1 ///////////////////////////////////////////////////////////////////////////
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.
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.
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.
25 use super::equate::Equate;
29 use super::type_variable::TypeVariableValue;
30 use super::unify_key::replace_if_possible;
31 use super::unify_key::{ConstVarValue, ConstVariableValue};
32 use super::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
33 use super::{InferCtxt, MiscVariable, TypeTrace};
35 use crate::traits::{Obligation, PredicateObligations};
38 use rustc_hir::def_id::DefId;
39 use rustc_middle::ty::error::TypeError;
40 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
41 use rustc_middle::ty::subst::SubstsRef;
42 use rustc_middle::ty::{self, InferConst, Ty, TyCtxt, TypeFoldable};
43 use rustc_middle::ty::{IntType, UintType};
44 use rustc_span::{Span, DUMMY_SP};
47 pub struct CombineFields<'infcx, 'tcx> {
48 pub infcx: &'infcx InferCtxt<'infcx, 'tcx>,
49 pub trace: TypeTrace<'tcx>,
50 pub cause: Option<ty::relate::Cause>,
51 pub param_env: ty::ParamEnv<'tcx>,
52 pub obligations: PredicateObligations<'tcx>,
55 #[derive(Copy, Clone, Debug)]
56 pub enum RelationDir {
62 impl<'infcx, 'tcx> InferCtxt<'infcx, 'tcx> {
63 pub fn super_combine_tys<R>(
68 ) -> RelateResult<'tcx, Ty<'tcx>>
70 R: TypeRelation<'tcx>,
72 let a_is_expected = relation.a_is_expected();
74 match (&a.kind, &b.kind) {
75 // Relate integral variables to other types
76 (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
79 .int_unification_table()
80 .unify_var_var(a_id, b_id)
81 .map_err(|e| int_unification_error(a_is_expected, e))?;
84 (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
85 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
87 (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
88 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
90 (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
91 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
93 (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
94 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
97 // Relate floating-point variables to other types
98 (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
101 .float_unification_table()
102 .unify_var_var(a_id, b_id)
103 .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
106 (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
107 self.unify_float_variable(a_is_expected, v_id, v)
109 (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
110 self.unify_float_variable(!a_is_expected, v_id, v)
113 // All other cases of inference are errors
114 (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
115 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
118 _ => ty::relate::super_relate_tys(relation, a, b),
122 pub fn super_combine_consts<R>(
125 a: &'tcx ty::Const<'tcx>,
126 b: &'tcx ty::Const<'tcx>,
127 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>>
129 R: ConstEquateRelation<'tcx>,
131 debug!("{}.consts({:?}, {:?})", relation.tag(), a, b);
136 let a = replace_if_possible(&mut self.inner.borrow_mut().const_unification_table(), a);
137 let b = replace_if_possible(&mut self.inner.borrow_mut().const_unification_table(), b);
139 let a_is_expected = relation.a_is_expected();
141 match (a.val, b.val) {
143 ty::ConstKind::Infer(InferConst::Var(a_vid)),
144 ty::ConstKind::Infer(InferConst::Var(b_vid)),
148 .const_unification_table()
149 .unify_var_var(a_vid, b_vid)
150 .map_err(|e| const_unification_error(a_is_expected, e))?;
154 // All other cases of inference with other variables are errors.
155 (ty::ConstKind::Infer(InferConst::Var(_)), ty::ConstKind::Infer(_))
156 | (ty::ConstKind::Infer(_), ty::ConstKind::Infer(InferConst::Var(_))) => {
157 bug!("tried to combine ConstKind::Infer/ConstKind::Infer(InferConst::Var)")
160 (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
161 return self.unify_const_variable(a_is_expected, vid, b);
164 (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
165 return self.unify_const_variable(!a_is_expected, vid, a);
167 (ty::ConstKind::Unevaluated(..), _) if self.tcx.lazy_normalization() => {
168 // FIXME(#59490): Need to remove the leak check to accomodate
169 // escaping bound variables here.
170 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
171 relation.const_equate_obligation(a, b);
175 (_, ty::ConstKind::Unevaluated(..)) if self.tcx.lazy_normalization() => {
176 // FIXME(#59490): Need to remove the leak check to accomodate
177 // escaping bound variables here.
178 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
179 relation.const_equate_obligation(a, b);
186 ty::relate::super_relate_consts(relation, a, b)
189 pub fn unify_const_variable(
191 vid_is_expected: bool,
192 vid: ty::ConstVid<'tcx>,
193 value: &'tcx ty::Const<'tcx>,
194 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
197 .const_unification_table()
201 origin: ConstVariableOrigin {
202 kind: ConstVariableOriginKind::ConstInference,
205 val: ConstVariableValue::Known { value },
208 .map_err(|e| const_unification_error(vid_is_expected, e))?;
212 fn unify_integral_variable(
214 vid_is_expected: bool,
216 val: ty::IntVarValue,
217 ) -> RelateResult<'tcx, Ty<'tcx>> {
220 .int_unification_table()
221 .unify_var_value(vid, Some(val))
222 .map_err(|e| int_unification_error(vid_is_expected, e))?;
224 IntType(v) => Ok(self.tcx.mk_mach_int(v)),
225 UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
229 fn unify_float_variable(
231 vid_is_expected: bool,
234 ) -> RelateResult<'tcx, Ty<'tcx>> {
237 .float_unification_table()
238 .unify_var_value(vid, Some(ty::FloatVarValue(val)))
239 .map_err(|e| float_unification_error(vid_is_expected, e))?;
240 Ok(self.tcx.mk_mach_float(val))
244 impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
245 pub fn tcx(&self) -> TyCtxt<'tcx> {
249 pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'tcx> {
250 Equate::new(self, a_is_expected)
253 pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'tcx> {
254 Sub::new(self, a_is_expected)
257 pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'tcx> {
258 Lub::new(self, a_is_expected)
261 pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'tcx> {
262 Glb::new(self, a_is_expected)
265 /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
266 /// The idea is that we should ensure that the type `a_ty` is equal
267 /// to, a subtype of, or a supertype of (respectively) the type
268 /// to which `b_vid` is bound.
270 /// Since `b_vid` has not yet been instantiated with a type, we
271 /// will first instantiate `b_vid` with a *generalized* version
272 /// of `a_ty`. Generalization introduces other inference
273 /// variables wherever subtyping could occur.
280 ) -> RelateResult<'tcx, ()> {
281 use self::RelationDir::*;
283 // Get the actual variable that b_vid has been inferred to
284 debug_assert!(self.infcx.inner.borrow_mut().type_variables().probe(b_vid).is_unknown());
286 debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
288 // Generalize type of `a_ty` appropriately depending on the
289 // direction. As an example, assume:
291 // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
292 // inference variable,
293 // - and `dir` == `SubtypeOf`.
295 // Then the generalized form `b_ty` would be `&'?2 ?3`, where
296 // `'?2` and `?3` are fresh region/type inference
297 // variables. (Down below, we will relate `a_ty <: b_ty`,
298 // adding constraints like `'x: '?2` and `?1 <: ?3`.)
299 let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
301 "instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
302 a_ty, dir, b_vid, b_ty
304 self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
307 self.obligations.push(Obligation::new(
308 self.trace.cause.clone(),
310 ty::PredicateKind::WellFormed(b_ty),
314 // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
316 // FIXME(#16847): This code is non-ideal because all these subtype
317 // relations wind up attributed to the same spans. We need
318 // to associate causes/spans with each of the relations in
319 // the stack to get this right.
321 EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
322 SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
324 self.sub(a_is_expected).relate_with_variance(ty::Contravariant, &a_ty, &b_ty)
331 /// Attempts to generalize `ty` for the type variable `for_vid`.
332 /// This checks for cycle -- that is, whether the type `ty`
333 /// references `for_vid`. The `dir` is the "direction" for which we
334 /// a performing the generalization (i.e., are we producing a type
335 /// that can be used as a supertype etc).
339 /// - `for_vid` is a "root vid"
345 ) -> RelateResult<'tcx, Generalization<'tcx>> {
346 debug!("generalize(ty={:?}, for_vid={:?}, dir={:?}", ty, for_vid, dir);
347 // Determine the ambient variance within which `ty` appears.
348 // The surrounding equation is:
352 // where `op` is either `==`, `<:`, or `:>`. This maps quite
354 let ambient_variance = match dir {
355 RelationDir::EqTo => ty::Invariant,
356 RelationDir::SubtypeOf => ty::Covariant,
357 RelationDir::SupertypeOf => ty::Contravariant,
360 debug!("generalize: ambient_variance = {:?}", ambient_variance);
362 let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
363 v @ TypeVariableValue::Known { .. } => {
364 panic!("instantiating {:?} which has a known value {:?}", for_vid, v,)
366 TypeVariableValue::Unknown { universe } => universe,
369 debug!("generalize: for_universe = {:?}", for_universe);
371 let mut generalize = Generalizer {
373 span: self.trace.cause.span,
374 for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
379 param_env: self.param_env,
382 let ty = match generalize.relate(&ty, &ty) {
385 debug!("generalize: failure {:?}", e);
389 let needs_wf = generalize.needs_wf;
390 debug!("generalize: success {{ {:?}, {:?} }}", ty, needs_wf);
391 Ok(Generalization { ty, needs_wf })
394 pub fn add_const_equate_obligation(
397 a: &'tcx ty::Const<'tcx>,
398 b: &'tcx ty::Const<'tcx>,
400 let predicate = if a_is_expected {
401 ty::Predicate::ConstEquate(a, b)
403 ty::Predicate::ConstEquate(b, a)
405 self.obligations.push(Obligation::new(self.trace.cause.clone(), self.param_env, predicate));
409 struct Generalizer<'cx, 'tcx> {
410 infcx: &'cx InferCtxt<'cx, 'tcx>,
412 /// The span, used when creating new type variables and things.
415 /// The vid of the type variable that is in the process of being
416 /// instantiated; if we find this within the type we are folding,
417 /// that means we would have created a cyclic type.
418 for_vid_sub_root: ty::TyVid,
420 /// The universe of the type variable that is in the process of
421 /// being instantiated. Any fresh variables that we create in this
422 /// process should be in that same universe.
423 for_universe: ty::UniverseIndex,
425 /// Track the variance as we descend into the type.
426 ambient_variance: ty::Variance,
428 /// See the field `needs_wf` in `Generalization`.
431 /// The root type that we are generalizing. Used when reporting cycles.
434 param_env: ty::ParamEnv<'tcx>,
437 /// Result from a generalization operation. This includes
438 /// not only the generalized type, but also a bool flag
439 /// indicating whether further WF checks are needed.
440 struct Generalization<'tcx> {
443 /// If true, then the generalized type may not be well-formed,
444 /// even if the source type is well-formed, so we should add an
445 /// additional check to enforce that it is. This arises in
446 /// particular around 'bivariant' type parameters that are only
447 /// constrained by a where-clause. As an example, imagine a type:
449 /// struct Foo<A, B> where A: Iterator<Item = B> {
453 /// here, `A` will be covariant, but `B` is
454 /// unconstrained. However, whatever it is, for `Foo` to be WF, it
455 /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
456 /// then after generalization we will wind up with a type like
457 /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
458 /// ?D>` (or `>:`), we will wind up with the requirement that `?A
459 /// <: ?C`, but no particular relationship between `?B` and `?D`
460 /// (after all, we do not know the variance of the normalized form
461 /// of `A::Item` with respect to `A`). If we do nothing else, this
462 /// may mean that `?D` goes unconstrained (as in #41677). So, in
463 /// this scenario where we create a new type variable in a
464 /// bivariant context, we set the `needs_wf` flag to true. This
465 /// will force the calling code to check that `WF(Foo<?C, ?D>)`
466 /// holds, which in turn implies that `?C::Item == ?D`. So once
467 /// `?C` is constrained, that should suffice to restrict `?D`.
471 impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
472 fn tcx(&self) -> TyCtxt<'tcx> {
475 fn param_env(&self) -> ty::ParamEnv<'tcx> {
479 fn tag(&self) -> &'static str {
483 fn a_is_expected(&self) -> bool {
491 ) -> RelateResult<'tcx, ty::Binder<T>>
495 Ok(ty::Binder::bind(self.relate(a.skip_binder(), b.skip_binder())?))
498 fn relate_item_substs(
501 a_subst: SubstsRef<'tcx>,
502 b_subst: SubstsRef<'tcx>,
503 ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
504 if self.ambient_variance == ty::Variance::Invariant {
505 // Avoid fetching the variance if we are in an invariant
506 // context; no need, and it can induce dependency cycles
508 relate::relate_substs(self, None, a_subst, b_subst)
510 let opt_variances = self.tcx().variances_of(item_def_id);
511 relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
515 fn relate_with_variance<T: Relate<'tcx>>(
517 variance: ty::Variance,
520 ) -> RelateResult<'tcx, T> {
521 let old_ambient_variance = self.ambient_variance;
522 self.ambient_variance = self.ambient_variance.xform(variance);
524 let result = self.relate(a, b);
525 self.ambient_variance = old_ambient_variance;
529 fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
530 assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
532 debug!("generalize: t={:?}", t);
534 // Check to see whether the type we are generalizing references
535 // any other type variable related to `vid` via
536 // subtyping. This is basically our "occurs check", preventing
537 // us from creating infinitely sized types.
539 ty::Infer(ty::TyVar(vid)) => {
540 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
541 let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
542 if sub_vid == self.for_vid_sub_root {
543 // If sub-roots are equal, then `for_vid` and
544 // `vid` are related via subtyping.
545 Err(TypeError::CyclicTy(self.root_ty))
547 let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
549 TypeVariableValue::Known { value: u } => {
550 debug!("generalize: known value {:?}", u);
553 TypeVariableValue::Unknown { universe } => {
554 match self.ambient_variance {
555 // Invariant: no need to make a fresh type variable.
557 if self.for_universe.can_name(universe) {
562 // Bivariant: make a fresh var, but we
563 // may need a WF predicate. See
564 // comment on `needs_wf` field for
566 ty::Bivariant => self.needs_wf = true,
568 // Co/contravariant: this will be
569 // sufficiently constrained later on.
570 ty::Covariant | ty::Contravariant => (),
574 *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
575 let new_var_id = self
580 .new_var(self.for_universe, false, origin);
581 let u = self.tcx().mk_ty_var(new_var_id);
582 debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
588 ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
589 // No matter what mode we are in,
590 // integer/floating-point types must be equal to be
594 _ => relate::super_relate_tys(self, t, t),
601 r2: ty::Region<'tcx>,
602 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
603 assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
605 debug!("generalize: regions r={:?}", r);
608 // Never make variables for regions bound within the type itself,
609 // nor for erased regions.
610 ty::ReLateBound(..) | ty::ReErased => {
614 ty::RePlaceholder(..)
619 | ty::ReEarlyBound(..)
620 | ty::ReFree(..) => {
621 // see common code below
625 // If we are in an invariant context, we can re-use the region
626 // as is, unless it happens to be in some universe that we
627 // can't name. (In the case of a region *variable*, we could
628 // use it if we promoted it into our universe, but we don't
630 if let ty::Invariant = self.ambient_variance {
631 let r_universe = self.infcx.universe_of_region(r);
632 if self.for_universe.can_name(r_universe) {
637 // FIXME: This is non-ideal because we don't give a
638 // very descriptive origin for this region variable.
639 Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
644 c: &'tcx ty::Const<'tcx>,
645 c2: &'tcx ty::Const<'tcx>,
646 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
647 assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
650 ty::ConstKind::Infer(InferConst::Var(vid)) => {
651 let mut inner = self.infcx.inner.borrow_mut();
652 let variable_table = &mut inner.const_unification_table();
653 let var_value = variable_table.probe_value(vid);
654 match var_value.val {
655 ConstVariableValue::Known { value: u } => self.relate(&u, &u),
656 ConstVariableValue::Unknown { universe } => {
657 if self.for_universe.can_name(universe) {
660 let new_var_id = variable_table.new_key(ConstVarValue {
661 origin: var_value.origin,
662 val: ConstVariableValue::Unknown { universe: self.for_universe },
664 Ok(self.tcx().mk_const_var(new_var_id, c.ty))
669 ty::ConstKind::Unevaluated(..) if self.tcx().lazy_normalization() => Ok(c),
670 _ => relate::super_relate_consts(self, c, c),
675 pub trait ConstEquateRelation<'tcx>: TypeRelation<'tcx> {
676 /// Register an obligation that both constants must be equal to each other.
678 /// If they aren't equal then the relation doesn't hold.
679 fn const_equate_obligation(&mut self, a: &'tcx ty::Const<'tcx>, b: &'tcx ty::Const<'tcx>);
682 pub trait RelateResultCompare<'tcx, T> {
683 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T>
685 F: FnOnce() -> TypeError<'tcx>;
688 impl<'tcx, T: Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
689 fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T>
691 F: FnOnce() -> TypeError<'tcx>,
693 self.clone().and_then(|s| if s == t { self.clone() } else { Err(f()) })
697 pub fn const_unification_error<'tcx>(
699 (a, b): (&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>),
700 ) -> TypeError<'tcx> {
701 TypeError::ConstMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
704 fn int_unification_error<'tcx>(
706 v: (ty::IntVarValue, ty::IntVarValue),
707 ) -> TypeError<'tcx> {
709 TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
712 fn float_unification_error<'tcx>(
714 v: (ty::FloatVarValue, ty::FloatVarValue),
715 ) -> TypeError<'tcx> {
716 let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
717 TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))