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 crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
28 use crate::traits::PredicateObligation;
29 use rustc_data_structures::fx::FxHashMap;
30 use rustc_middle::traits::ObligationCause;
31 use rustc_middle::ty::error::TypeError;
32 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
33 use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor};
34 use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
37 use std::ops::ControlFlow;
40 pub enum NormalizationStrategy {
45 pub struct TypeRelating<'me, 'tcx, D>
47 D: TypeRelatingDelegate<'tcx>,
49 infcx: &'me InferCtxt<'tcx>,
51 /// Callback to use when we deduce an outlives relationship.
54 /// How are we relating `a` and `b`?
56 /// - Covariant means `a <: b`.
57 /// - Contravariant means `b <: a`.
58 /// - Invariant means `a == b.
59 /// - Bivariant means that it doesn't matter.
60 ambient_variance: ty::Variance,
62 ambient_variance_info: ty::VarianceDiagInfo<'tcx>,
64 /// When we pass through a set of binders (e.g., when looking into
65 /// a `fn` type), we push a new bound region scope onto here. This
66 /// will contain the instantiated region for each region in those
67 /// binders. When we then encounter a `ReLateBound(d, br)`, we can
68 /// use the De Bruijn index `d` to find the right scope, and then
69 /// bound region name `br` to find the specific instantiation from
70 /// within that scope. See `replace_bound_region`.
72 /// This field stores the instantiations for late-bound regions in
74 a_scopes: Vec<BoundRegionScope<'tcx>>,
76 /// Same as `a_scopes`, but for the `b` type.
77 b_scopes: Vec<BoundRegionScope<'tcx>>,
80 pub trait TypeRelatingDelegate<'tcx> {
81 fn param_env(&self) -> ty::ParamEnv<'tcx>;
82 fn span(&self) -> Span;
84 /// Push a constraint `sup: sub` -- this constraint must be
85 /// satisfied for the two types to be related. `sub` and `sup` may
86 /// be regions from the type or new variables created through the
90 sup: ty::Region<'tcx>,
91 sub: ty::Region<'tcx>,
92 info: ty::VarianceDiagInfo<'tcx>,
95 fn const_equate(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
96 fn register_opaque_type_obligations(
98 obligations: Vec<PredicateObligation<'tcx>>,
99 ) -> Result<(), TypeError<'tcx>>;
101 /// Creates a new universe index. Used when instantiating placeholders.
102 fn create_next_universe(&mut self) -> ty::UniverseIndex;
104 /// Creates a new region variable representing a higher-ranked
105 /// region that is instantiated existentially. This creates an
106 /// inference variable, typically.
108 /// So e.g., if you have `for<'a> fn(..) <: for<'b> fn(..)`, then
109 /// we will invoke this method to instantiate `'a` with an
110 /// inference variable (though `'b` would be instantiated first,
111 /// as a placeholder).
112 fn next_existential_region_var(&mut self, was_placeholder: bool) -> ty::Region<'tcx>;
114 /// Creates a new region variable representing a
115 /// higher-ranked region that is instantiated universally.
116 /// This creates a new region placeholder, typically.
118 /// So e.g., if you have `for<'a> fn(..) <: for<'b> fn(..)`, then
119 /// we will invoke this method to instantiate `'b` with a
120 /// placeholder region.
121 fn next_placeholder_region(&mut self, placeholder: ty::PlaceholderRegion) -> ty::Region<'tcx>;
123 /// Creates a new existential region in the given universe. This
124 /// is used when handling subtyping and type variables -- if we
125 /// have that `?X <: Foo<'a>`, for example, we would instantiate
126 /// `?X` with a type like `Foo<'?0>` where `'?0` is a fresh
127 /// existential variable created by this function. We would then
128 /// relate `Foo<'?0>` with `Foo<'a>` (and probably add an outlives
129 /// relation stating that `'?0: 'a`).
130 fn generalize_existential(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx>;
132 /// Define the normalization strategy to use, eager or lazy.
133 fn normalization() -> NormalizationStrategy;
135 /// Enables some optimizations if we do not expect inference variables
136 /// in the RHS of the relation.
137 fn forbid_inference_vars() -> bool;
140 #[derive(Clone, Debug, Default)]
141 struct BoundRegionScope<'tcx> {
142 map: FxHashMap<ty::BoundRegion, ty::Region<'tcx>>,
145 #[derive(Copy, Clone)]
146 struct UniversallyQuantified(bool);
148 impl<'me, 'tcx, D> TypeRelating<'me, 'tcx, D>
150 D: TypeRelatingDelegate<'tcx>,
152 pub fn new(infcx: &'me InferCtxt<'tcx>, delegate: D, ambient_variance: ty::Variance) -> Self {
157 ambient_variance_info: ty::VarianceDiagInfo::default(),
163 fn ambient_covariance(&self) -> bool {
164 match self.ambient_variance {
165 ty::Variance::Covariant | ty::Variance::Invariant => true,
166 ty::Variance::Contravariant | ty::Variance::Bivariant => false,
170 fn ambient_contravariance(&self) -> bool {
171 match self.ambient_variance {
172 ty::Variance::Contravariant | ty::Variance::Invariant => true,
173 ty::Variance::Covariant | ty::Variance::Bivariant => false,
179 value: ty::Binder<'tcx, impl Relate<'tcx>>,
180 universally_quantified: UniversallyQuantified,
181 ) -> BoundRegionScope<'tcx> {
182 let mut scope = BoundRegionScope::default();
184 // Create a callback that creates (via the delegate) either an
185 // existential or placeholder region as needed.
186 let mut next_region = {
187 let delegate = &mut self.delegate;
188 let mut lazy_universe = None;
189 move |br: ty::BoundRegion| {
190 if universally_quantified.0 {
191 // The first time this closure is called, create a
192 // new universe for the placeholders we will make
194 let universe = lazy_universe.unwrap_or_else(|| {
195 let universe = delegate.create_next_universe();
196 lazy_universe = Some(universe);
200 let placeholder = ty::PlaceholderRegion { universe, name: br.kind };
201 delegate.next_placeholder_region(placeholder)
203 delegate.next_existential_region_var(true)
208 value.skip_binder().visit_with(&mut ScopeInstantiator {
209 next_region: &mut next_region,
210 target_index: ty::INNERMOST,
211 bound_region_scope: &mut scope,
217 /// When we encounter binders during the type traversal, we record
218 /// the value to substitute for each of the things contained in
219 /// that binder. (This will be either a universal placeholder or
220 /// an existential inference variable.) Given the De Bruijn index
221 /// `debruijn` (and name `br`) of some binder we have now
222 /// encountered, this routine finds the value that we instantiated
223 /// the region with; to do so, it indexes backwards into the list
224 /// of ambient scopes `scopes`.
225 fn lookup_bound_region(
226 debruijn: ty::DebruijnIndex,
227 br: &ty::BoundRegion,
228 first_free_index: ty::DebruijnIndex,
229 scopes: &[BoundRegionScope<'tcx>],
230 ) -> ty::Region<'tcx> {
231 // The debruijn index is a "reverse index" into the
232 // scopes listing. So when we have INNERMOST (0), we
233 // want the *last* scope pushed, and so forth.
234 let debruijn_index = debruijn.index() - first_free_index.index();
235 let scope = &scopes[scopes.len() - debruijn_index - 1];
237 // Find this bound region in that scope to map to a
238 // particular region.
242 /// If `r` is a bound region, find the scope in which it is bound
243 /// (from `scopes`) and return the value that we instantiated it
244 /// with. Otherwise just return `r`.
245 fn replace_bound_region(
248 first_free_index: ty::DebruijnIndex,
249 scopes: &[BoundRegionScope<'tcx>],
250 ) -> ty::Region<'tcx> {
251 debug!("replace_bound_regions(scopes={:?})", scopes);
252 if let ty::ReLateBound(debruijn, br) = *r {
253 Self::lookup_bound_region(debruijn, &br, first_free_index, scopes)
259 /// Push a new outlives requirement into our output set of
263 sup: ty::Region<'tcx>,
264 sub: ty::Region<'tcx>,
265 info: ty::VarianceDiagInfo<'tcx>,
267 debug!("push_outlives({:?}: {:?})", sup, sub);
269 self.delegate.push_outlives(sup, sub, info);
272 /// Relate a projection type and some value type lazily. This will always
273 /// succeed, but we push an additional `ProjectionEq` goal depending
274 /// on the value type:
275 /// - if the value type is any type `T` which is not a projection, we push
276 /// `ProjectionEq(projection = T)`.
277 /// - if the value type is another projection `other_projection`, we create
278 /// a new inference variable `?U` and push the two goals
279 /// `ProjectionEq(projection = ?U)`, `ProjectionEq(other_projection = ?U)`.
280 fn relate_projection_ty(
282 projection_ty: ty::ProjectionTy<'tcx>,
285 use rustc_span::DUMMY_SP;
287 match *value_ty.kind() {
288 ty::Projection(other_projection_ty) => {
289 let var = self.infcx.next_ty_var(TypeVariableOrigin {
290 kind: TypeVariableOriginKind::MiscVariable,
293 // FIXME(lazy-normalization): This will always ICE, because the recursive
294 // call will end up in the _ arm below.
295 self.relate_projection_ty(projection_ty, var);
296 self.relate_projection_ty(other_projection_ty, var);
300 _ => bug!("should never be invoked with eager normalization"),
304 /// Relate a type inference variable with a value type. This works
305 /// by creating a "generalization" G of the value where all the
306 /// lifetimes are replaced with fresh inference values. This
307 /// generalization G becomes the value of the inference variable,
308 /// and is then related in turn to the value. So e.g. if you had
309 /// `vid = ?0` and `value = &'a u32`, we might first instantiate
310 /// `?0` to a type like `&'0 u32` where `'0` is a fresh variable,
311 /// and then relate `&'0 u32` with `&'a u32` (resulting in
312 /// relations between `'0` and `'a`).
314 /// The variable `pair` can be either a `(vid, ty)` or `(ty, vid)`
315 /// -- in other words, it is always an (unresolved) inference
316 /// variable `vid` and a type `ty` that are being related, but the
317 /// vid may appear either as the "a" type or the "b" type,
318 /// depending on where it appears in the tuple. The trait
319 /// `VidValuePair` lets us work with the vid/type while preserving
320 /// the "sidedness" when necessary -- the sidedness is relevant in
321 /// particular for the variance and set of in-scope things.
322 fn relate_ty_var<PAIR: VidValuePair<'tcx>>(
325 ) -> RelateResult<'tcx, Ty<'tcx>> {
326 debug!("relate_ty_var({:?})", pair);
328 let vid = pair.vid();
329 let value_ty = pair.value_ty();
331 // FIXME(invariance) -- this logic assumes invariance, but that is wrong.
332 // This only presently applies to chalk integration, as NLL
333 // doesn't permit type variables to appear on both sides (and
334 // doesn't use lazy norm).
335 match *value_ty.kind() {
336 ty::Infer(ty::TyVar(value_vid)) => {
337 // Two type variables: just equate them.
338 self.infcx.inner.borrow_mut().type_variables().equate(vid, value_vid);
342 ty::Projection(projection_ty) if D::normalization() == NormalizationStrategy::Lazy => {
343 return Ok(self.relate_projection_ty(projection_ty, self.infcx.tcx.mk_ty_var(vid)));
349 let generalized_ty = self.generalize_value(value_ty, vid)?;
350 debug!("relate_ty_var: generalized_ty = {:?}", generalized_ty);
352 if D::forbid_inference_vars() {
353 // In NLL, we don't have type inference variables
354 // floating around, so we can do this rather imprecise
355 // variant of the occurs-check.
356 assert!(!generalized_ty.has_non_region_infer());
359 self.infcx.inner.borrow_mut().type_variables().instantiate(vid, generalized_ty);
361 // The generalized values we extract from `canonical_var_values` have
362 // been fully instantiated and hence the set of scopes we have
363 // doesn't matter -- just to be sure, put an empty vector
365 let old_a_scopes = std::mem::take(pair.vid_scopes(self));
367 // Relate the generalized kind to the original one.
368 let result = pair.relate_generalized_ty(self, generalized_ty);
370 // Restore the old scopes now.
371 *pair.vid_scopes(self) = old_a_scopes;
373 debug!("relate_ty_var: complete, result = {:?}", result);
377 fn generalize_value<T: Relate<'tcx>>(
381 ) -> RelateResult<'tcx, T> {
382 let universe = self.infcx.probe_ty_var(for_vid).unwrap_err();
384 let mut generalizer = TypeGeneralizer {
386 delegate: &mut self.delegate,
387 first_free_index: ty::INNERMOST,
388 ambient_variance: self.ambient_variance,
389 for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
393 generalizer.relate(value, value)
396 fn relate_opaques(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
397 let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
398 let mut generalize = |ty, ty_is_expected| {
399 let var = self.infcx.next_ty_var_id_in_universe(
401 kind: TypeVariableOriginKind::MiscVariable,
402 span: self.delegate.span(),
404 ty::UniverseIndex::ROOT,
407 self.relate_ty_var((ty, var))
409 self.relate_ty_var((var, ty))
412 let (a, b) = match (a.kind(), b.kind()) {
413 (&ty::Opaque(..), _) => (a, generalize(b, false)?),
414 (_, &ty::Opaque(..)) => (generalize(a, true)?, b),
417 let cause = ObligationCause::dummy_with_span(self.delegate.span());
418 let obligations = self
420 .handle_opaque_type(a, b, true, &cause, self.delegate.param_env())?
422 self.delegate.register_opaque_type_obligations(obligations)?;
423 trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
428 /// When we instantiate an inference variable with a value in
429 /// `relate_ty_var`, we always have the pair of a `TyVid` and a `Ty`,
430 /// but the ordering may vary (depending on whether the inference
431 /// variable was found on the `a` or `b` sides). Therefore, this trait
432 /// allows us to factor out common code, while preserving the order
434 trait VidValuePair<'tcx>: Debug {
435 /// Extract the inference variable (which could be either the
436 /// first or second part of the tuple).
437 fn vid(&self) -> ty::TyVid;
439 /// Extract the value it is being related to (which will be the
440 /// opposite part of the tuple from the vid).
441 fn value_ty(&self) -> Ty<'tcx>;
443 /// Extract the scopes that apply to whichever side of the tuple
444 /// the vid was found on. See the comment where this is called
445 /// for more details on why we want them.
446 fn vid_scopes<'r, D: TypeRelatingDelegate<'tcx>>(
448 relate: &'r mut TypeRelating<'_, 'tcx, D>,
449 ) -> &'r mut Vec<BoundRegionScope<'tcx>>;
451 /// Given a generalized type G that should replace the vid, relate
452 /// G to the value, putting G on whichever side the vid would have
454 fn relate_generalized_ty<D>(
456 relate: &mut TypeRelating<'_, 'tcx, D>,
457 generalized_ty: Ty<'tcx>,
458 ) -> RelateResult<'tcx, Ty<'tcx>>
460 D: TypeRelatingDelegate<'tcx>;
463 impl<'tcx> VidValuePair<'tcx> for (ty::TyVid, Ty<'tcx>) {
464 fn vid(&self) -> ty::TyVid {
468 fn value_ty(&self) -> Ty<'tcx> {
472 fn vid_scopes<'r, D>(
474 relate: &'r mut TypeRelating<'_, 'tcx, D>,
475 ) -> &'r mut Vec<BoundRegionScope<'tcx>>
477 D: TypeRelatingDelegate<'tcx>,
482 fn relate_generalized_ty<D>(
484 relate: &mut TypeRelating<'_, 'tcx, D>,
485 generalized_ty: Ty<'tcx>,
486 ) -> RelateResult<'tcx, Ty<'tcx>>
488 D: TypeRelatingDelegate<'tcx>,
490 relate.relate(generalized_ty, self.value_ty())
494 // In this case, the "vid" is the "b" type.
495 impl<'tcx> VidValuePair<'tcx> for (Ty<'tcx>, ty::TyVid) {
496 fn vid(&self) -> ty::TyVid {
500 fn value_ty(&self) -> Ty<'tcx> {
504 fn vid_scopes<'r, D>(
506 relate: &'r mut TypeRelating<'_, 'tcx, D>,
507 ) -> &'r mut Vec<BoundRegionScope<'tcx>>
509 D: TypeRelatingDelegate<'tcx>,
514 fn relate_generalized_ty<D>(
516 relate: &mut TypeRelating<'_, 'tcx, D>,
517 generalized_ty: Ty<'tcx>,
518 ) -> RelateResult<'tcx, Ty<'tcx>>
520 D: TypeRelatingDelegate<'tcx>,
522 relate.relate(self.value_ty(), generalized_ty)
526 impl<'tcx, D> TypeRelation<'tcx> for TypeRelating<'_, 'tcx, D>
528 D: TypeRelatingDelegate<'tcx>,
530 fn tcx(&self) -> TyCtxt<'tcx> {
534 fn param_env(&self) -> ty::ParamEnv<'tcx> {
535 self.delegate.param_env()
538 fn tag(&self) -> &'static str {
542 fn a_is_expected(&self) -> bool {
546 #[instrument(skip(self, info), level = "trace", ret)]
547 fn relate_with_variance<T: Relate<'tcx>>(
549 variance: ty::Variance,
550 info: ty::VarianceDiagInfo<'tcx>,
553 ) -> RelateResult<'tcx, T> {
554 let old_ambient_variance = self.ambient_variance;
555 self.ambient_variance = self.ambient_variance.xform(variance);
556 self.ambient_variance_info = self.ambient_variance_info.xform(info);
558 debug!(?self.ambient_variance);
559 // In a bivariant context this always succeeds.
561 if self.ambient_variance == ty::Variance::Bivariant { a } else { self.relate(a, b)? };
563 self.ambient_variance = old_ambient_variance;
568 #[instrument(skip(self), level = "debug")]
569 fn tys(&mut self, a: Ty<'tcx>, mut b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
570 let infcx = self.infcx;
572 let a = self.infcx.shallow_resolve(a);
574 if !D::forbid_inference_vars() {
575 b = self.infcx.shallow_resolve(b);
579 // Subtle: if a or b has a bound variable that we are lazily
580 // substituting, then even if a == b, it could be that the values we
581 // will substitute for those bound variables are *not* the same, and
582 // hence returning `Ok(a)` is incorrect.
583 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
588 match (a.kind(), b.kind()) {
589 (_, &ty::Infer(ty::TyVar(vid))) => {
590 if D::forbid_inference_vars() {
591 // Forbid inference variables in the RHS.
592 bug!("unexpected inference var {:?}", b)
594 self.relate_ty_var((a, vid))
598 (&ty::Infer(ty::TyVar(vid)), _) => self.relate_ty_var((vid, b)),
600 (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
601 infcx.super_combine_tys(self, a, b).or_else(|err| {
602 self.tcx().sess.delay_span_bug(
603 self.delegate.span(),
604 "failure to relate an opaque to itself should result in an error later on",
606 if a_def_id.is_local() { self.relate_opaques(a, b) } else { Err(err) }
609 (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..)) if did.is_local() => {
610 self.relate_opaques(a, b)
613 (&ty::Projection(projection_ty), _)
614 if D::normalization() == NormalizationStrategy::Lazy =>
616 Ok(self.relate_projection_ty(projection_ty, b))
619 (_, &ty::Projection(projection_ty))
620 if D::normalization() == NormalizationStrategy::Lazy =>
622 Ok(self.relate_projection_ty(projection_ty, a))
626 debug!(?a, ?b, ?self.ambient_variance);
628 // Will also handle unification of `IntVar` and `FloatVar`.
629 self.infcx.super_combine_tys(self, a, b)
634 #[instrument(skip(self), level = "trace")]
639 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
640 debug!(?self.ambient_variance);
642 let v_a = self.replace_bound_region(a, ty::INNERMOST, &self.a_scopes);
643 let v_b = self.replace_bound_region(b, ty::INNERMOST, &self.b_scopes);
648 if self.ambient_covariance() {
649 // Covariance: a <= b. Hence, `b: a`.
650 self.push_outlives(v_b, v_a, self.ambient_variance_info);
653 if self.ambient_contravariance() {
654 // Contravariant: b <= a. Hence, `a: b`.
655 self.push_outlives(v_a, v_b, self.ambient_variance_info);
664 mut b: ty::Const<'tcx>,
665 ) -> RelateResult<'tcx, ty::Const<'tcx>> {
666 let a = self.infcx.shallow_resolve(a);
668 if !D::forbid_inference_vars() {
669 b = self.infcx.shallow_resolve(b);
673 ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
674 // Forbid inference variables in the RHS.
675 self.infcx.tcx.sess.delay_span_bug(
676 self.delegate.span(),
677 format!("unexpected inference var {:?}", b,),
681 // FIXME(invariance): see the related FIXME above.
682 _ => self.infcx.super_combine_consts(self, a, b),
686 #[instrument(skip(self), level = "trace")]
689 a: ty::Binder<'tcx, T>,
690 b: ty::Binder<'tcx, T>,
691 ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
698 // for<'a> fn(&'a u32) -> &'a u32 <:
699 // fn(&'b u32) -> &'b u32
705 // fn(&'a u32) -> &'a u32 <:
706 // for<'b> fn(&'b u32) -> &'b u32
709 // We therefore proceed as follows:
711 // - Instantiate binders on `b` universally, yielding a universe U1.
712 // - Instantiate binders on `a` existentially in U1.
714 debug!(?self.ambient_variance);
716 if let (Some(a), Some(b)) = (a.no_bound_vars(), b.no_bound_vars()) {
717 // Fast path for the common case.
719 return Ok(ty::Binder::dummy(a));
722 if self.ambient_covariance() {
723 // Covariance, so we want `for<..> A <: for<..> B` --
724 // therefore we compare any instantiation of A (i.e., A
725 // instantiated with existentials) against every
726 // instantiation of B (i.e., B instantiated with
729 let b_scope = self.create_scope(b, UniversallyQuantified(true));
730 let a_scope = self.create_scope(a, UniversallyQuantified(false));
732 debug!(?a_scope, "(existential)");
733 debug!(?b_scope, "(universal)");
735 self.b_scopes.push(b_scope);
736 self.a_scopes.push(a_scope);
738 // Reset the ambient variance to covariant. This is needed
739 // to correctly handle cases like
741 // for<'a> fn(&'a u32, &'a u32) == for<'b, 'c> fn(&'b u32, &'c u32)
743 // Somewhat surprisingly, these two types are actually
744 // **equal**, even though the one on the right looks more
745 // polymorphic. The reason is due to subtyping. To see it,
746 // consider that each function can call the other:
748 // - The left function can call the right with `'b` and
749 // `'c` both equal to `'a`
751 // - The right function can call the left with `'a` set to
752 // `{P}`, where P is the point in the CFG where the call
753 // itself occurs. Note that `'b` and `'c` must both
754 // include P. At the point, the call works because of
755 // subtyping (i.e., `&'b u32 <: &{P} u32`).
756 let variance = std::mem::replace(&mut self.ambient_variance, ty::Variance::Covariant);
758 self.relate(a.skip_binder(), b.skip_binder())?;
760 self.ambient_variance = variance;
762 self.b_scopes.pop().unwrap();
763 self.a_scopes.pop().unwrap();
766 if self.ambient_contravariance() {
767 // Contravariance, so we want `for<..> A :> for<..> B`
768 // -- therefore we compare every instantiation of A (i.e.,
769 // A instantiated with universals) against any
770 // instantiation of B (i.e., B instantiated with
771 // existentials). Opposite of above.
773 let a_scope = self.create_scope(a, UniversallyQuantified(true));
774 let b_scope = self.create_scope(b, UniversallyQuantified(false));
776 debug!(?a_scope, "(universal)");
777 debug!(?b_scope, "(existential)");
779 self.a_scopes.push(a_scope);
780 self.b_scopes.push(b_scope);
782 // Reset ambient variance to contravariance. See the
783 // covariant case above for an explanation.
785 std::mem::replace(&mut self.ambient_variance, ty::Variance::Contravariant);
787 self.relate(a.skip_binder(), b.skip_binder())?;
789 self.ambient_variance = variance;
791 self.b_scopes.pop().unwrap();
792 self.a_scopes.pop().unwrap();
799 impl<'tcx, D> ConstEquateRelation<'tcx> for TypeRelating<'_, 'tcx, D>
801 D: TypeRelatingDelegate<'tcx>,
803 fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
804 self.delegate.const_equate(a, b);
808 /// When we encounter a binder like `for<..> fn(..)`, we actually have
809 /// to walk the `fn` value to find all the values bound by the `for`
810 /// (these are not explicitly present in the ty representation right
811 /// now). This visitor handles that: it descends the type, tracking
812 /// binder depth, and finds late-bound regions targeting the
813 /// `for<..`>. For each of those, it creates an entry in
814 /// `bound_region_scope`.
815 struct ScopeInstantiator<'me, 'tcx> {
816 next_region: &'me mut dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
817 // The debruijn index of the scope we are instantiating.
818 target_index: ty::DebruijnIndex,
819 bound_region_scope: &'me mut BoundRegionScope<'tcx>,
822 impl<'me, 'tcx> TypeVisitor<'tcx> for ScopeInstantiator<'me, 'tcx> {
823 fn visit_binder<T: TypeVisitable<'tcx>>(
825 t: &ty::Binder<'tcx, T>,
826 ) -> ControlFlow<Self::BreakTy> {
827 self.target_index.shift_in(1);
828 t.super_visit_with(self);
829 self.target_index.shift_out(1);
831 ControlFlow::CONTINUE
834 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
835 let ScopeInstantiator { bound_region_scope, next_region, .. } = self;
838 ty::ReLateBound(debruijn, br) if debruijn == self.target_index => {
839 bound_region_scope.map.entry(br).or_insert_with(|| next_region(br));
845 ControlFlow::CONTINUE
849 /// The "type generalizer" is used when handling inference variables.
851 /// The basic strategy for handling a constraint like `?A <: B` is to
852 /// apply a "generalization strategy" to the type `B` -- this replaces
853 /// all the lifetimes in the type `B` with fresh inference
854 /// variables. (You can read more about the strategy in this [blog
857 /// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
858 /// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
859 /// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
860 /// establishes `'0: 'x` as a constraint.
862 /// As a side-effect of this generalization procedure, we also replace
863 /// all the bound regions that we have traversed with concrete values,
864 /// so that the resulting generalized type is independent from the
867 /// [blog post]: https://is.gd/0hKvIr
868 struct TypeGeneralizer<'me, 'tcx, D>
870 D: TypeRelatingDelegate<'tcx>,
872 infcx: &'me InferCtxt<'tcx>,
874 delegate: &'me mut D,
876 /// After we generalize this type, we are going to relate it to
877 /// some other type. What will be the variance at this point?
878 ambient_variance: ty::Variance,
880 first_free_index: ty::DebruijnIndex,
882 /// The vid of the type variable that is in the process of being
883 /// instantiated. If we find this within the value we are folding,
884 /// that means we would have created a cyclic value.
885 for_vid_sub_root: ty::TyVid,
887 /// The universe of the type variable that is in the process of being
888 /// instantiated. If we find anything that this universe cannot name,
889 /// we reject the relation.
890 universe: ty::UniverseIndex,
893 impl<'tcx, D> TypeRelation<'tcx> for TypeGeneralizer<'_, 'tcx, D>
895 D: TypeRelatingDelegate<'tcx>,
897 fn tcx(&self) -> TyCtxt<'tcx> {
901 fn param_env(&self) -> ty::ParamEnv<'tcx> {
902 self.delegate.param_env()
905 fn tag(&self) -> &'static str {
909 fn a_is_expected(&self) -> bool {
913 fn relate_with_variance<T: Relate<'tcx>>(
915 variance: ty::Variance,
916 _info: ty::VarianceDiagInfo<'tcx>,
919 ) -> RelateResult<'tcx, T> {
921 "TypeGeneralizer::relate_with_variance(variance={:?}, a={:?}, b={:?})",
925 let old_ambient_variance = self.ambient_variance;
926 self.ambient_variance = self.ambient_variance.xform(variance);
929 "TypeGeneralizer::relate_with_variance: ambient_variance = {:?}",
930 self.ambient_variance
933 let r = self.relate(a, b)?;
935 self.ambient_variance = old_ambient_variance;
937 debug!("TypeGeneralizer::relate_with_variance: r={:?}", r);
942 fn tys(&mut self, a: Ty<'tcx>, _: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
943 use crate::infer::type_variable::TypeVariableValue;
945 debug!("TypeGeneralizer::tys(a={:?})", a);
948 ty::Infer(ty::TyVar(_)) | ty::Infer(ty::IntVar(_)) | ty::Infer(ty::FloatVar(_))
949 if D::forbid_inference_vars() =>
951 bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
954 ty::Infer(ty::TyVar(vid)) => {
955 let mut inner = self.infcx.inner.borrow_mut();
956 let variables = &mut inner.type_variables();
957 let vid = variables.root_var(vid);
958 let sub_vid = variables.sub_root_var(vid);
959 if sub_vid == self.for_vid_sub_root {
960 // If sub-roots are equal, then `for_vid` and
961 // `vid` are related via subtyping.
962 debug!("TypeGeneralizer::tys: occurs check failed");
963 Err(TypeError::Mismatch)
965 match variables.probe(vid) {
966 TypeVariableValue::Known { value: u } => {
970 TypeVariableValue::Unknown { universe: _universe } => {
971 if self.ambient_variance == ty::Bivariant {
972 // FIXME: we may need a WF predicate (related to #54105).
975 let origin = *variables.var_origin(vid);
977 // Replacing with a new variable in the universe `self.universe`,
978 // it will be unified later with the original type variable in
979 // the universe `_universe`.
980 let new_var_id = variables.new_var(self.universe, origin);
982 let u = self.tcx().mk_ty_var(new_var_id);
983 debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
990 ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
991 // No matter what mode we are in,
992 // integer/floating-point types must be equal to be
997 ty::Placeholder(placeholder) => {
998 if self.universe.cannot_name(placeholder.universe) {
1000 "TypeGeneralizer::tys: root universe {:?} cannot name\
1001 placeholder in universe {:?}",
1002 self.universe, placeholder.universe
1004 Err(TypeError::Mismatch)
1010 _ => relate::super_relate_tys(self, a, a),
1016 a: ty::Region<'tcx>,
1017 _: ty::Region<'tcx>,
1018 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
1019 debug!("TypeGeneralizer::regions(a={:?})", a);
1021 if let ty::ReLateBound(debruijn, _) = *a && debruijn < self.first_free_index {
1025 // For now, we just always create a fresh region variable to
1026 // replace all the regions in the source type. In the main
1027 // type checker, we special case the case where the ambient
1028 // variance is `Invariant` and try to avoid creating a fresh
1029 // region variable, but since this comes up so much less in
1030 // NLL (only when users use `_` etc) it is much less
1033 // As an aside, since these new variables are created in
1034 // `self.universe` universe, this also serves to enforce the
1035 // universe scoping rules.
1037 // FIXME(#54105) -- if the ambient variance is bivariant,
1038 // though, we may however need to check well-formedness or
1039 // risk a problem like #41677 again.
1041 let replacement_region_vid = self.delegate.generalize_existential(self.universe);
1043 Ok(replacement_region_vid)
1050 ) -> RelateResult<'tcx, ty::Const<'tcx>> {
1052 ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
1053 bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
1055 ty::ConstKind::Infer(InferConst::Var(vid)) => {
1056 let mut inner = self.infcx.inner.borrow_mut();
1057 let variable_table = &mut inner.const_unification_table();
1058 let var_value = variable_table.probe_value(vid);
1059 match var_value.val.known() {
1060 Some(u) => self.relate(u, u),
1062 let new_var_id = variable_table.new_key(ConstVarValue {
1063 origin: var_value.origin,
1064 val: ConstVariableValue::Unknown { universe: self.universe },
1066 Ok(self.tcx().mk_const_var(new_var_id, a.ty()))
1070 ty::ConstKind::Unevaluated(..) if self.tcx().lazy_normalization() => Ok(a),
1071 _ => relate::super_relate_consts(self, a, a),
1077 a: ty::Binder<'tcx, T>,
1078 _: ty::Binder<'tcx, T>,
1079 ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
1083 debug!("TypeGeneralizer::binders(a={:?})", a);
1085 self.first_free_index.shift_in(1);
1086 let result = self.relate(a.skip_binder(), a.skip_binder())?;
1087 self.first_free_index.shift_out(1);
1088 Ok(a.rebind(result))