1 //! This code is kind of an alternate way of doing subtyping,
2 //! supertyping, and type equating, distinct from the `combine.rs`
3 //! code but very similar in its effect and design. Eventually the two
4 //! ought to be merged. This code is intended for use in NLL and chalk.
6 //! Here are the key differences:
8 //! - This code may choose to bypass some checks (e.g., the occurs check)
9 //! in the case where we know that there are no unbound type inference
10 //! variables. This is the case for NLL, because at NLL time types are fully
11 //! inferred up-to regions.
12 //! - This code uses "universes" to handle higher-ranked regions and
13 //! not the leak-check. This is "more correct" than what rustc does
14 //! and we are generally migrating in this direction, but NLL had to
17 //! Also, this code assumes that there are no bound types at all, not even
18 //! free ones. This is ok because:
19 //! - we are not relating anything quantified over some type variable
20 //! - we will have instantiated all the bound type vars already (the one
21 //! thing we relate in chalk are basically domain goals and their
24 use crate::infer::InferCtxt;
25 use crate::traits::DomainGoal;
26 use crate::ty::error::TypeError;
27 use crate::ty::fold::{TypeFoldable, TypeVisitor};
28 use crate::ty::relate::{self, Relate, RelateResult, TypeRelation};
29 use crate::ty::subst::Kind;
30 use crate::ty::{self, Ty, TyCtxt, InferConst};
31 use crate::mir::interpret::ConstValue;
32 use rustc_data_structures::fx::FxHashMap;
35 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
36 pub enum NormalizationStrategy {
41 pub struct TypeRelating<'me, 'tcx, D>
43 D: TypeRelatingDelegate<'tcx>,
45 infcx: &'me InferCtxt<'me, 'tcx>,
47 /// Callback to use when we deduce an outlives relationship
50 /// How are we relating `a` and `b`?
52 /// - Covariant means `a <: b`.
53 /// - Contravariant means `b <: a`.
54 /// - Invariant means `a == b.
55 /// - Bivariant means that it doesn't matter.
56 ambient_variance: ty::Variance,
58 /// When we pass through a set of binders (e.g., when looking into
59 /// a `fn` type), we push a new bound region scope onto here. This
60 /// will contain the instantiated region for each region in those
61 /// binders. When we then encounter a `ReLateBound(d, br)`, we can
62 /// use the De Bruijn index `d` to find the right scope, and then
63 /// bound region name `br` to find the specific instantiation from
64 /// within that scope. See `replace_bound_region`.
66 /// This field stores the instantiations for late-bound regions in
68 a_scopes: Vec<BoundRegionScope<'tcx>>,
70 /// Same as `a_scopes`, but for the `b` type.
71 b_scopes: Vec<BoundRegionScope<'tcx>>,
74 pub trait TypeRelatingDelegate<'tcx> {
75 /// Push a constraint `sup: sub` -- this constraint must be
76 /// satisfied for the two types to be related. `sub` and `sup` may
77 /// be regions from the type or new variables created through the
79 fn push_outlives(&mut self, sup: ty::Region<'tcx>, sub: ty::Region<'tcx>);
81 /// Push a domain goal that will need to be proved for the two types to
82 /// be related. Used for lazy normalization.
83 fn push_domain_goal(&mut self, domain_goal: DomainGoal<'tcx>);
85 /// Creates a new universe index. Used when instantiating placeholders.
86 fn create_next_universe(&mut self) -> ty::UniverseIndex;
88 /// Creates a new region variable representing a higher-ranked
89 /// region that is instantiated existentially. This creates an
90 /// inference variable, typically.
92 /// So e.g., if you have `for<'a> fn(..) <: for<'b> fn(..)`, then
93 /// we will invoke this method to instantiate `'a` with an
94 /// inference variable (though `'b` would be instantiated first,
95 /// as a placeholder).
96 fn next_existential_region_var(&mut self) -> ty::Region<'tcx>;
98 /// Creates a new region variable representing a
99 /// higher-ranked region that is instantiated universally.
100 /// This creates a new region placeholder, typically.
102 /// So e.g., if you have `for<'a> fn(..) <: for<'b> fn(..)`, then
103 /// we will invoke this method to instantiate `'b` with a
104 /// placeholder region.
105 fn next_placeholder_region(&mut self, placeholder: ty::PlaceholderRegion) -> ty::Region<'tcx>;
107 /// Creates a new existential region in the given universe. This
108 /// is used when handling subtyping and type variables -- if we
109 /// have that `?X <: Foo<'a>`, for example, we would instantiate
110 /// `?X` with a type like `Foo<'?0>` where `'?0` is a fresh
111 /// existential variable created by this function. We would then
112 /// relate `Foo<'?0>` with `Foo<'a>` (and probably add an outlives
113 /// relation stating that `'?0: 'a`).
114 fn generalize_existential(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx>;
116 /// Define the normalization strategy to use, eager or lazy.
117 fn normalization() -> NormalizationStrategy;
119 /// Enables some optimizations if we do not expect inference variables
120 /// in the RHS of the relation.
121 fn forbid_inference_vars() -> bool;
124 #[derive(Clone, Debug)]
125 struct ScopesAndKind<'tcx> {
126 scopes: Vec<BoundRegionScope<'tcx>>,
130 #[derive(Clone, Debug, Default)]
131 struct BoundRegionScope<'tcx> {
132 map: FxHashMap<ty::BoundRegion, ty::Region<'tcx>>,
135 #[derive(Copy, Clone)]
136 struct UniversallyQuantified(bool);
138 impl<'me, 'tcx, D> TypeRelating<'me, 'tcx, D>
140 D: TypeRelatingDelegate<'tcx>,
143 infcx: &'me InferCtxt<'me, 'tcx>,
145 ambient_variance: ty::Variance,
156 fn ambient_covariance(&self) -> bool {
157 match self.ambient_variance {
158 ty::Variance::Covariant | ty::Variance::Invariant => true,
159 ty::Variance::Contravariant | ty::Variance::Bivariant => false,
163 fn ambient_contravariance(&self) -> bool {
164 match self.ambient_variance {
165 ty::Variance::Contravariant | ty::Variance::Invariant => true,
166 ty::Variance::Covariant | ty::Variance::Bivariant => false,
172 value: &ty::Binder<impl TypeFoldable<'tcx>>,
173 universally_quantified: UniversallyQuantified,
174 ) -> BoundRegionScope<'tcx> {
175 let mut scope = BoundRegionScope::default();
177 // Create a callback that creates (via the delegate) either an
178 // existential or placeholder region as needed.
179 let mut next_region = {
180 let delegate = &mut self.delegate;
181 let mut lazy_universe = None;
182 move |br: ty::BoundRegion| {
183 if universally_quantified.0 {
184 // The first time this closure is called, create a
185 // new universe for the placeholders we will make
187 let universe = lazy_universe.unwrap_or_else(|| {
188 let universe = delegate.create_next_universe();
189 lazy_universe = Some(universe);
193 let placeholder = ty::PlaceholderRegion { universe, name: br };
194 delegate.next_placeholder_region(placeholder)
196 delegate.next_existential_region_var()
201 value.skip_binder().visit_with(&mut ScopeInstantiator {
202 next_region: &mut next_region,
203 target_index: ty::INNERMOST,
204 bound_region_scope: &mut scope,
210 /// When we encounter binders during the type traversal, we record
211 /// the value to substitute for each of the things contained in
212 /// that binder. (This will be either a universal placeholder or
213 /// an existential inference variable.) Given the De Bruijn index
214 /// `debruijn` (and name `br`) of some binder we have now
215 /// encountered, this routine finds the value that we instantiated
216 /// the region with; to do so, it indexes backwards into the list
217 /// of ambient scopes `scopes`.
218 fn lookup_bound_region(
219 debruijn: ty::DebruijnIndex,
220 br: &ty::BoundRegion,
221 first_free_index: ty::DebruijnIndex,
222 scopes: &[BoundRegionScope<'tcx>],
223 ) -> ty::Region<'tcx> {
224 // The debruijn index is a "reverse index" into the
225 // scopes listing. So when we have INNERMOST (0), we
226 // want the *last* scope pushed, and so forth.
227 let debruijn_index = debruijn.index() - first_free_index.index();
228 let scope = &scopes[scopes.len() - debruijn_index - 1];
230 // Find this bound region in that scope to map to a
231 // particular region.
235 /// If `r` is a bound region, find the scope in which it is bound
236 /// (from `scopes`) and return the value that we instantiated it
237 /// with. Otherwise just return `r`.
238 fn replace_bound_region(
241 first_free_index: ty::DebruijnIndex,
242 scopes: &[BoundRegionScope<'tcx>],
243 ) -> ty::Region<'tcx> {
244 debug!("replace_bound_regions(scopes={:?})", scopes);
245 if let ty::ReLateBound(debruijn, br) = r {
246 Self::lookup_bound_region(*debruijn, br, first_free_index, scopes)
252 /// Push a new outlives requirement into our output set of
254 fn push_outlives(&mut self, sup: ty::Region<'tcx>, sub: ty::Region<'tcx>) {
255 debug!("push_outlives({:?}: {:?})", sup, sub);
257 self.delegate.push_outlives(sup, sub);
260 /// Relate a projection type and some value type lazily. This will always
261 /// succeed, but we push an additional `ProjectionEq` goal depending
262 /// on the value type:
263 /// - if the value type is any type `T` which is not a projection, we push
264 /// `ProjectionEq(projection = T)`.
265 /// - if the value type is another projection `other_projection`, we create
266 /// a new inference variable `?U` and push the two goals
267 /// `ProjectionEq(projection = ?U)`, `ProjectionEq(other_projection = ?U)`.
268 fn relate_projection_ty(
270 projection_ty: ty::ProjectionTy<'tcx>,
273 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
274 use crate::traits::WhereClause;
275 use syntax_pos::DUMMY_SP;
278 ty::Projection(other_projection_ty) => {
279 let var = self.infcx.next_ty_var(TypeVariableOrigin {
280 kind: TypeVariableOriginKind::MiscVariable,
283 self.relate_projection_ty(projection_ty, var);
284 self.relate_projection_ty(other_projection_ty, var);
289 let projection = ty::ProjectionPredicate {
294 .push_domain_goal(DomainGoal::Holds(WhereClause::ProjectionEq(projection)));
300 /// Relate a type inference variable with a value type. This works
301 /// by creating a "generalization" G of the value where all the
302 /// lifetimes are replaced with fresh inference values. This
303 /// genearlization G becomes the value of the inference variable,
304 /// and is then related in turn to the value. So e.g. if you had
305 /// `vid = ?0` and `value = &'a u32`, we might first instantiate
306 /// `?0` to a type like `&'0 u32` where `'0` is a fresh variable,
307 /// and then relate `&'0 u32` with `&'a u32` (resulting in
308 /// relations between `'0` and `'a`).
310 /// The variable `pair` can be either a `(vid, ty)` or `(ty, vid)`
311 /// -- in other words, it is always a (unresolved) inference
312 /// variable `vid` and a type `ty` that are being related, but the
313 /// vid may appear either as the "a" type or the "b" type,
314 /// depending on where it appears in the tuple. The trait
315 /// `VidValuePair` lets us work with the vid/type while preserving
316 /// the "sidedness" when necessary -- the sidedness is relevant in
317 /// particular for the variance and set of in-scope things.
318 fn relate_ty_var<PAIR: VidValuePair<'tcx>>(
321 ) -> RelateResult<'tcx, Ty<'tcx>> {
322 debug!("relate_ty_var({:?})", pair);
324 let vid = pair.vid();
325 let value_ty = pair.value_ty();
327 // FIXME -- this logic assumes invariance, but that is wrong.
328 // This only presently applies to chalk integration, as NLL
329 // doesn't permit type variables to appear on both sides (and
330 // doesn't use lazy norm).
332 ty::Infer(ty::TyVar(value_vid)) => {
333 // Two type variables: just equate them.
337 .equate(vid, value_vid);
341 ty::Projection(projection_ty) if D::normalization() == NormalizationStrategy::Lazy => {
342 return Ok(self.relate_projection_ty(projection_ty, self.infcx.tcx.mk_ty_var(vid)));
348 let generalized_ty = self.generalize_value(value_ty, vid)?;
349 debug!("relate_ty_var: generalized_ty = {:?}", generalized_ty);
351 if D::forbid_inference_vars() {
352 // In NLL, we don't have type inference variables
353 // floating around, so we can do this rather imprecise
354 // variant of the occurs-check.
355 assert!(!generalized_ty.has_infer_types());
361 .instantiate(vid, generalized_ty);
363 // The generalized values we extract from `canonical_var_values` have
364 // been fully instantiated and hence the set of scopes we have
365 // doesn't matter -- just to be sure, put an empty vector
367 let old_a_scopes = ::std::mem::replace(pair.vid_scopes(self), vec![]);
369 // Relate the generalized kind to the original one.
370 let result = pair.relate_generalized_ty(self, generalized_ty);
372 // Restore the old scopes now.
373 *pair.vid_scopes(self) = old_a_scopes;
375 debug!("relate_ty_var: complete, result = {:?}", result);
379 fn generalize_value<T: Relate<'tcx>>(
383 ) -> RelateResult<'tcx, T> {
384 let universe = self.infcx.probe_ty_var(for_vid).unwrap_err();
386 let mut generalizer = TypeGeneralizer {
388 delegate: &mut self.delegate,
389 first_free_index: ty::INNERMOST,
390 ambient_variance: self.ambient_variance,
391 for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
395 generalizer.relate(&value, &value)
399 /// When we instantiate a inference variable with a value in
400 /// `relate_ty_var`, we always have the pair of a `TyVid` and a `Ty`,
401 /// but the ordering may vary (depending on whether the inference
402 /// variable was found on the `a` or `b` sides). Therefore, this trait
403 /// allows us to factor out common code, while preserving the order
405 trait VidValuePair<'tcx>: Debug {
406 /// Extract the inference variable (which could be either the
407 /// first or second part of the tuple).
408 fn vid(&self) -> ty::TyVid;
410 /// Extract the value it is being related to (which will be the
411 /// opposite part of the tuple from the vid).
412 fn value_ty(&self) -> Ty<'tcx>;
414 /// Extract the scopes that apply to whichever side of the tuple
415 /// the vid was found on. See the comment where this is called
416 /// for more details on why we want them.
417 fn vid_scopes<D: TypeRelatingDelegate<'tcx>>(
419 relate: &'r mut TypeRelating<'_, 'tcx, D>,
420 ) -> &'r mut Vec<BoundRegionScope<'tcx>>;
422 /// Given a generalized type G that should replace the vid, relate
423 /// G to the value, putting G on whichever side the vid would have
425 fn relate_generalized_ty<D>(
427 relate: &mut TypeRelating<'_, 'tcx, D>,
428 generalized_ty: Ty<'tcx>,
429 ) -> RelateResult<'tcx, Ty<'tcx>>
431 D: TypeRelatingDelegate<'tcx>;
434 impl VidValuePair<'tcx> for (ty::TyVid, Ty<'tcx>) {
435 fn vid(&self) -> ty::TyVid {
439 fn value_ty(&self) -> Ty<'tcx> {
445 relate: &'r mut TypeRelating<'_, 'tcx, D>,
446 ) -> &'r mut Vec<BoundRegionScope<'tcx>>
448 D: TypeRelatingDelegate<'tcx>,
453 fn relate_generalized_ty<D>(
455 relate: &mut TypeRelating<'_, 'tcx, D>,
456 generalized_ty: Ty<'tcx>,
457 ) -> RelateResult<'tcx, Ty<'tcx>>
459 D: TypeRelatingDelegate<'tcx>,
461 relate.relate(&generalized_ty, &self.value_ty())
465 // In this case, the "vid" is the "b" type.
466 impl VidValuePair<'tcx> for (Ty<'tcx>, ty::TyVid) {
467 fn vid(&self) -> ty::TyVid {
471 fn value_ty(&self) -> Ty<'tcx> {
477 relate: &'r mut TypeRelating<'_, 'tcx, D>,
478 ) -> &'r mut Vec<BoundRegionScope<'tcx>>
480 D: TypeRelatingDelegate<'tcx>,
485 fn relate_generalized_ty<D>(
487 relate: &mut TypeRelating<'_, 'tcx, D>,
488 generalized_ty: Ty<'tcx>,
489 ) -> RelateResult<'tcx, Ty<'tcx>>
491 D: TypeRelatingDelegate<'tcx>,
493 relate.relate(&self.value_ty(), &generalized_ty)
497 impl<D> TypeRelation<'tcx> for TypeRelating<'me, 'tcx, D>
499 D: TypeRelatingDelegate<'tcx>,
501 fn tcx(&self) -> TyCtxt<'tcx> {
505 fn tag(&self) -> &'static str {
509 fn a_is_expected(&self) -> bool {
513 fn relate_with_variance<T: Relate<'tcx>>(
515 variance: ty::Variance,
518 ) -> RelateResult<'tcx, T> {
520 "relate_with_variance(variance={:?}, a={:?}, b={:?})",
524 let old_ambient_variance = self.ambient_variance;
525 self.ambient_variance = self.ambient_variance.xform(variance);
528 "relate_with_variance: ambient_variance = {:?}",
529 self.ambient_variance
532 let r = self.relate(a, b)?;
534 self.ambient_variance = old_ambient_variance;
536 debug!("relate_with_variance: r={:?}", r);
541 fn tys(&mut self, a: Ty<'tcx>, mut b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
542 let a = self.infcx.shallow_resolve(a);
544 if !D::forbid_inference_vars() {
545 b = self.infcx.shallow_resolve(b);
548 match (&a.sty, &b.sty) {
549 (_, &ty::Infer(ty::TyVar(vid))) => {
550 if D::forbid_inference_vars() {
551 // Forbid inference variables in the RHS.
552 bug!("unexpected inference var {:?}", b)
554 self.relate_ty_var((a, vid))
558 (&ty::Infer(ty::TyVar(vid)), _) => self.relate_ty_var((vid, b)),
560 (&ty::Projection(projection_ty), _)
561 if D::normalization() == NormalizationStrategy::Lazy =>
563 Ok(self.relate_projection_ty(projection_ty, b))
566 (_, &ty::Projection(projection_ty))
567 if D::normalization() == NormalizationStrategy::Lazy =>
569 Ok(self.relate_projection_ty(projection_ty, a))
574 "tys(a={:?}, b={:?}, variance={:?})",
575 a, b, self.ambient_variance
578 // Will also handle unification of `IntVar` and `FloatVar`.
579 self.infcx.super_combine_tys(self, a, b)
588 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
590 "regions(a={:?}, b={:?}, variance={:?})",
591 a, b, self.ambient_variance
594 let v_a = self.replace_bound_region(a, ty::INNERMOST, &self.a_scopes);
595 let v_b = self.replace_bound_region(b, ty::INNERMOST, &self.b_scopes);
597 debug!("regions: v_a = {:?}", v_a);
598 debug!("regions: v_b = {:?}", v_b);
600 if self.ambient_covariance() {
601 // Covariance: a <= b. Hence, `b: a`.
602 self.push_outlives(v_b, v_a);
605 if self.ambient_contravariance() {
606 // Contravariant: b <= a. Hence, `a: b`.
607 self.push_outlives(v_a, v_b);
615 a: &'tcx ty::Const<'tcx>,
616 b: &'tcx ty::Const<'tcx>,
617 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
618 if let ty::Const { val: ConstValue::Infer(InferConst::Canonical(_, _)), .. } = a {
619 // FIXME(const_generics): I'm unsure how this branch should actually be handled,
620 // so this is probably not correct.
621 self.infcx.super_combine_consts(self, a, b)
623 debug!("consts(a={:?}, b={:?}, variance={:?})", a, b, self.ambient_variance);
624 relate::super_relate_consts(self, a, b)
632 ) -> RelateResult<'tcx, ty::Binder<T>>
639 // for<'a> fn(&'a u32) -> &'a u32 <:
640 // fn(&'b u32) -> &'b u32
646 // fn(&'a u32) -> &'a u32 <:
647 // for<'b> fn(&'b u32) -> &'b u32
650 // We therefore proceed as follows:
652 // - Instantiate binders on `b` universally, yielding a universe U1.
653 // - Instantiate binders on `a` existentially in U1.
656 "binders({:?}: {:?}, ambient_variance={:?})",
657 a, b, self.ambient_variance
660 if self.ambient_covariance() {
661 // Covariance, so we want `for<..> A <: for<..> B` --
662 // therefore we compare any instantiation of A (i.e., A
663 // instantiated with existentials) against every
664 // instantiation of B (i.e., B instantiated with
667 let b_scope = self.create_scope(b, UniversallyQuantified(true));
668 let a_scope = self.create_scope(a, UniversallyQuantified(false));
670 debug!("binders: a_scope = {:?} (existential)", a_scope);
671 debug!("binders: b_scope = {:?} (universal)", b_scope);
673 self.b_scopes.push(b_scope);
674 self.a_scopes.push(a_scope);
676 // Reset the ambient variance to covariant. This is needed
677 // to correctly handle cases like
679 // for<'a> fn(&'a u32, &'a u3) == for<'b, 'c> fn(&'b u32, &'c u32)
681 // Somewhat surprisingly, these two types are actually
682 // **equal**, even though the one on the right looks more
683 // polymorphic. The reason is due to subtyping. To see it,
684 // consider that each function can call the other:
686 // - The left function can call the right with `'b` and
687 // `'c` both equal to `'a`
689 // - The right function can call the left with `'a` set to
690 // `{P}`, where P is the point in the CFG where the call
691 // itself occurs. Note that `'b` and `'c` must both
692 // include P. At the point, the call works because of
693 // subtyping (i.e., `&'b u32 <: &{P} u32`).
694 let variance = ::std::mem::replace(&mut self.ambient_variance, ty::Variance::Covariant);
696 self.relate(a.skip_binder(), b.skip_binder())?;
698 self.ambient_variance = variance;
700 self.b_scopes.pop().unwrap();
701 self.a_scopes.pop().unwrap();
704 if self.ambient_contravariance() {
705 // Contravariance, so we want `for<..> A :> for<..> B`
706 // -- therefore we compare every instantiation of A (i.e.,
707 // A instantiated with universals) against any
708 // instantiation of B (i.e., B instantiated with
709 // existentials). Opposite of above.
711 let a_scope = self.create_scope(a, UniversallyQuantified(true));
712 let b_scope = self.create_scope(b, UniversallyQuantified(false));
714 debug!("binders: a_scope = {:?} (universal)", a_scope);
715 debug!("binders: b_scope = {:?} (existential)", b_scope);
717 self.a_scopes.push(a_scope);
718 self.b_scopes.push(b_scope);
720 // Reset ambient variance to contravariance. See the
721 // covariant case above for an explanation.
723 ::std::mem::replace(&mut self.ambient_variance, ty::Variance::Contravariant);
725 self.relate(a.skip_binder(), b.skip_binder())?;
727 self.ambient_variance = variance;
729 self.b_scopes.pop().unwrap();
730 self.a_scopes.pop().unwrap();
737 /// When we encounter a binder like `for<..> fn(..)`, we actually have
738 /// to walk the `fn` value to find all the values bound by the `for`
739 /// (these are not explicitly present in the ty representation right
740 /// now). This visitor handles that: it descends the type, tracking
741 /// binder depth, and finds late-bound regions targeting the
742 /// `for<..`>. For each of those, it creates an entry in
743 /// `bound_region_scope`.
744 struct ScopeInstantiator<'me, 'tcx> {
745 next_region: &'me mut dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
746 // The debruijn index of the scope we are instantiating.
747 target_index: ty::DebruijnIndex,
748 bound_region_scope: &'me mut BoundRegionScope<'tcx>,
751 impl<'me, 'tcx> TypeVisitor<'tcx> for ScopeInstantiator<'me, 'tcx> {
752 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> bool {
753 self.target_index.shift_in(1);
754 t.super_visit_with(self);
755 self.target_index.shift_out(1);
760 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
761 let ScopeInstantiator {
768 ty::ReLateBound(debruijn, br) if *debruijn == self.target_index => {
772 .or_insert_with(|| next_region(*br));
782 /// The "type generalize" is used when handling inference variables.
784 /// The basic strategy for handling a constraint like `?A <: B` is to
785 /// apply a "generalization strategy" to the type `B` -- this replaces
786 /// all the lifetimes in the type `B` with fresh inference
787 /// variables. (You can read more about the strategy in this [blog
790 /// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
791 /// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
792 /// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
793 /// establishes `'0: 'x` as a constraint.
795 /// As a side-effect of this generalization procedure, we also replace
796 /// all the bound regions that we have traversed with concrete values,
797 /// so that the resulting generalized type is independent from the
800 /// [blog post]: https://is.gd/0hKvIr
801 struct TypeGeneralizer<'me, 'tcx, D>
803 D: TypeRelatingDelegate<'tcx>,
805 infcx: &'me InferCtxt<'me, 'tcx>,
807 delegate: &'me mut D,
809 /// After we generalize this type, we are going to relative it to
810 /// some other type. What will be the variance at this point?
811 ambient_variance: ty::Variance,
813 first_free_index: ty::DebruijnIndex,
815 /// The vid of the type variable that is in the process of being
816 /// instantiated. If we find this within the value we are folding,
817 /// that means we would have created a cyclic value.
818 for_vid_sub_root: ty::TyVid,
820 /// The universe of the type variable that is in the process of being
821 /// instantiated. If we find anything that this universe cannot name,
822 /// we reject the relation.
823 universe: ty::UniverseIndex,
826 impl<D> TypeRelation<'tcx> for TypeGeneralizer<'me, 'tcx, D>
828 D: TypeRelatingDelegate<'tcx>,
830 fn tcx(&self) -> TyCtxt<'tcx> {
834 fn tag(&self) -> &'static str {
838 fn a_is_expected(&self) -> bool {
842 fn relate_with_variance<T: Relate<'tcx>>(
844 variance: ty::Variance,
847 ) -> RelateResult<'tcx, T> {
849 "TypeGeneralizer::relate_with_variance(variance={:?}, a={:?}, b={:?})",
853 let old_ambient_variance = self.ambient_variance;
854 self.ambient_variance = self.ambient_variance.xform(variance);
857 "TypeGeneralizer::relate_with_variance: ambient_variance = {:?}",
858 self.ambient_variance
861 let r = self.relate(a, b)?;
863 self.ambient_variance = old_ambient_variance;
865 debug!("TypeGeneralizer::relate_with_variance: r={:?}", r);
870 fn tys(&mut self, a: Ty<'tcx>, _: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
871 use crate::infer::type_variable::TypeVariableValue;
873 debug!("TypeGeneralizer::tys(a={:?})", a);
876 ty::Infer(ty::TyVar(_)) | ty::Infer(ty::IntVar(_)) | ty::Infer(ty::FloatVar(_))
877 if D::forbid_inference_vars() =>
880 "unexpected inference variable encountered in NLL generalization: {:?}",
885 ty::Infer(ty::TyVar(vid)) => {
886 let mut variables = self.infcx.type_variables.borrow_mut();
887 let vid = variables.root_var(vid);
888 let sub_vid = variables.sub_root_var(vid);
889 if sub_vid == self.for_vid_sub_root {
890 // If sub-roots are equal, then `for_vid` and
891 // `vid` are related via subtyping.
892 debug!("TypeGeneralizer::tys: occurs check failed");
893 return Err(TypeError::Mismatch);
895 match variables.probe(vid) {
896 TypeVariableValue::Known { value: u } => {
900 TypeVariableValue::Unknown {
903 if self.ambient_variance == ty::Bivariant {
904 // FIXME: we may need a WF predicate (related to #54105).
907 let origin = *variables.var_origin(vid);
909 // Replacing with a new variable in the universe `self.universe`,
910 // it will be unified later with the original type variable in
911 // the universe `_universe`.
912 let new_var_id = variables.new_var(self.universe, false, origin);
914 let u = self.tcx().mk_ty_var(new_var_id);
916 "generalize: replacing original vid={:?} with new={:?}",
925 ty::Infer(ty::IntVar(_)) | ty::Infer(ty::FloatVar(_)) => {
926 // No matter what mode we are in,
927 // integer/floating-point types must be equal to be
932 ty::Placeholder(placeholder) => {
933 if self.universe.cannot_name(placeholder.universe) {
935 "TypeGeneralizer::tys: root universe {:?} cannot name\
936 placeholder in universe {:?}",
937 self.universe, placeholder.universe
939 Err(TypeError::Mismatch)
945 _ => relate::super_relate_tys(self, a, a),
953 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
954 debug!("TypeGeneralizer::regions(a={:?})", a);
956 if let ty::ReLateBound(debruijn, _) = a {
957 if *debruijn < self.first_free_index {
962 // For now, we just always create a fresh region variable to
963 // replace all the regions in the source type. In the main
964 // type checker, we special case the case where the ambient
965 // variance is `Invariant` and try to avoid creating a fresh
966 // region variable, but since this comes up so much less in
967 // NLL (only when users use `_` etc) it is much less
970 // As an aside, since these new variables are created in
971 // `self.universe` universe, this also serves to enforce the
972 // universe scoping rules.
974 // FIXME(#54105) -- if the ambient variance is bivariant,
975 // though, we may however need to check well-formedness or
976 // risk a problem like #41677 again.
978 let replacement_region_vid = self.delegate.generalize_existential(self.universe);
980 Ok(replacement_region_vid)
985 a: &'tcx ty::Const<'tcx>,
986 _: &'tcx ty::Const<'tcx>,
987 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
988 debug!("TypeGeneralizer::consts(a={:?})", a);
990 if let ty::Const { val: ConstValue::Infer(InferConst::Canonical(_, _)), .. } = a {
992 "unexpected inference variable encountered in NLL generalization: {:?}",
996 relate::super_relate_consts(self, a, a)
1004 ) -> RelateResult<'tcx, ty::Binder<T>>
1008 debug!("TypeGeneralizer::binders(a={:?})", a);
1010 self.first_free_index.shift_in(1);
1011 let result = self.relate(a.skip_binder(), a.skip_binder())?;
1012 self.first_free_index.shift_out(1);
1013 Ok(ty::Binder::bind(result))