compiler/rustc_borrowck/src/diagnostics/region_name.rs

   1 use std::fmt::{self, Display};
   2 use std::iter;
   3
   4 use rustc_errors::Diagnostic;
   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, DefIdTree, 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 pub(crate) struct RegionName {
  19     /// The name of the region (interned).
  20     pub(crate) name: Symbol,
  21     /// Where the region comes from.
  22     pub(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 pub(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, &'static str),
  38     /// The region corresponding to an argument.
  39     AnonRegionFromArgument(RegionNameHighlight),
  40     /// The region corresponding to a closure upvar.
  41     AnonRegionFromUpvar(Span, Symbol),
  42     /// The region corresponding to the return type of a closure.
  43     AnonRegionFromOutput(RegionNameHighlight, &'static str),
  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     /// An anonymous region from an impl self type or trait
  49     AnonRegionFromImplSignature(Span, &'static str),
  50 }
  51
  52 /// Describes what to highlight to explain to the user that we're giving an anonymous region a
  53 /// synthesized name, and how to highlight it.
  54 #[derive(Debug, Clone)]
  55 pub(crate) enum RegionNameHighlight {
  56     /// The anonymous region corresponds to a reference that was found by traversing the type in the HIR.
  57     MatchedHirTy(Span),
  58     /// The anonymous region corresponds to a `'_` in the generics list of a struct/enum/union.
  59     MatchedAdtAndSegment(Span),
  60     /// The anonymous region corresponds to a region where the type annotation is completely missing
  61     /// from the code, e.g. in a closure arguments `|x| { ... }`, where `x` is a reference.
  62     CannotMatchHirTy(Span, String),
  63     /// The anonymous region corresponds to a region where the type annotation is completely missing
  64     /// from the code, and *even if* we print out the full name of the type, the region name won't
  65     /// be included. This currently occurs for opaque types like `impl Future`.
  66     Occluded(Span, String),
  67 }
  68
  69 impl RegionName {
  70     pub(crate) fn was_named(&self) -> bool {
  71         match self.source {
  72             RegionNameSource::NamedEarlyBoundRegion(..)
  73             | RegionNameSource::NamedFreeRegion(..)
  74             | RegionNameSource::Static => true,
  75             RegionNameSource::SynthesizedFreeEnvRegion(..)
  76             | RegionNameSource::AnonRegionFromArgument(..)
  77             | RegionNameSource::AnonRegionFromUpvar(..)
  78             | RegionNameSource::AnonRegionFromOutput(..)
  79             | RegionNameSource::AnonRegionFromYieldTy(..)
  80             | RegionNameSource::AnonRegionFromAsyncFn(..)
  81             | RegionNameSource::AnonRegionFromImplSignature(..) => false,
  82         }
  83     }
  84
  85     pub(crate) fn span(&self) -> Option<Span> {
  86         match self.source {
  87             RegionNameSource::Static => None,
  88             RegionNameSource::NamedEarlyBoundRegion(span)
  89             | RegionNameSource::NamedFreeRegion(span)
  90             | RegionNameSource::SynthesizedFreeEnvRegion(span, _)
  91             | RegionNameSource::AnonRegionFromUpvar(span, _)
  92             | RegionNameSource::AnonRegionFromYieldTy(span, _)
  93             | RegionNameSource::AnonRegionFromAsyncFn(span)
  94             | RegionNameSource::AnonRegionFromImplSignature(span, _) => Some(span),
  95             RegionNameSource::AnonRegionFromArgument(ref highlight)
  96             | RegionNameSource::AnonRegionFromOutput(ref highlight, _) => match *highlight {
  97                 RegionNameHighlight::MatchedHirTy(span)
  98                 | RegionNameHighlight::MatchedAdtAndSegment(span)
  99                 | RegionNameHighlight::CannotMatchHirTy(span, _)
 100                 | RegionNameHighlight::Occluded(span, _) => Some(span),
 101             },
 102         }
 103     }
 104
 105     pub(crate) fn highlight_region_name(&self, diag: &mut Diagnostic) {
 106         match &self.source {
 107             RegionNameSource::NamedFreeRegion(span)
 108             | RegionNameSource::NamedEarlyBoundRegion(span) => {
 109                 diag.span_label(*span, format!("lifetime `{self}` defined here"));
 110             }
 111             RegionNameSource::SynthesizedFreeEnvRegion(span, note) => {
 112                 diag.span_label(*span, format!("lifetime `{self}` represents this closure's body"));
 113                 diag.note(*note);
 114             }
 115             RegionNameSource::AnonRegionFromArgument(RegionNameHighlight::CannotMatchHirTy(
 116                 span,
 117                 type_name,
 118             )) => {
 119                 diag.span_label(*span, format!("has type `{type_name}`"));
 120             }
 121             RegionNameSource::AnonRegionFromArgument(RegionNameHighlight::MatchedHirTy(span))
 122             | RegionNameSource::AnonRegionFromOutput(RegionNameHighlight::MatchedHirTy(span), _)
 123             | RegionNameSource::AnonRegionFromAsyncFn(span) => {
 124                 diag.span_label(
 125                     *span,
 126                     format!("let's call the lifetime of this reference `{self}`"),
 127                 );
 128             }
 129             RegionNameSource::AnonRegionFromArgument(
 130                 RegionNameHighlight::MatchedAdtAndSegment(span),
 131             )
 132             | RegionNameSource::AnonRegionFromOutput(
 133                 RegionNameHighlight::MatchedAdtAndSegment(span),
 134                 _,
 135             ) => {
 136                 diag.span_label(*span, format!("let's call this `{self}`"));
 137             }
 138             RegionNameSource::AnonRegionFromArgument(RegionNameHighlight::Occluded(
 139                 span,
 140                 type_name,
 141             )) => {
 142                 diag.span_label(
 143                     *span,
 144                     format!("lifetime `{self}` appears in the type {type_name}"),
 145                 );
 146             }
 147             RegionNameSource::AnonRegionFromOutput(
 148                 RegionNameHighlight::Occluded(span, type_name),
 149                 mir_description,
 150             ) => {
 151                 diag.span_label(
 152                     *span,
 153                     format!(
 154                         "return type{mir_description} `{type_name}` contains a lifetime `{self}`"
 155                     ),
 156                 );
 157             }
 158             RegionNameSource::AnonRegionFromUpvar(span, upvar_name) => {
 159                 diag.span_label(
 160                     *span,
 161                     format!("lifetime `{self}` appears in the type of `{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{mir_description} is {type_name}"));
 169             }
 170             RegionNameSource::AnonRegionFromYieldTy(span, type_name) => {
 171                 diag.span_label(*span, format!("yield type is {type_name}"));
 172             }
 173             RegionNameSource::AnonRegionFromImplSignature(span, location) => {
 174                 diag.span_label(
 175                     *span,
 176                     format!("lifetime `{self}` appears in the `impl`'s {location}"),
 177                 );
 178             }
 179             RegionNameSource::Static => {}
 180         }
 181     }
 182 }
 183
 184 impl Display for RegionName {
 185     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 186         write!(f, "{}", self.name)
 187     }
 188 }
 189
 190 impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
 191     pub(crate) fn mir_def_id(&self) -> hir::def_id::LocalDefId {
 192         self.body.source.def_id().expect_local()
 193     }
 194
 195     pub(crate) fn mir_hir_id(&self) -> hir::HirId {
 196         self.infcx.tcx.hir().local_def_id_to_hir_id(self.mir_def_id())
 197     }
 198
 199     /// Generate a synthetic region named `'N`, where `N` is the next value of the counter. Then,
 200     /// increment the counter.
 201     ///
 202     /// This is _not_ idempotent. Call `give_region_a_name` when possible.
 203     fn synthesize_region_name(&self) -> Symbol {
 204         let c = self.next_region_name.replace_with(|counter| *counter + 1);
 205         Symbol::intern(&format!("'{:?}", c))
 206     }
 207
 208     /// Maps from an internal MIR region vid to something that we can
 209     /// report to the user. In some cases, the region vids will map
 210     /// directly to lifetimes that the user has a name for (e.g.,
 211     /// `'static`). But frequently they will not, in which case we
 212     /// have to find some way to identify the lifetime to the user. To
 213     /// that end, this function takes a "diagnostic" so that it can
 214     /// create auxiliary notes as needed.
 215     ///
 216     /// The names are memoized, so this is both cheap to recompute and idempotent.
 217     ///
 218     /// Example (function arguments):
 219     ///
 220     /// Suppose we are trying to give a name to the lifetime of the
 221     /// reference `x`:
 222     ///
 223     /// ```ignore (pseudo-rust)
 224     /// fn foo(x: &u32) { .. }
 225     /// ```
 226     ///
 227     /// This function would create a label like this:
 228     ///
 229     /// ```text
 230     ///  | fn foo(x: &u32) { .. }
 231     ///           ------- fully elaborated type of `x` is `&'1 u32`
 232     /// ```
 233     ///
 234     /// and then return the name `'1` for us to use.
 235     pub(crate) fn give_region_a_name(&self, fr: RegionVid) -> Option<RegionName> {
 236         debug!(
 237             "give_region_a_name(fr={:?}, counter={:?})",
 238             fr,
 239             self.next_region_name.try_borrow().unwrap()
 240         );
 241
 242         assert!(self.regioncx.universal_regions().is_universal_region(fr));
 243
 244         if let Some(value) = self.region_names.try_borrow_mut().unwrap().get(&fr) {
 245             return Some(value.clone());
 246         }
 247
 248         let value = self
 249             .give_name_from_error_region(fr)
 250             .or_else(|| self.give_name_if_anonymous_region_appears_in_arguments(fr))
 251             .or_else(|| self.give_name_if_anonymous_region_appears_in_upvars(fr))
 252             .or_else(|| self.give_name_if_anonymous_region_appears_in_output(fr))
 253             .or_else(|| self.give_name_if_anonymous_region_appears_in_yield_ty(fr))
 254             .or_else(|| self.give_name_if_anonymous_region_appears_in_impl_signature(fr));
 255
 256         if let Some(ref value) = value {
 257             self.region_names.try_borrow_mut().unwrap().insert(fr, value.clone());
 258         }
 259
 260         debug!("give_region_a_name: gave name {:?}", value);
 261         value
 262     }
 263
 264     /// Checks for the case where `fr` maps to something that the
 265     /// *user* has a name for. In that case, we'll be able to map
 266     /// `fr` to a `Region<'tcx>`, and that region will be one of
 267     /// named variants.
 268     #[instrument(level = "trace", skip(self))]
 269     fn give_name_from_error_region(&self, fr: RegionVid) -> Option<RegionName> {
 270         let error_region = self.to_error_region(fr)?;
 271
 272         let tcx = self.infcx.tcx;
 273
 274         debug!("give_region_a_name: error_region = {:?}", error_region);
 275         match *error_region {
 276             ty::ReEarlyBound(ebr) => {
 277                 if ebr.has_name() {
 278                     let span = tcx.hir().span_if_local(ebr.def_id).unwrap_or(DUMMY_SP);
 279                     Some(RegionName {
 280                         name: ebr.name,
 281                         source: RegionNameSource::NamedEarlyBoundRegion(span),
 282                     })
 283                 } else {
 284                     None
 285                 }
 286             }
 287
 288             ty::ReStatic => {
 289                 Some(RegionName { name: kw::StaticLifetime, source: RegionNameSource::Static })
 290             }
 291
 292             ty::ReFree(free_region) => match free_region.bound_region {
 293                 ty::BoundRegionKind::BrNamed(region_def_id, name) => {
 294                     // Get the span to point to, even if we don't use the name.
 295                     let span = tcx.hir().span_if_local(region_def_id).unwrap_or(DUMMY_SP);
 296                     debug!(
 297                         "bound region named: {:?}, is_named: {:?}",
 298                         name,
 299                         free_region.bound_region.is_named()
 300                     );
 301
 302                     if free_region.bound_region.is_named() {
 303                         // A named region that is actually named.
 304                         Some(RegionName { name, source: RegionNameSource::NamedFreeRegion(span) })
 305                     } else if let hir::IsAsync::Async = tcx.asyncness(self.mir_hir_id().owner) {
 306                         // If we spuriously thought that the region is named, we should let the
 307                         // system generate a true name for error messages. Currently this can
 308                         // happen if we have an elided name in an async fn for example: the
 309                         // compiler will generate a region named `'_`, but reporting such a name is
 310                         // not actually useful, so we synthesize a name for it instead.
 311                         let name = self.synthesize_region_name();
 312                         Some(RegionName {
 313                             name,
 314                             source: RegionNameSource::AnonRegionFromAsyncFn(span),
 315                         })
 316                     } else {
 317                         None
 318                     }
 319                 }
 320
 321                 ty::BoundRegionKind::BrEnv => {
 322                     let def_ty = self.regioncx.universal_regions().defining_ty;
 323
 324                     let DefiningTy::Closure(_, substs) = def_ty else {
 325                         // Can't have BrEnv in functions, constants or generators.
 326                         bug!("BrEnv outside of closure.");
 327                     };
 328                     let hir::ExprKind::Closure(&hir::Closure { fn_decl_span, .. })
 329                         = tcx.hir().expect_expr(self.mir_hir_id()).kind
 330                     else {
 331                         bug!("Closure is not defined by a closure expr");
 332                     };
 333                     let region_name = self.synthesize_region_name();
 334
 335                     let closure_kind_ty = substs.as_closure().kind_ty();
 336                     let note = match closure_kind_ty.to_opt_closure_kind() {
 337                         Some(ty::ClosureKind::Fn) => {
 338                             "closure implements `Fn`, so references to captured variables \
 339                                 can't escape the closure"
 340                         }
 341                         Some(ty::ClosureKind::FnMut) => {
 342                             "closure implements `FnMut`, so references to captured variables \
 343                                 can't escape the closure"
 344                         }
 345                         Some(ty::ClosureKind::FnOnce) => {
 346                             bug!("BrEnv in a `FnOnce` closure");
 347                         }
 348                         None => bug!("Closure kind not inferred in borrow check"),
 349                     };
 350
 351                     Some(RegionName {
 352                         name: region_name,
 353                         source: RegionNameSource::SynthesizedFreeEnvRegion(fn_decl_span, note),
 354                     })
 355                 }
 356
 357                 ty::BoundRegionKind::BrAnon(_) => None,
 358             },
 359
 360             ty::ReLateBound(..)
 361             | ty::ReVar(..)
 362             | ty::RePlaceholder(..)
 363             | ty::ReEmpty(_)
 364             | ty::ReErased => None,
 365         }
 366     }
 367
 368     /// Finds an argument that contains `fr` and label it with a fully
 369     /// elaborated type, returning something like `'1`. Result looks
 370     /// like:
 371     ///
 372     /// ```text
 373     ///  | fn foo(x: &u32) { .. }
 374     ///           ------- fully elaborated type of `x` is `&'1 u32`
 375     /// ```
 376     #[instrument(level = "trace", skip(self))]
 377     fn give_name_if_anonymous_region_appears_in_arguments(
 378         &self,
 379         fr: RegionVid,
 380     ) -> Option<RegionName> {
 381         let implicit_inputs = self.regioncx.universal_regions().defining_ty.implicit_inputs();
 382         let argument_index = self.regioncx.get_argument_index_for_region(self.infcx.tcx, fr)?;
 383
 384         let arg_ty = self.regioncx.universal_regions().unnormalized_input_tys
 385             [implicit_inputs + argument_index];
 386         let (_, span) = self.regioncx.get_argument_name_and_span_for_region(
 387             &self.body,
 388             &self.local_names,
 389             argument_index,
 390         );
 391
 392         let highlight = self
 393             .get_argument_hir_ty_for_highlighting(argument_index)
 394             .and_then(|arg_hir_ty| self.highlight_if_we_can_match_hir_ty(fr, arg_ty, arg_hir_ty))
 395             .unwrap_or_else(|| {
 396                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 397                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 398                 // will increment the counter, "reserving" the number we just used.
 399                 let counter = *self.next_region_name.try_borrow().unwrap();
 400                 self.highlight_if_we_cannot_match_hir_ty(fr, arg_ty, span, counter)
 401             });
 402
 403         Some(RegionName {
 404             name: self.synthesize_region_name(),
 405             source: RegionNameSource::AnonRegionFromArgument(highlight),
 406         })
 407     }
 408
 409     fn get_argument_hir_ty_for_highlighting(
 410         &self,
 411         argument_index: usize,
 412     ) -> Option<&hir::Ty<'tcx>> {
 413         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(self.mir_hir_id())?;
 414         let argument_hir_ty: &hir::Ty<'_> = fn_decl.inputs.get(argument_index)?;
 415         match argument_hir_ty.kind {
 416             // This indicates a variable with no type annotation, like
 417             // `|x|`... in that case, we can't highlight the type but
 418             // must highlight the variable.
 419             // NOTE(eddyb) this is handled in/by the sole caller
 420             // (`give_name_if_anonymous_region_appears_in_arguments`).
 421             hir::TyKind::Infer => None,
 422
 423             _ => Some(argument_hir_ty),
 424         }
 425     }
 426
 427     /// Attempts to highlight the specific part of a type in an argument
 428     /// that has no type annotation.
 429     /// For example, we might produce an annotation like this:
 430     ///
 431     /// ```text
 432     ///  |     foo(|a, b| b)
 433     ///  |          -  -
 434     ///  |          |  |
 435     ///  |          |  has type `&'1 u32`
 436     ///  |          has type `&'2 u32`
 437     /// ```
 438     fn highlight_if_we_cannot_match_hir_ty(
 439         &self,
 440         needle_fr: RegionVid,
 441         ty: Ty<'tcx>,
 442         span: Span,
 443         counter: usize,
 444     ) -> RegionNameHighlight {
 445         let mut highlight = RegionHighlightMode::new(self.infcx.tcx);
 446         highlight.highlighting_region_vid(needle_fr, counter);
 447         let type_name =
 448             self.infcx.extract_inference_diagnostics_data(ty.into(), Some(highlight)).name;
 449
 450         debug!(
 451             "highlight_if_we_cannot_match_hir_ty: type_name={:?} needle_fr={:?}",
 452             type_name, needle_fr
 453         );
 454         if type_name.contains(&format!("'{counter}")) {
 455             // Only add a label if we can confirm that a region was labelled.
 456             RegionNameHighlight::CannotMatchHirTy(span, type_name)
 457         } else {
 458             RegionNameHighlight::Occluded(span, type_name)
 459         }
 460     }
 461
 462     /// Attempts to highlight the specific part of a type annotation
 463     /// that contains the anonymous reference we want to give a name
 464     /// to. For example, we might produce an annotation like this:
 465     ///
 466     /// ```text
 467     ///  | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item = &T>> {
 468     ///  |                - let's call the lifetime of this reference `'1`
 469     /// ```
 470     ///
 471     /// the way this works is that we match up `ty`, which is
 472     /// a `Ty<'tcx>` (the internal form of the type) with
 473     /// `hir_ty`, a `hir::Ty` (the syntax of the type
 474     /// annotation). We are descending through the types stepwise,
 475     /// looking in to find the region `needle_fr` in the internal
 476     /// type. Once we find that, we can use the span of the `hir::Ty`
 477     /// to add the highlight.
 478     ///
 479     /// This is a somewhat imperfect process, so along the way we also
 480     /// keep track of the **closest** type we've found. If we fail to
 481     /// find the exact `&` or `'_` to highlight, then we may fall back
 482     /// to highlighting that closest type instead.
 483     fn highlight_if_we_can_match_hir_ty(
 484         &self,
 485         needle_fr: RegionVid,
 486         ty: Ty<'tcx>,
 487         hir_ty: &hir::Ty<'_>,
 488     ) -> Option<RegionNameHighlight> {
 489         let search_stack: &mut Vec<(Ty<'tcx>, &hir::Ty<'_>)> = &mut vec![(ty, hir_ty)];
 490
 491         while let Some((ty, hir_ty)) = search_stack.pop() {
 492             match (ty.kind(), &hir_ty.kind) {
 493                 // Check if the `ty` is `&'X ..` where `'X`
 494                 // is the region we are looking for -- if so, and we have a `&T`
 495                 // on the RHS, then we want to highlight the `&` like so:
 496                 //
 497                 //     &
 498                 //     - let's call the lifetime of this reference `'1`
 499                 (
 500                     ty::Ref(region, referent_ty, _),
 501                     hir::TyKind::Rptr(_lifetime, referent_hir_ty),
 502                 ) => {
 503                     if region.to_region_vid() == needle_fr {
 504                         // Just grab the first character, the `&`.
 505                         let source_map = self.infcx.tcx.sess.source_map();
 506                         let ampersand_span = source_map.start_point(hir_ty.span);
 507
 508                         return Some(RegionNameHighlight::MatchedHirTy(ampersand_span));
 509                     }
 510
 511                     // Otherwise, let's descend into the referent types.
 512                     search_stack.push((*referent_ty, &referent_hir_ty.ty));
 513                 }
 514
 515                 // Match up something like `Foo<'1>`
 516                 (
 517                     ty::Adt(_adt_def, substs),
 518                     hir::TyKind::Path(hir::QPath::Resolved(None, path)),
 519                 ) => {
 520                     match path.res {
 521                         // Type parameters of the type alias have no reason to
 522                         // be the same as those of the ADT.
 523                         // FIXME: We should be able to do something similar to
 524                         // match_adt_and_segment in this case.
 525                         Res::Def(DefKind::TyAlias, _) => (),
 526                         _ => {
 527                             if let Some(last_segment) = path.segments.last() {
 528                                 if let Some(highlight) = self.match_adt_and_segment(
 529                                     substs,
 530                                     needle_fr,
 531                                     last_segment,
 532                                     search_stack,
 533                                 ) {
 534                                     return Some(highlight);
 535                                 }
 536                             }
 537                         }
 538                     }
 539                 }
 540
 541                 // The following cases don't have lifetimes, so we
 542                 // just worry about trying to match up the rustc type
 543                 // with the HIR types:
 544                 (&ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
 545                     search_stack.extend(iter::zip(elem_tys, *elem_hir_tys));
 546                 }
 547
 548                 (ty::Slice(elem_ty), hir::TyKind::Slice(elem_hir_ty))
 549                 | (ty::Array(elem_ty, _), hir::TyKind::Array(elem_hir_ty, _)) => {
 550                     search_stack.push((*elem_ty, elem_hir_ty));
 551                 }
 552
 553                 (ty::RawPtr(mut_ty), hir::TyKind::Ptr(mut_hir_ty)) => {
 554                     search_stack.push((mut_ty.ty, &mut_hir_ty.ty));
 555                 }
 556
 557                 _ => {
 558                     // FIXME there are other cases that we could trace
 559                 }
 560             }
 561         }
 562
 563         None
 564     }
 565
 566     /// We've found an enum/struct/union type with the substitutions
 567     /// `substs` and -- in the HIR -- a path type with the final
 568     /// segment `last_segment`. Try to find a `'_` to highlight in
 569     /// the generic args (or, if not, to produce new zipped pairs of
 570     /// types+hir to search through).
 571     fn match_adt_and_segment<'hir>(
 572         &self,
 573         substs: SubstsRef<'tcx>,
 574         needle_fr: RegionVid,
 575         last_segment: &'hir hir::PathSegment<'hir>,
 576         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 577     ) -> Option<RegionNameHighlight> {
 578         // Did the user give explicit arguments? (e.g., `Foo<..>`)
 579         let args = last_segment.args.as_ref()?;
 580         let lifetime =
 581             self.try_match_adt_and_generic_args(substs, needle_fr, args, search_stack)?;
 582         match lifetime.name {
 583             hir::LifetimeName::Param(_, hir::ParamName::Plain(_) | hir::ParamName::Error)
 584             | hir::LifetimeName::Error
 585             | hir::LifetimeName::Static => {
 586                 let lifetime_span = lifetime.span;
 587                 Some(RegionNameHighlight::MatchedAdtAndSegment(lifetime_span))
 588             }
 589
 590             hir::LifetimeName::Param(_, hir::ParamName::Fresh)
 591             | hir::LifetimeName::ImplicitObjectLifetimeDefault
 592             | hir::LifetimeName::Infer => {
 593                 // In this case, the user left off the lifetime; so
 594                 // they wrote something like:
 595                 //
 596                 // ```
 597                 // x: Foo<T>
 598                 // ```
 599                 //
 600                 // where the fully elaborated form is `Foo<'_, '1,
 601                 // T>`. We don't consider this a match; instead we let
 602                 // the "fully elaborated" type fallback above handle
 603                 // it.
 604                 None
 605             }
 606         }
 607     }
 608
 609     /// We've found an enum/struct/union type with the substitutions
 610     /// `substs` and -- in the HIR -- a path with the generic
 611     /// arguments `args`. If `needle_fr` appears in the args, return
 612     /// the `hir::Lifetime` that corresponds to it. If not, push onto
 613     /// `search_stack` the types+hir to search through.
 614     fn try_match_adt_and_generic_args<'hir>(
 615         &self,
 616         substs: SubstsRef<'tcx>,
 617         needle_fr: RegionVid,
 618         args: &'hir hir::GenericArgs<'hir>,
 619         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 620     ) -> Option<&'hir hir::Lifetime> {
 621         for (kind, hir_arg) in iter::zip(substs, args.args) {
 622             match (kind.unpack(), hir_arg) {
 623                 (GenericArgKind::Lifetime(r), hir::GenericArg::Lifetime(lt)) => {
 624                     if r.to_region_vid() == needle_fr {
 625                         return Some(lt);
 626                     }
 627                 }
 628
 629                 (GenericArgKind::Type(ty), hir::GenericArg::Type(hir_ty)) => {
 630                     search_stack.push((ty, hir_ty));
 631                 }
 632
 633                 (GenericArgKind::Const(_ct), hir::GenericArg::Const(_hir_ct)) => {
 634                     // Lifetimes cannot be found in consts, so we don't need
 635                     // to search anything here.
 636                 }
 637
 638                 (
 639                     GenericArgKind::Lifetime(_)
 640                     | GenericArgKind::Type(_)
 641                     | GenericArgKind::Const(_),
 642                     _,
 643                 ) => {
 644                     // HIR lowering sometimes doesn't catch this in erroneous
 645                     // programs, so we need to use delay_span_bug here. See #82126.
 646                     self.infcx.tcx.sess.delay_span_bug(
 647                         hir_arg.span(),
 648                         &format!("unmatched subst and hir arg: found {:?} vs {:?}", kind, hir_arg),
 649                     );
 650                 }
 651             }
 652         }
 653
 654         None
 655     }
 656
 657     /// Finds a closure upvar that contains `fr` and label it with a
 658     /// fully elaborated type, returning something like `'1`. Result
 659     /// looks like:
 660     ///
 661     /// ```text
 662     ///  | let x = Some(&22);
 663     ///        - fully elaborated type of `x` is `Option<&'1 u32>`
 664     /// ```
 665     #[instrument(level = "trace", skip(self))]
 666     fn give_name_if_anonymous_region_appears_in_upvars(&self, fr: RegionVid) -> Option<RegionName> {
 667         let upvar_index = self.regioncx.get_upvar_index_for_region(self.infcx.tcx, fr)?;
 668         let (upvar_name, upvar_span) = self.regioncx.get_upvar_name_and_span_for_region(
 669             self.infcx.tcx,
 670             &self.upvars,
 671             upvar_index,
 672         );
 673         let region_name = self.synthesize_region_name();
 674
 675         Some(RegionName {
 676             name: region_name,
 677             source: RegionNameSource::AnonRegionFromUpvar(upvar_span, upvar_name),
 678         })
 679     }
 680
 681     /// Checks for arguments appearing in the (closure) return type. It
 682     /// must be a closure since, in a free fn, such an argument would
 683     /// have to either also appear in an argument (if using elision)
 684     /// or be early bound (named, not in argument).
 685     #[instrument(level = "trace", skip(self))]
 686     fn give_name_if_anonymous_region_appears_in_output(&self, fr: RegionVid) -> Option<RegionName> {
 687         let tcx = self.infcx.tcx;
 688         let hir = tcx.hir();
 689
 690         let return_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 691         debug!("give_name_if_anonymous_region_appears_in_output: return_ty = {:?}", return_ty);
 692         if !tcx.any_free_region_meets(&return_ty, |r| r.to_region_vid() == fr) {
 693             return None;
 694         }
 695
 696         let mir_hir_id = self.mir_hir_id();
 697
 698         let (return_span, mir_description, hir_ty) = match hir.get(mir_hir_id) {
 699             hir::Node::Expr(hir::Expr {
 700                 kind: hir::ExprKind::Closure(&hir::Closure { fn_decl, body, fn_decl_span, .. }),
 701                 ..
 702             }) => {
 703                 let (mut span, mut hir_ty) = match fn_decl.output {
 704                     hir::FnRetTy::DefaultReturn(_) => {
 705                         (tcx.sess.source_map().end_point(fn_decl_span), None)
 706                     }
 707                     hir::FnRetTy::Return(hir_ty) => (fn_decl.output.span(), Some(hir_ty)),
 708                 };
 709                 let mir_description = match hir.body(body).generator_kind {
 710                     Some(hir::GeneratorKind::Async(gen)) => match gen {
 711                         hir::AsyncGeneratorKind::Block => " of async block",
 712                         hir::AsyncGeneratorKind::Closure => " of async closure",
 713                         hir::AsyncGeneratorKind::Fn => {
 714                             let parent_item = hir.get_by_def_id(hir.get_parent_item(mir_hir_id));
 715                             let output = &parent_item
 716                                 .fn_decl()
 717                                 .expect("generator lowered from async fn should be in fn")
 718                                 .output;
 719                             span = output.span();
 720                             if let hir::FnRetTy::Return(ret) = output {
 721                                 hir_ty = Some(self.get_future_inner_return_ty(*ret));
 722                             }
 723                             " of async function"
 724                         }
 725                     },
 726                     Some(hir::GeneratorKind::Gen) => " of generator",
 727                     None => " of closure",
 728                 };
 729                 (span, mir_description, hir_ty)
 730             }
 731             node => match node.fn_decl() {
 732                 Some(fn_decl) => {
 733                     let hir_ty = match fn_decl.output {
 734                         hir::FnRetTy::DefaultReturn(_) => None,
 735                         hir::FnRetTy::Return(ty) => Some(ty),
 736                     };
 737                     (fn_decl.output.span(), "", hir_ty)
 738                 }
 739                 None => (self.body.span, "", None),
 740             },
 741         };
 742
 743         let highlight = hir_ty
 744             .and_then(|hir_ty| self.highlight_if_we_can_match_hir_ty(fr, return_ty, hir_ty))
 745             .unwrap_or_else(|| {
 746                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 747                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 748                 // will increment the counter, "reserving" the number we just used.
 749                 let counter = *self.next_region_name.try_borrow().unwrap();
 750                 self.highlight_if_we_cannot_match_hir_ty(fr, return_ty, return_span, counter)
 751             });
 752
 753         Some(RegionName {
 754             name: self.synthesize_region_name(),
 755             source: RegionNameSource::AnonRegionFromOutput(highlight, mir_description),
 756         })
 757     }
 758
 759     /// From the [`hir::Ty`] of an async function's lowered return type,
 760     /// retrieve the `hir::Ty` representing the type the user originally wrote.
 761     ///
 762     /// e.g. given the function:
 763     ///
 764     /// ```
 765     /// async fn foo() -> i32 { 2 }
 766     /// ```
 767     ///
 768     /// this function, given the lowered return type of `foo`, an [`OpaqueDef`] that implements `Future<Output=i32>`,
 769     /// returns the `i32`.
 770     ///
 771     /// [`OpaqueDef`]: hir::TyKind::OpaqueDef
 772     fn get_future_inner_return_ty(&self, hir_ty: &'tcx hir::Ty<'tcx>) -> &'tcx hir::Ty<'tcx> {
 773         let hir = self.infcx.tcx.hir();
 774
 775         let hir::TyKind::OpaqueDef(id, _) = hir_ty.kind else {
 776             span_bug!(
 777                 hir_ty.span,
 778                 "lowered return type of async fn is not OpaqueDef: {:?}",
 779                 hir_ty
 780             );
 781         };
 782         let opaque_ty = hir.item(id);
 783         if let hir::ItemKind::OpaqueTy(hir::OpaqueTy {
 784             bounds:
 785                 [
 786                     hir::GenericBound::LangItemTrait(
 787                         hir::LangItem::Future,
 788                         _,
 789                         _,
 790                         hir::GenericArgs {
 791                             bindings:
 792                                 [
 793                                     hir::TypeBinding {
 794                                         ident: Ident { name: sym::Output, .. },
 795                                         kind:
 796                                             hir::TypeBindingKind::Equality { term: hir::Term::Ty(ty) },
 797                                         ..
 798                                     },
 799                                 ],
 800                             ..
 801                         },
 802                     ),
 803                 ],
 804             ..
 805         }) = opaque_ty.kind
 806         {
 807             ty
 808         } else {
 809             span_bug!(
 810                 hir_ty.span,
 811                 "bounds from lowered return type of async fn did not match expected format: {:?}",
 812                 opaque_ty
 813             );
 814         }
 815     }
 816
 817     #[instrument(level = "trace", skip(self))]
 818     fn give_name_if_anonymous_region_appears_in_yield_ty(
 819         &self,
 820         fr: RegionVid,
 821     ) -> Option<RegionName> {
 822         // Note: generators from `async fn` yield `()`, so we don't have to
 823         // worry about them here.
 824         let yield_ty = self.regioncx.universal_regions().yield_ty?;
 825         debug!("give_name_if_anonymous_region_appears_in_yield_ty: yield_ty = {:?}", yield_ty);
 826
 827         let tcx = self.infcx.tcx;
 828
 829         if !tcx.any_free_region_meets(&yield_ty, |r| r.to_region_vid() == fr) {
 830             return None;
 831         }
 832
 833         let mut highlight = RegionHighlightMode::new(tcx);
 834         highlight.highlighting_region_vid(fr, *self.next_region_name.try_borrow().unwrap());
 835         let type_name =
 836             self.infcx.extract_inference_diagnostics_data(yield_ty.into(), Some(highlight)).name;
 837
 838         let yield_span = match tcx.hir().get(self.mir_hir_id()) {
 839             hir::Node::Expr(hir::Expr {
 840                 kind: hir::ExprKind::Closure(&hir::Closure { fn_decl_span, .. }),
 841                 ..
 842             }) => tcx.sess.source_map().end_point(fn_decl_span),
 843             _ => self.body.span,
 844         };
 845
 846         debug!(
 847             "give_name_if_anonymous_region_appears_in_yield_ty: \
 848              type_name = {:?}, yield_span = {:?}",
 849             yield_span, type_name,
 850         );
 851
 852         Some(RegionName {
 853             name: self.synthesize_region_name(),
 854             source: RegionNameSource::AnonRegionFromYieldTy(yield_span, type_name),
 855         })
 856     }
 857
 858     fn give_name_if_anonymous_region_appears_in_impl_signature(
 859         &self,
 860         fr: RegionVid,
 861     ) -> Option<RegionName> {
 862         let ty::ReEarlyBound(region) = *self.to_error_region(fr)? else {
 863             return None;
 864         };
 865         if region.has_name() {
 866             return None;
 867         };
 868
 869         let tcx = self.infcx.tcx;
 870         let body_parent_did = tcx.opt_parent(self.mir_def_id().to_def_id())?;
 871         if tcx.parent(region.def_id) != body_parent_did
 872             || tcx.def_kind(body_parent_did) != DefKind::Impl
 873         {
 874             return None;
 875         }
 876
 877         let mut found = false;
 878         tcx.fold_regions(tcx.type_of(body_parent_did), |r: ty::Region<'tcx>, _| {
 879             if *r == ty::ReEarlyBound(region) {
 880                 found = true;
 881             }
 882             r
 883         });
 884
 885         Some(RegionName {
 886             name: self.synthesize_region_name(),
 887             source: RegionNameSource::AnonRegionFromImplSignature(
 888                 tcx.def_span(region.def_id),
 889                 // FIXME(compiler-errors): Does this ever actually show up
 890                 // anywhere other than the self type? I couldn't create an
 891                 // example of a `'_` in the impl's trait being referenceable.
 892                 if found { "self type" } else { "header" },
 893             ),
 894         })
 895     }
 896 }