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             .or_else(|| self.give_name_if_anonymous_region_appears_in_arg_position_impl_trait(fr));
 256
 257         if let Some(ref value) = value {
 258             self.region_names.try_borrow_mut().unwrap().insert(fr, value.clone());
 259         }
 260
 261         debug!("give_region_a_name: gave name {:?}", value);
 262         value
 263     }
 264
 265     /// Checks for the case where `fr` maps to something that the
 266     /// *user* has a name for. In that case, we'll be able to map
 267     /// `fr` to a `Region<'tcx>`, and that region will be one of
 268     /// named variants.
 269     #[instrument(level = "trace", skip(self))]
 270     fn give_name_from_error_region(&self, fr: RegionVid) -> Option<RegionName> {
 271         let error_region = self.to_error_region(fr)?;
 272
 273         let tcx = self.infcx.tcx;
 274
 275         debug!("give_region_a_name: error_region = {:?}", error_region);
 276         match *error_region {
 277             ty::ReEarlyBound(ebr) => {
 278                 if ebr.has_name() {
 279                     let span = tcx.hir().span_if_local(ebr.def_id).unwrap_or(DUMMY_SP);
 280                     Some(RegionName {
 281                         name: ebr.name,
 282                         source: RegionNameSource::NamedEarlyBoundRegion(span),
 283                     })
 284                 } else {
 285                     None
 286                 }
 287             }
 288
 289             ty::ReStatic => {
 290                 Some(RegionName { name: kw::StaticLifetime, source: RegionNameSource::Static })
 291             }
 292
 293             ty::ReFree(free_region) => match free_region.bound_region {
 294                 ty::BoundRegionKind::BrNamed(region_def_id, name) => {
 295                     // Get the span to point to, even if we don't use the name.
 296                     let span = tcx.hir().span_if_local(region_def_id).unwrap_or(DUMMY_SP);
 297                     debug!(
 298                         "bound region named: {:?}, is_named: {:?}",
 299                         name,
 300                         free_region.bound_region.is_named()
 301                     );
 302
 303                     if free_region.bound_region.is_named() {
 304                         // A named region that is actually named.
 305                         Some(RegionName { name, source: RegionNameSource::NamedFreeRegion(span) })
 306                     } else if let hir::IsAsync::Async = tcx.asyncness(self.mir_hir_id().owner) {
 307                         // If we spuriously thought that the region is named, we should let the
 308                         // system generate a true name for error messages. Currently this can
 309                         // happen if we have an elided name in an async fn for example: the
 310                         // compiler will generate a region named `'_`, but reporting such a name is
 311                         // not actually useful, so we synthesize a name for it instead.
 312                         let name = self.synthesize_region_name();
 313                         Some(RegionName {
 314                             name,
 315                             source: RegionNameSource::AnonRegionFromAsyncFn(span),
 316                         })
 317                     } else {
 318                         None
 319                     }
 320                 }
 321
 322                 ty::BoundRegionKind::BrEnv => {
 323                     let def_ty = self.regioncx.universal_regions().defining_ty;
 324
 325                     let DefiningTy::Closure(_, substs) = def_ty else {
 326                         // Can't have BrEnv in functions, constants or generators.
 327                         bug!("BrEnv outside of closure.");
 328                     };
 329                     let hir::ExprKind::Closure(&hir::Closure { fn_decl_span, .. })
 330                         = tcx.hir().expect_expr(self.mir_hir_id()).kind
 331                     else {
 332                         bug!("Closure is not defined by a closure expr");
 333                     };
 334                     let region_name = self.synthesize_region_name();
 335
 336                     let closure_kind_ty = substs.as_closure().kind_ty();
 337                     let note = match closure_kind_ty.to_opt_closure_kind() {
 338                         Some(ty::ClosureKind::Fn) => {
 339                             "closure implements `Fn`, so references to captured variables \
 340                                 can't escape the closure"
 341                         }
 342                         Some(ty::ClosureKind::FnMut) => {
 343                             "closure implements `FnMut`, so references to captured variables \
 344                                 can't escape the closure"
 345                         }
 346                         Some(ty::ClosureKind::FnOnce) => {
 347                             bug!("BrEnv in a `FnOnce` closure");
 348                         }
 349                         None => bug!("Closure kind not inferred in borrow check"),
 350                     };
 351
 352                     Some(RegionName {
 353                         name: region_name,
 354                         source: RegionNameSource::SynthesizedFreeEnvRegion(fn_decl_span, note),
 355                     })
 356                 }
 357
 358                 ty::BoundRegionKind::BrAnon(_) => None,
 359             },
 360
 361             ty::ReLateBound(..) | ty::ReVar(..) | ty::RePlaceholder(..) | ty::ReErased => None,
 362         }
 363     }
 364
 365     /// Finds an argument that contains `fr` and label it with a fully
 366     /// elaborated type, returning something like `'1`. Result looks
 367     /// like:
 368     ///
 369     /// ```text
 370     ///  | fn foo(x: &u32) { .. }
 371     ///           ------- fully elaborated type of `x` is `&'1 u32`
 372     /// ```
 373     #[instrument(level = "trace", skip(self))]
 374     fn give_name_if_anonymous_region_appears_in_arguments(
 375         &self,
 376         fr: RegionVid,
 377     ) -> Option<RegionName> {
 378         let implicit_inputs = self.regioncx.universal_regions().defining_ty.implicit_inputs();
 379         let argument_index = self.regioncx.get_argument_index_for_region(self.infcx.tcx, fr)?;
 380
 381         let arg_ty = self.regioncx.universal_regions().unnormalized_input_tys
 382             [implicit_inputs + argument_index];
 383         let (_, span) = self.regioncx.get_argument_name_and_span_for_region(
 384             &self.body,
 385             &self.local_names,
 386             argument_index,
 387         );
 388
 389         let highlight = self
 390             .get_argument_hir_ty_for_highlighting(argument_index)
 391             .and_then(|arg_hir_ty| self.highlight_if_we_can_match_hir_ty(fr, arg_ty, arg_hir_ty))
 392             .unwrap_or_else(|| {
 393                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 394                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 395                 // will increment the counter, "reserving" the number we just used.
 396                 let counter = *self.next_region_name.try_borrow().unwrap();
 397                 self.highlight_if_we_cannot_match_hir_ty(fr, arg_ty, span, counter)
 398             });
 399
 400         Some(RegionName {
 401             name: self.synthesize_region_name(),
 402             source: RegionNameSource::AnonRegionFromArgument(highlight),
 403         })
 404     }
 405
 406     fn get_argument_hir_ty_for_highlighting(
 407         &self,
 408         argument_index: usize,
 409     ) -> Option<&hir::Ty<'tcx>> {
 410         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(self.mir_hir_id())?;
 411         let argument_hir_ty: &hir::Ty<'_> = fn_decl.inputs.get(argument_index)?;
 412         match argument_hir_ty.kind {
 413             // This indicates a variable with no type annotation, like
 414             // `|x|`... in that case, we can't highlight the type but
 415             // must highlight the variable.
 416             // NOTE(eddyb) this is handled in/by the sole caller
 417             // (`give_name_if_anonymous_region_appears_in_arguments`).
 418             hir::TyKind::Infer => None,
 419
 420             _ => Some(argument_hir_ty),
 421         }
 422     }
 423
 424     /// Attempts to highlight the specific part of a type in an argument
 425     /// that has no type annotation.
 426     /// For example, we might produce an annotation like this:
 427     ///
 428     /// ```text
 429     ///  |     foo(|a, b| b)
 430     ///  |          -  -
 431     ///  |          |  |
 432     ///  |          |  has type `&'1 u32`
 433     ///  |          has type `&'2 u32`
 434     /// ```
 435     fn highlight_if_we_cannot_match_hir_ty(
 436         &self,
 437         needle_fr: RegionVid,
 438         ty: Ty<'tcx>,
 439         span: Span,
 440         counter: usize,
 441     ) -> RegionNameHighlight {
 442         let mut highlight = RegionHighlightMode::new(self.infcx.tcx);
 443         highlight.highlighting_region_vid(needle_fr, counter);
 444         let type_name =
 445             self.infcx.extract_inference_diagnostics_data(ty.into(), Some(highlight)).name;
 446
 447         debug!(
 448             "highlight_if_we_cannot_match_hir_ty: type_name={:?} needle_fr={:?}",
 449             type_name, needle_fr
 450         );
 451         if type_name.contains(&format!("'{counter}")) {
 452             // Only add a label if we can confirm that a region was labelled.
 453             RegionNameHighlight::CannotMatchHirTy(span, type_name)
 454         } else {
 455             RegionNameHighlight::Occluded(span, type_name)
 456         }
 457     }
 458
 459     /// Attempts to highlight the specific part of a type annotation
 460     /// that contains the anonymous reference we want to give a name
 461     /// to. For example, we might produce an annotation like this:
 462     ///
 463     /// ```text
 464     ///  | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item = &T>> {
 465     ///  |                - let's call the lifetime of this reference `'1`
 466     /// ```
 467     ///
 468     /// the way this works is that we match up `ty`, which is
 469     /// a `Ty<'tcx>` (the internal form of the type) with
 470     /// `hir_ty`, a `hir::Ty` (the syntax of the type
 471     /// annotation). We are descending through the types stepwise,
 472     /// looking in to find the region `needle_fr` in the internal
 473     /// type. Once we find that, we can use the span of the `hir::Ty`
 474     /// to add the highlight.
 475     ///
 476     /// This is a somewhat imperfect process, so along the way we also
 477     /// keep track of the **closest** type we've found. If we fail to
 478     /// find the exact `&` or `'_` to highlight, then we may fall back
 479     /// to highlighting that closest type instead.
 480     fn highlight_if_we_can_match_hir_ty(
 481         &self,
 482         needle_fr: RegionVid,
 483         ty: Ty<'tcx>,
 484         hir_ty: &hir::Ty<'_>,
 485     ) -> Option<RegionNameHighlight> {
 486         let search_stack: &mut Vec<(Ty<'tcx>, &hir::Ty<'_>)> = &mut vec![(ty, hir_ty)];
 487
 488         while let Some((ty, hir_ty)) = search_stack.pop() {
 489             match (ty.kind(), &hir_ty.kind) {
 490                 // Check if the `ty` is `&'X ..` where `'X`
 491                 // is the region we are looking for -- if so, and we have a `&T`
 492                 // on the RHS, then we want to highlight the `&` like so:
 493                 //
 494                 //     &
 495                 //     - let's call the lifetime of this reference `'1`
 496                 (
 497                     ty::Ref(region, referent_ty, _),
 498                     hir::TyKind::Rptr(_lifetime, referent_hir_ty),
 499                 ) => {
 500                     if region.to_region_vid() == needle_fr {
 501                         // Just grab the first character, the `&`.
 502                         let source_map = self.infcx.tcx.sess.source_map();
 503                         let ampersand_span = source_map.start_point(hir_ty.span);
 504
 505                         return Some(RegionNameHighlight::MatchedHirTy(ampersand_span));
 506                     }
 507
 508                     // Otherwise, let's descend into the referent types.
 509                     search_stack.push((*referent_ty, &referent_hir_ty.ty));
 510                 }
 511
 512                 // Match up something like `Foo<'1>`
 513                 (
 514                     ty::Adt(_adt_def, substs),
 515                     hir::TyKind::Path(hir::QPath::Resolved(None, path)),
 516                 ) => {
 517                     match path.res {
 518                         // Type parameters of the type alias have no reason to
 519                         // be the same as those of the ADT.
 520                         // FIXME: We should be able to do something similar to
 521                         // match_adt_and_segment in this case.
 522                         Res::Def(DefKind::TyAlias, _) => (),
 523                         _ => {
 524                             if let Some(last_segment) = path.segments.last() {
 525                                 if let Some(highlight) = self.match_adt_and_segment(
 526                                     substs,
 527                                     needle_fr,
 528                                     last_segment,
 529                                     search_stack,
 530                                 ) {
 531                                     return Some(highlight);
 532                                 }
 533                             }
 534                         }
 535                     }
 536                 }
 537
 538                 // The following cases don't have lifetimes, so we
 539                 // just worry about trying to match up the rustc type
 540                 // with the HIR types:
 541                 (&ty::Tuple(elem_tys), hir::TyKind::Tup(elem_hir_tys)) => {
 542                     search_stack.extend(iter::zip(elem_tys, *elem_hir_tys));
 543                 }
 544
 545                 (ty::Slice(elem_ty), hir::TyKind::Slice(elem_hir_ty))
 546                 | (ty::Array(elem_ty, _), hir::TyKind::Array(elem_hir_ty, _)) => {
 547                     search_stack.push((*elem_ty, elem_hir_ty));
 548                 }
 549
 550                 (ty::RawPtr(mut_ty), hir::TyKind::Ptr(mut_hir_ty)) => {
 551                     search_stack.push((mut_ty.ty, &mut_hir_ty.ty));
 552                 }
 553
 554                 _ => {
 555                     // FIXME there are other cases that we could trace
 556                 }
 557             }
 558         }
 559
 560         None
 561     }
 562
 563     /// We've found an enum/struct/union type with the substitutions
 564     /// `substs` and -- in the HIR -- a path type with the final
 565     /// segment `last_segment`. Try to find a `'_` to highlight in
 566     /// the generic args (or, if not, to produce new zipped pairs of
 567     /// types+hir to search through).
 568     fn match_adt_and_segment<'hir>(
 569         &self,
 570         substs: SubstsRef<'tcx>,
 571         needle_fr: RegionVid,
 572         last_segment: &'hir hir::PathSegment<'hir>,
 573         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 574     ) -> Option<RegionNameHighlight> {
 575         // Did the user give explicit arguments? (e.g., `Foo<..>`)
 576         let args = last_segment.args.as_ref()?;
 577         let lifetime =
 578             self.try_match_adt_and_generic_args(substs, needle_fr, args, search_stack)?;
 579         match lifetime.name {
 580             hir::LifetimeName::Param(_, hir::ParamName::Plain(_) | hir::ParamName::Error)
 581             | hir::LifetimeName::Error
 582             | hir::LifetimeName::Static => {
 583                 let lifetime_span = lifetime.span;
 584                 Some(RegionNameHighlight::MatchedAdtAndSegment(lifetime_span))
 585             }
 586
 587             hir::LifetimeName::Param(_, hir::ParamName::Fresh)
 588             | hir::LifetimeName::ImplicitObjectLifetimeDefault
 589             | hir::LifetimeName::Infer => {
 590                 // In this case, the user left off the lifetime; so
 591                 // they wrote something like:
 592                 //
 593                 // ```
 594                 // x: Foo<T>
 595                 // ```
 596                 //
 597                 // where the fully elaborated form is `Foo<'_, '1,
 598                 // T>`. We don't consider this a match; instead we let
 599                 // the "fully elaborated" type fallback above handle
 600                 // it.
 601                 None
 602             }
 603         }
 604     }
 605
 606     /// We've found an enum/struct/union type with the substitutions
 607     /// `substs` and -- in the HIR -- a path with the generic
 608     /// arguments `args`. If `needle_fr` appears in the args, return
 609     /// the `hir::Lifetime` that corresponds to it. If not, push onto
 610     /// `search_stack` the types+hir to search through.
 611     fn try_match_adt_and_generic_args<'hir>(
 612         &self,
 613         substs: SubstsRef<'tcx>,
 614         needle_fr: RegionVid,
 615         args: &'hir hir::GenericArgs<'hir>,
 616         search_stack: &mut Vec<(Ty<'tcx>, &'hir hir::Ty<'hir>)>,
 617     ) -> Option<&'hir hir::Lifetime> {
 618         for (kind, hir_arg) in iter::zip(substs, args.args) {
 619             match (kind.unpack(), hir_arg) {
 620                 (GenericArgKind::Lifetime(r), hir::GenericArg::Lifetime(lt)) => {
 621                     if r.to_region_vid() == needle_fr {
 622                         return Some(lt);
 623                     }
 624                 }
 625
 626                 (GenericArgKind::Type(ty), hir::GenericArg::Type(hir_ty)) => {
 627                     search_stack.push((ty, hir_ty));
 628                 }
 629
 630                 (GenericArgKind::Const(_ct), hir::GenericArg::Const(_hir_ct)) => {
 631                     // Lifetimes cannot be found in consts, so we don't need
 632                     // to search anything here.
 633                 }
 634
 635                 (
 636                     GenericArgKind::Lifetime(_)
 637                     | GenericArgKind::Type(_)
 638                     | GenericArgKind::Const(_),
 639                     _,
 640                 ) => {
 641                     // HIR lowering sometimes doesn't catch this in erroneous
 642                     // programs, so we need to use delay_span_bug here. See #82126.
 643                     self.infcx.tcx.sess.delay_span_bug(
 644                         hir_arg.span(),
 645                         &format!("unmatched subst and hir arg: found {:?} vs {:?}", kind, hir_arg),
 646                     );
 647                 }
 648             }
 649         }
 650
 651         None
 652     }
 653
 654     /// Finds a closure upvar that contains `fr` and label it with a
 655     /// fully elaborated type, returning something like `'1`. Result
 656     /// looks like:
 657     ///
 658     /// ```text
 659     ///  | let x = Some(&22);
 660     ///        - fully elaborated type of `x` is `Option<&'1 u32>`
 661     /// ```
 662     #[instrument(level = "trace", skip(self))]
 663     fn give_name_if_anonymous_region_appears_in_upvars(&self, fr: RegionVid) -> Option<RegionName> {
 664         let upvar_index = self.regioncx.get_upvar_index_for_region(self.infcx.tcx, fr)?;
 665         let (upvar_name, upvar_span) = self.regioncx.get_upvar_name_and_span_for_region(
 666             self.infcx.tcx,
 667             &self.upvars,
 668             upvar_index,
 669         );
 670         let region_name = self.synthesize_region_name();
 671
 672         Some(RegionName {
 673             name: region_name,
 674             source: RegionNameSource::AnonRegionFromUpvar(upvar_span, upvar_name),
 675         })
 676     }
 677
 678     /// Checks for arguments appearing in the (closure) return type. It
 679     /// must be a closure since, in a free fn, such an argument would
 680     /// have to either also appear in an argument (if using elision)
 681     /// or be early bound (named, not in argument).
 682     #[instrument(level = "trace", skip(self))]
 683     fn give_name_if_anonymous_region_appears_in_output(&self, fr: RegionVid) -> Option<RegionName> {
 684         let tcx = self.infcx.tcx;
 685         let hir = tcx.hir();
 686
 687         let return_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 688         debug!("give_name_if_anonymous_region_appears_in_output: return_ty = {:?}", return_ty);
 689         if !tcx.any_free_region_meets(&return_ty, |r| r.to_region_vid() == fr) {
 690             return None;
 691         }
 692
 693         let mir_hir_id = self.mir_hir_id();
 694
 695         let (return_span, mir_description, hir_ty) = match hir.get(mir_hir_id) {
 696             hir::Node::Expr(hir::Expr {
 697                 kind: hir::ExprKind::Closure(&hir::Closure { fn_decl, body, fn_decl_span, .. }),
 698                 ..
 699             }) => {
 700                 let (mut span, mut hir_ty) = match fn_decl.output {
 701                     hir::FnRetTy::DefaultReturn(_) => {
 702                         (tcx.sess.source_map().end_point(fn_decl_span), None)
 703                     }
 704                     hir::FnRetTy::Return(hir_ty) => (fn_decl.output.span(), Some(hir_ty)),
 705                 };
 706                 let mir_description = match hir.body(body).generator_kind {
 707                     Some(hir::GeneratorKind::Async(gen)) => match gen {
 708                         hir::AsyncGeneratorKind::Block => " of async block",
 709                         hir::AsyncGeneratorKind::Closure => " of async closure",
 710                         hir::AsyncGeneratorKind::Fn => {
 711                             let parent_item =
 712                                 hir.get_by_def_id(hir.get_parent_item(mir_hir_id).def_id);
 713                             let output = &parent_item
 714                                 .fn_decl()
 715                                 .expect("generator lowered from async fn should be in fn")
 716                                 .output;
 717                             span = output.span();
 718                             if let hir::FnRetTy::Return(ret) = output {
 719                                 hir_ty = Some(self.get_future_inner_return_ty(*ret));
 720                             }
 721                             " of async function"
 722                         }
 723                     },
 724                     Some(hir::GeneratorKind::Gen) => " of generator",
 725                     None => " of closure",
 726                 };
 727                 (span, mir_description, hir_ty)
 728             }
 729             node => match node.fn_decl() {
 730                 Some(fn_decl) => {
 731                     let hir_ty = match fn_decl.output {
 732                         hir::FnRetTy::DefaultReturn(_) => None,
 733                         hir::FnRetTy::Return(ty) => Some(ty),
 734                     };
 735                     (fn_decl.output.span(), "", hir_ty)
 736                 }
 737                 None => (self.body.span, "", None),
 738             },
 739         };
 740
 741         let highlight = hir_ty
 742             .and_then(|hir_ty| self.highlight_if_we_can_match_hir_ty(fr, return_ty, hir_ty))
 743             .unwrap_or_else(|| {
 744                 // `highlight_if_we_cannot_match_hir_ty` needs to know the number we will give to
 745                 // the anonymous region. If it succeeds, the `synthesize_region_name` call below
 746                 // will increment the counter, "reserving" the number we just used.
 747                 let counter = *self.next_region_name.try_borrow().unwrap();
 748                 self.highlight_if_we_cannot_match_hir_ty(fr, return_ty, return_span, counter)
 749             });
 750
 751         Some(RegionName {
 752             name: self.synthesize_region_name(),
 753             source: RegionNameSource::AnonRegionFromOutput(highlight, mir_description),
 754         })
 755     }
 756
 757     /// From the [`hir::Ty`] of an async function's lowered return type,
 758     /// retrieve the `hir::Ty` representing the type the user originally wrote.
 759     ///
 760     /// e.g. given the function:
 761     ///
 762     /// ```
 763     /// async fn foo() -> i32 { 2 }
 764     /// ```
 765     ///
 766     /// this function, given the lowered return type of `foo`, an [`OpaqueDef`] that implements `Future<Output=i32>`,
 767     /// returns the `i32`.
 768     ///
 769     /// [`OpaqueDef`]: hir::TyKind::OpaqueDef
 770     fn get_future_inner_return_ty(&self, hir_ty: &'tcx hir::Ty<'tcx>) -> &'tcx hir::Ty<'tcx> {
 771         let hir = self.infcx.tcx.hir();
 772
 773         let hir::TyKind::OpaqueDef(id, _, _) = hir_ty.kind else {
 774             span_bug!(
 775                 hir_ty.span,
 776                 "lowered return type of async fn is not OpaqueDef: {:?}",
 777                 hir_ty
 778             );
 779         };
 780         let opaque_ty = hir.item(id);
 781         if let hir::ItemKind::OpaqueTy(hir::OpaqueTy {
 782             bounds:
 783                 [
 784                     hir::GenericBound::LangItemTrait(
 785                         hir::LangItem::Future,
 786                         _,
 787                         _,
 788                         hir::GenericArgs {
 789                             bindings:
 790                                 [
 791                                     hir::TypeBinding {
 792                                         ident: Ident { name: sym::Output, .. },
 793                                         kind:
 794                                             hir::TypeBindingKind::Equality { term: hir::Term::Ty(ty) },
 795                                         ..
 796                                     },
 797                                 ],
 798                             ..
 799                         },
 800                     ),
 801                 ],
 802             ..
 803         }) = opaque_ty.kind
 804         {
 805             ty
 806         } else {
 807             span_bug!(
 808                 hir_ty.span,
 809                 "bounds from lowered return type of async fn did not match expected format: {:?}",
 810                 opaque_ty
 811             );
 812         }
 813     }
 814
 815     #[instrument(level = "trace", skip(self))]
 816     fn give_name_if_anonymous_region_appears_in_yield_ty(
 817         &self,
 818         fr: RegionVid,
 819     ) -> Option<RegionName> {
 820         // Note: generators from `async fn` yield `()`, so we don't have to
 821         // worry about them here.
 822         let yield_ty = self.regioncx.universal_regions().yield_ty?;
 823         debug!("give_name_if_anonymous_region_appears_in_yield_ty: yield_ty = {:?}", yield_ty);
 824
 825         let tcx = self.infcx.tcx;
 826
 827         if !tcx.any_free_region_meets(&yield_ty, |r| r.to_region_vid() == fr) {
 828             return None;
 829         }
 830
 831         let mut highlight = RegionHighlightMode::new(tcx);
 832         highlight.highlighting_region_vid(fr, *self.next_region_name.try_borrow().unwrap());
 833         let type_name =
 834             self.infcx.extract_inference_diagnostics_data(yield_ty.into(), Some(highlight)).name;
 835
 836         let yield_span = match tcx.hir().get(self.mir_hir_id()) {
 837             hir::Node::Expr(hir::Expr {
 838                 kind: hir::ExprKind::Closure(&hir::Closure { fn_decl_span, .. }),
 839                 ..
 840             }) => tcx.sess.source_map().end_point(fn_decl_span),
 841             _ => self.body.span,
 842         };
 843
 844         debug!(
 845             "give_name_if_anonymous_region_appears_in_yield_ty: \
 846              type_name = {:?}, yield_span = {:?}",
 847             yield_span, type_name,
 848         );
 849
 850         Some(RegionName {
 851             name: self.synthesize_region_name(),
 852             source: RegionNameSource::AnonRegionFromYieldTy(yield_span, type_name),
 853         })
 854     }
 855
 856     fn give_name_if_anonymous_region_appears_in_impl_signature(
 857         &self,
 858         fr: RegionVid,
 859     ) -> Option<RegionName> {
 860         let ty::ReEarlyBound(region) = *self.to_error_region(fr)? else {
 861             return None;
 862         };
 863         if region.has_name() {
 864             return None;
 865         };
 866
 867         let tcx = self.infcx.tcx;
 868         let region_parent = tcx.parent(region.def_id);
 869         if tcx.def_kind(region_parent) != DefKind::Impl {
 870             return None;
 871         }
 872
 873         let found = tcx
 874             .any_free_region_meets(&tcx.type_of(region_parent), |r| *r == ty::ReEarlyBound(region));
 875
 876         Some(RegionName {
 877             name: self.synthesize_region_name(),
 878             source: RegionNameSource::AnonRegionFromImplSignature(
 879                 tcx.def_span(region.def_id),
 880                 // FIXME(compiler-errors): Does this ever actually show up
 881                 // anywhere other than the self type? I couldn't create an
 882                 // example of a `'_` in the impl's trait being referenceable.
 883                 if found { "self type" } else { "header" },
 884             ),
 885         })
 886     }
 887
 888     fn give_name_if_anonymous_region_appears_in_arg_position_impl_trait(
 889         &self,
 890         fr: RegionVid,
 891     ) -> Option<RegionName> {
 892         let ty::ReEarlyBound(region) = *self.to_error_region(fr)? else {
 893             return None;
 894         };
 895         if region.has_name() {
 896             return None;
 897         };
 898
 899         let predicates = self
 900             .infcx
 901             .tcx
 902             .predicates_of(self.body.source.def_id())
 903             .instantiate_identity(self.infcx.tcx)
 904             .predicates;
 905
 906         if let Some(upvar_index) = self
 907             .regioncx
 908             .universal_regions()
 909             .defining_ty
 910             .upvar_tys()
 911             .position(|ty| self.any_param_predicate_mentions(&predicates, ty, region))
 912         {
 913             let (upvar_name, upvar_span) = self.regioncx.get_upvar_name_and_span_for_region(
 914                 self.infcx.tcx,
 915                 &self.upvars,
 916                 upvar_index,
 917             );
 918             let region_name = self.synthesize_region_name();
 919
 920             Some(RegionName {
 921                 name: region_name,
 922                 source: RegionNameSource::AnonRegionFromUpvar(upvar_span, upvar_name),
 923             })
 924         } else if let Some(arg_index) = self
 925             .regioncx
 926             .universal_regions()
 927             .unnormalized_input_tys
 928             .iter()
 929             .position(|ty| self.any_param_predicate_mentions(&predicates, *ty, region))
 930         {
 931             let (arg_name, arg_span) = self.regioncx.get_argument_name_and_span_for_region(
 932                 self.body,
 933                 &self.local_names,
 934                 arg_index,
 935             );
 936             let region_name = self.synthesize_region_name();
 937
 938             Some(RegionName {
 939                 name: region_name,
 940                 source: RegionNameSource::AnonRegionFromArgument(
 941                     RegionNameHighlight::CannotMatchHirTy(arg_span, arg_name?.to_string()),
 942                 ),
 943             })
 944         } else {
 945             None
 946         }
 947     }
 948
 949     fn any_param_predicate_mentions(
 950         &self,
 951         predicates: &[ty::Predicate<'tcx>],
 952         ty: Ty<'tcx>,
 953         region: ty::EarlyBoundRegion,
 954     ) -> bool {
 955         let tcx = self.infcx.tcx;
 956         ty.walk().any(|arg| {
 957             if let ty::GenericArgKind::Type(ty) = arg.unpack()
 958                 && let ty::Param(_) = ty.kind()
 959             {
 960                 predicates.iter().any(|pred| {
 961                     match pred.kind().skip_binder() {
 962                         ty::PredicateKind::Trait(data) if data.self_ty() == ty => {}
 963                         ty::PredicateKind::Projection(data) if data.projection_ty.self_ty() == ty => {}
 964                         _ => return false,
 965                     }
 966                     tcx.any_free_region_meets(pred, |r| {
 967                         *r == ty::ReEarlyBound(region)
 968                     })
 969                 })
 970             } else {
 971                 false
 972             }
 973         })
 974     }
 975 }