1 use rustc_data_structures::fx::FxHashSet;
3 use rustc_hir::lang_items::LangItem;
4 use rustc_middle::ty::{self, Region, RegionVid, TypeFoldable, TypeSuperFoldable};
5 use rustc_trait_selection::traits::auto_trait::{self, AutoTraitResult};
12 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
13 enum RegionTarget<'tcx> {
18 #[derive(Default, Debug, Clone)]
19 struct RegionDeps<'tcx> {
20 larger: FxHashSet<RegionTarget<'tcx>>,
21 smaller: FxHashSet<RegionTarget<'tcx>>,
24 pub(crate) struct AutoTraitFinder<'a, 'tcx> {
25 pub(crate) cx: &'a mut core::DocContext<'tcx>,
28 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx>
30 'tcx: 'a, // should be an implied bound; rustc bug #98852.
32 pub(crate) fn new(cx: &'a mut core::DocContext<'tcx>) -> Self {
33 AutoTraitFinder { cx }
36 fn generate_for_trait(
40 param_env: ty::ParamEnv<'tcx>,
42 f: &auto_trait::AutoTraitFinder<'tcx>,
43 // If this is set, show only negative trait implementations, not positive ones.
44 discard_positive_impl: bool,
46 let tcx = self.cx.tcx;
47 let trait_ref = tcx.mk_trait_ref(trait_def_id, [ty]);
48 if !self.cx.generated_synthetics.insert((ty, trait_def_id)) {
49 debug!("get_auto_trait_impl_for({:?}): already generated, aborting", trait_ref);
53 let result = f.find_auto_trait_generics(ty, param_env, trait_def_id, |info| {
54 let region_data = info.region_data;
57 .generics_of(item_def_id)
60 .filter_map(|param| match param.kind {
61 ty::GenericParamDefKind::Lifetime => Some(param.name),
64 .map(|name| (name, Lifetime(name)))
66 let lifetime_predicates = Self::handle_lifetimes(®ion_data, &names_map);
67 let new_generics = self.param_env_to_generics(
75 "find_auto_trait_generics(item_def_id={:?}, trait_def_id={:?}): \
77 item_def_id, trait_def_id, new_generics
84 let new_generics = match result {
85 AutoTraitResult::PositiveImpl(new_generics) => {
86 polarity = ty::ImplPolarity::Positive;
87 if discard_positive_impl {
92 AutoTraitResult::NegativeImpl => {
93 polarity = ty::ImplPolarity::Negative;
95 // For negative impls, we use the generic params, but *not* the predicates,
96 // from the original type. Otherwise, the displayed impl appears to be a
97 // conditional negative impl, when it's really unconditional.
99 // For example, consider the struct Foo<T: Copy>(*mut T). Using
100 // the original predicates in our impl would cause us to generate
101 // `impl !Send for Foo<T: Copy>`, which makes it appear that Foo
102 // implements Send where T is not copy.
104 // Instead, we generate `impl !Send for Foo<T>`, which better
105 // expresses the fact that `Foo<T>` never implements `Send`,
106 // regardless of the choice of `T`.
107 let raw_generics = clean_ty_generics(
109 tcx.generics_of(item_def_id),
110 ty::GenericPredicates::default(),
112 let params = raw_generics.params;
114 Generics { params, where_predicates: ThinVec::new() }
116 AutoTraitResult::ExplicitImpl => return None,
121 attrs: Default::default(),
122 item_id: ItemId::Auto { trait_: trait_def_id, for_: item_def_id },
123 kind: Box::new(ImplItem(Box::new(Impl {
124 unsafety: hir::Unsafety::Normal,
125 generics: new_generics,
126 trait_: Some(clean_trait_ref_with_bindings(self.cx, trait_ref, ThinVec::new())),
127 for_: clean_middle_ty(ty, self.cx, None),
130 kind: ImplKind::Auto,
133 inline_stmt_id: None,
137 pub(crate) fn get_auto_trait_impls(&mut self, item_def_id: DefId) -> Vec<Item> {
138 let tcx = self.cx.tcx;
139 let param_env = tcx.param_env(item_def_id);
140 let ty = tcx.type_of(item_def_id);
141 let f = auto_trait::AutoTraitFinder::new(tcx);
143 debug!("get_auto_trait_impls({:?})", ty);
144 let auto_traits: Vec<_> = self.cx.auto_traits.iter().copied().collect();
145 let mut auto_traits: Vec<Item> = auto_traits
147 .filter_map(|trait_def_id| {
148 self.generate_for_trait(ty, trait_def_id, param_env, item_def_id, &f, false)
151 // We are only interested in case the type *doesn't* implement the Sized trait.
152 if !ty.is_sized(tcx, param_env) {
153 // In case `#![no_core]` is used, `sized_trait` returns nothing.
154 if let Some(item) = tcx.lang_items().sized_trait().and_then(|sized_trait_did| {
155 self.generate_for_trait(ty, sized_trait_did, param_env, item_def_id, &f, true)
157 auto_traits.push(item);
163 fn get_lifetime(region: Region<'_>, names_map: &FxHashMap<Symbol, Lifetime>) -> Lifetime {
166 names_map.get(&name).unwrap_or_else(|| {
167 panic!("Missing lifetime with name {:?} for {:?}", name.as_str(), region)
170 .unwrap_or(&Lifetime::statik())
174 /// This method calculates two things: Lifetime constraints of the form `'a: 'b`,
175 /// and region constraints of the form `RegionVid: 'a`
177 /// This is essentially a simplified version of lexical_region_resolve. However,
178 /// handle_lifetimes determines what *needs be* true in order for an impl to hold.
179 /// lexical_region_resolve, along with much of the rest of the compiler, is concerned
180 /// with determining if a given set up constraints/predicates *are* met, given some
181 /// starting conditions (e.g., user-provided code). For this reason, it's easier
182 /// to perform the calculations we need on our own, rather than trying to make
183 /// existing inference/solver code do what we want.
184 fn handle_lifetimes<'cx>(
185 regions: &RegionConstraintData<'cx>,
186 names_map: &FxHashMap<Symbol, Lifetime>,
187 ) -> ThinVec<WherePredicate> {
188 // Our goal is to 'flatten' the list of constraints by eliminating
189 // all intermediate RegionVids. At the end, all constraints should
190 // be between Regions (aka region variables). This gives us the information
191 // we need to create the Generics.
192 let mut finished: FxHashMap<_, Vec<_>> = Default::default();
194 let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();
196 // Flattening is done in two parts. First, we insert all of the constraints
197 // into a map. Each RegionTarget (either a RegionVid or a Region) maps
198 // to its smaller and larger regions. Note that 'larger' regions correspond
199 // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
200 for constraint in regions.constraints.keys() {
202 Constraint::VarSubVar(r1, r2) => {
204 let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
205 deps1.larger.insert(RegionTarget::RegionVid(r2));
208 let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
209 deps2.smaller.insert(RegionTarget::RegionVid(r1));
211 Constraint::RegSubVar(region, vid) => {
212 let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
213 deps.smaller.insert(RegionTarget::Region(region));
215 Constraint::VarSubReg(vid, region) => {
216 let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
217 deps.larger.insert(RegionTarget::Region(region));
219 Constraint::RegSubReg(r1, r2) => {
220 // The constraint is already in the form that we want, so we're done with it
221 // Desired order is 'larger, smaller', so flip then
222 if region_name(r1) != region_name(r2) {
224 .entry(region_name(r2).expect("no region_name found"))
232 // Here, we 'flatten' the map one element at a time.
233 // All of the element's sub and super regions are connected
234 // to each other. For example, if we have a graph that looks like this:
236 // (A, B) - C - (D, E)
237 // Where (A, B) are subregions, and (D,E) are super-regions
239 // then after deleting 'C', the graph will look like this:
240 // ... - A - (D, E ...)
241 // ... - B - (D, E, ...)
242 // (A, B, ...) - D - ...
243 // (A, B, ...) - E - ...
245 // where '...' signifies the existing sub and super regions of an entry
246 // When two adjacent ty::Regions are encountered, we've computed a final
247 // constraint, and add it to our list. Since we make sure to never re-add
248 // deleted items, this process will always finish.
249 while !vid_map.is_empty() {
250 let target = *vid_map.keys().next().expect("Keys somehow empty");
251 let deps = vid_map.remove(&target).expect("Entry somehow missing");
253 for smaller in deps.smaller.iter() {
254 for larger in deps.larger.iter() {
255 match (smaller, larger) {
256 (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
257 if region_name(r1) != region_name(r2) {
259 .entry(region_name(r2).expect("no region name found"))
261 .push(r1) // Larger, smaller
264 (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
265 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
266 let smaller_deps = v.into_mut();
267 smaller_deps.larger.insert(*larger);
268 smaller_deps.larger.remove(&target);
271 (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
272 if let Entry::Occupied(v) = vid_map.entry(*larger) {
273 let deps = v.into_mut();
274 deps.smaller.insert(*smaller);
275 deps.smaller.remove(&target);
278 (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
279 if let Entry::Occupied(v) = vid_map.entry(*smaller) {
280 let smaller_deps = v.into_mut();
281 smaller_deps.larger.insert(*larger);
282 smaller_deps.larger.remove(&target);
285 if let Entry::Occupied(v) = vid_map.entry(*larger) {
286 let larger_deps = v.into_mut();
287 larger_deps.smaller.insert(*smaller);
288 larger_deps.smaller.remove(&target);
296 let lifetime_predicates = names_map
298 .flat_map(|(name, lifetime)| {
299 let empty = Vec::new();
300 let bounds: FxHashSet<GenericBound> = finished
304 .map(|region| GenericBound::Outlives(Self::get_lifetime(*region, names_map)))
307 if bounds.is_empty() {
310 Some(WherePredicate::RegionPredicate {
311 lifetime: lifetime.clone(),
312 bounds: bounds.into_iter().collect(),
320 fn extract_for_generics(&self, pred: ty::Predicate<'tcx>) -> FxHashSet<GenericParamDef> {
321 let bound_predicate = pred.kind();
322 let tcx = self.cx.tcx;
323 let regions = match bound_predicate.skip_binder() {
324 ty::PredicateKind::Clause(ty::Clause::Trait(poly_trait_pred)) => {
325 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_trait_pred))
327 ty::PredicateKind::Clause(ty::Clause::Projection(poly_proj_pred)) => {
328 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_proj_pred))
330 _ => return FxHashSet::default(),
337 // We only care about named late bound regions, as we need to add them
338 // to the 'for<>' section
339 ty::BrNamed(_, name) => Some(GenericParamDef::lifetime(name)),
346 fn make_final_bounds(
348 ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
349 ty_to_fn: FxHashMap<Type, (PolyTrait, Option<Type>)>,
350 lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
351 ) -> Vec<WherePredicate> {
354 .flat_map(|(ty, mut bounds)| {
355 if let Some((ref poly_trait, ref output)) = ty_to_fn.get(&ty) {
356 let mut new_path = poly_trait.trait_.clone();
357 let last_segment = new_path.segments.pop().expect("segments were empty");
359 let (old_input, old_output) = match last_segment.args {
360 GenericArgs::AngleBracketed { args, .. } => {
363 .filter_map(|arg| match arg {
364 GenericArg::Type(ty) => Some(ty.clone()),
370 GenericArgs::Parenthesized { inputs, output } => (inputs, output),
373 let output = output.as_ref().cloned().map(Box::new);
374 if old_output.is_some() && old_output != output {
375 panic!("Output mismatch for {:?} {:?} {:?}", ty, old_output, output);
378 let new_params = GenericArgs::Parenthesized { inputs: old_input, output };
382 .push(PathSegment { name: last_segment.name, args: new_params });
384 bounds.insert(GenericBound::TraitBound(
387 generic_params: poly_trait.generic_params.clone(),
389 hir::TraitBoundModifier::None,
392 if bounds.is_empty() {
396 let mut bounds_vec = bounds.into_iter().collect();
397 self.sort_where_bounds(&mut bounds_vec);
399 Some(WherePredicate::BoundPredicate {
402 bound_params: Vec::new(),
406 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
407 |(lifetime, bounds)| {
408 let mut bounds_vec = bounds.into_iter().collect();
409 self.sort_where_bounds(&mut bounds_vec);
410 WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
417 /// Converts the calculated `ParamEnv` and lifetime information to a [`clean::Generics`](Generics), suitable for
418 /// display on the docs page. Cleaning the `Predicates` produces sub-optimal [`WherePredicate`]s,
419 /// so we fix them up:
421 /// * Multiple bounds for the same type are coalesced into one: e.g., `T: Copy`, `T: Debug`
422 /// becomes `T: Copy + Debug`
423 /// * `Fn` bounds are handled specially - instead of leaving it as `T: Fn(), <T as Fn::Output> =
424 /// K`, we use the dedicated syntax `T: Fn() -> K`
425 /// * We explicitly add a `?Sized` bound if we didn't find any `Sized` predicates for a type
426 fn param_env_to_generics(
429 param_env: ty::ParamEnv<'tcx>,
430 mut existing_predicates: ThinVec<WherePredicate>,
431 vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
434 "param_env_to_generics(item_def_id={:?}, param_env={:?}, \
435 existing_predicates={:?})",
436 item_def_id, param_env, existing_predicates
439 let tcx = self.cx.tcx;
441 // The `Sized` trait must be handled specially, since we only display it when
442 // it is *not* required (i.e., '?Sized')
443 let sized_trait = tcx.require_lang_item(LangItem::Sized, None);
445 let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
447 let orig_bounds: FxHashSet<_> = tcx.param_env(item_def_id).caller_bounds().iter().collect();
448 let clean_where_predicates = param_env
452 !orig_bounds.contains(p)
453 || match p.kind().skip_binder() {
454 ty::PredicateKind::Clause(ty::Clause::Trait(pred)) => {
455 pred.def_id() == sized_trait
460 .map(|p| p.fold_with(&mut replacer));
462 let raw_generics = clean_ty_generics(
464 tcx.generics_of(item_def_id),
465 tcx.explicit_predicates_of(item_def_id),
467 let mut generic_params = raw_generics.params;
469 debug!("param_env_to_generics({:?}): generic_params={:?}", item_def_id, generic_params);
471 let mut has_sized = FxHashSet::default();
472 let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
473 let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
474 let mut ty_to_traits: FxHashMap<Type, FxHashSet<Path>> = Default::default();
476 let mut ty_to_fn: FxHashMap<Type, (PolyTrait, Option<Type>)> = Default::default();
478 // FIXME: This code shares much of the logic found in `clean_ty_generics` and
479 // `simplify::where_clause`. Consider deduplicating it to avoid diverging
481 // Further, the code below does not merge (partially re-sugared) bounds like
482 // `Tr<A = T>` & `Tr<B = U>` and it does not render higher-ranked parameters
483 // originating from equality predicates.
484 for p in clean_where_predicates {
485 let (orig_p, p) = (p, clean_predicate(p, self.cx));
491 WherePredicate::BoundPredicate { ty, mut bounds, .. } => {
492 // Writing a projection trait bound of the form
493 // <T as Trait>::Name : ?Sized
494 // is illegal, because ?Sized bounds can only
495 // be written in the (here, nonexistent) definition
497 // Therefore, we make sure that we never add a ?Sized
498 // bound for projections
499 if let Type::QPath { .. } = ty {
500 has_sized.insert(ty.clone());
503 if bounds.is_empty() {
507 let mut for_generics = self.extract_for_generics(orig_p);
509 assert!(bounds.len() == 1);
510 let mut b = bounds.pop().expect("bounds were empty");
512 if b.is_sized_bound(self.cx) {
513 has_sized.insert(ty.clone());
519 .map(|bounds| bounds.contains(&strip_path_generics(trait_)))
523 // If we've already added a projection bound for the same type, don't add
524 // this, as it would be a duplicate
526 // Handle any 'Fn/FnOnce/FnMut' bounds specially,
527 // as we want to combine them with any 'Output' qpaths
530 let is_fn = match b {
531 GenericBound::TraitBound(ref mut p, _) => {
532 // Insert regions into the for_generics hash map first, to ensure
533 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
534 for_generics.extend(p.generic_params.drain(..));
535 p.generic_params.extend(for_generics);
536 self.is_fn_trait(&p.trait_)
541 let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
546 .and_modify(|e| *e = (poly_trait.clone(), e.1.clone()))
547 .or_insert(((poly_trait.clone()), None));
549 ty_to_bounds.entry(ty.clone()).or_default();
551 ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
555 WherePredicate::RegionPredicate { lifetime, bounds } => {
556 lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
558 WherePredicate::EqPredicate { lhs, rhs, bound_params } => {
560 Type::QPath(box QPathData {
561 ref assoc, ref self_type, ref trait_, ..
563 let ty = &*self_type;
564 let mut new_trait = trait_.clone();
566 if self.is_fn_trait(trait_) && assoc.name == sym::Output {
570 *e = (e.0.clone(), Some(rhs.ty().unwrap().clone()))
574 trait_: trait_.clone(),
575 generic_params: Vec::new(),
577 Some(rhs.ty().unwrap().clone()),
582 let args = &mut new_trait
585 .expect("segments were empty")
589 // Convert something like '<T as Iterator::Item> = u8'
590 // to 'T: Iterator<Item=u8>'
591 GenericArgs::AngleBracketed { ref mut bindings, .. } => {
592 bindings.push(TypeBinding {
593 assoc: assoc.clone(),
594 kind: TypeBindingKind::Equality { term: *rhs },
597 GenericArgs::Parenthesized { .. } => {
598 existing_predicates.push(WherePredicate::EqPredicate {
603 continue; // If something other than a Fn ends up
604 // with parentheses, leave it alone
608 let bounds = ty_to_bounds.entry(ty.clone()).or_default();
610 bounds.insert(GenericBound::TraitBound(
611 PolyTrait { trait_: new_trait, generic_params: Vec::new() },
612 hir::TraitBoundModifier::None,
615 // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
616 // that we don't see a
617 // duplicate bound like `T: Iterator + Iterator<Item=u8>`
619 bounds.remove(&GenericBound::TraitBound(
620 PolyTrait { trait_: trait_.clone(), generic_params: Vec::new() },
621 hir::TraitBoundModifier::None,
623 // Avoid creating any new duplicate bounds later in the outer
625 ty_to_traits.entry(ty.clone()).or_default().insert(trait_.clone());
627 _ => panic!("Unexpected LHS {:?} for {:?}", lhs, item_def_id),
633 let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
635 existing_predicates.extend(final_bounds);
637 for param in generic_params.iter_mut() {
639 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
640 // We never want something like `impl<T=Foo>`.
642 let generic_ty = Type::Generic(param.name);
643 if !has_sized.contains(&generic_ty) {
644 bounds.insert(0, GenericBound::maybe_sized(self.cx));
647 GenericParamDefKind::Lifetime { .. } => {}
648 GenericParamDefKind::Const { ref mut default, .. } => {
649 // We never want something like `impl<const N: usize = 10>`
655 self.sort_where_predicates(&mut existing_predicates);
657 Generics { params: generic_params, where_predicates: existing_predicates }
660 /// Ensure that the predicates are in a consistent order. The precise
661 /// ordering doesn't actually matter, but it's important that
662 /// a given set of predicates always appears in the same order -
663 /// both for visual consistency between 'rustdoc' runs, and to
664 /// make writing tests much easier
666 fn sort_where_predicates(&self, predicates: &mut [WherePredicate]) {
667 // We should never have identical bounds - and if we do,
668 // they're visually identical as well. Therefore, using
669 // an unstable sort is fine.
670 self.unstable_debug_sort(predicates);
673 /// Ensure that the bounds are in a consistent order. The precise
674 /// ordering doesn't actually matter, but it's important that
675 /// a given set of bounds always appears in the same order -
676 /// both for visual consistency between 'rustdoc' runs, and to
677 /// make writing tests much easier
679 fn sort_where_bounds(&self, bounds: &mut Vec<GenericBound>) {
680 // We should never have identical bounds - and if we do,
681 // they're visually identical as well. Therefore, using
682 // an unstable sort is fine.
683 self.unstable_debug_sort(bounds);
686 /// This might look horrendously hacky, but it's actually not that bad.
688 /// For performance reasons, we use several different FxHashMaps
689 /// in the process of computing the final set of where predicates.
690 /// However, the iteration order of a HashMap is completely unspecified.
691 /// In fact, the iteration of an FxHashMap can even vary between platforms,
692 /// since FxHasher has different behavior for 32-bit and 64-bit platforms.
694 /// Obviously, it's extremely undesirable for documentation rendering
695 /// to be dependent on the platform it's run on. Apart from being confusing
696 /// to end users, it makes writing tests much more difficult, as predicates
697 /// can appear in any order in the final result.
699 /// To solve this problem, we sort WherePredicates and GenericBounds
700 /// by their Debug string. The thing to keep in mind is that we don't really
701 /// care what the final order is - we're synthesizing an impl or bound
702 /// ourselves, so any order can be considered equally valid. By sorting the
703 /// predicates and bounds, however, we ensure that for a given codebase, all
704 /// auto-trait impls always render in exactly the same way.
706 /// Using the Debug implementation for sorting prevents us from needing to
707 /// write quite a bit of almost entirely useless code (e.g., how should two
708 /// Types be sorted relative to each other). It also allows us to solve the
709 /// problem for both WherePredicates and GenericBounds at the same time. This
710 /// approach is probably somewhat slower, but the small number of items
711 /// involved (impls rarely have more than a few bounds) means that it
712 /// shouldn't matter in practice.
713 fn unstable_debug_sort<T: Debug>(&self, vec: &mut [T]) {
714 vec.sort_by_cached_key(|x| format!("{:?}", x))
717 fn is_fn_trait(&self, path: &Path) -> bool {
718 let tcx = self.cx.tcx;
719 let did = path.def_id();
720 did == tcx.require_lang_item(LangItem::Fn, None)
721 || did == tcx.require_lang_item(LangItem::FnMut, None)
722 || did == tcx.require_lang_item(LangItem::FnOnce, None)
726 fn region_name(region: Region<'_>) -> Option<Symbol> {
728 ty::ReEarlyBound(r) => Some(r.name),
733 /// Replaces all [`ty::RegionVid`]s in a type with [`ty::Region`]s, using the provided map.
734 struct RegionReplacer<'a, 'tcx> {
735 vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
739 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
740 fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
744 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
746 ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
749 .unwrap_or_else(|| r.super_fold_with(self))