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(..) | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReErased => None,
 361         }
 362     }
 363
 364     /// Finds an argument that contains `fr` and label it with a fully
 365     /// elaborated type, returning something like `'1`. Result looks
 366     /// like:
 367     ///
 368     /// ```text
 369     ///  | fn foo(x: &u32) { .. }
 370     ///           ------- fully elaborated type of `x` is `&'1 u32`
 371     /// ```
 372     #[instrument(level = "trace", skip(self))]
 373     fn give_name_if_anonymous_region_appears_in_arguments(
 374         &self,
 375         fr: RegionVid,
 376     ) -> Option<RegionName> {
 377         let implicit_inputs = self.regioncx.universal_regions().defining_ty.implicit_inputs();
 378         let argument_index = self.regioncx.get_argument_index_for_region(self.infcx.tcx, fr)?;
 379
 380         let arg_ty = self.regioncx.universal_regions().unnormalized_input_tys
 381             [implicit_inputs + argument_index];
 382         let (_, span) = self.regioncx.get_argument_name_and_span_for_region(
 383             &self.body,
 384             &self.local_names,
 385             argument_index,
 386         );
 387
 388         let highlight = self
 389             .get_argument_hir_ty_for_highlighting(argument_index)
 390             .and_then(|arg_hir_ty| self.highlight_if_we_can_match_hir_ty(fr, arg_ty, arg_hir_ty))
 391             .unwrap_or_else(|| {
 392                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 393                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 394                 // will increment the counter, "reserving" the number we just used.
 395                 let counter = *self.next_region_name.try_borrow().unwrap();
 396                 self.highlight_if_we_cannot_match_hir_ty(fr, arg_ty, span, counter)
 397             });
 398
 399         Some(RegionName {
 400             name: self.synthesize_region_name(),
 401             source: RegionNameSource::AnonRegionFromArgument(highlight),
 402         })
 403     }
 404
 405     fn get_argument_hir_ty_for_highlighting(
 406         &self,
 407         argument_index: usize,
 408     ) -> Option<&hir::Ty<'tcx>> {
 409         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(self.mir_hir_id())?;
 410         let argument_hir_ty: &hir::Ty<'_> = fn_decl.inputs.get(argument_index)?;
 411         match argument_hir_ty.kind {
 412             // This indicates a variable with no type annotation, like
 413             // `|x|`... in that case, we can't highlight the type but
 414             // must highlight the variable.
 415             // NOTE(eddyb) this is handled in/by the sole caller
 416             // (`give_name_if_anonymous_region_appears_in_arguments`).
 417             hir::TyKind::Infer => None,
 418
 419             _ => Some(argument_hir_ty),
 420         }
 421     }
 422
 423     /// Attempts to highlight the specific part of a type in an argument
 424     /// that has no type annotation.
 425     /// For example, we might produce an annotation like this:
 426     ///
 427     /// ```text
 428     ///  |     foo(|a, b| b)
 429     ///  |          -  -
 430     ///  |          |  |
 431     ///  |          |  has type `&'1 u32`
 432     ///  |          has type `&'2 u32`
 433     /// ```
 434     fn highlight_if_we_cannot_match_hir_ty(
 435         &self,
 436         needle_fr: RegionVid,
 437         ty: Ty<'tcx>,
 438         span: Span,
 439         counter: usize,
 440     ) -> RegionNameHighlight {
 441         let mut highlight = RegionHighlightMode::new(self.infcx.tcx);
 442         highlight.highlighting_region_vid(needle_fr, counter);
 443         let type_name =
 444             self.infcx.extract_inference_diagnostics_data(ty.into(), Some(highlight)).name;
 445
 446         debug!(
 447             "highlight_if_we_cannot_match_hir_ty: type_name={:?} needle_fr={:?}",
 448             type_name, needle_fr
 449         );
 450         if type_name.contains(&format!("'{counter}")) {
 451             // Only add a label if we can confirm that a region was labelled.
 452             RegionNameHighlight::CannotMatchHirTy(span, type_name)
 453         } else {
 454             RegionNameHighlight::Occluded(span, type_name)
 455         }
 456     }
 457
 458     /// Attempts to highlight the specific part of a type annotation
 459     /// that contains the anonymous reference we want to give a name
 460     /// to. For example, we might produce an annotation like this:
 461     ///
 462     /// ```text
 463     ///  | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item = &T>> {
 464     ///  |                - let's call the lifetime of this reference `'1`
 465     /// ```
 466     ///
 467     /// the way this works is that we match up `ty`, which is
 468     /// a `Ty<'tcx>` (the internal form of the type) with
 469     /// `hir_ty`, a `hir::Ty` (the syntax of the type
 470     /// annotation). We are descending through the types stepwise,
 471     /// looking in to find the region `needle_fr` in the internal
 472     /// type. Once we find that, we can use the span of the `hir::Ty`
 473     /// to add the highlight.
 474     ///
 475     /// This is a somewhat imperfect process, so along the way we also
 476     /// keep track of the **closest** type we've found. If we fail to
 477     /// find the exact `&` or `'_` to highlight, then we may fall back
 478     /// to highlighting that closest type instead.
 479     fn highlight_if_we_can_match_hir_ty(
 480         &self,
 481         needle_fr: RegionVid,
 482         ty: Ty<'tcx>,
 483         hir_ty: &hir::Ty<'_>,
 484     ) -> Option<RegionNameHighlight> {
 485         let search_stack: &mut Vec<(Ty<'tcx>, &hir::Ty<'_>)> = &mut vec![(ty, hir_ty)];
 486
 487         while let Some((ty, hir_ty)) = search_stack.pop() {
 488             match (ty.kind(), &hir_ty.kind) {
 489                 // Check if the `ty` is `&'X ..` where `'X`
 490                 // is the region we are looking for -- if so, and we have a `&T`
 491                 // on the RHS, then we want to highlight the `&` like so:
 492                 //
 493                 //     &
 494                 //     - let's call the lifetime of this reference `'1`
 495                 (
 496                     ty::Ref(region, referent_ty, _),
 497                     hir::TyKind::Rptr(_lifetime, referent_hir_ty),
 498                 ) => {
 499                     if region.to_region_vid() == needle_fr {
 500                         // Just grab the first character, the `&`.
 501                         let source_map = self.infcx.tcx.sess.source_map();
 502                         let ampersand_span = source_map.start_point(hir_ty.span);
 503
 504                         return Some(RegionNameHighlight::MatchedHirTy(ampersand_span));
 505                     }
 506
 507                     // Otherwise, let's descend into the referent types.
 508                     search_stack.push((*referent_ty, &referent_hir_ty.ty));
 509                 }
 510
 511                 // Match up something like `Foo<'1>`
 512                 (
 513                     ty::Adt(_adt_def, substs),
 514                     hir::TyKind::Path(hir::QPath::Resolved(None, path)),
 515                 ) => {
 516                     match path.res {
 517                         // Type parameters of the type alias have no reason to
 518                         // be the same as those of the ADT.
 519                         // FIXME: We should be able to do something similar to
 520                         // match_adt_and_segment in this case.
 521                         Res::Def(DefKind::TyAlias, _) => (),
 522                         _ => {
 523                             if let Some(last_segment) = path.segments.last() {
 524                                 if let Some(highlight) = self.match_adt_and_segment(
 525                                     substs,
 526                                     needle_fr,
 527                                     last_segment,
 528                                     search_stack,
 529                                 ) {
 530                                     return Some(highlight);
 531                                 }
 532                             }
 533                         }
 534                     }
 535                 }
 536
 537                 // The following cases don't have lifetimes, so we
 538                 // just worry about trying to match up the rustc type
 539                 // with the HIR types:
 540                 (&ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
 541                     search_stack.extend(iter::zip(elem_tys, *elem_hir_tys));
 542                 }
 543
 544                 (ty::Slice(elem_ty), hir::TyKind::Slice(elem_hir_ty))
 545                 | (ty::Array(elem_ty, _), hir::TyKind::Array(elem_hir_ty, _)) => {
 546                     search_stack.push((*elem_ty, elem_hir_ty));
 547                 }
 548
 549                 (ty::RawPtr(mut_ty), hir::TyKind::Ptr(mut_hir_ty)) => {
 550                     search_stack.push((mut_ty.ty, &mut_hir_ty.ty));
 551                 }
 552
 553                 _ => {
 554                     // FIXME there are other cases that we could trace
 555                 }
 556             }
 557         }
 558
 559         None
 560     }
 561
 562     /// We've found an enum/struct/union type with the substitutions
 563     /// `substs` and -- in the HIR -- a path type with the final
 564     /// segment `last_segment`. Try to find a `'_` to highlight in
 565     /// the generic args (or, if not, to produce new zipped pairs of
 566     /// types+hir to search through).
 567     fn match_adt_and_segment<'hir>(
 568         &self,
 569         substs: SubstsRef<'tcx>,
 570         needle_fr: RegionVid,
 571         last_segment: &'hir hir::PathSegment<'hir>,
 572         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 573     ) -> Option<RegionNameHighlight> {
 574         // Did the user give explicit arguments? (e.g., `Foo<..>`)
 575         let args = last_segment.args.as_ref()?;
 576         let lifetime =
 577             self.try_match_adt_and_generic_args(substs, needle_fr, args, search_stack)?;
 578         match lifetime.name {
 579             hir::LifetimeName::Param(_, hir::ParamName::Plain(_) | hir::ParamName::Error)
 580             | hir::LifetimeName::Error
 581             | hir::LifetimeName::Static => {
 582                 let lifetime_span = lifetime.span;
 583                 Some(RegionNameHighlight::MatchedAdtAndSegment(lifetime_span))
 584             }
 585
 586             hir::LifetimeName::Param(_, hir::ParamName::Fresh)
 587             | hir::LifetimeName::ImplicitObjectLifetimeDefault
 588             | hir::LifetimeName::Infer => {
 589                 // In this case, the user left off the lifetime; so
 590                 // they wrote something like:
 591                 //
 592                 // ```
 593                 // x: Foo<T>
 594                 // ```
 595                 //
 596                 // where the fully elaborated form is `Foo<'_, '1,
 597                 // T>`. We don't consider this a match; instead we let
 598                 // the "fully elaborated" type fallback above handle
 599                 // it.
 600                 None
 601             }
 602         }
 603     }
 604
 605     /// We've found an enum/struct/union type with the substitutions
 606     /// `substs` and -- in the HIR -- a path with the generic
 607     /// arguments `args`. If `needle_fr` appears in the args, return
 608     /// the `hir::Lifetime` that corresponds to it. If not, push onto
 609     /// `search_stack` the types+hir to search through.
 610     fn try_match_adt_and_generic_args<'hir>(
 611         &self,
 612         substs: SubstsRef<'tcx>,
 613         needle_fr: RegionVid,
 614         args: &'hir hir::GenericArgs<'hir>,
 615         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 616     ) -> Option<&'hir hir::Lifetime> {
 617         for (kind, hir_arg) in iter::zip(substs, args.args) {
 618             match (kind.unpack(), hir_arg) {
 619                 (GenericArgKind::Lifetime(r), hir::GenericArg::Lifetime(lt)) => {
 620                     if r.to_region_vid() == needle_fr {
 621                         return Some(lt);
 622                     }
 623                 }
 624
 625                 (GenericArgKind::Type(ty), hir::GenericArg::Type(hir_ty)) => {
 626                     search_stack.push((ty, hir_ty));
 627                 }
 628
 629                 (GenericArgKind::Const(_ct), hir::GenericArg::Const(_hir_ct)) => {
 630                     // Lifetimes cannot be found in consts, so we don't need
 631                     // to search anything here.
 632                 }
 633
 634                 (
 635                     GenericArgKind::Lifetime(_)
 636                     | GenericArgKind::Type(_)
 637                     | GenericArgKind::Const(_),
 638                     _,
 639                 ) => {
 640                     // HIR lowering sometimes doesn't catch this in erroneous
 641                     // programs, so we need to use delay_span_bug here. See #82126.
 642                     self.infcx.tcx.sess.delay_span_bug(
 643                         hir_arg.span(),
 644                         &format!("unmatched subst and hir arg: found {:?} vs {:?}", kind, hir_arg),
 645                     );
 646                 }
 647             }
 648         }
 649
 650         None
 651     }
 652
 653     /// Finds a closure upvar that contains `fr` and label it with a
 654     /// fully elaborated type, returning something like `'1`. Result
 655     /// looks like:
 656     ///
 657     /// ```text
 658     ///  | let x = Some(&22);
 659     ///        - fully elaborated type of `x` is `Option<&'1 u32>`
 660     /// ```
 661     #[instrument(level = "trace", skip(self))]
 662     fn give_name_if_anonymous_region_appears_in_upvars(&self, fr: RegionVid) -> Option<RegionName> {
 663         let upvar_index = self.regioncx.get_upvar_index_for_region(self.infcx.tcx, fr)?;
 664         let (upvar_name, upvar_span) = self.regioncx.get_upvar_name_and_span_for_region(
 665             self.infcx.tcx,
 666             &self.upvars,
 667             upvar_index,
 668         );
 669         let region_name = self.synthesize_region_name();
 670
 671         Some(RegionName {
 672             name: region_name,
 673             source: RegionNameSource::AnonRegionFromUpvar(upvar_span, upvar_name),
 674         })
 675     }
 676
 677     /// Checks for arguments appearing in the (closure) return type. It
 678     /// must be a closure since, in a free fn, such an argument would
 679     /// have to either also appear in an argument (if using elision)
 680     /// or be early bound (named, not in argument).
 681     #[instrument(level = "trace", skip(self))]
 682     fn give_name_if_anonymous_region_appears_in_output(&self, fr: RegionVid) -> Option<RegionName> {
 683         let tcx = self.infcx.tcx;
 684         let hir = tcx.hir();
 685
 686         let return_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 687         debug!("give_name_if_anonymous_region_appears_in_output: return_ty = {:?}", return_ty);
 688         if !tcx.any_free_region_meets(&return_ty, |r| r.to_region_vid() == fr) {
 689             return None;
 690         }
 691
 692         let mir_hir_id = self.mir_hir_id();
 693
 694         let (return_span, mir_description, hir_ty) = match hir.get(mir_hir_id) {
 695             hir::Node::Expr(hir::Expr {
 696                 kind: hir::ExprKind::Closure(&hir::Closure { fn_decl, body, fn_decl_span, .. }),
 697                 ..
 698             }) => {
 699                 let (mut span, mut hir_ty) = match fn_decl.output {
 700                     hir::FnRetTy::DefaultReturn(_) => {
 701                         (tcx.sess.source_map().end_point(fn_decl_span), None)
 702                     }
 703                     hir::FnRetTy::Return(hir_ty) => (fn_decl.output.span(), Some(hir_ty)),
 704                 };
 705                 let mir_description = match hir.body(body).generator_kind {
 706                     Some(hir::GeneratorKind::Async(gen)) => match gen {
 707                         hir::AsyncGeneratorKind::Block => " of async block",
 708                         hir::AsyncGeneratorKind::Closure => " of async closure",
 709                         hir::AsyncGeneratorKind::Fn => {
 710                             let parent_item = hir.get_by_def_id(hir.get_parent_item(mir_hir_id));
 711                             let output = &parent_item
 712                                 .fn_decl()
 713                                 .expect("generator lowered from async fn should be in fn")
 714                                 .output;
 715                             span = output.span();
 716                             if let hir::FnRetTy::Return(ret) = output {
 717                                 hir_ty = Some(self.get_future_inner_return_ty(*ret));
 718                             }
 719                             " of async function"
 720                         }
 721                     },
 722                     Some(hir::GeneratorKind::Gen) => " of generator",
 723                     None => " of closure",
 724                 };
 725                 (span, mir_description, hir_ty)
 726             }
 727             node => match node.fn_decl() {
 728                 Some(fn_decl) => {
 729                     let hir_ty = match fn_decl.output {
 730                         hir::FnRetTy::DefaultReturn(_) => None,
 731                         hir::FnRetTy::Return(ty) => Some(ty),
 732                     };
 733                     (fn_decl.output.span(), "", hir_ty)
 734                 }
 735                 None => (self.body.span, "", None),
 736             },
 737         };
 738
 739         let highlight = hir_ty
 740             .and_then(|hir_ty| self.highlight_if_we_can_match_hir_ty(fr, return_ty, hir_ty))
 741             .unwrap_or_else(|| {
 742                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 743                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 744                 // will increment the counter, "reserving" the number we just used.
 745                 let counter = *self.next_region_name.try_borrow().unwrap();
 746                 self.highlight_if_we_cannot_match_hir_ty(fr, return_ty, return_span, counter)
 747             });
 748
 749         Some(RegionName {
 750             name: self.synthesize_region_name(),
 751             source: RegionNameSource::AnonRegionFromOutput(highlight, mir_description),
 752         })
 753     }
 754
 755     /// From the [`hir::Ty`] of an async function's lowered return type,
 756     /// retrieve the `hir::Ty` representing the type the user originally wrote.
 757     ///
 758     /// e.g. given the function:
 759     ///
 760     /// ```
 761     /// async fn foo() -> i32 { 2 }
 762     /// ```
 763     ///
 764     /// this function, given the lowered return type of `foo`, an [`OpaqueDef`] that implements `Future<Output=i32>`,
 765     /// returns the `i32`.
 766     ///
 767     /// [`OpaqueDef`]: hir::TyKind::OpaqueDef
 768     fn get_future_inner_return_ty(&self, hir_ty: &'tcx hir::Ty<'tcx>) -> &'tcx hir::Ty<'tcx> {
 769         let hir = self.infcx.tcx.hir();
 770
 771         let hir::TyKind::OpaqueDef(id, _, _) = hir_ty.kind else {
 772             span_bug!(
 773                 hir_ty.span,
 774                 "lowered return type of async fn is not OpaqueDef: {:?}",
 775                 hir_ty
 776             );
 777         };
 778         let opaque_ty = hir.item(id);
 779         if let hir::ItemKind::OpaqueTy(hir::OpaqueTy {
 780             bounds:
 781                 [
 782                     hir::GenericBound::LangItemTrait(
 783                         hir::LangItem::Future,
 784                         _,
 785                         _,
 786                         hir::GenericArgs {
 787                             bindings:
 788                                 [
 789                                     hir::TypeBinding {
 790                                         ident: Ident { name: sym::Output, .. },
 791                                         kind:
 792                                             hir::TypeBindingKind::Equality { term: hir::Term::Ty(ty) },
 793                                         ..
 794                                     },
 795                                 ],
 796                             ..
 797                         },
 798                     ),
 799                 ],
 800             ..
 801         }) = opaque_ty.kind
 802         {
 803             ty
 804         } else {
 805             span_bug!(
 806                 hir_ty.span,
 807                 "bounds from lowered return type of async fn did not match expected format: {:?}",
 808                 opaque_ty
 809             );
 810         }
 811     }
 812
 813     #[instrument(level = "trace", skip(self))]
 814     fn give_name_if_anonymous_region_appears_in_yield_ty(
 815         &self,
 816         fr: RegionVid,
 817     ) -> Option<RegionName> {
 818         // Note: generators from `async fn` yield `()`, so we don't have to
 819         // worry about them here.
 820         let yield_ty = self.regioncx.universal_regions().yield_ty?;
 821         debug!("give_name_if_anonymous_region_appears_in_yield_ty: yield_ty = {:?}", yield_ty);
 822
 823         let tcx = self.infcx.tcx;
 824
 825         if !tcx.any_free_region_meets(&yield_ty, |r| r.to_region_vid() == fr) {
 826             return None;
 827         }
 828
 829         let mut highlight = RegionHighlightMode::new(tcx);
 830         highlight.highlighting_region_vid(fr, *self.next_region_name.try_borrow().unwrap());
 831         let type_name =
 832             self.infcx.extract_inference_diagnostics_data(yield_ty.into(), Some(highlight)).name;
 833
 834         let yield_span = match tcx.hir().get(self.mir_hir_id()) {
 835             hir::Node::Expr(hir::Expr {
 836                 kind: hir::ExprKind::Closure(&hir::Closure { fn_decl_span, .. }),
 837                 ..
 838             }) => tcx.sess.source_map().end_point(fn_decl_span),
 839             _ => self.body.span,
 840         };
 841
 842         debug!(
 843             "give_name_if_anonymous_region_appears_in_yield_ty: \
 844              type_name = {:?}, yield_span = {:?}",
 845             yield_span, type_name,
 846         );
 847
 848         Some(RegionName {
 849             name: self.synthesize_region_name(),
 850             source: RegionNameSource::AnonRegionFromYieldTy(yield_span, type_name),
 851         })
 852     }
 853
 854     fn give_name_if_anonymous_region_appears_in_impl_signature(
 855         &self,
 856         fr: RegionVid,
 857     ) -> Option<RegionName> {
 858         let ty::ReEarlyBound(region) = *self.to_error_region(fr)? else {
 859             return None;
 860         };
 861         if region.has_name() {
 862             return None;
 863         };
 864
 865         let tcx = self.infcx.tcx;
 866         let body_parent_did = tcx.opt_parent(self.mir_def_id().to_def_id())?;
 867         if tcx.parent(region.def_id) != body_parent_did
 868             || tcx.def_kind(body_parent_did) != DefKind::Impl
 869         {
 870             return None;
 871         }
 872
 873         let mut found = false;
 874         tcx.fold_regions(tcx.type_of(body_parent_did), |r: ty::Region<'tcx>, _| {
 875             if *r == ty::ReEarlyBound(region) {
 876                 found = true;
 877             }
 878             r
 879         });
 880
 881         Some(RegionName {
 882             name: self.synthesize_region_name(),
 883             source: RegionNameSource::AnonRegionFromImplSignature(
 884                 tcx.def_span(region.def_id),
 885                 // FIXME(compiler-errors): Does this ever actually show up
 886                 // anywhere other than the self type? I couldn't create an
 887                 // example of a `'_` in the impl's trait being referenceable.
 888                 if found { "self type" } else { "header" },
 889             ),
 890         })
 891     }
 892 }