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use std::mem;
use rustc_data_structures::sso::SsoHashMap;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir::def_id::DefId;
use rustc_middle::bug;
use rustc_middle::infer::unify_key::ConstVariableValue;
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::visit::MaxUniverse;
use rustc_middle::ty::{
self, AliasRelationDirection, InferConst, Term, Ty, TyCtxt, TypeVisitable, TypeVisitableExt,
};
use rustc_span::Span;
use tracing::{debug, instrument, warn};
use super::{
PredicateEmittingRelation, Relate, RelateResult, StructurallyRelateAliases, TypeRelation,
};
use crate::infer::type_variable::TypeVariableValue;
use crate::infer::{relate, InferCtxt, RegionVariableOrigin};
impl<'tcx> InferCtxt<'tcx> {
/// The idea is that we should ensure that the type variable `target_vid`
/// is equal to, a subtype of, or a supertype of `source_ty`.
///
/// For this, we will instantiate `target_vid` with a *generalized* version
/// of `source_ty`. Generalization introduces other inference variables wherever
/// subtyping could occur. This also does the occurs checks, detecting whether
/// instantiating `target_vid` would result in a cyclic type. We eagerly error
/// in this case.
///
/// This is *not* expected to be used anywhere except for an implementation of
/// `TypeRelation`. Do not use this, and instead please use `At::eq`, for all
/// other usecases (i.e. setting the value of a type var).
#[instrument(level = "debug", skip(self, relation))]
pub fn instantiate_ty_var<R: PredicateEmittingRelation<InferCtxt<'tcx>>>(
&self,
relation: &mut R,
target_is_expected: bool,
target_vid: ty::TyVid,
instantiation_variance: ty::Variance,
source_ty: Ty<'tcx>,
) -> RelateResult<'tcx, ()> {
debug_assert!(self.inner.borrow_mut().type_variables().probe(target_vid).is_unknown());
// Generalize `source_ty` depending on the current variance. As an example, assume
// `?target <: &'x ?1`, where `'x` is some free region and `?1` is an inference
// variable.
//
// Then the `generalized_ty` would be `&'?2 ?3`, where `'?2` and `?3` are fresh
// region/type inference variables.
//
// We then relate `generalized_ty <: source_ty`,adding constraints like `'x: '?2` and `?1 <: ?3`.
let Generalization { value_may_be_infer: generalized_ty, has_unconstrained_ty_var } = self
.generalize(
relation.span(),
relation.structurally_relate_aliases(),
target_vid,
instantiation_variance,
source_ty,
)?;
// Constrain `b_vid` to the generalized type `generalized_ty`.
if let &ty::Infer(ty::TyVar(generalized_vid)) = generalized_ty.kind() {
self.inner.borrow_mut().type_variables().equate(target_vid, generalized_vid);
} else {
self.inner.borrow_mut().type_variables().instantiate(target_vid, generalized_ty);
}
// See the comment on `Generalization::has_unconstrained_ty_var`.
if has_unconstrained_ty_var {
relation.register_predicates([ty::ClauseKind::WellFormed(generalized_ty.into())]);
}
// Finally, relate `generalized_ty` to `source_ty`, as described in previous comment.
//
// FIXME(#16847): This code is non-ideal because all these subtype
// relations wind up attributed to the same spans. We need
// to associate causes/spans with each of the relations in
// the stack to get this right.
if generalized_ty.is_ty_var() {
// This happens for cases like `<?0 as Trait>::Assoc == ?0`.
// We can't instantiate `?0` here as that would result in a
// cyclic type. We instead delay the unification in case
// the alias can be normalized to something which does not
// mention `?0`.
if self.next_trait_solver() {
let (lhs, rhs, direction) = match instantiation_variance {
ty::Invariant => {
(generalized_ty.into(), source_ty.into(), AliasRelationDirection::Equate)
}
ty::Covariant => {
(generalized_ty.into(), source_ty.into(), AliasRelationDirection::Subtype)
}
ty::Contravariant => {
(source_ty.into(), generalized_ty.into(), AliasRelationDirection::Subtype)
}
ty::Bivariant => unreachable!("bivariant generalization"),
};
relation.register_predicates([ty::PredicateKind::AliasRelate(lhs, rhs, direction)]);
} else {
match source_ty.kind() {
&ty::Alias(ty::Projection, data) => {
// FIXME: This does not handle subtyping correctly, we could
// instead create a new inference variable `?normalized_source`, emitting
// `Projection(normalized_source, ?ty_normalized)` and `?normalized_source <: generalized_ty`.
relation.register_predicates([ty::ProjectionPredicate {
projection_term: data.into(),
term: generalized_ty.into(),
}]);
}
// The old solver only accepts projection predicates for associated types.
ty::Alias(ty::Inherent | ty::Weak | ty::Opaque, _) => {
return Err(TypeError::CyclicTy(source_ty));
}
_ => bug!("generalized `{source_ty:?} to infer, not an alias"),
}
}
} else {
// NOTE: The `instantiation_variance` is not the same variance as
// used by the relation. When instantiating `b`, `target_is_expected`
// is flipped and the `instantion_variance` is also flipped. To
// constrain the `generalized_ty` while using the original relation,
// we therefore only have to flip the arguments.
//
// ```ignore (not code)
// ?a rel B
// instantiate_ty_var(?a, B) # expected and variance not flipped
// B' rel B
// ```
// or
// ```ignore (not code)
// A rel ?b
// instantiate_ty_var(?b, A) # expected and variance flipped
// A rel A'
// ```
if target_is_expected {
relation.relate(generalized_ty, source_ty)?;
} else {
debug!("flip relation");
relation.relate(source_ty, generalized_ty)?;
}
}
Ok(())
}
/// Instantiates the const variable `target_vid` with the given constant.
///
/// This also tests if the given const `ct` contains an inference variable which was previously
/// unioned with `target_vid`. If this is the case, inferring `target_vid` to `ct`
/// would result in an infinite type as we continuously replace an inference variable
/// in `ct` with `ct` itself.
///
/// This is especially important as unevaluated consts use their parents generics.
/// They therefore often contain unused args, making these errors far more likely.
///
/// A good example of this is the following:
///
/// ```compile_fail,E0308
/// #![feature(generic_const_exprs)]
///
/// fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
/// todo!()
/// }
///
/// fn main() {
/// let mut arr = Default::default();
/// arr = bind(arr);
/// }
/// ```
///
/// Here `3 + 4` ends up as `ConstKind::Unevaluated` which uses the generics
/// of `fn bind` (meaning that its args contain `N`).
///
/// `bind(arr)` now infers that the type of `arr` must be `[u8; N]`.
/// The assignment `arr = bind(arr)` now tries to equate `N` with `3 + 4`.
///
/// As `3 + 4` contains `N` in its args, this must not succeed.
///
/// See `tests/ui/const-generics/occurs-check/` for more examples where this is relevant.
#[instrument(level = "debug", skip(self, relation))]
pub(super) fn instantiate_const_var<R: PredicateEmittingRelation<InferCtxt<'tcx>>>(
&self,
relation: &mut R,
target_is_expected: bool,
target_vid: ty::ConstVid,
source_ct: ty::Const<'tcx>,
) -> RelateResult<'tcx, ()> {
// FIXME(generic_const_exprs): Occurs check failures for unevaluated
// constants and generic expressions are not yet handled correctly.
let Generalization { value_may_be_infer: generalized_ct, has_unconstrained_ty_var } = self
.generalize(
relation.span(),
relation.structurally_relate_aliases(),
target_vid,
ty::Invariant,
source_ct,
)?;
debug_assert!(!generalized_ct.is_ct_infer());
if has_unconstrained_ty_var {
bug!("unconstrained ty var when generalizing `{source_ct:?}`");
}
self.inner
.borrow_mut()
.const_unification_table()
.union_value(target_vid, ConstVariableValue::Known { value: generalized_ct });
// Make sure that the order is correct when relating the
// generalized const and the source.
if target_is_expected {
relation.relate_with_variance(
ty::Invariant,
ty::VarianceDiagInfo::default(),
generalized_ct,
source_ct,
)?;
} else {
relation.relate_with_variance(
ty::Invariant,
ty::VarianceDiagInfo::default(),
source_ct,
generalized_ct,
)?;
}
Ok(())
}
/// Attempts to generalize `source_term` for the type variable `target_vid`.
/// This checks for cycles -- that is, whether `source_term` references `target_vid`.
fn generalize<T: Into<Term<'tcx>> + Relate<TyCtxt<'tcx>>>(
&self,
span: Span,
structurally_relate_aliases: StructurallyRelateAliases,
target_vid: impl Into<ty::TermVid>,
ambient_variance: ty::Variance,
source_term: T,
) -> RelateResult<'tcx, Generalization<T>> {
assert!(!source_term.has_escaping_bound_vars());
let (for_universe, root_vid) = match target_vid.into() {
ty::TermVid::Ty(ty_vid) => {
(self.probe_ty_var(ty_vid).unwrap_err(), ty::TermVid::Ty(self.root_var(ty_vid)))
}
ty::TermVid::Const(ct_vid) => (
self.probe_const_var(ct_vid).unwrap_err(),
ty::TermVid::Const(
self.inner.borrow_mut().const_unification_table().find(ct_vid).vid,
),
),
};
let mut generalizer = Generalizer {
infcx: self,
span,
structurally_relate_aliases,
root_vid,
for_universe,
ambient_variance,
root_term: source_term.into(),
in_alias: false,
has_unconstrained_ty_var: false,
cache: Default::default(),
};
let value_may_be_infer = generalizer.relate(source_term, source_term)?;
let has_unconstrained_ty_var = generalizer.has_unconstrained_ty_var;
Ok(Generalization { value_may_be_infer, has_unconstrained_ty_var })
}
}
/// The "generalizer" is used when handling inference variables.
///
/// The basic strategy for handling a constraint like `?A <: B` is to
/// apply a "generalization strategy" to the term `B` -- this replaces
/// all the lifetimes in the term `B` with fresh inference variables.
/// (You can read more about the strategy in this [blog post].)
///
/// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
/// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
/// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
/// establishes `'0: 'x` as a constraint.
///
/// [blog post]: https://is.gd/0hKvIr
struct Generalizer<'me, 'tcx> {
infcx: &'me InferCtxt<'tcx>,
span: Span,
/// Whether aliases should be related structurally. If not, we have to
/// be careful when generalizing aliases.
structurally_relate_aliases: StructurallyRelateAliases,
/// The vid of the type variable that is in the process of being
/// instantiated. If we find this within the value we are folding,
/// that means we would have created a cyclic value.
root_vid: ty::TermVid,
/// The universe of the type variable that is in the process of being
/// instantiated. If we find anything that this universe cannot name,
/// we reject the relation.
for_universe: ty::UniverseIndex,
/// After we generalize this type, we are going to relate it to
/// some other type. What will be the variance at this point?
ambient_variance: ty::Variance,
/// The root term (const or type) we're generalizing. Used for cycle errors.
root_term: Term<'tcx>,
cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
/// This is set once we're generalizing the arguments of an alias.
///
/// This is necessary to correctly handle
/// `<T as Bar<<?0 as Foo>::Assoc>::Assoc == ?0`. This equality can
/// hold by either normalizing the outer or the inner associated type.
in_alias: bool,
/// See the field `has_unconstrained_ty_var` in `Generalization`.
has_unconstrained_ty_var: bool,
}
impl<'tcx> Generalizer<'_, 'tcx> {
/// Create an error that corresponds to the term kind in `root_term`
fn cyclic_term_error(&self) -> TypeError<'tcx> {
match self.root_term.unpack() {
ty::TermKind::Ty(ty) => TypeError::CyclicTy(ty),
ty::TermKind::Const(ct) => TypeError::CyclicConst(ct),
}
}
/// Create a new type variable in the universe of the target when
/// generalizing an alias. This has to set `has_unconstrained_ty_var`
/// if we're currently in a bivariant context.
fn next_ty_var_for_alias(&mut self) -> Ty<'tcx> {
self.has_unconstrained_ty_var |= self.ambient_variance == ty::Bivariant;
self.infcx.next_ty_var_in_universe(self.span, self.for_universe)
}
/// An occurs check failure inside of an alias does not mean
/// that the types definitely don't unify. We may be able
/// to normalize the alias after all.
///
/// We handle this by lazily equating the alias and generalizing
/// it to an inference variable. In the new solver, we always
/// generalize to an infer var unless the alias contains escaping
/// bound variables.
///
/// Correctly handling aliases with escaping bound variables is
/// difficult and currently incomplete in two opposite ways:
/// - if we get an occurs check failure in the alias, replace it with a new infer var.
/// This causes us to later emit an alias-relate goal and is incomplete in case the
/// alias normalizes to type containing one of the bound variables.
/// - if the alias contains an inference variable not nameable by `for_universe`, we
/// continue generalizing the alias. This ends up pulling down the universe of the
/// inference variable and is incomplete in case the alias would normalize to a type
/// which does not mention that inference variable.
fn generalize_alias_ty(
&mut self,
alias: ty::AliasTy<'tcx>,
) -> Result<Ty<'tcx>, TypeError<'tcx>> {
// We do not eagerly replace aliases with inference variables if they have
// escaping bound vars, see the method comment for details. However, when we
// are inside of an alias with escaping bound vars replacing nested aliases
// with inference variables can cause incorrect ambiguity.
//
// cc trait-system-refactor-initiative#110
if self.infcx.next_trait_solver() && !alias.has_escaping_bound_vars() && !self.in_alias {
return Ok(self.next_ty_var_for_alias());
}
let is_nested_alias = mem::replace(&mut self.in_alias, true);
let result = match self.relate(alias, alias) {
Ok(alias) => Ok(alias.to_ty(self.cx())),
Err(e) => {
if is_nested_alias {
return Err(e);
} else {
let mut visitor = MaxUniverse::new();
alias.visit_with(&mut visitor);
let infer_replacement_is_complete =
self.for_universe.can_name(visitor.max_universe())
&& !alias.has_escaping_bound_vars();
if !infer_replacement_is_complete {
warn!("may incompletely handle alias type: {alias:?}");
}
debug!("generalization failure in alias");
Ok(self.next_ty_var_for_alias())
}
}
};
self.in_alias = is_nested_alias;
result
}
}
impl<'tcx> TypeRelation<TyCtxt<'tcx>> for Generalizer<'_, 'tcx> {
fn cx(&self) -> TyCtxt<'tcx> {
self.infcx.tcx
}
fn relate_item_args(
&mut self,
item_def_id: DefId,
a_arg: ty::GenericArgsRef<'tcx>,
b_arg: ty::GenericArgsRef<'tcx>,
) -> RelateResult<'tcx, ty::GenericArgsRef<'tcx>> {
if self.ambient_variance == ty::Invariant {
// Avoid fetching the variance if we are in an invariant
// context; no need, and it can induce dependency cycles
// (e.g., #41849).
relate::relate_args_invariantly(self, a_arg, b_arg)
} else {
let tcx = self.cx();
let opt_variances = tcx.variances_of(item_def_id);
relate::relate_args_with_variances(
self,
item_def_id,
opt_variances,
a_arg,
b_arg,
false,
)
}
}
#[instrument(level = "debug", skip(self, variance, b), ret)]
fn relate_with_variance<T: Relate<TyCtxt<'tcx>>>(
&mut self,
variance: ty::Variance,
_info: ty::VarianceDiagInfo<TyCtxt<'tcx>>,
a: T,
b: T,
) -> RelateResult<'tcx, T> {
let old_ambient_variance = self.ambient_variance;
self.ambient_variance = self.ambient_variance.xform(variance);
debug!(?self.ambient_variance, "new ambient variance");
// Recursive calls to `relate` can overflow the stack. For example a deeper version of
// `ui/associated-consts/issue-93775.rs`.
let r = ensure_sufficient_stack(|| self.relate(a, b));
self.ambient_variance = old_ambient_variance;
r
}
#[instrument(level = "debug", skip(self, t2), ret)]
fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
assert_eq!(t, t2); // we are misusing TypeRelation here; both LHS and RHS ought to be ==
if let Some(&result) = self.cache.get(&t) {
return Ok(result);
}
// Check to see whether the type we are generalizing references
// any other type variable related to `vid` via
// subtyping. This is basically our "occurs check", preventing
// us from creating infinitely sized types.
let g = match *t.kind() {
ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("unexpected infer type: {t}")
}
ty::Infer(ty::TyVar(vid)) => {
let mut inner = self.infcx.inner.borrow_mut();
let vid = inner.type_variables().root_var(vid);
if ty::TermVid::Ty(vid) == self.root_vid {
// If sub-roots are equal, then `root_vid` and
// `vid` are related via subtyping.
Err(self.cyclic_term_error())
} else {
let probe = inner.type_variables().probe(vid);
match probe {
TypeVariableValue::Known { value: u } => {
drop(inner);
self.relate(u, u)
}
TypeVariableValue::Unknown { universe } => {
match self.ambient_variance {
// Invariant: no need to make a fresh type variable
// if we can name the universe.
ty::Invariant => {
if self.for_universe.can_name(universe) {
return Ok(t);
}
}
// Bivariant: make a fresh var, but remember that
// it is unconstrained. See the comment in
// `Generalization`.
ty::Bivariant => self.has_unconstrained_ty_var = true,
// Co/contravariant: this will be
// sufficiently constrained later on.
ty::Covariant | ty::Contravariant => (),
}
let origin = inner.type_variables().var_origin(vid);
let new_var_id =
inner.type_variables().new_var(self.for_universe, origin);
// If we're in the new solver and create a new inference
// variable inside of an alias we eagerly constrain that
// inference variable to prevent unexpected ambiguity errors.
//
// This is incomplete as it pulls down the universe of the
// original inference variable, even though the alias could
// normalize to a type which does not refer to that type at
// all. I don't expect this to cause unexpected errors in
// practice.
//
// We only need to do so for type and const variables, as
// region variables do not impact normalization, and will get
// correctly constrained by `AliasRelate` later on.
//
// cc trait-system-refactor-initiative#108
if self.infcx.next_trait_solver()
&& !self.infcx.intercrate
&& self.in_alias
{
inner.type_variables().equate(vid, new_var_id);
}
debug!("replacing original vid={:?} with new={:?}", vid, new_var_id);
Ok(Ty::new_var(self.cx(), new_var_id))
}
}
}
}
ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
// No matter what mode we are in,
// integer/floating-point types must be equal to be
// relatable.
Ok(t)
}
ty::Placeholder(placeholder) => {
if self.for_universe.can_name(placeholder.universe) {
Ok(t)
} else {
debug!(
"root universe {:?} cannot name placeholder in universe {:?}",
self.for_universe, placeholder.universe
);
Err(TypeError::Mismatch)
}
}
ty::Alias(_, data) => match self.structurally_relate_aliases {
StructurallyRelateAliases::No => self.generalize_alias_ty(data),
StructurallyRelateAliases::Yes => relate::structurally_relate_tys(self, t, t),
},
_ => relate::structurally_relate_tys(self, t, t),
}?;
self.cache.insert(t, g);
Ok(g)
}
#[instrument(level = "debug", skip(self, r2), ret)]
fn regions(
&mut self,
r: ty::Region<'tcx>,
r2: ty::Region<'tcx>,
) -> RelateResult<'tcx, ty::Region<'tcx>> {
assert_eq!(r, r2); // we are misusing TypeRelation here; both LHS and RHS ought to be ==
match *r {
// Never make variables for regions bound within the type itself,
// nor for erased regions.
ty::ReBound(..) | ty::ReErased => {
return Ok(r);
}
// It doesn't really matter for correctness if we generalize ReError,
// since we're already on a doomed compilation path.
ty::ReError(_) => {
return Ok(r);
}
ty::RePlaceholder(..)
| ty::ReVar(..)
| ty::ReStatic
| ty::ReEarlyParam(..)
| ty::ReLateParam(..) => {
// see common code below
}
}
// If we are in an invariant context, we can re-use the region
// as is, unless it happens to be in some universe that we
// can't name.
if let ty::Invariant = self.ambient_variance {
let r_universe = self.infcx.universe_of_region(r);
if self.for_universe.can_name(r_universe) {
return Ok(r);
}
}
Ok(self.infcx.next_region_var_in_universe(
RegionVariableOrigin::MiscVariable(self.span),
self.for_universe,
))
}
#[instrument(level = "debug", skip(self, c2), ret)]
fn consts(
&mut self,
c: ty::Const<'tcx>,
c2: ty::Const<'tcx>,
) -> RelateResult<'tcx, ty::Const<'tcx>> {
assert_eq!(c, c2); // we are misusing TypeRelation here; both LHS and RHS ought to be ==
match c.kind() {
ty::ConstKind::Infer(InferConst::Var(vid)) => {
// If root const vids are equal, then `root_vid` and
// `vid` are related and we'd be inferring an infinitely
// deep const.
if ty::TermVid::Const(
self.infcx.inner.borrow_mut().const_unification_table().find(vid).vid,
) == self.root_vid
{
return Err(self.cyclic_term_error());
}
let mut inner = self.infcx.inner.borrow_mut();
let variable_table = &mut inner.const_unification_table();
match variable_table.probe_value(vid) {
ConstVariableValue::Known { value: u } => {
drop(inner);
self.relate(u, u)
}
ConstVariableValue::Unknown { origin, universe } => {
if self.for_universe.can_name(universe) {
Ok(c)
} else {
let new_var_id = variable_table
.new_key(ConstVariableValue::Unknown {
origin,
universe: self.for_universe,
})
.vid;
// See the comment for type inference variables
// for more details.
if self.infcx.next_trait_solver()
&& !self.infcx.intercrate
&& self.in_alias
{
variable_table.union(vid, new_var_id);
}
Ok(ty::Const::new_var(self.cx(), new_var_id))
}
}
}
}
ty::ConstKind::Infer(InferConst::EffectVar(_)) => Ok(c),
// FIXME: Unevaluated constants are also not rigid, so the current
// approach of always relating them structurally is incomplete.
//
// FIXME: remove this branch once `structurally_relate_consts` is fully
// structural.
ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, args }) => {
let args = self.relate_with_variance(
ty::Invariant,
ty::VarianceDiagInfo::default(),
args,
args,
)?;
Ok(ty::Const::new_unevaluated(self.cx(), ty::UnevaluatedConst { def, args }))
}
ty::ConstKind::Placeholder(placeholder) => {
if self.for_universe.can_name(placeholder.universe) {
Ok(c)
} else {
debug!(
"root universe {:?} cannot name placeholder in universe {:?}",
self.for_universe, placeholder.universe
);
Err(TypeError::Mismatch)
}
}
_ => relate::structurally_relate_consts(self, c, c),
}
}
#[instrument(level = "debug", skip(self), ret)]
fn binders<T>(
&mut self,
a: ty::Binder<'tcx, T>,
_: ty::Binder<'tcx, T>,
) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
where
T: Relate<TyCtxt<'tcx>>,
{
let result = self.relate(a.skip_binder(), a.skip_binder())?;
Ok(a.rebind(result))
}
}
/// Result from a generalization operation. This includes
/// not only the generalized type, but also a bool flag
/// indicating whether further WF checks are needed.
#[derive(Debug)]
struct Generalization<T> {
/// When generalizing `<?0 as Trait>::Assoc` or
/// `<T as Bar<<?0 as Foo>::Assoc>>::Assoc`
/// for `?0` generalization returns an inference
/// variable.
///
/// This has to be handled wotj care as it can
/// otherwise very easily result in infinite
/// recursion.
pub value_may_be_infer: T,
/// In general, we do not check whether all types which occur during
/// type checking are well-formed. We only check wf of user-provided types
/// and when actually using a type, e.g. for method calls.
///
/// This means that when subtyping, we may end up with unconstrained
/// inference variables if a generalized type has bivariant parameters.
/// A parameter may only be bivariant if it is constrained by a projection
/// bound in a where-clause. As an example, imagine a type:
///
/// struct Foo<A, B> where A: Iterator<Item = B> {
/// data: A
/// }
///
/// here, `A` will be covariant, but `B` is unconstrained.
///
/// However, whatever it is, for `Foo` to be WF, it must be equal to `A::Item`.
/// If we have an input `Foo<?A, ?B>`, then after generalization we will wind
/// up with a type like `Foo<?C, ?D>`. When we enforce `Foo<?A, ?B> <: Foo<?C, ?D>`,
/// we will wind up with the requirement that `?A <: ?C`, but no particular
/// relationship between `?B` and `?D` (after all, these types may be completely
/// different). If we do nothing else, this may mean that `?D` goes unconstrained
/// (as in #41677). To avoid this we emit a `WellFormed` obligation in these cases.
pub has_unconstrained_ty_var: bool,
}