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().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 }
 291
 292 pub enum UnderspecifiedArgKind {
 293     Type { prefix: Cow<'static, str> },
 294     Const { is_parameter: bool },
 295 }
 296
 297 impl InferenceDiagnosticsData {
 298     /// Generate a label for a generic argument which can't be inferred. When not
 299     /// much is known about the argument, `use_diag` may be used to describe the
 300     /// labeled value.
 301     fn cannot_infer_msg(&self, use_diag: Option<&UseDiagnostic<'_>>) -> String {
 302         if self.name == "_" && matches!(self.kind, UnderspecifiedArgKind::Type { .. }) {
 303             if let Some(use_diag) = use_diag {
 304                 return format!("cannot infer type of {}", use_diag.descr());
 305             }
 306
 307             return "cannot infer type".to_string();
 308         }
 309
 310         let suffix = match (&self.parent, use_diag) {
 311             (Some(parent), _) => format!(" declared on the {} `{}`", parent.prefix, parent.name),
 312             (None, Some(use_diag)) => format!(" in {}", use_diag.type_descr()),
 313             (None, None) => String::new(),
 314         };
 315
 316         // For example: "cannot infer type for type parameter `T`"
 317         format!("cannot infer {} `{}`{}", self.kind.prefix_string(), self.name, suffix)
 318     }
 319 }
 320
 321 impl InferenceDiagnosticsParentData {
 322     fn for_def_id(tcx: TyCtxt<'_>, def_id: DefId) -> Option<InferenceDiagnosticsParentData> {
 323         let parent_def_id = tcx.parent(def_id)?;
 324
 325         let parent_name =
 326             tcx.def_key(parent_def_id).disambiguated_data.data.get_opt_name()?.to_string();
 327
 328         Some(InferenceDiagnosticsParentData {
 329             prefix: tcx.def_kind(parent_def_id).descr(parent_def_id),
 330             name: parent_name,
 331         })
 332     }
 333 }
 334
 335 impl UnderspecifiedArgKind {
 336     fn prefix_string(&self) -> Cow<'static, str> {
 337         match self {
 338             Self::Type { prefix } => format!("type for {}", prefix).into(),
 339             Self::Const { is_parameter: true } => "the value of const parameter".into(),
 340             Self::Const { is_parameter: false } => "the value of the constant".into(),
 341         }
 342     }
 343 }
 344
 345 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 346     /// Extracts data used by diagnostic for either types or constants
 347     /// which were stuck during inference.
 348     pub fn extract_inference_diagnostics_data(
 349         &self,
 350         arg: GenericArg<'tcx>,
 351         highlight: Option<ty::print::RegionHighlightMode>,
 352     ) -> InferenceDiagnosticsData {
 353         match arg.unpack() {
 354             GenericArgKind::Type(ty) => {
 355                 if let ty::Infer(ty::TyVar(ty_vid)) = *ty.kind() {
 356                     let mut inner = self.inner.borrow_mut();
 357                     let ty_vars = &inner.type_variables();
 358                     let var_origin = ty_vars.var_origin(ty_vid);
 359                     if let TypeVariableOriginKind::TypeParameterDefinition(name, def_id) =
 360                         var_origin.kind
 361                     {
 362                         if name != kw::SelfUpper {
 363                             return InferenceDiagnosticsData {
 364                                 name: name.to_string(),
 365                                 span: Some(var_origin.span),
 366                                 kind: UnderspecifiedArgKind::Type {
 367                                     prefix: "type parameter".into(),
 368                                 },
 369                                 parent: def_id.and_then(|def_id| {
 370                                     InferenceDiagnosticsParentData::for_def_id(self.tcx, def_id)
 371                                 }),
 372                             };
 373                         }
 374                     }
 375                 }
 376
 377                 let mut s = String::new();
 378                 let mut printer = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS);
 379                 if let Some(highlight) = highlight {
 380                     printer.region_highlight_mode = highlight;
 381                 }
 382                 let _ = ty.print(printer);
 383                 InferenceDiagnosticsData {
 384                     name: s,
 385                     span: None,
 386                     kind: UnderspecifiedArgKind::Type { prefix: ty.prefix_string(self.tcx) },
 387                     parent: None,
 388                 }
 389             }
 390             GenericArgKind::Const(ct) => {
 391                 if let ty::ConstKind::Infer(InferConst::Var(vid)) = ct.val {
 392                     let origin =
 393                         self.inner.borrow_mut().const_unification_table().probe_value(vid).origin;
 394                     if let ConstVariableOriginKind::ConstParameterDefinition(name, def_id) =
 395                         origin.kind
 396                     {
 397                         return InferenceDiagnosticsData {
 398                             name: name.to_string(),
 399                             span: Some(origin.span),
 400                             kind: UnderspecifiedArgKind::Const { is_parameter: true },
 401                             parent: InferenceDiagnosticsParentData::for_def_id(self.tcx, def_id),
 402                         };
 403                     }
 404
 405                     debug_assert!(!origin.span.is_dummy());
 406                     let mut s = String::new();
 407                     let mut printer =
 408                         ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::ValueNS);
 409                     if let Some(highlight) = highlight {
 410                         printer.region_highlight_mode = highlight;
 411                     }
 412                     let _ = ct.print(printer);
 413                     InferenceDiagnosticsData {
 414                         name: s,
 415                         span: Some(origin.span),
 416                         kind: UnderspecifiedArgKind::Const { is_parameter: false },
 417                         parent: None,
 418                     }
 419                 } else {
 420                     bug!("unexpect const: {:?}", ct);
 421                 }
 422             }
 423             GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),
 424         }
 425     }
 426
 427     pub fn emit_inference_failure_err(
 428         &self,
 429         body_id: Option<hir::BodyId>,
 430         span: Span,
 431         arg: GenericArg<'tcx>,
 432         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 433         error_code: TypeAnnotationNeeded,
 434     ) -> DiagnosticBuilder<'tcx> {
 435         let arg = self.resolve_vars_if_possible(arg);
 436         let arg_data = self.extract_inference_diagnostics_data(arg, None);
 437
 438         let mut local_visitor = FindHirNodeVisitor::new(&self, arg, span);
 439         let ty_to_string = |ty: Ty<'tcx>| -> String {
 440             let mut s = String::new();
 441             let mut printer = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS);
 442             let mut inner = self.inner.borrow_mut();
 443             let ty_vars = inner.type_variables();
 444             let getter = move |ty_vid| {
 445                 let var_origin = ty_vars.var_origin(ty_vid);
 446                 if let TypeVariableOriginKind::TypeParameterDefinition(name, _) = var_origin.kind {
 447                     return Some(name.to_string());
 448                 }
 449                 None
 450             };
 451             printer.name_resolver = Some(Box::new(&getter));
 452             let _ = if let ty::FnDef(..) = ty.kind() {
 453                 // We don't want the regular output for `fn`s because it includes its path in
 454                 // invalid pseudo-syntax, we want the `fn`-pointer output instead.
 455                 ty.fn_sig(self.tcx).print(printer)
 456             } else {
 457                 ty.print(printer)
 458             };
 459             s
 460         };
 461
 462         if let Some(body_id) = body_id {
 463             let expr = self.tcx.hir().expect_expr(body_id.hir_id);
 464             local_visitor.visit_expr(expr);
 465         }
 466         let err_span = if let Some(pattern) = local_visitor.found_arg_pattern {
 467             pattern.span
 468         } else if let Some(span) = arg_data.span {
 469             // `span` here lets us point at `sum` instead of the entire right hand side expr:
 470             // error[E0282]: type annotations needed
 471             //  --> file2.rs:3:15
 472             //   |
 473             // 3 |     let _ = x.sum() as f64;
 474             //   |               ^^^ cannot infer type for `S`
 475             span
 476         } else if let Some(ExprKind::MethodCall(_, call_span, _, _)) =
 477             local_visitor.found_method_call.map(|e| &e.kind)
 478         {
 479             // Point at the call instead of the whole expression:
 480             // error[E0284]: type annotations needed
 481             //  --> file.rs:2:5
 482             //   |
 483             // 2 |     vec![Ok(2)].into_iter().collect()?;
 484             //   |                             ^^^^^^^ cannot infer type
 485             //   |
 486             //   = note: cannot resolve `<_ as std::ops::Try>::Ok == _`
 487             if span.contains(*call_span) { *call_span } else { span }
 488         } else {
 489             span
 490         };
 491
 492         let is_named_and_not_impl_trait = |ty: Ty<'_>| {
 493             &ty.to_string() != "_" &&
 494                 // FIXME: Remove this check after `impl_trait_in_bindings` is stabilized. #63527
 495                 (!ty.is_impl_trait() || self.tcx.features().impl_trait_in_bindings)
 496         };
 497
 498         let ty_msg = match (local_visitor.found_node_ty, local_visitor.found_exact_method_call) {
 499             (_, Some(_)) => String::new(),
 500             (Some(ty), _) if ty.is_closure() => {
 501                 let substs =
 502                     if let ty::Closure(_, substs) = *ty.kind() { substs } else { unreachable!() };
 503                 let fn_sig = substs.as_closure().sig();
 504                 let args = closure_args(&fn_sig);
 505                 let ret = fn_sig.output().skip_binder().to_string();
 506                 format!(" for the closure `fn({}) -> {}`", args, ret)
 507             }
 508             (Some(ty), _) if is_named_and_not_impl_trait(ty) => {
 509                 let ty = ty_to_string(ty);
 510                 format!(" for `{}`", ty)
 511             }
 512             _ => String::new(),
 513         };
 514
 515         // When `arg_data.name` corresponds to a type argument, show the path of the full type we're
 516         // trying to infer. In the following example, `ty_msg` contains
 517         // " for `std::result::Result<i32, E>`":
 518         // ```
 519         // error[E0282]: type annotations needed for `std::result::Result<i32, E>`
 520         //  --> file.rs:L:CC
 521         //   |
 522         // L |     let b = Ok(4);
 523         //   |         -   ^^ cannot infer type for `E` in `std::result::Result<i32, E>`
 524         //   |         |
 525         //   |         consider giving `b` the explicit type `std::result::Result<i32, E>`, where
 526         //   |         the type parameter `E` is specified
 527         // ```
 528         let error_code = error_code.into();
 529         let mut err = self.tcx.sess.struct_span_err_with_code(
 530             err_span,
 531             &format!("type annotations needed{}", ty_msg),
 532             error_code,
 533         );
 534
 535         let use_diag = local_visitor.found_use_diagnostic.as_ref();
 536         if let Some(use_diag) = use_diag {
 537             if use_diag.applies_to(err_span) {
 538                 use_diag.attach_note(&mut err);
 539             }
 540         }
 541
 542         let suffix = match local_visitor.found_node_ty {
 543             Some(ty) if ty.is_closure() => {
 544                 let substs =
 545                     if let ty::Closure(_, substs) = *ty.kind() { substs } else { unreachable!() };
 546                 let fn_sig = substs.as_closure().sig();
 547                 let ret = fn_sig.output().skip_binder().to_string();
 548
 549                 let closure_decl_and_body_id =
 550                     local_visitor.found_closure.and_then(|closure| match &closure.kind {
 551                         ExprKind::Closure(_, decl, body_id, ..) => Some((decl, *body_id)),
 552                         _ => None,
 553                     });
 554
 555                 if let Some((decl, body_id)) = closure_decl_and_body_id {
 556                     closure_return_type_suggestion(
 557                         &mut err,
 558                         &decl.output,
 559                         self.tcx.hir().body(body_id),
 560                         &ret,
 561                     );
 562                     // We don't want to give the other suggestions when the problem is the
 563                     // closure return type.
 564                     err.span_label(
 565                         span,
 566                         arg_data.cannot_infer_msg(use_diag.filter(|d| d.applies_to(span))),
 567                     );
 568                     return err;
 569                 }
 570
 571                 // This shouldn't be reachable, but just in case we leave a reasonable fallback.
 572                 let args = closure_args(&fn_sig);
 573                 // This suggestion is incomplete, as the user will get further type inference
 574                 // errors due to the `_` placeholders and the introduction of `Box`, but it does
 575                 // nudge them in the right direction.
 576                 format!("a boxed closure type like `Box<dyn Fn({}) -> {}>`", args, ret)
 577             }
 578             Some(ty) if is_named_and_not_impl_trait(ty) && arg_data.name == "_" => {
 579                 let ty = ty_to_string(ty);
 580                 format!("the explicit type `{}`, with the type parameters specified", ty)
 581             }
 582             Some(ty) if is_named_and_not_impl_trait(ty) && ty.to_string() != arg_data.name => {
 583                 let ty = ty_to_string(ty);
 584                 format!(
 585                     "the explicit type `{}`, where the type parameter `{}` is specified",
 586                     ty, arg_data.name,
 587                 )
 588             }
 589             _ => "a type".to_string(),
 590         };
 591
 592         if let Some(e) = local_visitor.found_exact_method_call {
 593             if let ExprKind::MethodCall(segment, ..) = &e.kind {
 594                 // Suggest specifying type params or point out the return type of the call:
 595                 //
 596                 // error[E0282]: type annotations needed
 597                 //   --> $DIR/type-annotations-needed-expr.rs:2:39
 598                 //    |
 599                 // LL |     let _ = x.into_iter().sum() as f64;
 600                 //    |                           ^^^
 601                 //    |                           |
 602                 //    |                           cannot infer type for `S`
 603                 //    |                           help: consider specifying the type argument in
 604                 //    |                           the method call: `sum::<S>`
 605                 //    |
 606                 //    = note: type must be known at this point
 607                 //
 608                 // or
 609                 //
 610                 // error[E0282]: type annotations needed
 611                 //   --> $DIR/issue-65611.rs:59:20
 612                 //    |
 613                 // LL |     let x = buffer.last().unwrap().0.clone();
 614                 //    |             -------^^^^--
 615                 //    |             |      |
 616                 //    |             |      cannot infer type for `T`
 617                 //    |             this method call resolves to `std::option::Option<&T>`
 618                 //    |
 619                 //    = note: type must be known at this point
 620                 self.annotate_method_call(segment, e, &mut err);
 621             }
 622         } else if let Some(pattern) = local_visitor.found_arg_pattern {
 623             // We don't want to show the default label for closures.
 624             //
 625             // So, before clearing, the output would look something like this:
 626             // ```
 627             // let x = |_| {  };
 628             //          -  ^^^^ cannot infer type for `[_; 0]`
 629             //          |
 630             //          consider giving this closure parameter a type
 631             // ```
 632             //
 633             // After clearing, it looks something like this:
 634             // ```
 635             // let x = |_| {  };
 636             //          ^ consider giving this closure parameter the type `[_; 0]`
 637             //            with the type parameter `_` specified
 638             // ```
 639             err.span_label(
 640                 pattern.span,
 641                 format!("consider giving this closure parameter {}", suffix),
 642             );
 643         } else if let Some(pattern) = local_visitor.found_local_pattern {
 644             let msg = if let Some(simple_ident) = pattern.simple_ident() {
 645                 match pattern.span.desugaring_kind() {
 646                     None => format!("consider giving `{}` {}", simple_ident, suffix),
 647                     Some(DesugaringKind::ForLoop(_)) => {
 648                         "the element type for this iterator is not specified".to_string()
 649                     }
 650                     _ => format!("this needs {}", suffix),
 651                 }
 652             } else {
 653                 format!("consider giving this pattern {}", suffix)
 654             };
 655             err.span_label(pattern.span, msg);
 656         } else if let Some(e) = local_visitor.found_method_call {
 657             if let ExprKind::MethodCall(segment, _, exprs, _) = &e.kind {
 658                 // Suggest impl candidates:
 659                 //
 660                 // error[E0283]: type annotations needed
 661                 //   --> $DIR/E0283.rs:35:24
 662                 //    |
 663                 // LL |     let bar = foo_impl.into() * 1u32;
 664                 //    |               ---------^^^^--
 665                 //    |               |        |
 666                 //    |               |        cannot infer type for type parameter `T` declared on the trait `Into`
 667                 //    |               this method call resolves to `T`
 668                 //    |               help: specify type like: `<Impl as Into<u32>>::into(foo_impl)`
 669                 //    |
 670                 //    = note: cannot satisfy `Impl: Into<_>`
 671                 if !impl_candidates.is_empty() && e.span.contains(span) {
 672                     if let Some(expr) = exprs.first() {
 673                         if let ExprKind::Path(hir::QPath::Resolved(_, path)) = expr.kind {
 674                             if let [path_segment] = path.segments {
 675                                 let candidate_len = impl_candidates.len();
 676                                 let suggestions = impl_candidates.iter().map(|candidate| {
 677                                     format!(
 678                                         "{}::{}({})",
 679                                         candidate, segment.ident, path_segment.ident
 680                                     )
 681                                 });
 682                                 err.span_suggestions(
 683                                     e.span,
 684                                     &format!(
 685                                         "use the fully qualified path for the potential candidate{}",
 686                                         pluralize!(candidate_len),
 687                                     ),
 688                                     suggestions,
 689                                     Applicability::MaybeIncorrect,
 690                                 );
 691                             }
 692                         }
 693                     };
 694                 }
 695                 // Suggest specifying type params or point out the return type of the call:
 696                 //
 697                 // error[E0282]: type annotations needed
 698                 //   --> $DIR/type-annotations-needed-expr.rs:2:39
 699                 //    |
 700                 // LL |     let _ = x.into_iter().sum() as f64;
 701                 //    |                           ^^^
 702                 //    |                           |
 703                 //    |                           cannot infer type for `S`
 704                 //    |                           help: consider specifying the type argument in
 705                 //    |                           the method call: `sum::<S>`
 706                 //    |
 707                 //    = note: type must be known at this point
 708                 //
 709                 // or
 710                 //
 711                 // error[E0282]: type annotations needed
 712                 //   --> $DIR/issue-65611.rs:59:20
 713                 //    |
 714                 // LL |     let x = buffer.last().unwrap().0.clone();
 715                 //    |             -------^^^^--
 716                 //    |             |      |
 717                 //    |             |      cannot infer type for `T`
 718                 //    |             this method call resolves to `std::option::Option<&T>`
 719                 //    |
 720                 //    = note: type must be known at this point
 721                 self.annotate_method_call(segment, e, &mut err);
 722             }
 723         }
 724         // Instead of the following:
 725         // error[E0282]: type annotations needed
 726         //  --> file2.rs:3:15
 727         //   |
 728         // 3 |     let _ = x.sum() as f64;
 729         //   |             --^^^--------- cannot infer type for `S`
 730         //   |
 731         //   = note: type must be known at this point
 732         // We want:
 733         // error[E0282]: type annotations needed
 734         //  --> file2.rs:3:15
 735         //   |
 736         // 3 |     let _ = x.sum() as f64;
 737         //   |               ^^^ cannot infer type for `S`
 738         //   |
 739         //   = note: type must be known at this point
 740         let span = arg_data.span.unwrap_or(err_span);
 741
 742         // Avoid multiple labels pointing at `span`.
 743         if !err
 744             .span
 745             .span_labels()
 746             .iter()
 747             .any(|span_label| span_label.label.is_some() && span_label.span == span)
 748             && local_visitor.found_arg_pattern.is_none()
 749         {
 750             // FIXME(const_generics): we would like to handle const arguments
 751             // as part of the normal diagnostics flow below, but there appear to
 752             // be subtleties in doing so, so for now we special-case const args
 753             // here.
 754             if let (UnderspecifiedArgKind::Const { .. }, Some(parent_data)) =
 755                 (&arg_data.kind, &arg_data.parent)
 756             {
 757                 err.span_suggestion_verbose(
 758                     span,
 759                     "consider specifying the const argument",
 760                     format!("{}::<{}>", parent_data.name, arg_data.name),
 761                     Applicability::MaybeIncorrect,
 762                 );
 763             }
 764
 765             err.span_label(
 766                 span,
 767                 arg_data.cannot_infer_msg(use_diag.filter(|d| d.applies_to(span))),
 768             );
 769         }
 770
 771         err
 772     }
 773
 774     fn trait_def_from_hir_fn(&self, hir_id: hir::HirId) -> Option<DefId> {
 775         // The DefId will be the method's trait item ID unless this is an inherent impl
 776         if let Some((DefKind::AssocFn, def_id)) =
 777             self.in_progress_typeck_results?.borrow().type_dependent_def(hir_id)
 778         {
 779             return self
 780                 .tcx
 781                 .parent(def_id)
 782                 .filter(|&parent_def_id| self.tcx.is_trait(parent_def_id));
 783         }
 784
 785         None
 786     }
 787
 788     /// If the `FnSig` for the method call can be found and type arguments are identified as
 789     /// needed, suggest annotating the call, otherwise point out the resulting type of the call.
 790     fn annotate_method_call(
 791         &self,
 792         segment: &hir::PathSegment<'_>,
 793         e: &Expr<'_>,
 794         err: &mut DiagnosticBuilder<'_>,
 795     ) {
 796         if let (Some(typeck_results), None) = (self.in_progress_typeck_results, &segment.args) {
 797             let borrow = typeck_results.borrow();
 798             if let Some((DefKind::AssocFn, did)) = borrow.type_dependent_def(e.hir_id) {
 799                 let generics = self.tcx.generics_of(did);
 800                 if !generics.params.is_empty() {
 801                     err.span_suggestion_verbose(
 802                         segment.ident.span.shrink_to_hi(),
 803                         &format!(
 804                             "consider specifying the type argument{} in the method call",
 805                             pluralize!(generics.params.len()),
 806                         ),
 807                         format!(
 808                             "::<{}>",
 809                             generics
 810                                 .params
 811                                 .iter()
 812                                 .map(|p| p.name.to_string())
 813                                 .collect::<Vec<String>>()
 814                                 .join(", ")
 815                         ),
 816                         Applicability::HasPlaceholders,
 817                     );
 818                 } else {
 819                     let sig = self.tcx.fn_sig(did);
 820                     let bound_output = sig.output();
 821                     let output = bound_output.skip_binder();
 822                     err.span_label(e.span, &format!("this method call resolves to `{}`", output));
 823                     let kind = output.kind();
 824                     if let ty::Projection(proj) = kind {
 825                         if let Some(span) = self.tcx.hir().span_if_local(proj.item_def_id) {
 826                             err.span_label(span, &format!("`{}` defined here", output));
 827                         }
 828                     }
 829                 }
 830             }
 831         }
 832     }
 833
 834     pub fn need_type_info_err_in_generator(
 835         &self,
 836         kind: hir::GeneratorKind,
 837         span: Span,
 838         ty: Ty<'tcx>,
 839     ) -> DiagnosticBuilder<'tcx> {
 840         let ty = self.resolve_vars_if_possible(ty);
 841         let data = self.extract_inference_diagnostics_data(ty.into(), None);
 842
 843         let mut err = struct_span_err!(
 844             self.tcx.sess,
 845             span,
 846             E0698,
 847             "type inside {} must be known in this context",
 848             kind,
 849         );
 850         err.span_label(span, data.cannot_infer_msg(None));
 851         err
 852     }
 853 }