compiler/rustc_borrowck/src/diagnostics/region_name.rs

   1 use std::fmt::{self, Display};
   2 use std::iter;
   3
   4 use rustc_errors::DiagnosticBuilder;
   5 use rustc_hir as hir;
   6 use rustc_hir::def::{DefKind, Res};
   7 use rustc_middle::ty::print::RegionHighlightMode;
   8 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef};
   9 use rustc_middle::ty::{self, RegionVid, Ty};
  10 use rustc_span::symbol::{kw, sym, Ident, Symbol};
  11 use rustc_span::{Span, DUMMY_SP};
  12
  13 use crate::{nll::ToRegionVid, universal_regions::DefiningTy, MirBorrowckCtxt};
  14
  15 /// A name for a particular region used in emitting diagnostics. This name could be a generated
  16 /// name like `'1`, a name used by the user like `'a`, or a name like `'static`.
  17 #[derive(Debug, Clone)]
  18 crate struct RegionName {
  19     /// The name of the region (interned).
  20     crate name: Symbol,
  21     /// Where the region comes from.
  22     crate source: RegionNameSource,
  23 }
  24
  25 /// Denotes the source of a region that is named by a `RegionName`. For example, a free region that
  26 /// was named by the user would get `NamedFreeRegion` and `'static` lifetime would get `Static`.
  27 /// This helps to print the right kinds of diagnostics.
  28 #[derive(Debug, Clone)]
  29 crate enum RegionNameSource {
  30     /// A bound (not free) region that was substituted at the def site (not an HRTB).
  31     NamedEarlyBoundRegion(Span),
  32     /// A free region that the user has a name (`'a`) for.
  33     NamedFreeRegion(Span),
  34     /// The `'static` region.
  35     Static,
  36     /// The free region corresponding to the environment of a closure.
  37     SynthesizedFreeEnvRegion(Span, String),
  38     /// The region corresponding to an argument.
  39     AnonRegionFromArgument(RegionNameHighlight),
  40     /// The region corresponding to a closure upvar.
  41     AnonRegionFromUpvar(Span, String),
  42     /// The region corresponding to the return type of a closure.
  43     AnonRegionFromOutput(RegionNameHighlight, String),
  44     /// The region from a type yielded by a generator.
  45     AnonRegionFromYieldTy(Span, String),
  46     /// An anonymous region from an async fn.
  47     AnonRegionFromAsyncFn(Span),
  48 }
  49
  50 /// Describes what to highlight to explain to the user that we're giving an anonymous region a
  51 /// synthesized name, and how to highlight it.
  52 #[derive(Debug, Clone)]
  53 crate enum RegionNameHighlight {
  54     /// The anonymous region corresponds to a reference that was found by traversing the type in the HIR.
  55     MatchedHirTy(Span),
  56     /// The anonymous region corresponds to a `'_` in the generics list of a struct/enum/union.
  57     MatchedAdtAndSegment(Span),
  58     /// The anonymous region corresponds to a region where the type annotation is completely missing
  59     /// from the code, e.g. in a closure arguments `|x| { ... }`, where `x` is a reference.
  60     CannotMatchHirTy(Span, String),
  61     /// The anonymous region corresponds to a region where the type annotation is completely missing
  62     /// from the code, and *even if* we print out the full name of the type, the region name won't
  63     /// be included. This currently occurs for opaque types like `impl Future`.
  64     Occluded(Span, String),
  65 }
  66
  67 impl RegionName {
  68     crate fn was_named(&self) -> bool {
  69         match self.source {
  70             RegionNameSource::NamedEarlyBoundRegion(..)
  71             | RegionNameSource::NamedFreeRegion(..)
  72             | RegionNameSource::Static => true,
  73             RegionNameSource::SynthesizedFreeEnvRegion(..)
  74             | RegionNameSource::AnonRegionFromArgument(..)
  75             | RegionNameSource::AnonRegionFromUpvar(..)
  76             | RegionNameSource::AnonRegionFromOutput(..)
  77             | RegionNameSource::AnonRegionFromYieldTy(..)
  78             | RegionNameSource::AnonRegionFromAsyncFn(..) => false,
  79         }
  80     }
  81
  82     crate fn span(&self) -> Option<Span> {
  83         match self.source {
  84             RegionNameSource::Static => None,
  85             RegionNameSource::NamedEarlyBoundRegion(span)
  86             | RegionNameSource::NamedFreeRegion(span)
  87             | RegionNameSource::SynthesizedFreeEnvRegion(span, _)
  88             | RegionNameSource::AnonRegionFromUpvar(span, _)
  89             | RegionNameSource::AnonRegionFromYieldTy(span, _)
  90             | RegionNameSource::AnonRegionFromAsyncFn(span) => Some(span),
  91             RegionNameSource::AnonRegionFromArgument(ref highlight)
  92             | RegionNameSource::AnonRegionFromOutput(ref highlight, _) => match *highlight {
  93                 RegionNameHighlight::MatchedHirTy(span)
  94                 | RegionNameHighlight::MatchedAdtAndSegment(span)
  95                 | RegionNameHighlight::CannotMatchHirTy(span, _)
  96                 | RegionNameHighlight::Occluded(span, _) => Some(span),
  97             },
  98         }
  99     }
 100
 101     crate fn highlight_region_name(&self, diag: &mut DiagnosticBuilder<'_>) {
 102         match &self.source {
 103             RegionNameSource::NamedFreeRegion(span)
 104             | RegionNameSource::NamedEarlyBoundRegion(span) => {
 105                 diag.span_label(*span, format!("lifetime `{}` defined here", self));
 106             }
 107             RegionNameSource::SynthesizedFreeEnvRegion(span, note) => {
 108                 diag.span_label(
 109                     *span,
 110                     format!("lifetime `{}` represents this closure's body", self),
 111                 );
 112                 diag.note(&note);
 113             }
 114             RegionNameSource::AnonRegionFromArgument(RegionNameHighlight::CannotMatchHirTy(
 115                 span,
 116                 type_name,
 117             )) => {
 118                 diag.span_label(*span, format!("has type `{}`", type_name));
 119             }
 120             RegionNameSource::AnonRegionFromArgument(RegionNameHighlight::MatchedHirTy(span))
 121             | RegionNameSource::AnonRegionFromOutput(RegionNameHighlight::MatchedHirTy(span), _)
 122             | RegionNameSource::AnonRegionFromAsyncFn(span) => {
 123                 diag.span_label(
 124                     *span,
 125                     format!("let's call the lifetime of this reference `{}`", self),
 126                 );
 127             }
 128             RegionNameSource::AnonRegionFromArgument(
 129                 RegionNameHighlight::MatchedAdtAndSegment(span),
 130             )
 131             | RegionNameSource::AnonRegionFromOutput(
 132                 RegionNameHighlight::MatchedAdtAndSegment(span),
 133                 _,
 134             ) => {
 135                 diag.span_label(*span, format!("let's call this `{}`", self));
 136             }
 137             RegionNameSource::AnonRegionFromArgument(RegionNameHighlight::Occluded(
 138                 span,
 139                 type_name,
 140             )) => {
 141                 diag.span_label(
 142                     *span,
 143                     format!("lifetime `{}` appears in the type {}", self, type_name),
 144                 );
 145             }
 146             RegionNameSource::AnonRegionFromOutput(
 147                 RegionNameHighlight::Occluded(span, type_name),
 148                 mir_description,
 149             ) => {
 150                 diag.span_label(
 151                     *span,
 152                     format!(
 153                         "return type{} `{}` contains a lifetime `{}`",
 154                         mir_description, type_name, self
 155                     ),
 156                 );
 157             }
 158             RegionNameSource::AnonRegionFromUpvar(span, upvar_name) => {
 159                 diag.span_label(
 160                     *span,
 161                     format!("lifetime `{}` appears in the type of `{}`", self, upvar_name),
 162                 );
 163             }
 164             RegionNameSource::AnonRegionFromOutput(
 165                 RegionNameHighlight::CannotMatchHirTy(span, type_name),
 166                 mir_description,
 167             ) => {
 168                 diag.span_label(*span, format!("return type{} is {}", mir_description, type_name));
 169             }
 170             RegionNameSource::AnonRegionFromYieldTy(span, type_name) => {
 171                 diag.span_label(*span, format!("yield type is {}", type_name));
 172             }
 173             RegionNameSource::Static => {}
 174         }
 175     }
 176 }
 177
 178 impl Display for RegionName {
 179     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 180         write!(f, "{}", self.name)
 181     }
 182 }
 183
 184 impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
 185     crate fn mir_def_id(&self) -> hir::def_id::LocalDefId {
 186         self.body.source.def_id().as_local().unwrap()
 187     }
 188
 189     crate fn mir_hir_id(&self) -> hir::HirId {
 190         self.infcx.tcx.hir().local_def_id_to_hir_id(self.mir_def_id())
 191     }
 192
 193     /// Generate a synthetic region named `'N`, where `N` is the next value of the counter. Then,
 194     /// increment the counter.
 195     ///
 196     /// This is _not_ idempotent. Call `give_region_a_name` when possible.
 197     fn synthesize_region_name(&self) -> Symbol {
 198         let c = self.next_region_name.replace_with(|counter| *counter + 1);
 199         Symbol::intern(&format!("'{:?}", c))
 200     }
 201
 202     /// Maps from an internal MIR region vid to something that we can
 203     /// report to the user. In some cases, the region vids will map
 204     /// directly to lifetimes that the user has a name for (e.g.,
 205     /// `'static`). But frequently they will not, in which case we
 206     /// have to find some way to identify the lifetime to the user. To
 207     /// that end, this function takes a "diagnostic" so that it can
 208     /// create auxiliary notes as needed.
 209     ///
 210     /// The names are memoized, so this is both cheap to recompute and idempotent.
 211     ///
 212     /// Example (function arguments):
 213     ///
 214     /// Suppose we are trying to give a name to the lifetime of the
 215     /// reference `x`:
 216     ///
 217     /// ```
 218     /// fn foo(x: &u32) { .. }
 219     /// ```
 220     ///
 221     /// This function would create a label like this:
 222     ///
 223     /// ```text
 224     ///  | fn foo(x: &u32) { .. }
 225     ///           ------- fully elaborated type of `x` is `&'1 u32`
 226     /// ```
 227     ///
 228     /// and then return the name `'1` for us to use.
 229     crate fn give_region_a_name(&self, fr: RegionVid) -> Option<RegionName> {
 230         debug!(
 231             "give_region_a_name(fr={:?}, counter={:?})",
 232             fr,
 233             self.next_region_name.try_borrow().unwrap()
 234         );
 235
 236         assert!(self.regioncx.universal_regions().is_universal_region(fr));
 237
 238         if let Some(value) = self.region_names.try_borrow_mut().unwrap().get(&fr) {
 239             return Some(value.clone());
 240         }
 241
 242         let value = self
 243             .give_name_from_error_region(fr)
 244             .or_else(|| self.give_name_if_anonymous_region_appears_in_arguments(fr))
 245             .or_else(|| self.give_name_if_anonymous_region_appears_in_upvars(fr))
 246             .or_else(|| self.give_name_if_anonymous_region_appears_in_output(fr))
 247             .or_else(|| self.give_name_if_anonymous_region_appears_in_yield_ty(fr));
 248
 249         if let Some(ref value) = value {
 250             self.region_names.try_borrow_mut().unwrap().insert(fr, value.clone());
 251         }
 252
 253         debug!("give_region_a_name: gave name {:?}", value);
 254         value
 255     }
 256
 257     /// Checks for the case where `fr` maps to something that the
 258     /// *user* has a name for. In that case, we'll be able to map
 259     /// `fr` to a `Region<'tcx>`, and that region will be one of
 260     /// named variants.
 261     fn give_name_from_error_region(&self, fr: RegionVid) -> Option<RegionName> {
 262         let error_region = self.to_error_region(fr)?;
 263
 264         let tcx = self.infcx.tcx;
 265
 266         debug!("give_region_a_name: error_region = {:?}", error_region);
 267         match error_region {
 268             ty::ReEarlyBound(ebr) => {
 269                 if ebr.has_name() {
 270                     let span = tcx.hir().span_if_local(ebr.def_id).unwrap_or(DUMMY_SP);
 271                     Some(RegionName {
 272                         name: ebr.name,
 273                         source: RegionNameSource::NamedEarlyBoundRegion(span),
 274                     })
 275                 } else {
 276                     None
 277                 }
 278             }
 279
 280             ty::ReStatic => {
 281                 Some(RegionName { name: kw::StaticLifetime, source: RegionNameSource::Static })
 282             }
 283
 284             ty::ReFree(free_region) => match free_region.bound_region {
 285                 ty::BoundRegionKind::BrNamed(region_def_id, name) => {
 286                     // Get the span to point to, even if we don't use the name.
 287                     let span = tcx.hir().span_if_local(region_def_id).unwrap_or(DUMMY_SP);
 288                     debug!(
 289                         "bound region named: {:?}, is_named: {:?}",
 290                         name,
 291                         free_region.bound_region.is_named()
 292                     );
 293
 294                     if free_region.bound_region.is_named() {
 295                         // A named region that is actually named.
 296                         Some(RegionName { name, source: RegionNameSource::NamedFreeRegion(span) })
 297                     } else {
 298                         // If we spuriously thought that the region is named, we should let the
 299                         // system generate a true name for error messages. Currently this can
 300                         // happen if we have an elided name in an async fn for example: the
 301                         // compiler will generate a region named `'_`, but reporting such a name is
 302                         // not actually useful, so we synthesize a name for it instead.
 303                         let name = self.synthesize_region_name();
 304                         Some(RegionName {
 305                             name,
 306                             source: RegionNameSource::AnonRegionFromAsyncFn(span),
 307                         })
 308                     }
 309                 }
 310
 311                 ty::BoundRegionKind::BrEnv => {
 312                     let def_ty = self.regioncx.universal_regions().defining_ty;
 313
 314                     let DefiningTy::Closure(_, substs) = def_ty else {
 315                         // Can't have BrEnv in functions, constants or generators.
 316                         bug!("BrEnv outside of closure.");
 317                     };
 318                     let hir::ExprKind::Closure(_, _, _, args_span, _) =
 319                         tcx.hir().expect_expr(self.mir_hir_id()).kind else {
 320                         bug!("Closure is not defined by a closure expr");
 321                     };
 322                     let region_name = self.synthesize_region_name();
 323
 324                     let closure_kind_ty = substs.as_closure().kind_ty();
 325                     let note = match closure_kind_ty.to_opt_closure_kind() {
 326                         Some(ty::ClosureKind::Fn) => {
 327                             "closure implements `Fn`, so references to captured variables \
 328                                 can't escape the closure"
 329                         }
 330                         Some(ty::ClosureKind::FnMut) => {
 331                             "closure implements `FnMut`, so references to captured variables \
 332                                 can't escape the closure"
 333                         }
 334                         Some(ty::ClosureKind::FnOnce) => {
 335                             bug!("BrEnv in a `FnOnce` closure");
 336                         }
 337                         None => bug!("Closure kind not inferred in borrow check"),
 338                     };
 339
 340                     Some(RegionName {
 341                         name: region_name,
 342                         source: RegionNameSource::SynthesizedFreeEnvRegion(
 343                             args_span,
 344                             note.to_string(),
 345                         ),
 346                     })
 347                 }
 348
 349                 ty::BoundRegionKind::BrAnon(_) => None,
 350             },
 351
 352             ty::ReLateBound(..)
 353             | ty::ReVar(..)
 354             | ty::RePlaceholder(..)
 355             | ty::ReEmpty(_)
 356             | ty::ReErased => None,
 357         }
 358     }
 359
 360     /// Finds an argument that contains `fr` and label it with a fully
 361     /// elaborated type, returning something like `'1`. Result looks
 362     /// like:
 363     ///
 364     /// ```text
 365     ///  | fn foo(x: &u32) { .. }
 366     ///           ------- fully elaborated type of `x` is `&'1 u32`
 367     /// ```
 368     fn give_name_if_anonymous_region_appears_in_arguments(
 369         &self,
 370         fr: RegionVid,
 371     ) -> Option<RegionName> {
 372         let implicit_inputs = self.regioncx.universal_regions().defining_ty.implicit_inputs();
 373         let argument_index = self.regioncx.get_argument_index_for_region(self.infcx.tcx, fr)?;
 374
 375         let arg_ty = self.regioncx.universal_regions().unnormalized_input_tys
 376             [implicit_inputs + argument_index];
 377         let (_, span) = self.regioncx.get_argument_name_and_span_for_region(
 378             &self.body,
 379             &self.local_names,
 380             argument_index,
 381         );
 382
 383         let highlight = self
 384             .get_argument_hir_ty_for_highlighting(argument_index)
 385             .and_then(|arg_hir_ty| self.highlight_if_we_can_match_hir_ty(fr, arg_ty, arg_hir_ty))
 386             .unwrap_or_else(|| {
 387                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 388                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 389                 // will increment the counter, "reserving" the number we just used.
 390                 let counter = *self.next_region_name.try_borrow().unwrap();
 391                 self.highlight_if_we_cannot_match_hir_ty(fr, arg_ty, span, counter)
 392             });
 393
 394         Some(RegionName {
 395             name: self.synthesize_region_name(),
 396             source: RegionNameSource::AnonRegionFromArgument(highlight),
 397         })
 398     }
 399
 400     fn get_argument_hir_ty_for_highlighting(
 401         &self,
 402         argument_index: usize,
 403     ) -> Option<&hir::Ty<'tcx>> {
 404         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(self.mir_hir_id())?;
 405         let argument_hir_ty: &hir::Ty<'_> = fn_decl.inputs.get(argument_index)?;
 406         match argument_hir_ty.kind {
 407             // This indicates a variable with no type annotation, like
 408             // `|x|`... in that case, we can't highlight the type but
 409             // must highlight the variable.
 410             // NOTE(eddyb) this is handled in/by the sole caller
 411             // (`give_name_if_anonymous_region_appears_in_arguments`).
 412             hir::TyKind::Infer => None,
 413
 414             _ => Some(argument_hir_ty),
 415         }
 416     }
 417
 418     /// Attempts to highlight the specific part of a type in an argument
 419     /// that has no type annotation.
 420     /// For example, we might produce an annotation like this:
 421     ///
 422     /// ```text
 423     ///  |     foo(|a, b| b)
 424     ///  |          -  -
 425     ///  |          |  |
 426     ///  |          |  has type `&'1 u32`
 427     ///  |          has type `&'2 u32`
 428     /// ```
 429     fn highlight_if_we_cannot_match_hir_ty(
 430         &self,
 431         needle_fr: RegionVid,
 432         ty: Ty<'tcx>,
 433         span: Span,
 434         counter: usize,
 435     ) -> RegionNameHighlight {
 436         let mut highlight = RegionHighlightMode::default();
 437         highlight.highlighting_region_vid(needle_fr, counter);
 438         let type_name =
 439             self.infcx.extract_inference_diagnostics_data(ty.into(), Some(highlight)).name;
 440
 441         debug!(
 442             "highlight_if_we_cannot_match_hir_ty: type_name={:?} needle_fr={:?}",
 443             type_name, needle_fr
 444         );
 445         if type_name.contains(&format!("'{}", counter)) {
 446             // Only add a label if we can confirm that a region was labelled.
 447             RegionNameHighlight::CannotMatchHirTy(span, type_name)
 448         } else {
 449             RegionNameHighlight::Occluded(span, type_name)
 450         }
 451     }
 452
 453     /// Attempts to highlight the specific part of a type annotation
 454     /// that contains the anonymous reference we want to give a name
 455     /// to. For example, we might produce an annotation like this:
 456     ///
 457     /// ```text
 458     ///  | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item = &T>> {
 459     ///  |                - let's call the lifetime of this reference `'1`
 460     /// ```
 461     ///
 462     /// the way this works is that we match up `ty`, which is
 463     /// a `Ty<'tcx>` (the internal form of the type) with
 464     /// `hir_ty`, a `hir::Ty` (the syntax of the type
 465     /// annotation). We are descending through the types stepwise,
 466     /// looking in to find the region `needle_fr` in the internal
 467     /// type. Once we find that, we can use the span of the `hir::Ty`
 468     /// to add the highlight.
 469     ///
 470     /// This is a somewhat imperfect process, so along the way we also
 471     /// keep track of the **closest** type we've found. If we fail to
 472     /// find the exact `&` or `'_` to highlight, then we may fall back
 473     /// to highlighting that closest type instead.
 474     fn highlight_if_we_can_match_hir_ty(
 475         &self,
 476         needle_fr: RegionVid,
 477         ty: Ty<'tcx>,
 478         hir_ty: &hir::Ty<'_>,
 479     ) -> Option<RegionNameHighlight> {
 480         let search_stack: &mut Vec<(Ty<'tcx>, &hir::Ty<'_>)> = &mut vec![(ty, hir_ty)];
 481
 482         while let Some((ty, hir_ty)) = search_stack.pop() {
 483             match (&ty.kind(), &hir_ty.kind) {
 484                 // Check if the `ty` is `&'X ..` where `'X`
 485                 // is the region we are looking for -- if so, and we have a `&T`
 486                 // on the RHS, then we want to highlight the `&` like so:
 487                 //
 488                 //     &
 489                 //     - let's call the lifetime of this reference `'1`
 490                 (
 491                     ty::Ref(region, referent_ty, _),
 492                     hir::TyKind::Rptr(_lifetime, referent_hir_ty),
 493                 ) => {
 494                     if region.to_region_vid() == needle_fr {
 495                         // Just grab the first character, the `&`.
 496                         let source_map = self.infcx.tcx.sess.source_map();
 497                         let ampersand_span = source_map.start_point(hir_ty.span);
 498
 499                         return Some(RegionNameHighlight::MatchedHirTy(ampersand_span));
 500                     }
 501
 502                     // Otherwise, let's descend into the referent types.
 503                     search_stack.push((referent_ty, &referent_hir_ty.ty));
 504                 }
 505
 506                 // Match up something like `Foo<'1>`
 507                 (
 508                     ty::Adt(_adt_def, substs),
 509                     hir::TyKind::Path(hir::QPath::Resolved(None, path)),
 510                 ) => {
 511                     match path.res {
 512                         // Type parameters of the type alias have no reason to
 513                         // be the same as those of the ADT.
 514                         // FIXME: We should be able to do something similar to
 515                         // match_adt_and_segment in this case.
 516                         Res::Def(DefKind::TyAlias, _) => (),
 517                         _ => {
 518                             if let Some(last_segment) = path.segments.last() {
 519                                 if let Some(highlight) = self.match_adt_and_segment(
 520                                     substs,
 521                                     needle_fr,
 522                                     last_segment,
 523                                     search_stack,
 524                                 ) {
 525                                     return Some(highlight);
 526                                 }
 527                             }
 528                         }
 529                     }
 530                 }
 531
 532                 // The following cases don't have lifetimes, so we
 533                 // just worry about trying to match up the rustc type
 534                 // with the HIR types:
 535                 (ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
 536                     search_stack
 537                         .extend(iter::zip(elem_tys.iter().map(|k| k.expect_ty()), *elem_hir_tys));
 538                 }
 539
 540                 (ty::Slice(elem_ty), hir::TyKind::Slice(elem_hir_ty))
 541                 | (ty::Array(elem_ty, _), hir::TyKind::Array(elem_hir_ty, _)) => {
 542                     search_stack.push((elem_ty, elem_hir_ty));
 543                 }
 544
 545                 (ty::RawPtr(mut_ty), hir::TyKind::Ptr(mut_hir_ty)) => {
 546                     search_stack.push((mut_ty.ty, &mut_hir_ty.ty));
 547                 }
 548
 549                 _ => {
 550                     // FIXME there are other cases that we could trace
 551                 }
 552             }
 553         }
 554
 555         None
 556     }
 557
 558     /// We've found an enum/struct/union type with the substitutions
 559     /// `substs` and -- in the HIR -- a path type with the final
 560     /// segment `last_segment`. Try to find a `'_` to highlight in
 561     /// the generic args (or, if not, to produce new zipped pairs of
 562     /// types+hir to search through).
 563     fn match_adt_and_segment<'hir>(
 564         &self,
 565         substs: SubstsRef<'tcx>,
 566         needle_fr: RegionVid,
 567         last_segment: &'hir hir::PathSegment<'hir>,
 568         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 569     ) -> Option<RegionNameHighlight> {
 570         // Did the user give explicit arguments? (e.g., `Foo<..>`)
 571         let args = last_segment.args.as_ref()?;
 572         let lifetime =
 573             self.try_match_adt_and_generic_args(substs, needle_fr, args, search_stack)?;
 574         match lifetime.name {
 575             hir::LifetimeName::Param(_)
 576             | hir::LifetimeName::Error
 577             | hir::LifetimeName::Static
 578             | hir::LifetimeName::Underscore => {
 579                 let lifetime_span = lifetime.span;
 580                 Some(RegionNameHighlight::MatchedAdtAndSegment(lifetime_span))
 581             }
 582
 583             hir::LifetimeName::ImplicitObjectLifetimeDefault | hir::LifetimeName::Implicit(_) => {
 584                 // In this case, the user left off the lifetime; so
 585                 // they wrote something like:
 586                 //
 587                 // ```
 588                 // x: Foo<T>
 589                 // ```
 590                 //
 591                 // where the fully elaborated form is `Foo<'_, '1,
 592                 // T>`. We don't consider this a match; instead we let
 593                 // the "fully elaborated" type fallback above handle
 594                 // it.
 595                 None
 596             }
 597         }
 598     }
 599
 600     /// We've found an enum/struct/union type with the substitutions
 601     /// `substs` and -- in the HIR -- a path with the generic
 602     /// arguments `args`. If `needle_fr` appears in the args, return
 603     /// the `hir::Lifetime` that corresponds to it. If not, push onto
 604     /// `search_stack` the types+hir to search through.
 605     fn try_match_adt_and_generic_args<'hir>(
 606         &self,
 607         substs: SubstsRef<'tcx>,
 608         needle_fr: RegionVid,
 609         args: &'hir hir::GenericArgs<'hir>,
 610         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 611     ) -> Option<&'hir hir::Lifetime> {
 612         for (kind, hir_arg) in iter::zip(substs, args.args) {
 613             match (kind.unpack(), hir_arg) {
 614                 (GenericArgKind::Lifetime(r), hir::GenericArg::Lifetime(lt)) => {
 615                     if r.to_region_vid() == needle_fr {
 616                         return Some(lt);
 617                     }
 618                 }
 619
 620                 (GenericArgKind::Type(ty), hir::GenericArg::Type(hir_ty)) => {
 621                     search_stack.push((ty, hir_ty));
 622                 }
 623
 624                 (GenericArgKind::Const(_ct), hir::GenericArg::Const(_hir_ct)) => {
 625                     // Lifetimes cannot be found in consts, so we don't need
 626                     // to search anything here.
 627                 }
 628
 629                 (
 630                     GenericArgKind::Lifetime(_)
 631                     | GenericArgKind::Type(_)
 632                     | GenericArgKind::Const(_),
 633                     _,
 634                 ) => {
 635                     // HIR lowering sometimes doesn't catch this in erroneous
 636                     // programs, so we need to use delay_span_bug here. See #82126.
 637                     self.infcx.tcx.sess.delay_span_bug(
 638                         hir_arg.span(),
 639                         &format!("unmatched subst and hir arg: found {:?} vs {:?}", kind, hir_arg),
 640                     );
 641                 }
 642             }
 643         }
 644
 645         None
 646     }
 647
 648     /// Finds a closure upvar that contains `fr` and label it with a
 649     /// fully elaborated type, returning something like `'1`. Result
 650     /// looks like:
 651     ///
 652     /// ```text
 653     ///  | let x = Some(&22);
 654     ///        - fully elaborated type of `x` is `Option<&'1 u32>`
 655     /// ```
 656     fn give_name_if_anonymous_region_appears_in_upvars(&self, fr: RegionVid) -> Option<RegionName> {
 657         let upvar_index = self.regioncx.get_upvar_index_for_region(self.infcx.tcx, fr)?;
 658         let (upvar_name, upvar_span) = self.regioncx.get_upvar_name_and_span_for_region(
 659             self.infcx.tcx,
 660             &self.upvars,
 661             upvar_index,
 662         );
 663         let region_name = self.synthesize_region_name();
 664
 665         Some(RegionName {
 666             name: region_name,
 667             source: RegionNameSource::AnonRegionFromUpvar(upvar_span, upvar_name.to_string()),
 668         })
 669     }
 670
 671     /// Checks for arguments appearing in the (closure) return type. It
 672     /// must be a closure since, in a free fn, such an argument would
 673     /// have to either also appear in an argument (if using elision)
 674     /// or be early bound (named, not in argument).
 675     fn give_name_if_anonymous_region_appears_in_output(&self, fr: RegionVid) -> Option<RegionName> {
 676         let tcx = self.infcx.tcx;
 677         let hir = tcx.hir();
 678
 679         let return_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 680         debug!("give_name_if_anonymous_region_appears_in_output: return_ty = {:?}", return_ty);
 681         if !tcx.any_free_region_meets(&return_ty, |r| r.to_region_vid() == fr) {
 682             return None;
 683         }
 684
 685         let mir_hir_id = self.mir_hir_id();
 686
 687         let (return_span, mir_description, hir_ty) = match hir.get(mir_hir_id) {
 688             hir::Node::Expr(hir::Expr {
 689                 kind: hir::ExprKind::Closure(_, return_ty, body_id, span, _),
 690                 ..
 691             }) => {
 692                 let (mut span, mut hir_ty) = match return_ty.output {
 693                     hir::FnRetTy::DefaultReturn(_) => {
 694                         (tcx.sess.source_map().end_point(*span), None)
 695                     }
 696                     hir::FnRetTy::Return(hir_ty) => (return_ty.output.span(), Some(hir_ty)),
 697                 };
 698                 let mir_description = match hir.body(*body_id).generator_kind {
 699                     Some(hir::GeneratorKind::Async(gen)) => match gen {
 700                         hir::AsyncGeneratorKind::Block => " of async block",
 701                         hir::AsyncGeneratorKind::Closure => " of async closure",
 702                         hir::AsyncGeneratorKind::Fn => {
 703                             let parent_item = hir.get_by_def_id(hir.get_parent_item(mir_hir_id));
 704                             let output = &parent_item
 705                                 .fn_decl()
 706                                 .expect("generator lowered from async fn should be in fn")
 707                                 .output;
 708                             span = output.span();
 709                             if let hir::FnRetTy::Return(ret) = output {
 710                                 hir_ty = Some(self.get_future_inner_return_ty(*ret));
 711                             }
 712                             " of async function"
 713                         }
 714                     },
 715                     Some(hir::GeneratorKind::Gen) => " of generator",
 716                     None => " of closure",
 717                 };
 718                 (span, mir_description, hir_ty)
 719             }
 720             node => match node.fn_decl() {
 721                 Some(fn_decl) => {
 722                     let hir_ty = match fn_decl.output {
 723                         hir::FnRetTy::DefaultReturn(_) => None,
 724                         hir::FnRetTy::Return(ty) => Some(ty),
 725                     };
 726                     (fn_decl.output.span(), "", hir_ty)
 727                 }
 728                 None => (self.body.span, "", None),
 729             },
 730         };
 731
 732         let highlight = hir_ty
 733             .and_then(|hir_ty| self.highlight_if_we_can_match_hir_ty(fr, return_ty, hir_ty))
 734             .unwrap_or_else(|| {
 735                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 736                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 737                 // will increment the counter, "reserving" the number we just used.
 738                 let counter = *self.next_region_name.try_borrow().unwrap();
 739                 self.highlight_if_we_cannot_match_hir_ty(fr, return_ty, return_span, counter)
 740             });
 741
 742         Some(RegionName {
 743             name: self.synthesize_region_name(),
 744             source: RegionNameSource::AnonRegionFromOutput(highlight, mir_description.to_string()),
 745         })
 746     }
 747
 748     /// From the [`hir::Ty`] of an async function's lowered return type,
 749     /// retrieve the `hir::Ty` representing the type the user originally wrote.
 750     ///
 751     /// e.g. given the function:
 752     ///
 753     /// ```
 754     /// async fn foo() -> i32 {}
 755     /// ```
 756     ///
 757     /// this function, given the lowered return type of `foo`, an [`OpaqueDef`] that implements `Future<Output=i32>`,
 758     /// returns the `i32`.
 759     ///
 760     /// [`OpaqueDef`]: hir::TyKind::OpaqueDef
 761     fn get_future_inner_return_ty(&self, hir_ty: &'tcx hir::Ty<'tcx>) -> &'tcx hir::Ty<'tcx> {
 762         let hir = self.infcx.tcx.hir();
 763
 764         let hir::TyKind::OpaqueDef(id, _) = hir_ty.kind else {
 765             span_bug!(
 766                 hir_ty.span,
 767                 "lowered return type of async fn is not OpaqueDef: {:?}",
 768                 hir_ty
 769             );
 770         };
 771         let opaque_ty = hir.item(id);
 772         if let hir::ItemKind::OpaqueTy(hir::OpaqueTy {
 773             bounds:
 774                 [
 775                     hir::GenericBound::LangItemTrait(
 776                         hir::LangItem::Future,
 777                         _,
 778                         _,
 779                         hir::GenericArgs {
 780                             bindings:
 781                                 [
 782                                     hir::TypeBinding {
 783                                         ident: Ident { name: sym::Output, .. },
 784                                         kind:
 785                                             hir::TypeBindingKind::Equality { term: hir::Term::Ty(ty) },
 786                                         ..
 787                                     },
 788                                 ],
 789                             ..
 790                         },
 791                     ),
 792                 ],
 793             ..
 794         }) = opaque_ty.kind
 795         {
 796             ty
 797         } else {
 798             span_bug!(
 799                 hir_ty.span,
 800                 "bounds from lowered return type of async fn did not match expected format: {:?}",
 801                 opaque_ty
 802             );
 803         }
 804     }
 805
 806     fn give_name_if_anonymous_region_appears_in_yield_ty(
 807         &self,
 808         fr: RegionVid,
 809     ) -> Option<RegionName> {
 810         // Note: generators from `async fn` yield `()`, so we don't have to
 811         // worry about them here.
 812         let yield_ty = self.regioncx.universal_regions().yield_ty?;
 813         debug!("give_name_if_anonymous_region_appears_in_yield_ty: yield_ty = {:?}", yield_ty,);
 814
 815         let tcx = self.infcx.tcx;
 816
 817         if !tcx.any_free_region_meets(&yield_ty, |r| r.to_region_vid() == fr) {
 818             return None;
 819         }
 820
 821         let mut highlight = RegionHighlightMode::default();
 822         highlight.highlighting_region_vid(fr, *self.next_region_name.try_borrow().unwrap());
 823         let type_name =
 824             self.infcx.extract_inference_diagnostics_data(yield_ty.into(), Some(highlight)).name;
 825
 826         let yield_span = match tcx.hir().get(self.mir_hir_id()) {
 827             hir::Node::Expr(hir::Expr {
 828                 kind: hir::ExprKind::Closure(_, _, _, span, _), ..
 829             }) => (tcx.sess.source_map().end_point(*span)),
 830             _ => self.body.span,
 831         };
 832
 833         debug!(
 834             "give_name_if_anonymous_region_appears_in_yield_ty: \
 835              type_name = {:?}, yield_span = {:?}",
 836             yield_span, type_name,
 837         );
 838
 839         Some(RegionName {
 840             name: self.synthesize_region_name(),
 841             source: RegionNameSource::AnonRegionFromYieldTy(yield_span, type_name),
 842         })
 843     }
 844 }