceDiagnosticsData {
 400                             name: name.to_string(),
 401                             span: Some(origin.span),
 402                             kind: UnderspecifiedArgKind::Const { is_parameter: true },
 403                             parent: InferenceDiagnosticsParentData::for_def_id(self.tcx, def_id),
 404                         };
 405                     }
 406
 407                     debug_assert!(!origin.span.is_dummy());
 408                     let mut s = String::new();
 409                     let mut printer =
 410                         ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::ValueNS);
 411                     if let Some(highlight) = highlight {
 412                         printer.region_highlight_mode = highlight;
 413                     }
 414                     let _ = ct.print(printer);
 415                     InferenceDiagnosticsData {
 416                         name: s,
 417                         span: Some(origin.span),
 418                         kind: UnderspecifiedArgKind::Const { is_parameter: false },
 419                         parent: None,
 420                     }
 421                 } else {
 422                     bug!("unexpect const: {:?}", ct);
 423                 }
 424             }
 425             GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),
 426         }
 427     }
 428
 429     pub fn emit_inference_failure_err(
 430         &self,
 431         body_id: Option<hir::BodyId>,
 432         span: Span,
 433         arg: GenericArg<'tcx>,
 434         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 435         error_code: TypeAnnotationNeeded,
 436     ) -> DiagnosticBuilder<'tcx> {
 437         let arg = self.resolve_vars_if_possible(arg);
 438         let arg_data = self.extract_inference_diagnostics_data(arg, None);
 439
 440         let mut local_visitor = FindHirNodeVisitor::new(&self, arg, span);
 441         let ty_to_string = |ty: Ty<'tcx>| -> String {
 442             let mut s = String::new();
 443             let mut printer = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS);
 444             let mut inner = self.inner.borrow_mut();
 445             let ty_vars = inner.type_variables();
 446             let getter = move |ty_vid| {
 447                 let var_origin = ty_vars.var_origin(ty_vid);
 448                 if let TypeVariableOriginKind::TypeParameterDefinition(name, _) = var_origin.kind {
 449                     return Some(name.to_string());
 450                 }
 451                 None
 452             };
 453             printer.name_resolver = Some(Box::new(&getter));
 454             let _ = if let ty::FnDef(..) = ty.kind() {
 455                 // We don't want the regular output for `fn`s because it includes its path in
 456                 // invalid pseudo-syntax, we want the `fn`-pointer output instead.
 457                 ty.fn_sig(self.tcx).print(printer)
 458             } else {
 459                 ty.print(printer)
 460             };
 461             s
 462         };
 463
 464         if let Some(body_id) = body_id {
 465             let expr = self.tcx.hir().expect_expr(body_id.hir_id);
 466             local_visitor.visit_expr(expr);
 467         }
 468         let err_span = if let Some(pattern) = local_visitor.found_arg_pattern {
 469             pattern.span
 470         } else if let Some(span) = arg_data.span {
 471             // `span` here lets us point at `sum` instead of the entire right hand side expr:
 472             // error[E0282]: type annotations needed
 473             //  --> file2.rs:3:15
 474             //   |
 475             // 3 |     let _ = x.sum() as f64;
 476             //   |               ^^^ cannot infer type for `S`
 477             span
 478         } else if let Some(ExprKind::MethodCall(_, call_span, _, _)) =
 479             local_visitor.found_method_call.map(|e| &e.kind)
 480         {
 481             // Point at the call instead of the whole expression:
 482             // error[E0284]: type annotations needed
 483             //  --> file.rs:2:5
 484             //   |
 485             // 2 |     vec![Ok(2)].into_iter().collect()?;
 486             //   |                             ^^^^^^^ cannot infer type
 487             //   |
 488             //   = note: cannot resolve `<_ as std::ops::Try>::Ok == _`
 489             if span.contains(*call_span) { *call_span } else { span }
 490         } else {
 491             span
 492         };
 493
 494         let is_named_and_not_impl_trait =
 495             |ty: Ty<'_>| &ty.to_string() != "_" && !ty.is_impl_trait();
 496
 497         let ty_msg = match (local_visitor.found_node_ty, local_visitor.found_exact_method_call) {
 498             (_, Some(_)) => String::new(),
 499             (Some(ty), _) if ty.is_closure() => {
 500                 let substs =
 501                     if let ty::Closure(_, substs) = *ty.kind() { substs } else { unreachable!() };
 502                 let fn_sig = substs.as_closure().sig();
 503                 let args = closure_args(&fn_sig);
 504                 let ret = fn_sig.output().skip_binder().to_string();
 505                 format!(" for the closure `fn({}) -> {}`", args, ret)
 506             }
 507             (Some(ty), _) if is_named_and_not_impl_trait(ty) => {
 508                 let ty = ty_to_string(ty);
 509                 format!(" for `{}`", ty)
 510             }
 511             _ => String::new(),
 512         };
 513
 514         // When `arg_data.name` corresponds to a type argument, show the path of the full type we're
 515         // trying to infer. In the following example, `ty_msg` contains
 516         // " for `std::result::Result<i32, E>`":
 517         // ```
 518         // error[E0282]: type annotations needed for `std::result::Result<i32, E>`
 519         //  --> file.rs:L:CC
 520         //   |
 521         // L |     let b = Ok(4);
 522         //   |         -   ^^ cannot infer type for `E` in `std::result::Result<i32, E>`
 523         //   |         |
 524         //   |         consider giving `b` the explicit type `std::result::Result<i32, E>`, where
 525         //   |         the type parameter `E` is specified
 526         // ```
 527         let error_code = error_code.into();
 528         let mut err = self.tcx.sess.struct_span_err_with_code(
 529             err_span,
 530             &format!("type annotations needed{}", ty_msg),
 531             error_code,
 532         );
 533
 534         let use_diag = local_visitor.found_use_diagnostic.as_ref();
 535         if let Some(use_diag) = use_diag {
 536             if use_diag.applies_to(err_span) {
 537                 use_diag.attach_note(&mut err);
 538             }
 539         }
 540
 541         let suffix = match local_visitor.found_node_ty {
 542             Some(ty) if ty.is_closure() => {
 543                 let substs =
 544                     if let ty::Closure(_, substs) = *ty.kind() { substs } else { unreachable!() };
 545                 let fn_sig = substs.as_closure().sig();
 546                 let ret = fn_sig.output().skip_binder().to_string();
 547
 548                 let closure_decl_and_body_id =
 549                     local_visitor.found_closure.and_then(|closure| match &closure.kind {
 550                         ExprKind::Closure(_, decl, body_id, ..) => Some((decl, *body_id)),
 551                         _ => None,
 552                     });
 553
 554                 if let Some((decl, body_id)) = closure_decl_and_body_id {
 555                     closure_return_type_suggestion(
 556                         &mut err,
 557                         &decl.output,
 558                         self.tcx.hir().body(body_id),
 559                         &ret,
 560                     );
 561                     // We don't want to give the other suggestions when the problem is the
 562                     // closure return type.
 563                     err.span_label(
 564                         span,
 565                         arg_data.cannot_infer_msg(use_diag.filter(|d| d.applies_to(span))),
 566                     );
 567                     return err;
 568                 }
 569
 570                 // This shouldn't be reachable, but just in case we leave a reasonable fallback.
 571                 let args = closure_args(&fn_sig);
 572                 // This suggestion is incomplete, as the user will get further type inference
 573                 // errors due to the `_` placeholders and the introduction of `Box`, but it does
 574                 // nudge them in the right direction.
 575                 format!("a boxed closure type like `Box<dyn Fn({}) -> {}>`", args, ret)
 576             }
 577             Some(ty) if is_named_and_not_impl_trait(ty) && arg_data.name == "_" => {
 578                 let ty = ty_to_string(ty);
 579                 format!("the explicit type `{}`, with the type parameters specified", ty)
 580             }
 581             Some(ty) if is_named_and_not_impl_trait(ty) && ty.to_string() != arg_data.name => {
 582                 let ty = ty_to_string(ty);
 583                 format!(
 584                     "the explicit type `{}`, where the type parameter `{}` is specified",
 585                     ty, arg_data.name,
 586                 )
 587             }
 588             _ => "a type".to_string(),
 589         };
 590
 591         if let Some(e) = local_visitor.found_exact_method_call {
 592             if let ExprKind::MethodCall(segment, ..) = &e.kind {
 593                 // Suggest specifying type params or point out the return type of the call:
 594                 //
 595                 // error[E0282]: type annotations needed
 596                 //   --> $DIR/type-annotations-needed-expr.rs:2:39
 597                 //    |
 598                 // LL |     let _ = x.into_iter().sum() as f64;
 599                 //    |                           ^^^
 600                 //    |                           |
 601                 //    |                           cannot infer type for `S`
 602                 //    |                           help: consider specifying the type argument in
 603                 //    |                           the method call: `sum::<S>`
 604                 //    |
 605                 //    = note: type must be known at this point
 606                 //
 607                 // or
 608                 //
 609                 // error[E0282]: type annotations needed
 610                 //   --> $DIR/issue-65611.rs:59:20
 611                 //    |
 612                 // LL |     let x = buffer.last().unwrap().0.clone();
 613                 //    |             -------^^^^--
 614                 //    |             |      |
 615                 //    |             |      cannot infer type for `T`
 616                 //    |             this method call resolves to `std::option::Option<&T>`
 617                 //    |
 618                 //    = note: type must be known at this point
 619                 self.annotate_method_call(segment, e, &mut err);
 620             }
 621         } else if let Some(pattern) = local_visitor.found_arg_pattern {
 622             // We don't want to show the default label for closures.
 623             //
 624             // So, before clearing, the output would look something like this:
 625             // ```
 626             // let x = |_| {  };
 627             //          -  ^^^^ cannot infer type for `[_; 0]`
 628             //          |
 629             //          consider giving this closure parameter a type
 630             // ```
 631             //
 632             // After clearing, it looks something like this:
 633             // ```
 634             // let x = |_| {  };
 635             //          ^ consider giving this closure parameter the type `[_; 0]`
 636             //            with the type parameter `_` specified
 637             // ```
 638             err.span_label(
 639                 pattern.span,
 640                 format!("consider giving this closure parameter {}", suffix),
 641             );
 642         } else if let Some(pattern) = local_visitor.found_local_pattern {
 643             let msg = if let Some(simple_ident) = pattern.simple_ident() {
 644                 match pattern.span.desugaring_kind() {
 645                     None => format!("consider giving `{}` {}", simple_ident, suffix),
 646                     Some(DesugaringKind::ForLoop(_)) => {
 647                         "the element type for this iterator is not specified".to_string()
 648                     }
 649                     _ => format!("this needs {}", suffix),
 650                 }
 651             } else {
 652                 format!("consider giving this pattern {}", suffix)
 653             };
 654             err.span_label(pattern.span, msg);
 655         } else if let Some(e) = local_visitor.found_method_call {
 656             if let ExprKind::MethodCall(segment, _, exprs, _) = &e.kind {
 657                 // Suggest impl candidates:
 658                 //
 659                 // error[E0283]: type annotations needed
 660                 //   --> $DIR/E0283.rs:35:24
 661                 //    |
 662                 // LL |     let bar = foo_impl.into() * 1u32;
 663                 //    |               ---------^^^^--
 664                 //    |               |        |
 665                 //    |               |        cannot infer type for type parameter `T` declared on the trait `Into`
 666                 //    |               this method call resolves to `T`
 667                 //    |               help: specify type like: `<Impl as Into<u32>>::into(foo_impl)`
 668                 //    |
 669                 //    = note: cannot satisfy `Impl: Into<_>`
 670                 if !impl_candidates.is_empty() && e.span.contains(span) {
 671                     if let Some(expr) = exprs.first() {
 672                         if let ExprKind::Path(hir::QPath::Resolved(_, path)) = expr.kind {
 673                             if let [path_segment] = path.segments {
 674                                 let candidate_len = impl_candidates.len();
 675                                 let suggestions = impl_candidates.iter().map(|candidate| {
 676                                     format!(
 677                                         "{}::{}({})",
 678                                         candidate, segment.ident, path_segment.ident
 679                                     )
 680                                 });
 681                                 err.span_suggestions(
 682                                     e.span,
 683                                     &format!(
 684                                         "use the fully qualified path for the potential candidate{}",
 685                                         pluralize!(candidate_len),
 686                                     ),
 687                                     suggestions,
 688                                     Applicability::MaybeIncorrect,
 689                                 );
 690                             }
 691                         }
 692                     };
 693                 }
 694                 // Suggest specifying type params or point out the return type of the call:
 695                 //
 696                 // error[E0282]: type annotations needed
 697                 //   --> $DIR/type-annotations-needed-expr.rs:2:39
 698                 //    |
 699                 // LL |     let _ = x.into_iter().sum() as f64;
 700                 //    |                           ^^^
 701                 //    |                           |
 702                 //    |                           cannot infer type for `S`
 703                 //    |                           help: consider specifying the type argument in
 704                 //    |                           the method call: `sum::<S>`
 705                 //    |
 706                 //    = note: type must be known at this point
 707                 //
 708                 // or
 709                 //
 710                 // error[E0282]: type annotations needed
 711                 //   --> $DIR/issue-65611.rs:59:20
 712                 //    |
 713                 // LL |     let x = buffer.last().unwrap().0.clone();
 714                 //    |             -------^^^^--
 715                 //    |             |      |
 716                 //    |             |      cannot infer type for `T`
 717                 //    |             this method call resolves to `std::option::Option<&T>`
 718                 //    |
 719                 //    = note: type must be known at this point
 720                 self.annotate_method_call(segment, e, &mut err);
 721             }
 722         }
 723         // Instead of the following:
 724         // error[E0282]: type annotations needed
 725         //  --> file2.rs:3:15
 726         //   |
 727         // 3 |     let _ = x.sum() as f64;
 728         //   |             --^^^--------- cannot infer type for `S`
 729         //   |
 730         //   = note: type must be known at this point
 731         // We want:
 732         // error[E0282]: type annotations needed
 733         //  --> file2.rs:3:15
 734         //   |
 735         // 3 |     let _ = x.sum() as f64;
 736         //   |               ^^^ cannot infer type for `S`
 737         //   |
 738         //   = note: type must be known at this point
 739         let span = arg_data.span.unwrap_or(err_span);
 740
 741         // Avoid multiple labels pointing at `span`.
 742         if !err
 743             .span
 744             .span_labels()
 745             .iter()
 746             .any(|span_label| span_label.label.is_some() && span_label.span == span)
 747             && local_visitor.found_arg_pattern.is_none()
 748         {
 749             // FIXME(const_generics): we would like to handle const arguments
 750             // as part of the normal diagnostics flow below, but there appear to
 751             // be subtleties in doing so, so for now we special-case const args
 752             // here.
 753             if let (UnderspecifiedArgKind::Const { .. }, Some(parent_data)) =
 754                 (&arg_data.kind, &arg_data.parent)
 755             {
 756                 // (#83606): Do not emit a suggestion if the parent has an `impl Trait`
 757                 // as an argument otherwise it will cause the E0282 error.
 758                 if !self.tcx.generics_of(parent_data.def_id).has_impl_trait()
 759                     || self.tcx.features().explicit_generic_args_with_impl_trait
 760                 {
 761                     err.span_suggestion_verbose(
 762                         span,
 763                         "consider specifying the const argument",
 764                         format!("{}::<{}>", parent_data.name, arg_data.name),
 765                         Applicability::MaybeIncorrect,
 766                     );
 767                 }
 768             }
 769
 770             err.span_label(
 771                 span,
 772                 arg_data.cannot_infer_msg(use_diag.filter(|d| d.applies_to(span))),
 773             );
 774         }
 775
 776         err
 777     }
 778
 779     fn trait_def_from_hir_fn(&self, hir_id: hir::HirId) -> Option<DefId> {
 780         // The DefId will be the method's trait item ID unless this is an inherent impl
 781         if let Some((DefKind::AssocFn, def_id)) =
 782             self.in_progress_typeck_results?.borrow().type_dependent_def(hir_id)
 783         {
 784             return self
 785                 .tcx
 786                 .parent(def_id)
 787                 .filter(|&parent_def_id| self.tcx.is_trait(parent_def_id));
 788         }
 789
 790         None
 791     }
 792
 793     /// If the `FnSig` for the method call can be found and type arguments are identified as
 794     /// needed, suggest annotating the call, otherwise point out the resulting type of the call.
 795     fn annotate_method_call(
 796         &self,
 797         segment: &hir::PathSegment<'_>,
 798         e: &Expr<'_>,
 799         err: &mut DiagnosticBuilder<'_>,
 800     ) {
 801         if let (Some(typeck_results), None) = (self.in_progress_typeck_results, &segment.args) {
 802             let borrow = typeck_results.borrow();
 803             if let Some((DefKind::AssocFn, did)) = borrow.type_dependent_def(e.hir_id) {
 804                 let generics = self.tcx.generics_of(did);
 805                 if !generics.params.is_empty() && !generics.has_impl_trait() {
 806                     err.span_suggestion_verbose(
 807                         segment.ident.span.shrink_to_hi(),
 808                         &format!(
 809                             "consider specifying the type argument{} in the method call",
 810                             pluralize!(generics.params.len()),
 811                         ),
 812                         format!(
 813                             "::<{}>",
 814                             generics
 815                                 .params
 816                                 .iter()
 817                                 .map(|p| p.name.to_string())
 818                                 .collect::<Vec<String>>()
 819                                 .join(", ")
 820                         ),
 821                         Applicability::HasPlaceholders,
 822                     );
 823                 } else {
 824                     let sig = self.tcx.fn_sig(did);
 825                     let bound_output = sig.output();
 826                     let output = bound_output.skip_binder();
 827                     err.span_label(e.span, &format!("this method call resolves to `{}`", output));
 828                     let kind = output.kind();
 829                     if let ty::Projection(proj) = kind {
 830                         if let Some(span) = self.tcx.hir().span_if_local(proj.item_def_id) {
 831                             err.span_label(span, &format!("`{}` defined here", output));
 832                         }
 833                     }
 834                 }
 835             }
 836         }
 837     }
 838
 839     pub fn need_type_info_err_in_generator(
 840         &self,
 841         kind: hir::GeneratorKind,
 842         span: Span,
 843         ty: Ty<'tcx>,
 844     ) -> DiagnosticBuilder<'tcx> {
 845         let ty = self.resolve_vars_if_possible(ty);
 846         let data = self.extract_inference_diagnostics_data(ty.into(), None);
 847
 848         let mut err = struct_span_err!(
 849             self.tcx.sess,
 850             span,
 851             E0698,
 852             "type inside {} must be known in this context",
 853             kind,
 854         );
 855         err.span_label(span, data.cannot_infer_msg(None));
 856         err
 857     }
 858 }