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
::GenericArg
;
30 use crate::ty
::{self, Ty, TyCtxt, InferConst}
;
31 use crate::infer
::{ConstVariableValue, ConstVarValue}
;
32 use rustc_data_structures
::fx
::FxHashMap
;
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, 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)]
125 struct ScopesAndKind
<'tcx
> {
126 scopes
: Vec
<BoundRegionScope
<'tcx
>>,
127 kind
: GenericArg
<'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(true)
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
;
277 match value_ty
.kind
{
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(invariance) -- 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).
331 match value_ty
.kind
{
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
::take(pair
.vid_scopes(self));
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 // FIXME(oli-obk): not sure how to get the correct ParamEnv
506 fn param_env(&self) -> ty
::ParamEnv
<'tcx
> { ty::ParamEnv::empty() }
508 fn tag(&self) -> &'
static str {
512 fn a_is_expected(&self) -> bool
{
516 fn relate_with_variance
<T
: Relate
<'tcx
>>(
518 variance
: ty
::Variance
,
521 ) -> RelateResult
<'tcx
, T
> {
523 "relate_with_variance(variance={:?}, a={:?}, b={:?})",
527 let old_ambient_variance
= self.ambient_variance
;
528 self.ambient_variance
= self.ambient_variance
.xform(variance
);
531 "relate_with_variance: ambient_variance = {:?}",
532 self.ambient_variance
535 let r
= self.relate(a
, b
)?
;
537 self.ambient_variance
= old_ambient_variance
;
539 debug
!("relate_with_variance: r={:?}", r
);
544 fn tys(&mut self, a
: Ty
<'tcx
>, mut b
: Ty
<'tcx
>) -> RelateResult
<'tcx
, Ty
<'tcx
>> {
545 let a
= self.infcx
.shallow_resolve(a
);
547 if !D
::forbid_inference_vars() {
548 b
= self.infcx
.shallow_resolve(b
);
551 match (&a
.kind
, &b
.kind
) {
552 (_
, &ty
::Infer(ty
::TyVar(vid
))) => {
553 if D
::forbid_inference_vars() {
554 // Forbid inference variables in the RHS.
555 bug
!("unexpected inference var {:?}", b
)
557 self.relate_ty_var((a
, vid
))
561 (&ty
::Infer(ty
::TyVar(vid
)), _
) => self.relate_ty_var((vid
, b
)),
563 (&ty
::Projection(projection_ty
), _
)
564 if D
::normalization() == NormalizationStrategy
::Lazy
=>
566 Ok(self.relate_projection_ty(projection_ty
, b
))
569 (_
, &ty
::Projection(projection_ty
))
570 if D
::normalization() == NormalizationStrategy
::Lazy
=>
572 Ok(self.relate_projection_ty(projection_ty
, a
))
577 "tys(a={:?}, b={:?}, variance={:?})",
578 a
, b
, self.ambient_variance
581 // Will also handle unification of `IntVar` and `FloatVar`.
582 self.infcx
.super_combine_tys(self, a
, b
)
591 ) -> RelateResult
<'tcx
, ty
::Region
<'tcx
>> {
593 "regions(a={:?}, b={:?}, variance={:?})",
594 a
, b
, self.ambient_variance
597 let v_a
= self.replace_bound_region(a
, ty
::INNERMOST
, &self.a_scopes
);
598 let v_b
= self.replace_bound_region(b
, ty
::INNERMOST
, &self.b_scopes
);
600 debug
!("regions: v_a = {:?}", v_a
);
601 debug
!("regions: v_b = {:?}", v_b
);
603 if self.ambient_covariance() {
604 // Covariance: a <= b. Hence, `b: a`.
605 self.push_outlives(v_b
, v_a
);
608 if self.ambient_contravariance() {
609 // Contravariant: b <= a. Hence, `a: b`.
610 self.push_outlives(v_a
, v_b
);
618 a
: &'tcx ty
::Const
<'tcx
>,
619 mut b
: &'tcx ty
::Const
<'tcx
>,
620 ) -> RelateResult
<'tcx
, &'tcx ty
::Const
<'tcx
>> {
621 let a
= self.infcx
.shallow_resolve(a
);
623 if !D
::forbid_inference_vars() {
624 b
= self.infcx
.shallow_resolve(b
);
628 ty
::ConstKind
::Infer(InferConst
::Var(_
)) if D
::forbid_inference_vars() => {
629 // Forbid inference variables in the RHS.
630 bug
!("unexpected inference var {:?}", b
)
632 // FIXME(invariance): see the related FIXME above.
633 _
=> self.infcx
.super_combine_consts(self, a
, b
)
641 ) -> RelateResult
<'tcx
, ty
::Binder
<T
>>
648 // for<'a> fn(&'a u32) -> &'a u32 <:
649 // fn(&'b u32) -> &'b u32
655 // fn(&'a u32) -> &'a u32 <:
656 // for<'b> fn(&'b u32) -> &'b u32
659 // We therefore proceed as follows:
661 // - Instantiate binders on `b` universally, yielding a universe U1.
662 // - Instantiate binders on `a` existentially in U1.
665 "binders({:?}: {:?}, ambient_variance={:?})",
666 a
, b
, self.ambient_variance
669 if self.ambient_covariance() {
670 // Covariance, so we want `for<..> A <: for<..> B` --
671 // therefore we compare any instantiation of A (i.e., A
672 // instantiated with existentials) against every
673 // instantiation of B (i.e., B instantiated with
676 let b_scope
= self.create_scope(b
, UniversallyQuantified(true));
677 let a_scope
= self.create_scope(a
, UniversallyQuantified(false));
679 debug
!("binders: a_scope = {:?} (existential)", a_scope
);
680 debug
!("binders: b_scope = {:?} (universal)", b_scope
);
682 self.b_scopes
.push(b_scope
);
683 self.a_scopes
.push(a_scope
);
685 // Reset the ambient variance to covariant. This is needed
686 // to correctly handle cases like
688 // for<'a> fn(&'a u32, &'a u3) == for<'b, 'c> fn(&'b u32, &'c u32)
690 // Somewhat surprisingly, these two types are actually
691 // **equal**, even though the one on the right looks more
692 // polymorphic. The reason is due to subtyping. To see it,
693 // consider that each function can call the other:
695 // - The left function can call the right with `'b` and
696 // `'c` both equal to `'a`
698 // - The right function can call the left with `'a` set to
699 // `{P}`, where P is the point in the CFG where the call
700 // itself occurs. Note that `'b` and `'c` must both
701 // include P. At the point, the call works because of
702 // subtyping (i.e., `&'b u32 <: &{P} u32`).
703 let variance
= ::std
::mem
::replace(&mut self.ambient_variance
, ty
::Variance
::Covariant
);
705 self.relate(a
.skip_binder(), b
.skip_binder())?
;
707 self.ambient_variance
= variance
;
709 self.b_scopes
.pop().unwrap();
710 self.a_scopes
.pop().unwrap();
713 if self.ambient_contravariance() {
714 // Contravariance, so we want `for<..> A :> for<..> B`
715 // -- therefore we compare every instantiation of A (i.e.,
716 // A instantiated with universals) against any
717 // instantiation of B (i.e., B instantiated with
718 // existentials). Opposite of above.
720 let a_scope
= self.create_scope(a
, UniversallyQuantified(true));
721 let b_scope
= self.create_scope(b
, UniversallyQuantified(false));
723 debug
!("binders: a_scope = {:?} (universal)", a_scope
);
724 debug
!("binders: b_scope = {:?} (existential)", b_scope
);
726 self.a_scopes
.push(a_scope
);
727 self.b_scopes
.push(b_scope
);
729 // Reset ambient variance to contravariance. See the
730 // covariant case above for an explanation.
732 ::std
::mem
::replace(&mut self.ambient_variance
, ty
::Variance
::Contravariant
);
734 self.relate(a
.skip_binder(), b
.skip_binder())?
;
736 self.ambient_variance
= variance
;
738 self.b_scopes
.pop().unwrap();
739 self.a_scopes
.pop().unwrap();
746 /// When we encounter a binder like `for<..> fn(..)`, we actually have
747 /// to walk the `fn` value to find all the values bound by the `for`
748 /// (these are not explicitly present in the ty representation right
749 /// now). This visitor handles that: it descends the type, tracking
750 /// binder depth, and finds late-bound regions targeting the
751 /// `for<..`>. For each of those, it creates an entry in
752 /// `bound_region_scope`.
753 struct ScopeInstantiator
<'me
, 'tcx
> {
754 next_region
: &'me
mut dyn FnMut(ty
::BoundRegion
) -> ty
::Region
<'tcx
>,
755 // The debruijn index of the scope we are instantiating.
756 target_index
: ty
::DebruijnIndex
,
757 bound_region_scope
: &'me
mut BoundRegionScope
<'tcx
>,
760 impl<'me
, 'tcx
> TypeVisitor
<'tcx
> for ScopeInstantiator
<'me
, 'tcx
> {
761 fn visit_binder
<T
: TypeFoldable
<'tcx
>>(&mut self, t
: &ty
::Binder
<T
>) -> bool
{
762 self.target_index
.shift_in(1);
763 t
.super_visit_with(self);
764 self.target_index
.shift_out(1);
769 fn visit_region(&mut self, r
: ty
::Region
<'tcx
>) -> bool
{
770 let ScopeInstantiator
{
777 ty
::ReLateBound(debruijn
, br
) if *debruijn
== self.target_index
=> {
781 .or_insert_with(|| next_region(*br
));
791 /// The "type generalize" is used when handling inference variables.
793 /// The basic strategy for handling a constraint like `?A <: B` is to
794 /// apply a "generalization strategy" to the type `B` -- this replaces
795 /// all the lifetimes in the type `B` with fresh inference
796 /// variables. (You can read more about the strategy in this [blog
799 /// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
800 /// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
801 /// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
802 /// establishes `'0: 'x` as a constraint.
804 /// As a side-effect of this generalization procedure, we also replace
805 /// all the bound regions that we have traversed with concrete values,
806 /// so that the resulting generalized type is independent from the
809 /// [blog post]: https://is.gd/0hKvIr
810 struct TypeGeneralizer
<'me
, 'tcx
, D
>
812 D
: TypeRelatingDelegate
<'tcx
>,
814 infcx
: &'me InferCtxt
<'me
, 'tcx
>,
816 delegate
: &'me
mut D
,
818 /// After we generalize this type, we are going to relative it to
819 /// some other type. What will be the variance at this point?
820 ambient_variance
: ty
::Variance
,
822 first_free_index
: ty
::DebruijnIndex
,
824 /// The vid of the type variable that is in the process of being
825 /// instantiated. If we find this within the value we are folding,
826 /// that means we would have created a cyclic value.
827 for_vid_sub_root
: ty
::TyVid
,
829 /// The universe of the type variable that is in the process of being
830 /// instantiated. If we find anything that this universe cannot name,
831 /// we reject the relation.
832 universe
: ty
::UniverseIndex
,
835 impl<D
> TypeRelation
<'tcx
> for TypeGeneralizer
<'me
, 'tcx
, D
>
837 D
: TypeRelatingDelegate
<'tcx
>,
839 fn tcx(&self) -> TyCtxt
<'tcx
> {
843 // FIXME(oli-obk): not sure how to get the correct ParamEnv
844 fn param_env(&self) -> ty
::ParamEnv
<'tcx
> { ty::ParamEnv::empty() }
846 fn tag(&self) -> &'
static str {
850 fn a_is_expected(&self) -> bool
{
854 fn relate_with_variance
<T
: Relate
<'tcx
>>(
856 variance
: ty
::Variance
,
859 ) -> RelateResult
<'tcx
, T
> {
861 "TypeGeneralizer::relate_with_variance(variance={:?}, a={:?}, b={:?})",
865 let old_ambient_variance
= self.ambient_variance
;
866 self.ambient_variance
= self.ambient_variance
.xform(variance
);
869 "TypeGeneralizer::relate_with_variance: ambient_variance = {:?}",
870 self.ambient_variance
873 let r
= self.relate(a
, b
)?
;
875 self.ambient_variance
= old_ambient_variance
;
877 debug
!("TypeGeneralizer::relate_with_variance: r={:?}", r
);
882 fn tys(&mut self, a
: Ty
<'tcx
>, _
: Ty
<'tcx
>) -> RelateResult
<'tcx
, Ty
<'tcx
>> {
883 use crate::infer
::type_variable
::TypeVariableValue
;
885 debug
!("TypeGeneralizer::tys(a={:?})", a
);
888 ty
::Infer(ty
::TyVar(_
)) | ty
::Infer(ty
::IntVar(_
)) | ty
::Infer(ty
::FloatVar(_
))
889 if D
::forbid_inference_vars() =>
892 "unexpected inference variable encountered in NLL generalization: {:?}",
897 ty
::Infer(ty
::TyVar(vid
)) => {
898 let mut variables
= self.infcx
.type_variables
.borrow_mut();
899 let vid
= variables
.root_var(vid
);
900 let sub_vid
= variables
.sub_root_var(vid
);
901 if sub_vid
== self.for_vid_sub_root
{
902 // If sub-roots are equal, then `for_vid` and
903 // `vid` are related via subtyping.
904 debug
!("TypeGeneralizer::tys: occurs check failed");
905 return Err(TypeError
::Mismatch
);
907 match variables
.probe(vid
) {
908 TypeVariableValue
::Known { value: u }
=> {
912 TypeVariableValue
::Unknown
{
915 if self.ambient_variance
== ty
::Bivariant
{
916 // FIXME: we may need a WF predicate (related to #54105).
919 let origin
= *variables
.var_origin(vid
);
921 // Replacing with a new variable in the universe `self.universe`,
922 // it will be unified later with the original type variable in
923 // the universe `_universe`.
924 let new_var_id
= variables
.new_var(self.universe
, false, origin
);
926 let u
= self.tcx().mk_ty_var(new_var_id
);
928 "generalize: replacing original vid={:?} with new={:?}",
937 ty
::Infer(ty
::IntVar(_
)) | ty
::Infer(ty
::FloatVar(_
)) => {
938 // No matter what mode we are in,
939 // integer/floating-point types must be equal to be
944 ty
::Placeholder(placeholder
) => {
945 if self.universe
.cannot_name(placeholder
.universe
) {
947 "TypeGeneralizer::tys: root universe {:?} cannot name\
948 placeholder in universe {:?}",
949 self.universe
, placeholder
.universe
951 Err(TypeError
::Mismatch
)
957 _
=> relate
::super_relate_tys(self, a
, a
),
965 ) -> RelateResult
<'tcx
, ty
::Region
<'tcx
>> {
966 debug
!("TypeGeneralizer::regions(a={:?})", a
);
968 if let ty
::ReLateBound(debruijn
, _
) = a
{
969 if *debruijn
< self.first_free_index
{
974 // For now, we just always create a fresh region variable to
975 // replace all the regions in the source type. In the main
976 // type checker, we special case the case where the ambient
977 // variance is `Invariant` and try to avoid creating a fresh
978 // region variable, but since this comes up so much less in
979 // NLL (only when users use `_` etc) it is much less
982 // As an aside, since these new variables are created in
983 // `self.universe` universe, this also serves to enforce the
984 // universe scoping rules.
986 // FIXME(#54105) -- if the ambient variance is bivariant,
987 // though, we may however need to check well-formedness or
988 // risk a problem like #41677 again.
990 let replacement_region_vid
= self.delegate
.generalize_existential(self.universe
);
992 Ok(replacement_region_vid
)
997 a
: &'tcx ty
::Const
<'tcx
>,
998 _
: &'tcx ty
::Const
<'tcx
>,
999 ) -> RelateResult
<'tcx
, &'tcx ty
::Const
<'tcx
>> {
1001 ty
::ConstKind
::Infer(InferConst
::Var(_
)) if D
::forbid_inference_vars() => {
1003 "unexpected inference variable encountered in NLL generalization: {:?}",
1007 ty
::ConstKind
::Infer(InferConst
::Var(vid
)) => {
1008 let mut variable_table
= self.infcx
.const_unification_table
.borrow_mut();
1009 let var_value
= variable_table
.probe_value(vid
);
1010 match var_value
.val
.known() {
1011 Some(u
) => self.relate(&u
, &u
),
1013 let new_var_id
= variable_table
.new_key(ConstVarValue
{
1014 origin
: var_value
.origin
,
1015 val
: ConstVariableValue
::Unknown { universe: self.universe }
,
1017 Ok(self.tcx().mk_const_var(new_var_id
, a
.ty
))
1021 _
=> relate
::super_relate_consts(self, a
, a
),
1029 ) -> RelateResult
<'tcx
, ty
::Binder
<T
>>
1033 debug
!("TypeGeneralizer::binders(a={:?})", a
);
1035 self.first_free_index
.shift_in(1);
1036 let result
= self.relate(a
.skip_binder(), a
.skip_binder())?
;
1037 self.first_free_index
.shift_out(1);
1038 Ok(ty
::Binder
::bind(result
))