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::combine::ConstEquateRelation;
25 use crate::infer::InferCtxt;
26 use crate::infer::{ConstVarValue, ConstVariableValue};
27 use rustc_data_structures::fx::FxHashMap;
28 use rustc_middle::ty::error::TypeError;
29 use rustc_middle::ty::fold::{TypeFoldable, TypeVisitor};
30 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
31 use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
33 use std::ops::ControlFlow;
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 fn param_env(&self) -> ty::ParamEnv<'tcx>;
77 /// Push a constraint `sup: sub` -- this constraint must be
78 /// satisfied for the two types to be related. `sub` and `sup` may
79 /// be regions from the type or new variables created through the
81 fn push_outlives(&mut self, sup: ty::Region<'tcx>, sub: ty::Region<'tcx>);
83 fn const_equate(&mut self, a: &'tcx ty::Const<'tcx>, b: &'tcx ty::Const<'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, was_placeholder: bool) -> 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, Default)]
125 struct BoundRegionScope<'tcx> {
126 map: FxHashMap<ty::BoundRegion, ty::Region<'tcx>>,
129 #[derive(Copy, Clone)]
130 struct UniversallyQuantified(bool);
132 impl<'me, 'tcx, D> TypeRelating<'me, 'tcx, D>
134 D: TypeRelatingDelegate<'tcx>,
137 infcx: &'me InferCtxt<'me, 'tcx>,
139 ambient_variance: ty::Variance,
141 Self { infcx, delegate, ambient_variance, a_scopes: vec![], b_scopes: vec![] }
144 fn ambient_covariance(&self) -> bool {
145 match self.ambient_variance {
146 ty::Variance::Covariant | ty::Variance::Invariant => true,
147 ty::Variance::Contravariant | ty::Variance::Bivariant => false,
151 fn ambient_contravariance(&self) -> bool {
152 match self.ambient_variance {
153 ty::Variance::Contravariant | ty::Variance::Invariant => true,
154 ty::Variance::Covariant | ty::Variance::Bivariant => false,
160 value: ty::Binder<'tcx, impl Relate<'tcx>>,
161 universally_quantified: UniversallyQuantified,
162 ) -> BoundRegionScope<'tcx> {
163 let mut scope = BoundRegionScope::default();
165 // Create a callback that creates (via the delegate) either an
166 // existential or placeholder region as needed.
167 let mut next_region = {
168 let delegate = &mut self.delegate;
169 let mut lazy_universe = None;
170 move |br: ty::BoundRegion| {
171 if universally_quantified.0 {
172 // The first time this closure is called, create a
173 // new universe for the placeholders we will make
175 let universe = lazy_universe.unwrap_or_else(|| {
176 let universe = delegate.create_next_universe();
177 lazy_universe = Some(universe);
181 let placeholder = ty::PlaceholderRegion { universe, name: br.kind };
182 delegate.next_placeholder_region(placeholder)
184 delegate.next_existential_region_var(true)
189 value.skip_binder().visit_with(&mut ScopeInstantiator {
190 next_region: &mut next_region,
191 target_index: ty::INNERMOST,
192 bound_region_scope: &mut scope,
198 /// When we encounter binders during the type traversal, we record
199 /// the value to substitute for each of the things contained in
200 /// that binder. (This will be either a universal placeholder or
201 /// an existential inference variable.) Given the De Bruijn index
202 /// `debruijn` (and name `br`) of some binder we have now
203 /// encountered, this routine finds the value that we instantiated
204 /// the region with; to do so, it indexes backwards into the list
205 /// of ambient scopes `scopes`.
206 fn lookup_bound_region(
207 debruijn: ty::DebruijnIndex,
208 br: &ty::BoundRegion,
209 first_free_index: ty::DebruijnIndex,
210 scopes: &[BoundRegionScope<'tcx>],
211 ) -> ty::Region<'tcx> {
212 // The debruijn index is a "reverse index" into the
213 // scopes listing. So when we have INNERMOST (0), we
214 // want the *last* scope pushed, and so forth.
215 let debruijn_index = debruijn.index() - first_free_index.index();
216 let scope = &scopes[scopes.len() - debruijn_index - 1];
218 // Find this bound region in that scope to map to a
219 // particular region.
223 /// If `r` is a bound region, find the scope in which it is bound
224 /// (from `scopes`) and return the value that we instantiated it
225 /// with. Otherwise just return `r`.
226 fn replace_bound_region(
229 first_free_index: ty::DebruijnIndex,
230 scopes: &[BoundRegionScope<'tcx>],
231 ) -> ty::Region<'tcx> {
232 debug!("replace_bound_regions(scopes={:?})", scopes);
233 if let ty::ReLateBound(debruijn, br) = r {
234 Self::lookup_bound_region(*debruijn, br, first_free_index, scopes)
240 /// Push a new outlives requirement into our output set of
242 fn push_outlives(&mut self, sup: ty::Region<'tcx>, sub: ty::Region<'tcx>) {
243 debug!("push_outlives({:?}: {:?})", sup, sub);
245 self.delegate.push_outlives(sup, sub);
248 /// Relate a projection type and some value type lazily. This will always
249 /// succeed, but we push an additional `ProjectionEq` goal depending
250 /// on the value type:
251 /// - if the value type is any type `T` which is not a projection, we push
252 /// `ProjectionEq(projection = T)`.
253 /// - if the value type is another projection `other_projection`, we create
254 /// a new inference variable `?U` and push the two goals
255 /// `ProjectionEq(projection = ?U)`, `ProjectionEq(other_projection = ?U)`.
256 fn relate_projection_ty(
258 projection_ty: ty::ProjectionTy<'tcx>,
261 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
262 use rustc_span::DUMMY_SP;
264 match *value_ty.kind() {
265 ty::Projection(other_projection_ty) => {
266 let var = self.infcx.next_ty_var(TypeVariableOrigin {
267 kind: TypeVariableOriginKind::MiscVariable,
270 self.relate_projection_ty(projection_ty, var);
271 self.relate_projection_ty(other_projection_ty, var);
275 _ => bug!("should never be invoked with eager normalization"),
279 /// Relate a type inference variable with a value type. This works
280 /// by creating a "generalization" G of the value where all the
281 /// lifetimes are replaced with fresh inference values. This
282 /// generalization G becomes the value of the inference variable,
283 /// and is then related in turn to the value. So e.g. if you had
284 /// `vid = ?0` and `value = &'a u32`, we might first instantiate
285 /// `?0` to a type like `&'0 u32` where `'0` is a fresh variable,
286 /// and then relate `&'0 u32` with `&'a u32` (resulting in
287 /// relations between `'0` and `'a`).
289 /// The variable `pair` can be either a `(vid, ty)` or `(ty, vid)`
290 /// -- in other words, it is always a (unresolved) inference
291 /// variable `vid` and a type `ty` that are being related, but the
292 /// vid may appear either as the "a" type or the "b" type,
293 /// depending on where it appears in the tuple. The trait
294 /// `VidValuePair` lets us work with the vid/type while preserving
295 /// the "sidedness" when necessary -- the sidedness is relevant in
296 /// particular for the variance and set of in-scope things.
297 fn relate_ty_var<PAIR: VidValuePair<'tcx>>(
300 ) -> RelateResult<'tcx, Ty<'tcx>> {
301 debug!("relate_ty_var({:?})", pair);
303 let vid = pair.vid();
304 let value_ty = pair.value_ty();
306 // FIXME(invariance) -- this logic assumes invariance, but that is wrong.
307 // This only presently applies to chalk integration, as NLL
308 // doesn't permit type variables to appear on both sides (and
309 // doesn't use lazy norm).
310 match *value_ty.kind() {
311 ty::Infer(ty::TyVar(value_vid)) => {
312 // Two type variables: just equate them.
313 self.infcx.inner.borrow_mut().type_variables().equate(vid, value_vid);
317 ty::Projection(projection_ty) if D::normalization() == NormalizationStrategy::Lazy => {
318 return Ok(self.relate_projection_ty(projection_ty, self.infcx.tcx.mk_ty_var(vid)));
324 let generalized_ty = self.generalize_value(value_ty, vid)?;
325 debug!("relate_ty_var: generalized_ty = {:?}", generalized_ty);
327 if D::forbid_inference_vars() {
328 // In NLL, we don't have type inference variables
329 // floating around, so we can do this rather imprecise
330 // variant of the occurs-check.
331 assert!(!generalized_ty.has_infer_types_or_consts());
334 self.infcx.inner.borrow_mut().type_variables().instantiate(vid, generalized_ty);
336 // The generalized values we extract from `canonical_var_values` have
337 // been fully instantiated and hence the set of scopes we have
338 // doesn't matter -- just to be sure, put an empty vector
340 let old_a_scopes = std::mem::take(pair.vid_scopes(self));
342 // Relate the generalized kind to the original one.
343 let result = pair.relate_generalized_ty(self, generalized_ty);
345 // Restore the old scopes now.
346 *pair.vid_scopes(self) = old_a_scopes;
348 debug!("relate_ty_var: complete, result = {:?}", result);
352 fn generalize_value<T: Relate<'tcx>>(
356 ) -> RelateResult<'tcx, T> {
357 let universe = self.infcx.probe_ty_var(for_vid).unwrap_err();
359 let mut generalizer = TypeGeneralizer {
361 delegate: &mut self.delegate,
362 first_free_index: ty::INNERMOST,
363 ambient_variance: self.ambient_variance,
364 for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
368 generalizer.relate(value, value)
372 /// When we instantiate a inference variable with a value in
373 /// `relate_ty_var`, we always have the pair of a `TyVid` and a `Ty`,
374 /// but the ordering may vary (depending on whether the inference
375 /// variable was found on the `a` or `b` sides). Therefore, this trait
376 /// allows us to factor out common code, while preserving the order
378 trait VidValuePair<'tcx>: Debug {
379 /// Extract the inference variable (which could be either the
380 /// first or second part of the tuple).
381 fn vid(&self) -> ty::TyVid;
383 /// Extract the value it is being related to (which will be the
384 /// opposite part of the tuple from the vid).
385 fn value_ty(&self) -> Ty<'tcx>;
387 /// Extract the scopes that apply to whichever side of the tuple
388 /// the vid was found on. See the comment where this is called
389 /// for more details on why we want them.
390 fn vid_scopes<D: TypeRelatingDelegate<'tcx>>(
392 relate: &'r mut TypeRelating<'_, 'tcx, D>,
393 ) -> &'r mut Vec<BoundRegionScope<'tcx>>;
395 /// Given a generalized type G that should replace the vid, relate
396 /// G to the value, putting G on whichever side the vid would have
398 fn relate_generalized_ty<D>(
400 relate: &mut TypeRelating<'_, 'tcx, D>,
401 generalized_ty: Ty<'tcx>,
402 ) -> RelateResult<'tcx, Ty<'tcx>>
404 D: TypeRelatingDelegate<'tcx>;
407 impl VidValuePair<'tcx> for (ty::TyVid, Ty<'tcx>) {
408 fn vid(&self) -> ty::TyVid {
412 fn value_ty(&self) -> Ty<'tcx> {
418 relate: &'r mut TypeRelating<'_, 'tcx, D>,
419 ) -> &'r mut Vec<BoundRegionScope<'tcx>>
421 D: TypeRelatingDelegate<'tcx>,
426 fn relate_generalized_ty<D>(
428 relate: &mut TypeRelating<'_, 'tcx, D>,
429 generalized_ty: Ty<'tcx>,
430 ) -> RelateResult<'tcx, Ty<'tcx>>
432 D: TypeRelatingDelegate<'tcx>,
434 relate.relate(&generalized_ty, &self.value_ty())
438 // In this case, the "vid" is the "b" type.
439 impl VidValuePair<'tcx> for (Ty<'tcx>, ty::TyVid) {
440 fn vid(&self) -> ty::TyVid {
444 fn value_ty(&self) -> Ty<'tcx> {
450 relate: &'r mut TypeRelating<'_, 'tcx, D>,
451 ) -> &'r mut Vec<BoundRegionScope<'tcx>>
453 D: TypeRelatingDelegate<'tcx>,
458 fn relate_generalized_ty<D>(
460 relate: &mut TypeRelating<'_, 'tcx, D>,
461 generalized_ty: Ty<'tcx>,
462 ) -> RelateResult<'tcx, Ty<'tcx>>
464 D: TypeRelatingDelegate<'tcx>,
466 relate.relate(&self.value_ty(), &generalized_ty)
470 impl<D> TypeRelation<'tcx> for TypeRelating<'me, 'tcx, D>
472 D: TypeRelatingDelegate<'tcx>,
474 fn tcx(&self) -> TyCtxt<'tcx> {
478 fn param_env(&self) -> ty::ParamEnv<'tcx> {
479 self.delegate.param_env()
482 fn tag(&self) -> &'static str {
486 fn a_is_expected(&self) -> bool {
490 fn relate_with_variance<T: Relate<'tcx>>(
492 variance: ty::Variance,
495 ) -> RelateResult<'tcx, T> {
496 debug!("relate_with_variance(variance={:?}, a={:?}, b={:?})", variance, a, b);
498 let old_ambient_variance = self.ambient_variance;
499 self.ambient_variance = self.ambient_variance.xform(variance);
501 debug!("relate_with_variance: ambient_variance = {:?}", self.ambient_variance);
503 let r = self.relate(a, b)?;
505 self.ambient_variance = old_ambient_variance;
507 debug!("relate_with_variance: r={:?}", r);
512 fn tys(&mut self, a: Ty<'tcx>, mut b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
513 let a = self.infcx.shallow_resolve(a);
515 if !D::forbid_inference_vars() {
516 b = self.infcx.shallow_resolve(b);
520 // Subtle: if a or b has a bound variable that we are lazilly
521 // substituting, then even if a == b, it could be that the values we
522 // will substitute for those bound variables are *not* the same, and
523 // hence returning `Ok(a)` is incorrect.
524 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
529 match (a.kind(), b.kind()) {
530 (_, &ty::Infer(ty::TyVar(vid))) => {
531 if D::forbid_inference_vars() {
532 // Forbid inference variables in the RHS.
533 bug!("unexpected inference var {:?}", b)
535 self.relate_ty_var((a, vid))
539 (&ty::Infer(ty::TyVar(vid)), _) => self.relate_ty_var((vid, b)),
541 (&ty::Projection(projection_ty), _)
542 if D::normalization() == NormalizationStrategy::Lazy =>
544 Ok(self.relate_projection_ty(projection_ty, b))
547 (_, &ty::Projection(projection_ty))
548 if D::normalization() == NormalizationStrategy::Lazy =>
550 Ok(self.relate_projection_ty(projection_ty, a))
554 debug!("tys(a={:?}, b={:?}, variance={:?})", a, b, self.ambient_variance);
556 // Will also handle unification of `IntVar` and `FloatVar`.
557 self.infcx.super_combine_tys(self, a, b)
566 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
567 debug!("regions(a={:?}, b={:?}, variance={:?})", a, b, self.ambient_variance);
569 let v_a = self.replace_bound_region(a, ty::INNERMOST, &self.a_scopes);
570 let v_b = self.replace_bound_region(b, ty::INNERMOST, &self.b_scopes);
572 debug!("regions: v_a = {:?}", v_a);
573 debug!("regions: v_b = {:?}", v_b);
575 if self.ambient_covariance() {
576 // Covariance: a <= b. Hence, `b: a`.
577 self.push_outlives(v_b, v_a);
580 if self.ambient_contravariance() {
581 // Contravariant: b <= a. Hence, `a: b`.
582 self.push_outlives(v_a, v_b);
590 a: &'tcx ty::Const<'tcx>,
591 mut b: &'tcx ty::Const<'tcx>,
592 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
593 let a = self.infcx.shallow_resolve(a);
595 if !D::forbid_inference_vars() {
596 b = self.infcx.shallow_resolve(b);
600 ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
601 // Forbid inference variables in the RHS.
602 bug!("unexpected inference var {:?}", b)
604 // FIXME(invariance): see the related FIXME above.
605 _ => self.infcx.super_combine_consts(self, a, b),
611 a: ty::Binder<'tcx, T>,
612 b: ty::Binder<'tcx, T>,
613 ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
620 // for<'a> fn(&'a u32) -> &'a u32 <:
621 // fn(&'b u32) -> &'b u32
627 // fn(&'a u32) -> &'a u32 <:
628 // for<'b> fn(&'b u32) -> &'b u32
631 // We therefore proceed as follows:
633 // - Instantiate binders on `b` universally, yielding a universe U1.
634 // - Instantiate binders on `a` existentially in U1.
636 debug!("binders({:?}: {:?}, ambient_variance={:?})", a, b, self.ambient_variance);
638 if let (Some(a), Some(b)) = (a.no_bound_vars(), b.no_bound_vars()) {
639 // Fast path for the common case.
641 return Ok(ty::Binder::dummy(a));
644 if self.ambient_covariance() {
645 // Covariance, so we want `for<..> A <: for<..> B` --
646 // therefore we compare any instantiation of A (i.e., A
647 // instantiated with existentials) against every
648 // instantiation of B (i.e., B instantiated with
651 let b_scope = self.create_scope(b, UniversallyQuantified(true));
652 let a_scope = self.create_scope(a, UniversallyQuantified(false));
654 debug!("binders: a_scope = {:?} (existential)", a_scope);
655 debug!("binders: b_scope = {:?} (universal)", b_scope);
657 self.b_scopes.push(b_scope);
658 self.a_scopes.push(a_scope);
660 // Reset the ambient variance to covariant. This is needed
661 // to correctly handle cases like
663 // for<'a> fn(&'a u32, &'a u32) == for<'b, 'c> fn(&'b u32, &'c u32)
665 // Somewhat surprisingly, these two types are actually
666 // **equal**, even though the one on the right looks more
667 // polymorphic. The reason is due to subtyping. To see it,
668 // consider that each function can call the other:
670 // - The left function can call the right with `'b` and
671 // `'c` both equal to `'a`
673 // - The right function can call the left with `'a` set to
674 // `{P}`, where P is the point in the CFG where the call
675 // itself occurs. Note that `'b` and `'c` must both
676 // include P. At the point, the call works because of
677 // subtyping (i.e., `&'b u32 <: &{P} u32`).
678 let variance = std::mem::replace(&mut self.ambient_variance, ty::Variance::Covariant);
680 self.relate(a.skip_binder(), b.skip_binder())?;
682 self.ambient_variance = variance;
684 self.b_scopes.pop().unwrap();
685 self.a_scopes.pop().unwrap();
688 if self.ambient_contravariance() {
689 // Contravariance, so we want `for<..> A :> for<..> B`
690 // -- therefore we compare every instantiation of A (i.e.,
691 // A instantiated with universals) against any
692 // instantiation of B (i.e., B instantiated with
693 // existentials). Opposite of above.
695 let a_scope = self.create_scope(a, UniversallyQuantified(true));
696 let b_scope = self.create_scope(b, UniversallyQuantified(false));
698 debug!("binders: a_scope = {:?} (universal)", a_scope);
699 debug!("binders: b_scope = {:?} (existential)", b_scope);
701 self.a_scopes.push(a_scope);
702 self.b_scopes.push(b_scope);
704 // Reset ambient variance to contravariance. See the
705 // covariant case above for an explanation.
707 std::mem::replace(&mut self.ambient_variance, ty::Variance::Contravariant);
709 self.relate(a.skip_binder(), b.skip_binder())?;
711 self.ambient_variance = variance;
713 self.b_scopes.pop().unwrap();
714 self.a_scopes.pop().unwrap();
721 impl<'tcx, D> ConstEquateRelation<'tcx> for TypeRelating<'_, 'tcx, D>
723 D: TypeRelatingDelegate<'tcx>,
725 fn const_equate_obligation(&mut self, a: &'tcx ty::Const<'tcx>, b: &'tcx ty::Const<'tcx>) {
726 self.delegate.const_equate(a, b);
730 /// When we encounter a binder like `for<..> fn(..)`, we actually have
731 /// to walk the `fn` value to find all the values bound by the `for`
732 /// (these are not explicitly present in the ty representation right
733 /// now). This visitor handles that: it descends the type, tracking
734 /// binder depth, and finds late-bound regions targeting the
735 /// `for<..`>. For each of those, it creates an entry in
736 /// `bound_region_scope`.
737 struct ScopeInstantiator<'me, 'tcx> {
738 next_region: &'me mut dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
739 // The debruijn index of the scope we are instantiating.
740 target_index: ty::DebruijnIndex,
741 bound_region_scope: &'me mut BoundRegionScope<'tcx>,
744 impl<'me, 'tcx> TypeVisitor<'tcx> for ScopeInstantiator<'me, 'tcx> {
745 fn visit_binder<T: TypeFoldable<'tcx>>(
747 t: &ty::Binder<'tcx, T>,
748 ) -> ControlFlow<Self::BreakTy> {
749 self.target_index.shift_in(1);
750 t.super_visit_with(self);
751 self.target_index.shift_out(1);
753 ControlFlow::CONTINUE
756 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
757 let ScopeInstantiator { bound_region_scope, next_region, .. } = self;
760 ty::ReLateBound(debruijn, br) if *debruijn == self.target_index => {
761 bound_region_scope.map.entry(*br).or_insert_with(|| next_region(*br));
767 ControlFlow::CONTINUE
771 /// The "type generalizer" is used when handling inference variables.
773 /// The basic strategy for handling a constraint like `?A <: B` is to
774 /// apply a "generalization strategy" to the type `B` -- this replaces
775 /// all the lifetimes in the type `B` with fresh inference
776 /// variables. (You can read more about the strategy in this [blog
779 /// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
780 /// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
781 /// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
782 /// establishes `'0: 'x` as a constraint.
784 /// As a side-effect of this generalization procedure, we also replace
785 /// all the bound regions that we have traversed with concrete values,
786 /// so that the resulting generalized type is independent from the
789 /// [blog post]: https://is.gd/0hKvIr
790 struct TypeGeneralizer<'me, 'tcx, D>
792 D: TypeRelatingDelegate<'tcx>,
794 infcx: &'me InferCtxt<'me, 'tcx>,
796 delegate: &'me mut D,
798 /// After we generalize this type, we are going to relative it to
799 /// some other type. What will be the variance at this point?
800 ambient_variance: ty::Variance,
802 first_free_index: ty::DebruijnIndex,
804 /// The vid of the type variable that is in the process of being
805 /// instantiated. If we find this within the value we are folding,
806 /// that means we would have created a cyclic value.
807 for_vid_sub_root: ty::TyVid,
809 /// The universe of the type variable that is in the process of being
810 /// instantiated. If we find anything that this universe cannot name,
811 /// we reject the relation.
812 universe: ty::UniverseIndex,
815 impl<D> TypeRelation<'tcx> for TypeGeneralizer<'me, 'tcx, D>
817 D: TypeRelatingDelegate<'tcx>,
819 fn tcx(&self) -> TyCtxt<'tcx> {
823 fn param_env(&self) -> ty::ParamEnv<'tcx> {
824 self.delegate.param_env()
827 fn tag(&self) -> &'static str {
831 fn a_is_expected(&self) -> bool {
835 fn relate_with_variance<T: Relate<'tcx>>(
837 variance: ty::Variance,
840 ) -> RelateResult<'tcx, T> {
842 "TypeGeneralizer::relate_with_variance(variance={:?}, a={:?}, b={:?})",
846 let old_ambient_variance = self.ambient_variance;
847 self.ambient_variance = self.ambient_variance.xform(variance);
850 "TypeGeneralizer::relate_with_variance: ambient_variance = {:?}",
851 self.ambient_variance
854 let r = self.relate(a, b)?;
856 self.ambient_variance = old_ambient_variance;
858 debug!("TypeGeneralizer::relate_with_variance: r={:?}", r);
863 fn tys(&mut self, a: Ty<'tcx>, _: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
864 use crate::infer::type_variable::TypeVariableValue;
866 debug!("TypeGeneralizer::tys(a={:?})", a);
869 ty::Infer(ty::TyVar(_)) | ty::Infer(ty::IntVar(_)) | ty::Infer(ty::FloatVar(_))
870 if D::forbid_inference_vars() =>
872 bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
875 ty::Infer(ty::TyVar(vid)) => {
876 let mut inner = self.infcx.inner.borrow_mut();
877 let variables = &mut inner.type_variables();
878 let vid = variables.root_var(vid);
879 let sub_vid = variables.sub_root_var(vid);
880 if sub_vid == self.for_vid_sub_root {
881 // If sub-roots are equal, then `for_vid` and
882 // `vid` are related via subtyping.
883 debug!("TypeGeneralizer::tys: occurs check failed");
884 Err(TypeError::Mismatch)
886 match variables.probe(vid) {
887 TypeVariableValue::Known { value: u } => {
891 TypeVariableValue::Unknown { universe: _universe } => {
892 if self.ambient_variance == ty::Bivariant {
893 // FIXME: we may need a WF predicate (related to #54105).
896 let origin = *variables.var_origin(vid);
898 // Replacing with a new variable in the universe `self.universe`,
899 // it will be unified later with the original type variable in
900 // the universe `_universe`.
901 let new_var_id = variables.new_var(self.universe, false, origin);
903 let u = self.tcx().mk_ty_var(new_var_id);
904 debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
911 ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
912 // No matter what mode we are in,
913 // integer/floating-point types must be equal to be
918 ty::Placeholder(placeholder) => {
919 if self.universe.cannot_name(placeholder.universe) {
921 "TypeGeneralizer::tys: root universe {:?} cannot name\
922 placeholder in universe {:?}",
923 self.universe, placeholder.universe
925 Err(TypeError::Mismatch)
931 _ => relate::super_relate_tys(self, a, a),
939 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
940 debug!("TypeGeneralizer::regions(a={:?})", a);
942 if let ty::ReLateBound(debruijn, _) = a {
943 if *debruijn < self.first_free_index {
948 // For now, we just always create a fresh region variable to
949 // replace all the regions in the source type. In the main
950 // type checker, we special case the case where the ambient
951 // variance is `Invariant` and try to avoid creating a fresh
952 // region variable, but since this comes up so much less in
953 // NLL (only when users use `_` etc) it is much less
956 // As an aside, since these new variables are created in
957 // `self.universe` universe, this also serves to enforce the
958 // universe scoping rules.
960 // FIXME(#54105) -- if the ambient variance is bivariant,
961 // though, we may however need to check well-formedness or
962 // risk a problem like #41677 again.
964 let replacement_region_vid = self.delegate.generalize_existential(self.universe);
966 Ok(replacement_region_vid)
971 a: &'tcx ty::Const<'tcx>,
972 _: &'tcx ty::Const<'tcx>,
973 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
975 ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
976 bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
978 ty::ConstKind::Infer(InferConst::Var(vid)) => {
979 let mut inner = self.infcx.inner.borrow_mut();
980 let variable_table = &mut inner.const_unification_table();
981 let var_value = variable_table.probe_value(vid);
982 match var_value.val.known() {
983 Some(u) => self.relate(u, u),
985 let new_var_id = variable_table.new_key(ConstVarValue {
986 origin: var_value.origin,
987 val: ConstVariableValue::Unknown { universe: self.universe },
989 Ok(self.tcx().mk_const_var(new_var_id, a.ty))
993 ty::ConstKind::Unevaluated(..) if self.tcx().lazy_normalization() => Ok(a),
994 _ => relate::super_relate_consts(self, a, a),
1000 a: ty::Binder<'tcx, T>,
1001 _: ty::Binder<'tcx, T>,
1002 ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
1006 debug!("TypeGeneralizer::binders(a={:?})", a);
1008 self.first_free_index.shift_in(1);
1009 let result = self.relate(a.skip_binder(), a.skip_binder())?;
1010 self.first_free_index.shift_out(1);
1011 Ok(a.rebind(result))