src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs

   1 use std::fmt::{self, Display};
   2 use crate::borrow_check::nll::region_infer::RegionInferenceContext;
   3 use crate::borrow_check::nll::universal_regions::DefiningTy;
   4 use crate::borrow_check::nll::ToRegionVid;
   5 use crate::borrow_check::Upvar;
   6 use rustc::hir;
   7 use rustc::hir::def::{Res, DefKind};
   8 use rustc::hir::def_id::DefId;
   9 use rustc::infer::InferCtxt;
  10 use rustc::mir::Body;
  11 use rustc::ty::subst::{SubstsRef, UnpackedKind};
  12 use rustc::ty::{self, RegionKind, RegionVid, Ty, TyCtxt};
  13 use rustc::ty::print::RegionHighlightMode;
  14 use rustc_errors::DiagnosticBuilder;
  15 use syntax::symbol::kw;
  16 use syntax_pos::Span;
  17 use syntax_pos::symbol::InternedString;
  18
  19 #[derive(Debug)]
  20 crate struct RegionName {
  21     crate name: InternedString,
  22     crate source: RegionNameSource,
  23 }
  24
  25 #[derive(Debug)]
  26 crate enum RegionNameSource {
  27     NamedEarlyBoundRegion(Span),
  28     NamedFreeRegion(Span),
  29     Static,
  30     SynthesizedFreeEnvRegion(Span, String),
  31     CannotMatchHirTy(Span, String),
  32     MatchedHirTy(Span),
  33     MatchedAdtAndSegment(Span),
  34     AnonRegionFromUpvar(Span, String),
  35     AnonRegionFromOutput(Span, String, String),
  36     AnonRegionFromYieldTy(Span, String),
  37 }
  38
  39 impl RegionName {
  40     #[allow(dead_code)]
  41     crate fn was_named(&self) -> bool {
  42         match self.source {
  43             RegionNameSource::NamedEarlyBoundRegion(..) |
  44             RegionNameSource::NamedFreeRegion(..) |
  45             RegionNameSource::Static => true,
  46             RegionNameSource::SynthesizedFreeEnvRegion(..) |
  47             RegionNameSource::CannotMatchHirTy(..) |
  48             RegionNameSource::MatchedHirTy(..) |
  49             RegionNameSource::MatchedAdtAndSegment(..) |
  50             RegionNameSource::AnonRegionFromUpvar(..) |
  51             RegionNameSource::AnonRegionFromOutput(..) |
  52             RegionNameSource::AnonRegionFromYieldTy(..) => false,
  53         }
  54     }
  55
  56     #[allow(dead_code)]
  57     crate fn was_synthesized(&self) -> bool {
  58         !self.was_named()
  59     }
  60
  61     #[allow(dead_code)]
  62     crate fn name(&self) -> InternedString {
  63         self.name
  64     }
  65
  66     crate fn highlight_region_name(
  67         &self,
  68         diag: &mut DiagnosticBuilder<'_>
  69     ) {
  70         match &self.source {
  71             RegionNameSource::NamedFreeRegion(span) |
  72             RegionNameSource::NamedEarlyBoundRegion(span) => {
  73                 diag.span_label(
  74                     *span,
  75                     format!("lifetime `{}` defined here", self),
  76                 );
  77             },
  78             RegionNameSource::SynthesizedFreeEnvRegion(span, note) => {
  79                 diag.span_label(
  80                     *span,
  81                     format!("lifetime `{}` represents this closure's body", self),
  82                 );
  83                 diag.note(&note);
  84             },
  85             RegionNameSource::CannotMatchHirTy(span, type_name) => {
  86                 diag.span_label(*span, format!("has type `{}`", type_name));
  87             },
  88             RegionNameSource::MatchedHirTy(span) => {
  89                 diag.span_label(
  90                     *span,
  91                     format!("let's call the lifetime of this reference `{}`", self),
  92                 );
  93             },
  94             RegionNameSource::MatchedAdtAndSegment(span) => {
  95                 diag.span_label(*span, format!("let's call this `{}`", self));
  96             },
  97             RegionNameSource::AnonRegionFromUpvar(span, upvar_name) => {
  98                 diag.span_label(
  99                     *span,
 100                     format!("lifetime `{}` appears in the type of `{}`", self, upvar_name),
 101                 );
 102             },
 103             RegionNameSource::AnonRegionFromOutput(span, mir_description, type_name) => {
 104                 diag.span_label(
 105                     *span,
 106                     format!("return type{} is {}", mir_description, type_name),
 107                 );
 108             },
 109             RegionNameSource::AnonRegionFromYieldTy(span, type_name) => {
 110                 diag.span_label(
 111                     *span,
 112                     format!("yield type is {}", type_name),
 113                 );
 114             }
 115             RegionNameSource::Static => {},
 116         }
 117     }
 118 }
 119
 120 impl Display for RegionName {
 121     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 122         write!(f, "{}", self.name)
 123     }
 124 }
 125
 126 impl<'tcx> RegionInferenceContext<'tcx> {
 127     /// Maps from an internal MIR region vid to something that we can
 128     /// report to the user. In some cases, the region vids will map
 129     /// directly to lifetimes that the user has a name for (e.g.,
 130     /// `'static`). But frequently they will not, in which case we
 131     /// have to find some way to identify the lifetime to the user. To
 132     /// that end, this function takes a "diagnostic" so that it can
 133     /// create auxiliary notes as needed.
 134     ///
 135     /// Example (function arguments):
 136     ///
 137     /// Suppose we are trying to give a name to the lifetime of the
 138     /// reference `x`:
 139     ///
 140     /// ```
 141     /// fn foo(x: &u32) { .. }
 142     /// ```
 143     ///
 144     /// This function would create a label like this:
 145     ///
 146     /// ```
 147     ///  | fn foo(x: &u32) { .. }
 148     ///           ------- fully elaborated type of `x` is `&'1 u32`
 149     /// ```
 150     ///
 151     /// and then return the name `'1` for us to use.
 152     crate fn give_region_a_name(
 153         &self,
 154         infcx: &InferCtxt<'_, 'tcx>,
 155         body: &Body<'tcx>,
 156         upvars: &[Upvar],
 157         mir_def_id: DefId,
 158         fr: RegionVid,
 159         counter: &mut usize,
 160     ) -> Option<RegionName> {
 161         debug!("give_region_a_name(fr={:?}, counter={})", fr, counter);
 162
 163         assert!(self.universal_regions.is_universal_region(fr));
 164
 165         let value = self.give_name_from_error_region(infcx.tcx, mir_def_id, fr, counter)
 166             .or_else(|| {
 167                 self.give_name_if_anonymous_region_appears_in_arguments(
 168                     infcx, body, mir_def_id, fr, counter,
 169                 )
 170             })
 171             .or_else(|| {
 172                 self.give_name_if_anonymous_region_appears_in_upvars(
 173                     infcx.tcx, upvars, fr, counter,
 174                 )
 175             })
 176             .or_else(|| {
 177                 self.give_name_if_anonymous_region_appears_in_output(
 178                     infcx, body, mir_def_id, fr, counter,
 179                 )
 180             })
 181             .or_else(|| {
 182                 self.give_name_if_anonymous_region_appears_in_yield_ty(
 183                     infcx, body, mir_def_id, fr, counter,
 184                 )
 185             });
 186
 187         debug!("give_region_a_name: gave name {:?}", value);
 188         value
 189     }
 190
 191     /// Checks for the case where `fr` maps to something that the
 192     /// *user* has a name for. In that case, we'll be able to map
 193     /// `fr` to a `Region<'tcx>`, and that region will be one of
 194     /// named variants.
 195     fn give_name_from_error_region(
 196         &self,
 197         tcx: TyCtxt<'tcx>,
 198         mir_def_id: DefId,
 199         fr: RegionVid,
 200         counter: &mut usize,
 201     ) -> Option<RegionName> {
 202         let error_region = self.to_error_region(fr)?;
 203
 204         debug!("give_region_a_name: error_region = {:?}", error_region);
 205         match error_region {
 206             ty::ReEarlyBound(ebr) => {
 207                 if ebr.has_name() {
 208                     let span = self.get_named_span(tcx, error_region, ebr.name);
 209                     Some(RegionName {
 210                         name: ebr.name,
 211                         source: RegionNameSource::NamedEarlyBoundRegion(span)
 212                     })
 213                 } else {
 214                     None
 215                 }
 216             }
 217
 218             ty::ReStatic => Some(RegionName {
 219                 name: kw::StaticLifetime.as_interned_str(),
 220                 source: RegionNameSource::Static
 221             }),
 222
 223             ty::ReFree(free_region) => match free_region.bound_region {
 224                 ty::BoundRegion::BrNamed(_, name) => {
 225                     let span = self.get_named_span(tcx, error_region, name);
 226                     Some(RegionName {
 227                         name,
 228                         source: RegionNameSource::NamedFreeRegion(span),
 229                     })
 230                 },
 231
 232                 ty::BoundRegion::BrEnv => {
 233                     let mir_hir_id = tcx.hir()
 234                                         .as_local_hir_id(mir_def_id)
 235                                         .expect("non-local mir");
 236                     let def_ty = self.universal_regions.defining_ty;
 237
 238                     if let DefiningTy::Closure(def_id, substs) = def_ty {
 239                         let args_span = if let hir::ExprKind::Closure(_, _, _, span, _) =
 240                             tcx.hir().expect_expr(mir_hir_id).node
 241                         {
 242                             span
 243                         } else {
 244                             bug!("Closure is not defined by a closure expr");
 245                         };
 246                         let region_name = self.synthesize_region_name(counter);
 247
 248                         let closure_kind_ty = substs.closure_kind_ty(def_id, tcx);
 249                         let note = match closure_kind_ty.to_opt_closure_kind() {
 250                             Some(ty::ClosureKind::Fn) => {
 251                                 "closure implements `Fn`, so references to captured variables \
 252                                  can't escape the closure"
 253                             }
 254                             Some(ty::ClosureKind::FnMut) => {
 255                                 "closure implements `FnMut`, so references to captured variables \
 256                                  can't escape the closure"
 257                             }
 258                             Some(ty::ClosureKind::FnOnce) => {
 259                                 bug!("BrEnv in a `FnOnce` closure");
 260                             }
 261                             None => bug!("Closure kind not inferred in borrow check"),
 262                         };
 263
 264                         Some(RegionName {
 265                             name: region_name,
 266                             source: RegionNameSource::SynthesizedFreeEnvRegion(
 267                                 args_span,
 268                                 note.to_string()
 269                             ),
 270                         })
 271                     } else {
 272                         // Can't have BrEnv in functions, constants or generators.
 273                         bug!("BrEnv outside of closure.");
 274                     }
 275                 }
 276
 277                 ty::BoundRegion::BrAnon(_) => None,
 278             },
 279
 280             ty::ReLateBound(..)
 281             | ty::ReScope(..)
 282             | ty::ReVar(..)
 283             | ty::RePlaceholder(..)
 284             | ty::ReEmpty
 285             | ty::ReErased
 286             | ty::ReClosureBound(..) => None,
 287         }
 288     }
 289
 290     /// Gets a span of a named region to provide context for error messages that
 291     /// mention that span, for example:
 292     ///
 293     /// ```
 294     ///  |
 295     ///  | fn two_regions<'a, 'b, T>(cell: Cell<&'a ()>, t: T)
 296     ///  |                --  -- lifetime `'b` defined here
 297     ///  |                |
 298     ///  |                lifetime `'a` defined here
 299     ///  |
 300     ///  |     with_signature(cell, t, |cell, t| require(cell, t));
 301     ///  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'b` must
 302     ///  |                                                         outlive `'a`
 303     /// ```
 304     fn get_named_span(
 305         &self,
 306         tcx: TyCtxt<'tcx>,
 307         error_region: &RegionKind,
 308         name: InternedString,
 309     ) -> Span {
 310         let scope = error_region.free_region_binding_scope(tcx);
 311         let node = tcx.hir().as_local_hir_id(scope).unwrap_or(hir::DUMMY_HIR_ID);
 312
 313         let span = tcx.sess.source_map().def_span(tcx.hir().span(node));
 314         if let Some(param) = tcx.hir()
 315             .get_generics(scope)
 316             .and_then(|generics| generics.get_named(name))
 317         {
 318             param.span
 319         } else {
 320             span
 321         }
 322     }
 323
 324     /// Finds an argument that contains `fr` and label it with a fully
 325     /// elaborated type, returning something like `'1`. Result looks
 326     /// like:
 327     ///
 328     /// ```
 329     ///  | fn foo(x: &u32) { .. }
 330     ///           ------- fully elaborated type of `x` is `&'1 u32`
 331     /// ```
 332     fn give_name_if_anonymous_region_appears_in_arguments(
 333         &self,
 334         infcx: &InferCtxt<'_, 'tcx>,
 335         body: &Body<'tcx>,
 336         mir_def_id: DefId,
 337         fr: RegionVid,
 338         counter: &mut usize,
 339     ) -> Option<RegionName> {
 340         let implicit_inputs = self.universal_regions.defining_ty.implicit_inputs();
 341         let argument_index = self.get_argument_index_for_region(infcx.tcx, fr)?;
 342
 343         let arg_ty =
 344             self.universal_regions.unnormalized_input_tys[implicit_inputs + argument_index];
 345         if let Some(region_name) = self.give_name_if_we_can_match_hir_ty_from_argument(
 346             infcx,
 347             body,
 348             mir_def_id,
 349             fr,
 350             arg_ty,
 351             argument_index,
 352             counter,
 353         ) {
 354             return Some(region_name);
 355         }
 356
 357         self.give_name_if_we_cannot_match_hir_ty(infcx, body, fr, arg_ty, counter)
 358     }
 359
 360     fn give_name_if_we_can_match_hir_ty_from_argument(
 361         &self,
 362         infcx: &InferCtxt<'_, 'tcx>,
 363         body: &Body<'tcx>,
 364         mir_def_id: DefId,
 365         needle_fr: RegionVid,
 366         argument_ty: Ty<'tcx>,
 367         argument_index: usize,
 368         counter: &mut usize,
 369     ) -> Option<RegionName> {
 370         let mir_hir_id = infcx.tcx.hir().as_local_hir_id(mir_def_id)?;
 371         let fn_decl = infcx.tcx.hir().fn_decl_by_hir_id(mir_hir_id)?;
 372         let argument_hir_ty: &hir::Ty = &fn_decl.inputs[argument_index];
 373         match argument_hir_ty.node {
 374             // This indicates a variable with no type annotation, like
 375             // `|x|`... in that case, we can't highlight the type but
 376             // must highlight the variable.
 377             hir::TyKind::Infer => self.give_name_if_we_cannot_match_hir_ty(
 378                 infcx,
 379                 body,
 380                 needle_fr,
 381                 argument_ty,
 382                 counter,
 383             ),
 384
 385             _ => self.give_name_if_we_can_match_hir_ty(
 386                 infcx.tcx,
 387                 needle_fr,
 388                 argument_ty,
 389                 argument_hir_ty,
 390                 counter,
 391             ),
 392         }
 393     }
 394
 395     /// Attempts to highlight the specific part of a type in an argument
 396     /// that has no type annotation.
 397     /// For example, we might produce an annotation like this:
 398     ///
 399     /// ```
 400     ///  |     foo(|a, b| b)
 401     ///  |          -  -
 402     ///  |          |  |
 403     ///  |          |  has type `&'1 u32`
 404     ///  |          has type `&'2 u32`
 405     /// ```
 406     fn give_name_if_we_cannot_match_hir_ty(
 407         &self,
 408         infcx: &InferCtxt<'_, 'tcx>,
 409         body: &Body<'tcx>,
 410         needle_fr: RegionVid,
 411         argument_ty: Ty<'tcx>,
 412         counter: &mut usize,
 413     ) -> Option<RegionName> {
 414         let mut highlight = RegionHighlightMode::default();
 415         highlight.highlighting_region_vid(needle_fr, *counter);
 416         let type_name = infcx.extract_type_name(&argument_ty, Some(highlight)).0;
 417
 418         debug!(
 419             "give_name_if_we_cannot_match_hir_ty: type_name={:?} needle_fr={:?}",
 420             type_name, needle_fr
 421         );
 422         let assigned_region_name = if type_name.find(&format!("'{}", counter)).is_some() {
 423             // Only add a label if we can confirm that a region was labelled.
 424             let argument_index = self.get_argument_index_for_region(infcx.tcx, needle_fr)?;
 425             let (_, span) = self.get_argument_name_and_span_for_region(body, argument_index);
 426
 427             Some(RegionName {
 428                 // This counter value will already have been used, so this function will increment
 429                 // it so the next value will be used next and return the region name that would
 430                 // have been used.
 431                 name: self.synthesize_region_name(counter),
 432                 source: RegionNameSource::CannotMatchHirTy(span, type_name),
 433             })
 434         } else {
 435             None
 436         };
 437
 438         assigned_region_name
 439     }
 440
 441     /// Attempts to highlight the specific part of a type annotation
 442     /// that contains the anonymous reference we want to give a name
 443     /// to. For example, we might produce an annotation like this:
 444     ///
 445     /// ```
 446     ///  | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item = &T>> {
 447     ///  |                - let's call the lifetime of this reference `'1`
 448     /// ```
 449     ///
 450     /// the way this works is that we match up `argument_ty`, which is
 451     /// a `Ty<'tcx>` (the internal form of the type) with
 452     /// `argument_hir_ty`, a `hir::Ty` (the syntax of the type
 453     /// annotation). We are descending through the types stepwise,
 454     /// looking in to find the region `needle_fr` in the internal
 455     /// type. Once we find that, we can use the span of the `hir::Ty`
 456     /// to add the highlight.
 457     ///
 458     /// This is a somewhat imperfect process, so long the way we also
 459     /// keep track of the **closest** type we've found. If we fail to
 460     /// find the exact `&` or `'_` to highlight, then we may fall back
 461     /// to highlighting that closest type instead.
 462     fn give_name_if_we_can_match_hir_ty(
 463         &self,
 464         tcx: TyCtxt<'tcx>,
 465         needle_fr: RegionVid,
 466         argument_ty: Ty<'tcx>,
 467         argument_hir_ty: &hir::Ty,
 468         counter: &mut usize,
 469     ) -> Option<RegionName> {
 470         let search_stack: &mut Vec<(Ty<'tcx>, &hir::Ty)> =
 471             &mut vec![(argument_ty, argument_hir_ty)];
 472
 473         while let Some((ty, hir_ty)) = search_stack.pop() {
 474             match (&ty.sty, &hir_ty.node) {
 475                 // Check if the `argument_ty` is `&'X ..` where `'X`
 476                 // is the region we are looking for -- if so, and we have a `&T`
 477                 // on the RHS, then we want to highlight the `&` like so:
 478                 //
 479                 //     &
 480                 //     - let's call the lifetime of this reference `'1`
 481                 (
 482                     ty::Ref(region, referent_ty, _),
 483                     hir::TyKind::Rptr(_lifetime, referent_hir_ty),
 484                 ) => {
 485                     if region.to_region_vid() == needle_fr {
 486                         let region_name = self.synthesize_region_name(counter);
 487
 488                         // Just grab the first character, the `&`.
 489                         let source_map = tcx.sess.source_map();
 490                         let ampersand_span = source_map.start_point(hir_ty.span);
 491
 492                         return Some(RegionName {
 493                             name: region_name,
 494                             source: RegionNameSource::MatchedHirTy(ampersand_span),
 495                         });
 496                     }
 497
 498                     // Otherwise, let's descend into the referent types.
 499                     search_stack.push((referent_ty, &referent_hir_ty.ty));
 500                 }
 501
 502                 // Match up something like `Foo<'1>`
 503                 (
 504                     ty::Adt(_adt_def, substs),
 505                     hir::TyKind::Path(hir::QPath::Resolved(None, path)),
 506                 ) => {
 507                     match path.res {
 508                         // Type parameters of the type alias have no reason to
 509                         // be the same as those of the ADT.
 510                         // FIXME: We should be able to do something similar to
 511                         // match_adt_and_segment in this case.
 512                         Res::Def(DefKind::TyAlias, _) => (),
 513                         _ => if let Some(last_segment) = path.segments.last() {
 514                             if let Some(name) = self.match_adt_and_segment(
 515                                 substs,
 516                                 needle_fr,
 517                                 last_segment,
 518                                 counter,
 519                                 search_stack,
 520                             ) {
 521                                 return Some(name);
 522                             }
 523                         }
 524                     }
 525                 }
 526
 527                 // The following cases don't have lifetimes, so we
 528                 // just worry about trying to match up the rustc type
 529                 // with the HIR types:
 530                 (ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
 531                     search_stack.extend(elem_tys.iter().map(|k| k.expect_ty()).zip(elem_hir_tys));
 532                 }
 533
 534                 (ty::Slice(elem_ty), hir::TyKind::Slice(elem_hir_ty))
 535                 | (ty::Array(elem_ty, _), hir::TyKind::Array(elem_hir_ty, _)) => {
 536                     search_stack.push((elem_ty, elem_hir_ty));
 537                 }
 538
 539                 (ty::RawPtr(mut_ty), hir::TyKind::Ptr(mut_hir_ty)) => {
 540                     search_stack.push((mut_ty.ty, &mut_hir_ty.ty));
 541                 }
 542
 543                 _ => {
 544                     // FIXME there are other cases that we could trace
 545                 }
 546             }
 547         }
 548
 549         return None;
 550     }
 551
 552     /// We've found an enum/struct/union type with the substitutions
 553     /// `substs` and -- in the HIR -- a path type with the final
 554     /// segment `last_segment`. Try to find a `'_` to highlight in
 555     /// the generic args (or, if not, to produce new zipped pairs of
 556     /// types+hir to search through).
 557     fn match_adt_and_segment<'hir>(
 558         &self,
 559         substs: SubstsRef<'tcx>,
 560         needle_fr: RegionVid,
 561         last_segment: &'hir hir::PathSegment,
 562         counter: &mut usize,
 563         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty)>,
 564     ) -> Option<RegionName> {
 565         // Did the user give explicit arguments? (e.g., `Foo<..>`)
 566         let args = last_segment.args.as_ref()?;
 567         let lifetime = self.try_match_adt_and_generic_args(substs, needle_fr, args, search_stack)?;
 568         match lifetime.name {
 569             hir::LifetimeName::Param(_)
 570             | hir::LifetimeName::Error
 571             | hir::LifetimeName::Static
 572             | hir::LifetimeName::Underscore => {
 573                 let region_name = self.synthesize_region_name(counter);
 574                 let ampersand_span = lifetime.span;
 575                 Some(RegionName {
 576                     name: region_name,
 577                     source: RegionNameSource::MatchedAdtAndSegment(ampersand_span),
 578                 })
 579             }
 580
 581             hir::LifetimeName::ImplicitObjectLifetimeDefault
 582             | hir::LifetimeName::Implicit => {
 583                 // In this case, the user left off the lifetime; so
 584                 // they wrote something like:
 585                 //
 586                 // ```
 587                 // x: Foo<T>
 588                 // ```
 589                 //
 590                 // where the fully elaborated form is `Foo<'_, '1,
 591                 // T>`. We don't consider this a match; instead we let
 592                 // the "fully elaborated" type fallback above handle
 593                 // it.
 594                 None
 595             }
 596         }
 597     }
 598
 599     /// We've found an enum/struct/union type with the substitutions
 600     /// `substs` and -- in the HIR -- a path with the generic
 601     /// arguments `args`. If `needle_fr` appears in the args, return
 602     /// the `hir::Lifetime` that corresponds to it. If not, push onto
 603     /// `search_stack` the types+hir to search through.
 604     fn try_match_adt_and_generic_args<'hir>(
 605         &self,
 606         substs: SubstsRef<'tcx>,
 607         needle_fr: RegionVid,
 608         args: &'hir hir::GenericArgs,
 609         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty)>,
 610     ) -> Option<&'hir hir::Lifetime> {
 611         for (kind, hir_arg) in substs.iter().zip(&args.args) {
 612             match (kind.unpack(), hir_arg) {
 613                 (UnpackedKind::Lifetime(r), hir::GenericArg::Lifetime(lt)) => {
 614                     if r.to_region_vid() == needle_fr {
 615                         return Some(lt);
 616                     }
 617                 }
 618
 619                 (UnpackedKind::Type(ty), hir::GenericArg::Type(hir_ty)) => {
 620                     search_stack.push((ty, hir_ty));
 621                 }
 622
 623                 (UnpackedKind::Const(_ct), hir::GenericArg::Const(_hir_ct)) => {
 624                     // Lifetimes cannot be found in consts, so we don't need
 625                     // to search anything here.
 626                 }
 627
 628                 (UnpackedKind::Lifetime(_), _)
 629                 | (UnpackedKind::Type(_), _)
 630                 | (UnpackedKind::Const(_), _) => {
 631                     // I *think* that HIR lowering should ensure this
 632                     // doesn't happen, even in erroneous
 633                     // programs. Else we should use delay-span-bug.
 634                     span_bug!(
 635                         hir_arg.span(),
 636                         "unmatched subst and hir arg: found {:?} vs {:?}",
 637                         kind,
 638                         hir_arg,
 639                     );
 640                 }
 641             }
 642         }
 643
 644         None
 645     }
 646
 647     /// Finds a closure upvar that contains `fr` and label it with a
 648     /// fully elaborated type, returning something like `'1`. Result
 649     /// looks like:
 650     ///
 651     /// ```
 652     ///  | let x = Some(&22);
 653     ///        - fully elaborated type of `x` is `Option<&'1 u32>`
 654     /// ```
 655     fn give_name_if_anonymous_region_appears_in_upvars(
 656         &self,
 657         tcx: TyCtxt<'tcx>,
 658         upvars: &[Upvar],
 659         fr: RegionVid,
 660         counter: &mut usize,
 661     ) -> Option<RegionName> {
 662         let upvar_index = self.get_upvar_index_for_region(tcx, fr)?;
 663         let (upvar_name, upvar_span) =
 664             self.get_upvar_name_and_span_for_region(tcx, upvars, upvar_index);
 665         let region_name = self.synthesize_region_name(counter);
 666
 667         Some(RegionName {
 668             name: region_name,
 669             source: RegionNameSource::AnonRegionFromUpvar(upvar_span, upvar_name.to_string()),
 670         })
 671     }
 672
 673     /// Checks for arguments appearing in the (closure) return type. It
 674     /// must be a closure since, in a free fn, such an argument would
 675     /// have to either also appear in an argument (if using elision)
 676     /// or be early bound (named, not in argument).
 677     fn give_name_if_anonymous_region_appears_in_output(
 678         &self,
 679         infcx: &InferCtxt<'_, 'tcx>,
 680         body: &Body<'tcx>,
 681         mir_def_id: DefId,
 682         fr: RegionVid,
 683         counter: &mut usize,
 684     ) -> Option<RegionName> {
 685         let tcx = infcx.tcx;
 686
 687         let return_ty = self.universal_regions.unnormalized_output_ty;
 688         debug!(
 689             "give_name_if_anonymous_region_appears_in_output: return_ty = {:?}",
 690             return_ty
 691         );
 692         if !tcx.any_free_region_meets(&return_ty, |r| r.to_region_vid() == fr) {
 693             return None;
 694         }
 695
 696         let mut highlight = RegionHighlightMode::default();
 697         highlight.highlighting_region_vid(fr, *counter);
 698         let type_name = infcx.extract_type_name(&return_ty, Some(highlight)).0;
 699
 700         let mir_hir_id = tcx.hir().as_local_hir_id(mir_def_id).expect("non-local mir");
 701
 702         let (return_span, mir_description) = match tcx.hir().get(mir_hir_id) {
 703             hir::Node::Expr(hir::Expr {
 704                 node: hir::ExprKind::Closure(_, return_ty, _, span, gen_move),
 705                 ..
 706             }) => (
 707                 match return_ty.output {
 708                     hir::FunctionRetTy::DefaultReturn(_) => tcx.sess.source_map().end_point(*span),
 709                     hir::FunctionRetTy::Return(_) => return_ty.output.span(),
 710                 },
 711                 if gen_move.is_some() {
 712                     " of generator"
 713                 } else {
 714                     " of closure"
 715                 },
 716             ),
 717             hir::Node::ImplItem(hir::ImplItem {
 718                 node: hir::ImplItemKind::Method(method_sig, _),
 719                 ..
 720             }) => (method_sig.decl.output.span(), ""),
 721             _ => (body.span, ""),
 722         };
 723
 724         Some(RegionName {
 725             // This counter value will already have been used, so this function will increment it
 726             // so the next value will be used next and return the region name that would have been
 727             // used.
 728             name: self.synthesize_region_name(counter),
 729             source: RegionNameSource::AnonRegionFromOutput(
 730                 return_span,
 731                 mir_description.to_string(),
 732                 type_name
 733             ),
 734         })
 735     }
 736
 737     fn give_name_if_anonymous_region_appears_in_yield_ty(
 738         &self,
 739         infcx: &InferCtxt<'_, 'tcx>,
 740         body: &Body<'tcx>,
 741         mir_def_id: DefId,
 742         fr: RegionVid,
 743         counter: &mut usize,
 744     ) -> Option<RegionName> {
 745         // Note: generators from `async fn` yield `()`, so we don't have to
 746         // worry about them here.
 747         let yield_ty = self.universal_regions.yield_ty?;
 748         debug!(
 749             "give_name_if_anonymous_region_appears_in_yield_ty: yield_ty = {:?}",
 750             yield_ty,
 751         );
 752
 753         let tcx = infcx.tcx;
 754
 755         if !tcx.any_free_region_meets(&yield_ty, |r| r.to_region_vid() == fr) {
 756             return None;
 757         }
 758
 759         let mut highlight = RegionHighlightMode::default();
 760         highlight.highlighting_region_vid(fr, *counter);
 761         let type_name = infcx.extract_type_name(&yield_ty, Some(highlight)).0;
 762
 763         let mir_hir_id = tcx.hir().as_local_hir_id(mir_def_id).expect("non-local mir");
 764
 765         let yield_span = match tcx.hir().get(mir_hir_id) {
 766             hir::Node::Expr(hir::Expr {
 767                 node: hir::ExprKind::Closure(_, _, _, span, _),
 768                 ..
 769             }) => (
 770                 tcx.sess.source_map().end_point(*span)
 771             ),
 772             _ => body.span,
 773         };
 774
 775         debug!(
 776             "give_name_if_anonymous_region_appears_in_yield_ty: \
 777              type_name = {:?}, yield_span = {:?}",
 778             yield_span,
 779             type_name,
 780         );
 781
 782         Some(RegionName {
 783             name: self.synthesize_region_name(counter),
 784             source: RegionNameSource::AnonRegionFromYieldTy(yield_span, type_name),
 785         })
 786     }
 787
 788     /// Creates a synthetic region named `'1`, incrementing the
 789     /// counter.
 790     fn synthesize_region_name(&self, counter: &mut usize) -> InternedString {
 791         let c = *counter;
 792         *counter += 1;
 793
 794         InternedString::intern(&format!("'{:?}", c))
 795     }
 796 }