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1 ///////////////////////////////////////////////////////////////////////////
2 // # Type combining
3 //
4 // There are four type combiners: equate, sub, lub, and glb.  Each
5 // implements the trait `Combine` and contains methods for combining
6 // two instances of various things and yielding a new instance.  These
7 // combiner methods always yield a `Result<T>`.  There is a lot of
8 // common code for these operations, implemented as default methods on
9 // the `Combine` trait.
10 //
11 // Each operation may have side-effects on the inference context,
12 // though these can be unrolled using snapshots. On success, the
13 // LUB/GLB operations return the appropriate bound. The Eq and Sub
14 // operations generally return the first operand.
15 //
16 // ## Contravariance
17 //
18 // When you are relating two things which have a contravariant
19 // relationship, you should use `contratys()` or `contraregions()`,
20 // rather than inversing the order of arguments!  This is necessary
21 // because the order of arguments is not relevant for LUB and GLB.  It
22 // is also useful to track which value is the "expected" value in
23 // terms of error reporting.
24
25 use super::equate::Equate;
26 use super::glb::Glb;
27 use super::lub::Lub;
28 use super::sub::Sub;
29 use super::type_variable::TypeVariableValue;
30 use super::{InferCtxt, MiscVariable, TypeTrace};
31 use crate::traits::{Obligation, PredicateObligations};
32 use rustc_data_structures::sso::SsoHashMap;
33 use rustc_hir::def_id::DefId;
34 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
35 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
36 use rustc_middle::traits::ObligationCause;
37 use rustc_middle::ty::error::{ExpectedFound, TypeError};
38 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
39 use rustc_middle::ty::subst::SubstsRef;
40 use rustc_middle::ty::{self, InferConst, ToPredicate, Ty, TyCtxt, TypeFoldable};
41 use rustc_middle::ty::{IntType, UintType};
42 use rustc_span::{Span, DUMMY_SP};
43
44 #[derive(Clone)]
45 pub struct CombineFields<'infcx, 'tcx> {
46     pub infcx: &'infcx InferCtxt<'infcx, 'tcx>,
47     pub trace: TypeTrace<'tcx>,
48     pub cause: Option<ty::relate::Cause>,
49     pub param_env: ty::ParamEnv<'tcx>,
50     pub obligations: PredicateObligations<'tcx>,
51     /// Whether we should define opaque types
52     /// or just treat them opaquely.
53     /// Currently only used to prevent predicate
54     /// matching from matching anything against opaque
55     /// types.
56     pub define_opaque_types: bool,
57 }
58
59 #[derive(Copy, Clone, Debug)]
60 pub enum RelationDir {
61     SubtypeOf,
62     SupertypeOf,
63     EqTo,
64 }
65
66 impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
67     pub fn super_combine_tys<R>(
68         &self,
69         relation: &mut R,
70         a: Ty<'tcx>,
71         b: Ty<'tcx>,
72     ) -> RelateResult<'tcx, Ty<'tcx>>
73     where
74         R: TypeRelation<'tcx>,
75     {
76         let a_is_expected = relation.a_is_expected();
77
78         match (a.kind(), b.kind()) {
79             // Relate integral variables to other types
80             (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
81                 self.inner
82                     .borrow_mut()
83                     .int_unification_table()
84                     .unify_var_var(a_id, b_id)
85                     .map_err(|e| int_unification_error(a_is_expected, e))?;
86                 Ok(a)
87             }
88             (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
89                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
90             }
91             (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
92                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
93             }
94             (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
95                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
96             }
97             (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
98                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
99             }
100
101             // Relate floating-point variables to other types
102             (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
103                 self.inner
104                     .borrow_mut()
105                     .float_unification_table()
106                     .unify_var_var(a_id, b_id)
107                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
108                 Ok(a)
109             }
110             (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
111                 self.unify_float_variable(a_is_expected, v_id, v)
112             }
113             (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
114                 self.unify_float_variable(!a_is_expected, v_id, v)
115             }
116
117             // All other cases of inference are errors
118             (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
119                 Err(TypeError::Sorts(ty::relate::expected_found(relation, a, b)))
120             }
121
122             _ => ty::relate::super_relate_tys(relation, a, b),
123         }
124     }
125
126     pub fn super_combine_consts<R>(
127         &self,
128         relation: &mut R,
129         a: ty::Const<'tcx>,
130         b: ty::Const<'tcx>,
131     ) -> RelateResult<'tcx, ty::Const<'tcx>>
132     where
133         R: ConstEquateRelation<'tcx>,
134     {
135         debug!("{}.consts({:?}, {:?})", relation.tag(), a, b);
136         if a == b {
137             return Ok(a);
138         }
139
140         let a = self.shallow_resolve(a);
141         let b = self.shallow_resolve(b);
142
143         let a_is_expected = relation.a_is_expected();
144
145         match (a.val(), b.val()) {
146             (
147                 ty::ConstKind::Infer(InferConst::Var(a_vid)),
148                 ty::ConstKind::Infer(InferConst::Var(b_vid)),
149             ) => {
150                 self.inner
151                     .borrow_mut()
152                     .const_unification_table()
153                     .unify_var_var(a_vid, b_vid)
154                     .map_err(|e| const_unification_error(a_is_expected, e))?;
155                 return Ok(a);
156             }
157
158             // All other cases of inference with other variables are errors.
159             (ty::ConstKind::Infer(InferConst::Var(_)), ty::ConstKind::Infer(_))
160             | (ty::ConstKind::Infer(_), ty::ConstKind::Infer(InferConst::Var(_))) => {
161                 bug!("tried to combine ConstKind::Infer/ConstKind::Infer(InferConst::Var)")
162             }
163
164             (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
165                 return self.unify_const_variable(relation.param_env(), vid, b, a_is_expected);
166             }
167
168             (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
169                 return self.unify_const_variable(relation.param_env(), vid, a, !a_is_expected);
170             }
171             (ty::ConstKind::Unevaluated(..), _) if self.tcx.lazy_normalization() => {
172                 // FIXME(#59490): Need to remove the leak check to accommodate
173                 // escaping bound variables here.
174                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
175                     relation.const_equate_obligation(a, b);
176                 }
177                 return Ok(b);
178             }
179             (_, ty::ConstKind::Unevaluated(..)) if self.tcx.lazy_normalization() => {
180                 // FIXME(#59490): Need to remove the leak check to accommodate
181                 // escaping bound variables here.
182                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
183                     relation.const_equate_obligation(a, b);
184                 }
185                 return Ok(a);
186             }
187             _ => {}
188         }
189
190         ty::relate::super_relate_consts(relation, a, b)
191     }
192
193     /// Unifies the const variable `target_vid` with the given constant.
194     ///
195     /// This also tests if the given const `ct` contains an inference variable which was previously
196     /// unioned with `target_vid`. If this is the case, inferring `target_vid` to `ct`
197     /// would result in an infinite type as we continuously replace an inference variable
198     /// in `ct` with `ct` itself.
199     ///
200     /// This is especially important as unevaluated consts use their parents generics.
201     /// They therefore often contain unused substs, making these errors far more likely.
202     ///
203     /// A good example of this is the following:
204     ///
205     /// ```rust
206     /// #![feature(generic_const_exprs)]
207     ///
208     /// fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
209     ///     todo!()
210     /// }
211     ///
212     /// fn main() {
213     ///     let mut arr = Default::default();
214     ///     arr = bind(arr);
215     /// }
216     /// ```
217     ///
218     /// Here `3 + 4` ends up as `ConstKind::Unevaluated` which uses the generics
219     /// of `fn bind` (meaning that its substs contain `N`).
220     ///
221     /// `bind(arr)` now infers that the type of `arr` must be `[u8; N]`.
222     /// The assignment `arr = bind(arr)` now tries to equate `N` with `3 + 4`.
223     ///
224     /// As `3 + 4` contains `N` in its substs, this must not succeed.
225     ///
226     /// See `src/test/ui/const-generics/occurs-check/` for more examples where this is relevant.
227     #[instrument(level = "debug", skip(self))]
228     fn unify_const_variable(
229         &self,
230         param_env: ty::ParamEnv<'tcx>,
231         target_vid: ty::ConstVid<'tcx>,
232         ct: ty::Const<'tcx>,
233         vid_is_expected: bool,
234     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
235         let (for_universe, span) = {
236             let mut inner = self.inner.borrow_mut();
237             let variable_table = &mut inner.const_unification_table();
238             let var_value = variable_table.probe_value(target_vid);
239             match var_value.val {
240                 ConstVariableValue::Known { value } => {
241                     bug!("instantiating {:?} which has a known value {:?}", target_vid, value)
242                 }
243                 ConstVariableValue::Unknown { universe } => (universe, var_value.origin.span),
244             }
245         };
246         let value = ConstInferUnifier { infcx: self, span, param_env, for_universe, target_vid }
247             .relate(ct, ct)?;
248
249         self.inner
250             .borrow_mut()
251             .const_unification_table()
252             .unify_var_value(
253                 target_vid,
254                 ConstVarValue {
255                     origin: ConstVariableOrigin {
256                         kind: ConstVariableOriginKind::ConstInference,
257                         span: DUMMY_SP,
258                     },
259                     val: ConstVariableValue::Known { value },
260                 },
261             )
262             .map(|()| value)
263             .map_err(|e| const_unification_error(vid_is_expected, e))
264     }
265
266     fn unify_integral_variable(
267         &self,
268         vid_is_expected: bool,
269         vid: ty::IntVid,
270         val: ty::IntVarValue,
271     ) -> RelateResult<'tcx, Ty<'tcx>> {
272         self.inner
273             .borrow_mut()
274             .int_unification_table()
275             .unify_var_value(vid, Some(val))
276             .map_err(|e| int_unification_error(vid_is_expected, e))?;
277         match val {
278             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
279             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
280         }
281     }
282
283     fn unify_float_variable(
284         &self,
285         vid_is_expected: bool,
286         vid: ty::FloatVid,
287         val: ty::FloatTy,
288     ) -> RelateResult<'tcx, Ty<'tcx>> {
289         self.inner
290             .borrow_mut()
291             .float_unification_table()
292             .unify_var_value(vid, Some(ty::FloatVarValue(val)))
293             .map_err(|e| float_unification_error(vid_is_expected, e))?;
294         Ok(self.tcx.mk_mach_float(val))
295     }
296 }
297
298 impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
299     pub fn tcx(&self) -> TyCtxt<'tcx> {
300         self.infcx.tcx
301     }
302
303     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'tcx> {
304         Equate::new(self, a_is_expected)
305     }
306
307     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'tcx> {
308         Sub::new(self, a_is_expected)
309     }
310
311     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'tcx> {
312         Lub::new(self, a_is_expected)
313     }
314
315     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'tcx> {
316         Glb::new(self, a_is_expected)
317     }
318
319     /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
320     /// The idea is that we should ensure that the type `a_ty` is equal
321     /// to, a subtype of, or a supertype of (respectively) the type
322     /// to which `b_vid` is bound.
323     ///
324     /// Since `b_vid` has not yet been instantiated with a type, we
325     /// will first instantiate `b_vid` with a *generalized* version
326     /// of `a_ty`. Generalization introduces other inference
327     /// variables wherever subtyping could occur.
328     #[instrument(skip(self), level = "debug")]
329     pub fn instantiate(
330         &mut self,
331         a_ty: Ty<'tcx>,
332         dir: RelationDir,
333         b_vid: ty::TyVid,
334         a_is_expected: bool,
335     ) -> RelateResult<'tcx, ()> {
336         use self::RelationDir::*;
337
338         // Get the actual variable that b_vid has been inferred to
339         debug_assert!(self.infcx.inner.borrow_mut().type_variables().probe(b_vid).is_unknown());
340
341         // Generalize type of `a_ty` appropriately depending on the
342         // direction.  As an example, assume:
343         //
344         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
345         //   inference variable,
346         // - and `dir` == `SubtypeOf`.
347         //
348         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
349         // `'?2` and `?3` are fresh region/type inference
350         // variables. (Down below, we will relate `a_ty <: b_ty`,
351         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
352         let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
353         debug!(?b_ty);
354         self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
355
356         if needs_wf {
357             self.obligations.push(Obligation::new(
358                 self.trace.cause.clone(),
359                 self.param_env,
360                 ty::Binder::dummy(ty::PredicateKind::WellFormed(b_ty.into()))
361                     .to_predicate(self.infcx.tcx),
362             ));
363         }
364
365         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
366         //
367         // FIXME(#16847): This code is non-ideal because all these subtype
368         // relations wind up attributed to the same spans. We need
369         // to associate causes/spans with each of the relations in
370         // the stack to get this right.
371         match dir {
372             EqTo => self.equate(a_is_expected).relate(a_ty, b_ty),
373             SubtypeOf => self.sub(a_is_expected).relate(a_ty, b_ty),
374             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
375                 ty::Contravariant,
376                 ty::VarianceDiagInfo::default(),
377                 a_ty,
378                 b_ty,
379             ),
380         }?;
381
382         Ok(())
383     }
384
385     /// Attempts to generalize `ty` for the type variable `for_vid`.
386     /// This checks for cycle -- that is, whether the type `ty`
387     /// references `for_vid`. The `dir` is the "direction" for which we
388     /// a performing the generalization (i.e., are we producing a type
389     /// that can be used as a supertype etc).
390     ///
391     /// Preconditions:
392     ///
393     /// - `for_vid` is a "root vid"
394     #[instrument(skip(self), level = "trace")]
395     fn generalize(
396         &self,
397         ty: Ty<'tcx>,
398         for_vid: ty::TyVid,
399         dir: RelationDir,
400     ) -> RelateResult<'tcx, Generalization<'tcx>> {
401         // Determine the ambient variance within which `ty` appears.
402         // The surrounding equation is:
403         //
404         //     ty [op] ty2
405         //
406         // where `op` is either `==`, `<:`, or `:>`. This maps quite
407         // naturally.
408         let ambient_variance = match dir {
409             RelationDir::EqTo => ty::Invariant,
410             RelationDir::SubtypeOf => ty::Covariant,
411             RelationDir::SupertypeOf => ty::Contravariant,
412         };
413
414         trace!(?ambient_variance);
415
416         let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
417             v @ TypeVariableValue::Known { .. } => {
418                 bug!("instantiating {:?} which has a known value {:?}", for_vid, v,)
419             }
420             TypeVariableValue::Unknown { universe } => universe,
421         };
422
423         trace!(?for_universe);
424         trace!(?self.trace);
425
426         let mut generalize = Generalizer {
427             infcx: self.infcx,
428             cause: &self.trace.cause,
429             for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
430             for_universe,
431             ambient_variance,
432             needs_wf: false,
433             root_ty: ty,
434             param_env: self.param_env,
435             cache: SsoHashMap::new(),
436         };
437
438         let ty = match generalize.relate(ty, ty) {
439             Ok(ty) => ty,
440             Err(e) => {
441                 debug!(?e, "failure");
442                 return Err(e);
443             }
444         };
445         let needs_wf = generalize.needs_wf;
446         trace!(?ty, ?needs_wf, "success");
447         Ok(Generalization { ty, needs_wf })
448     }
449
450     pub fn add_const_equate_obligation(
451         &mut self,
452         a_is_expected: bool,
453         a: ty::Const<'tcx>,
454         b: ty::Const<'tcx>,
455     ) {
456         let predicate = if a_is_expected {
457             ty::PredicateKind::ConstEquate(a, b)
458         } else {
459             ty::PredicateKind::ConstEquate(b, a)
460         };
461         self.obligations.push(Obligation::new(
462             self.trace.cause.clone(),
463             self.param_env,
464             ty::Binder::dummy(predicate).to_predicate(self.tcx()),
465         ));
466     }
467 }
468
469 struct Generalizer<'cx, 'tcx> {
470     infcx: &'cx InferCtxt<'cx, 'tcx>,
471
472     /// The span, used when creating new type variables and things.
473     cause: &'cx ObligationCause<'tcx>,
474
475     /// The vid of the type variable that is in the process of being
476     /// instantiated; if we find this within the type we are folding,
477     /// that means we would have created a cyclic type.
478     for_vid_sub_root: ty::TyVid,
479
480     /// The universe of the type variable that is in the process of
481     /// being instantiated. Any fresh variables that we create in this
482     /// process should be in that same universe.
483     for_universe: ty::UniverseIndex,
484
485     /// Track the variance as we descend into the type.
486     ambient_variance: ty::Variance,
487
488     /// See the field `needs_wf` in `Generalization`.
489     needs_wf: bool,
490
491     /// The root type that we are generalizing. Used when reporting cycles.
492     root_ty: Ty<'tcx>,
493
494     param_env: ty::ParamEnv<'tcx>,
495
496     cache: SsoHashMap<Ty<'tcx>, RelateResult<'tcx, Ty<'tcx>>>,
497 }
498
499 /// Result from a generalization operation. This includes
500 /// not only the generalized type, but also a bool flag
501 /// indicating whether further WF checks are needed.
502 struct Generalization<'tcx> {
503     ty: Ty<'tcx>,
504
505     /// If true, then the generalized type may not be well-formed,
506     /// even if the source type is well-formed, so we should add an
507     /// additional check to enforce that it is. This arises in
508     /// particular around 'bivariant' type parameters that are only
509     /// constrained by a where-clause. As an example, imagine a type:
510     ///
511     ///     struct Foo<A, B> where A: Iterator<Item = B> {
512     ///         data: A
513     ///     }
514     ///
515     /// here, `A` will be covariant, but `B` is
516     /// unconstrained. However, whatever it is, for `Foo` to be WF, it
517     /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
518     /// then after generalization we will wind up with a type like
519     /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
520     /// ?D>` (or `>:`), we will wind up with the requirement that `?A
521     /// <: ?C`, but no particular relationship between `?B` and `?D`
522     /// (after all, we do not know the variance of the normalized form
523     /// of `A::Item` with respect to `A`). If we do nothing else, this
524     /// may mean that `?D` goes unconstrained (as in #41677). So, in
525     /// this scenario where we create a new type variable in a
526     /// bivariant context, we set the `needs_wf` flag to true. This
527     /// will force the calling code to check that `WF(Foo<?C, ?D>)`
528     /// holds, which in turn implies that `?C::Item == ?D`. So once
529     /// `?C` is constrained, that should suffice to restrict `?D`.
530     needs_wf: bool,
531 }
532
533 impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
534     fn tcx(&self) -> TyCtxt<'tcx> {
535         self.infcx.tcx
536     }
537     fn param_env(&self) -> ty::ParamEnv<'tcx> {
538         self.param_env
539     }
540
541     fn tag(&self) -> &'static str {
542         "Generalizer"
543     }
544
545     fn a_is_expected(&self) -> bool {
546         true
547     }
548
549     fn binders<T>(
550         &mut self,
551         a: ty::Binder<'tcx, T>,
552         b: ty::Binder<'tcx, T>,
553     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
554     where
555         T: Relate<'tcx>,
556     {
557         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
558     }
559
560     fn relate_item_substs(
561         &mut self,
562         item_def_id: DefId,
563         a_subst: SubstsRef<'tcx>,
564         b_subst: SubstsRef<'tcx>,
565     ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
566         if self.ambient_variance == ty::Variance::Invariant {
567             // Avoid fetching the variance if we are in an invariant
568             // context; no need, and it can induce dependency cycles
569             // (e.g., #41849).
570             relate::relate_substs(self, None, a_subst, b_subst)
571         } else {
572             let tcx = self.tcx();
573             let opt_variances = tcx.variances_of(item_def_id);
574             relate::relate_substs(self, Some((item_def_id, &opt_variances)), a_subst, b_subst)
575         }
576     }
577
578     fn relate_with_variance<T: Relate<'tcx>>(
579         &mut self,
580         variance: ty::Variance,
581         _info: ty::VarianceDiagInfo<'tcx>,
582         a: T,
583         b: T,
584     ) -> RelateResult<'tcx, T> {
585         let old_ambient_variance = self.ambient_variance;
586         self.ambient_variance = self.ambient_variance.xform(variance);
587
588         let result = self.relate(a, b);
589         self.ambient_variance = old_ambient_variance;
590         result
591     }
592
593     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
594         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
595
596         if let Some(result) = self.cache.get(&t) {
597             return result.clone();
598         }
599         debug!("generalize: t={:?}", t);
600
601         // Check to see whether the type we are generalizing references
602         // any other type variable related to `vid` via
603         // subtyping. This is basically our "occurs check", preventing
604         // us from creating infinitely sized types.
605         let result = match *t.kind() {
606             ty::Infer(ty::TyVar(vid)) => {
607                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
608                 let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
609                 if sub_vid == self.for_vid_sub_root {
610                     // If sub-roots are equal, then `for_vid` and
611                     // `vid` are related via subtyping.
612                     Err(TypeError::CyclicTy(self.root_ty))
613                 } else {
614                     let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
615                     match probe {
616                         TypeVariableValue::Known { value: u } => {
617                             debug!("generalize: known value {:?}", u);
618                             self.relate(u, u)
619                         }
620                         TypeVariableValue::Unknown { universe } => {
621                             match self.ambient_variance {
622                                 // Invariant: no need to make a fresh type variable.
623                                 ty::Invariant => {
624                                     if self.for_universe.can_name(universe) {
625                                         return Ok(t);
626                                     }
627                                 }
628
629                                 // Bivariant: make a fresh var, but we
630                                 // may need a WF predicate. See
631                                 // comment on `needs_wf` field for
632                                 // more info.
633                                 ty::Bivariant => self.needs_wf = true,
634
635                                 // Co/contravariant: this will be
636                                 // sufficiently constrained later on.
637                                 ty::Covariant | ty::Contravariant => (),
638                             }
639
640                             let origin =
641                                 *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
642                             let new_var_id = self
643                                 .infcx
644                                 .inner
645                                 .borrow_mut()
646                                 .type_variables()
647                                 .new_var(self.for_universe, origin);
648                             let u = self.tcx().mk_ty_var(new_var_id);
649
650                             // Record that we replaced `vid` with `new_var_id` as part of a generalization
651                             // operation. This is needed to detect cyclic types. To see why, see the
652                             // docs in the `type_variables` module.
653                             self.infcx.inner.borrow_mut().type_variables().sub(vid, new_var_id);
654                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
655                             Ok(u)
656                         }
657                     }
658                 }
659             }
660             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
661                 // No matter what mode we are in,
662                 // integer/floating-point types must be equal to be
663                 // relatable.
664                 Ok(t)
665             }
666             _ => relate::super_relate_tys(self, t, t),
667         };
668
669         self.cache.insert(t, result.clone());
670         return result;
671     }
672
673     fn regions(
674         &mut self,
675         r: ty::Region<'tcx>,
676         r2: ty::Region<'tcx>,
677     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
678         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
679
680         debug!("generalize: regions r={:?}", r);
681
682         match *r {
683             // Never make variables for regions bound within the type itself,
684             // nor for erased regions.
685             ty::ReLateBound(..) | ty::ReErased => {
686                 return Ok(r);
687             }
688
689             ty::RePlaceholder(..)
690             | ty::ReVar(..)
691             | ty::ReEmpty(_)
692             | ty::ReStatic
693             | ty::ReEarlyBound(..)
694             | ty::ReFree(..) => {
695                 // see common code below
696             }
697         }
698
699         // If we are in an invariant context, we can re-use the region
700         // as is, unless it happens to be in some universe that we
701         // can't name. (In the case of a region *variable*, we could
702         // use it if we promoted it into our universe, but we don't
703         // bother.)
704         if let ty::Invariant = self.ambient_variance {
705             let r_universe = self.infcx.universe_of_region(r);
706             if self.for_universe.can_name(r_universe) {
707                 return Ok(r);
708             }
709         }
710
711         // FIXME: This is non-ideal because we don't give a
712         // very descriptive origin for this region variable.
713         Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.cause.span), self.for_universe))
714     }
715
716     fn consts(
717         &mut self,
718         c: ty::Const<'tcx>,
719         c2: ty::Const<'tcx>,
720     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
721         assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
722
723         match c.val() {
724             ty::ConstKind::Infer(InferConst::Var(vid)) => {
725                 let mut inner = self.infcx.inner.borrow_mut();
726                 let variable_table = &mut inner.const_unification_table();
727                 let var_value = variable_table.probe_value(vid);
728                 match var_value.val {
729                     ConstVariableValue::Known { value: u } => {
730                         drop(inner);
731                         self.relate(u, u)
732                     }
733                     ConstVariableValue::Unknown { universe } => {
734                         if self.for_universe.can_name(universe) {
735                             Ok(c)
736                         } else {
737                             let new_var_id = variable_table.new_key(ConstVarValue {
738                                 origin: var_value.origin,
739                                 val: ConstVariableValue::Unknown { universe: self.for_universe },
740                             });
741                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
742                         }
743                     }
744                 }
745             }
746             ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
747                 if self.tcx().lazy_normalization() =>
748             {
749                 assert_eq!(promoted, None);
750                 let substs = self.relate_with_variance(
751                     ty::Variance::Invariant,
752                     ty::VarianceDiagInfo::default(),
753                     substs,
754                     substs,
755                 )?;
756                 Ok(self.tcx().mk_const(ty::ConstS {
757                     ty: c.ty(),
758                     val: ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }),
759                 }))
760             }
761             _ => relate::super_relate_consts(self, c, c),
762         }
763     }
764 }
765
766 pub trait ConstEquateRelation<'tcx>: TypeRelation<'tcx> {
767     /// Register an obligation that both constants must be equal to each other.
768     ///
769     /// If they aren't equal then the relation doesn't hold.
770     fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
771 }
772
773 pub trait RelateResultCompare<'tcx, T> {
774     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T>
775     where
776         F: FnOnce() -> TypeError<'tcx>;
777 }
778
779 impl<'tcx, T: Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
780     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T>
781     where
782         F: FnOnce() -> TypeError<'tcx>,
783     {
784         self.clone().and_then(|s| if s == t { self.clone() } else { Err(f()) })
785     }
786 }
787
788 pub fn const_unification_error<'tcx>(
789     a_is_expected: bool,
790     (a, b): (ty::Const<'tcx>, ty::Const<'tcx>),
791 ) -> TypeError<'tcx> {
792     TypeError::ConstMismatch(ExpectedFound::new(a_is_expected, a, b))
793 }
794
795 fn int_unification_error<'tcx>(
796     a_is_expected: bool,
797     v: (ty::IntVarValue, ty::IntVarValue),
798 ) -> TypeError<'tcx> {
799     let (a, b) = v;
800     TypeError::IntMismatch(ExpectedFound::new(a_is_expected, a, b))
801 }
802
803 fn float_unification_error<'tcx>(
804     a_is_expected: bool,
805     v: (ty::FloatVarValue, ty::FloatVarValue),
806 ) -> TypeError<'tcx> {
807     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
808     TypeError::FloatMismatch(ExpectedFound::new(a_is_expected, a, b))
809 }
810
811 struct ConstInferUnifier<'cx, 'tcx> {
812     infcx: &'cx InferCtxt<'cx, 'tcx>,
813
814     span: Span,
815
816     param_env: ty::ParamEnv<'tcx>,
817
818     for_universe: ty::UniverseIndex,
819
820     /// The vid of the const variable that is in the process of being
821     /// instantiated; if we find this within the const we are folding,
822     /// that means we would have created a cyclic const.
823     target_vid: ty::ConstVid<'tcx>,
824 }
825
826 // We use `TypeRelation` here to propagate `RelateResult` upwards.
827 //
828 // Both inputs are expected to be the same.
829 impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
830     fn tcx(&self) -> TyCtxt<'tcx> {
831         self.infcx.tcx
832     }
833
834     fn param_env(&self) -> ty::ParamEnv<'tcx> {
835         self.param_env
836     }
837
838     fn tag(&self) -> &'static str {
839         "ConstInferUnifier"
840     }
841
842     fn a_is_expected(&self) -> bool {
843         true
844     }
845
846     fn relate_with_variance<T: Relate<'tcx>>(
847         &mut self,
848         _variance: ty::Variance,
849         _info: ty::VarianceDiagInfo<'tcx>,
850         a: T,
851         b: T,
852     ) -> RelateResult<'tcx, T> {
853         // We don't care about variance here.
854         self.relate(a, b)
855     }
856
857     fn binders<T>(
858         &mut self,
859         a: ty::Binder<'tcx, T>,
860         b: ty::Binder<'tcx, T>,
861     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
862     where
863         T: Relate<'tcx>,
864     {
865         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
866     }
867
868     #[tracing::instrument(level = "debug", skip(self))]
869     fn tys(&mut self, t: Ty<'tcx>, _t: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
870         debug_assert_eq!(t, _t);
871         debug!("ConstInferUnifier: t={:?}", t);
872
873         match t.kind() {
874             &ty::Infer(ty::TyVar(vid)) => {
875                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
876                 let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
877                 match probe {
878                     TypeVariableValue::Known { value: u } => {
879                         debug!("ConstOccursChecker: known value {:?}", u);
880                         self.tys(u, u)
881                     }
882                     TypeVariableValue::Unknown { universe } => {
883                         if self.for_universe.can_name(universe) {
884                             return Ok(t);
885                         }
886
887                         let origin =
888                             *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
889                         let new_var_id = self
890                             .infcx
891                             .inner
892                             .borrow_mut()
893                             .type_variables()
894                             .new_var(self.for_universe, origin);
895                         let u = self.tcx().mk_ty_var(new_var_id);
896                         debug!(
897                             "ConstInferUnifier: replacing original vid={:?} with new={:?}",
898                             vid, u
899                         );
900                         Ok(u)
901                     }
902                 }
903             }
904             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => Ok(t),
905             _ => relate::super_relate_tys(self, t, t),
906         }
907     }
908
909     fn regions(
910         &mut self,
911         r: ty::Region<'tcx>,
912         _r: ty::Region<'tcx>,
913     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
914         debug_assert_eq!(r, _r);
915         debug!("ConstInferUnifier: r={:?}", r);
916
917         match *r {
918             // Never make variables for regions bound within the type itself,
919             // nor for erased regions.
920             ty::ReLateBound(..) | ty::ReErased => {
921                 return Ok(r);
922             }
923
924             ty::RePlaceholder(..)
925             | ty::ReVar(..)
926             | ty::ReEmpty(_)
927             | ty::ReStatic
928             | ty::ReEarlyBound(..)
929             | ty::ReFree(..) => {
930                 // see common code below
931             }
932         }
933
934         let r_universe = self.infcx.universe_of_region(r);
935         if self.for_universe.can_name(r_universe) {
936             return Ok(r);
937         } else {
938             // FIXME: This is non-ideal because we don't give a
939             // very descriptive origin for this region variable.
940             Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
941         }
942     }
943
944     #[tracing::instrument(level = "debug", skip(self))]
945     fn consts(
946         &mut self,
947         c: ty::Const<'tcx>,
948         _c: ty::Const<'tcx>,
949     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
950         debug_assert_eq!(c, _c);
951         debug!("ConstInferUnifier: c={:?}", c);
952
953         match c.val() {
954             ty::ConstKind::Infer(InferConst::Var(vid)) => {
955                 // Check if the current unification would end up
956                 // unifying `target_vid` with a const which contains
957                 // an inference variable which is unioned with `target_vid`.
958                 //
959                 // Not doing so can easily result in stack overflows.
960                 if self
961                     .infcx
962                     .inner
963                     .borrow_mut()
964                     .const_unification_table()
965                     .unioned(self.target_vid, vid)
966                 {
967                     return Err(TypeError::CyclicConst(c));
968                 }
969
970                 let var_value =
971                     self.infcx.inner.borrow_mut().const_unification_table().probe_value(vid);
972                 match var_value.val {
973                     ConstVariableValue::Known { value: u } => self.consts(u, u),
974                     ConstVariableValue::Unknown { universe } => {
975                         if self.for_universe.can_name(universe) {
976                             Ok(c)
977                         } else {
978                             let new_var_id =
979                                 self.infcx.inner.borrow_mut().const_unification_table().new_key(
980                                     ConstVarValue {
981                                         origin: var_value.origin,
982                                         val: ConstVariableValue::Unknown {
983                                             universe: self.for_universe,
984                                         },
985                                     },
986                                 );
987                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
988                         }
989                     }
990                 }
991             }
992             ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
993                 if self.tcx().lazy_normalization() =>
994             {
995                 assert_eq!(promoted, None);
996                 let substs = self.relate_with_variance(
997                     ty::Variance::Invariant,
998                     ty::VarianceDiagInfo::default(),
999                     substs,
1000                     substs,
1001                 )?;
1002                 Ok(self.tcx().mk_const(ty::ConstS {
1003                     ty: c.ty(),
1004                     val: ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }),
1005                 }))
1006             }
1007             _ => relate::super_relate_consts(self, c, c),
1008         }
1009     }
1010 }