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Auto merge of rust-lang#120544 - BoxyUwU:enter_forall, r=lcnr
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Introduce `enter_forall` to supercede `instantiate_binder_with_placeholders`

r? `@lcnr`

Long term we'd like to experiment with decrementing the universe count after "exiting" binders so that we do not end up creating infer vars in non-root universes even when they logically reside in the root universe. The fact that we dont do this currently results in a number of issues in the new trait solver where we consider goals to be ambiguous because otherwise it would require lowering the universe of an infer var. i.e. the goal  `?x.0 eq <T as Trait<?y.1>>::Assoc` where the alias is rigid would not be able to instantiate `?x` with the alias as there would be a universe error.

This PR is the first-ish sort of step towards being able to implement this as eventually we would want to decrement the universe in `enter_forall`. Unfortunately its Difficult to actually implement decrementing universes nicely so this is a separate step which moves us closer to the long term goal ✨
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bors committed Feb 8, 2024
2 parents 1280928 + f867742 commit c29082f
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Showing 21 changed files with 627 additions and 547 deletions.
19 changes: 10 additions & 9 deletions compiler/rustc_infer/src/infer/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1032,21 +1032,22 @@ impl<'tcx> InferCtxt<'tcx> {
_ => {}
}

let ty::SubtypePredicate { a_is_expected, a, b } =
self.instantiate_binder_with_placeholders(predicate);

Ok(self.at(cause, param_env).sub_exp(DefineOpaqueTypes::No, a_is_expected, a, b))
self.enter_forall(predicate, |ty::SubtypePredicate { a_is_expected, a, b }| {
Ok(self.at(cause, param_env).sub_exp(DefineOpaqueTypes::No, a_is_expected, a, b))
})
}

pub fn region_outlives_predicate(
&self,
cause: &traits::ObligationCause<'tcx>,
predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
) {
let ty::OutlivesPredicate(r_a, r_b) = self.instantiate_binder_with_placeholders(predicate);
let origin =
SubregionOrigin::from_obligation_cause(cause, || RelateRegionParamBound(cause.span));
self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
self.enter_forall(predicate, |ty::OutlivesPredicate(r_a, r_b)| {
let origin = SubregionOrigin::from_obligation_cause(cause, || {
RelateRegionParamBound(cause.span)
});
self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
})
}

/// Number of type variables created so far.
Expand Down Expand Up @@ -1455,7 +1456,7 @@ impl<'tcx> InferCtxt<'tcx> {
// Use this method if you'd like to find some substitution of the binder's
// variables (e.g. during a method call). If there isn't a [`BoundRegionConversionTime`]
// that corresponds to your use case, consider whether or not you should
// use [`InferCtxt::instantiate_binder_with_placeholders`] instead.
// use [`InferCtxt::enter_forall`] instead.
pub fn instantiate_binder_with_fresh_vars<T>(
&self,
span: Span,
Expand Down
64 changes: 46 additions & 18 deletions compiler/rustc_infer/src/infer/relate/higher_ranked.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -38,24 +38,25 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
// First, we instantiate each bound region in the supertype with a
// fresh placeholder region. Note that this automatically creates
// a new universe if needed.
let sup_prime = self.infcx.instantiate_binder_with_placeholders(sup);
self.infcx.enter_forall(sup, |sup_prime| {
// Next, we instantiate each bound region in the subtype
// with a fresh region variable. These region variables --
// but no other preexisting region variables -- can name
// the placeholders.
let sub_prime =
self.infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, sub);
debug!("a_prime={:?}", sub_prime);
debug!("b_prime={:?}", sup_prime);

// Next, we instantiate each bound region in the subtype
// with a fresh region variable. These region variables --
// but no other preexisting region variables -- can name
// the placeholders.
let sub_prime = self.infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, sub);

debug!("a_prime={:?}", sub_prime);
debug!("b_prime={:?}", sup_prime);

// Compare types now that bound regions have been replaced.
let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?;

debug!("OK result={result:?}");
// NOTE: returning the result here would be dangerous as it contains
// placeholders which **must not** be named afterwards.
Ok(())
// Compare types now that bound regions have been replaced.
let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime);
if result.is_ok() {
debug!("OK result={result:?}");
}
// NOTE: returning the result here would be dangerous as it contains
// placeholders which **must not** be named afterwards.
result.map(|_| ())
})
}
}

Expand All @@ -68,9 +69,11 @@ impl<'tcx> InferCtxt<'tcx> {
/// This is the first step of checking subtyping when higher-ranked things are involved.
/// For more details visit the relevant sections of the [rustc dev guide].
///
/// `fn enter_forall` should be preferred over this method.
///
/// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
#[instrument(level = "debug", skip(self), ret)]
pub fn instantiate_binder_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
pub fn enter_forall_and_leak_universe<T>(&self, binder: ty::Binder<'tcx, T>) -> T
where
T: TypeFoldable<TyCtxt<'tcx>> + Copy,
{
Expand Down Expand Up @@ -106,6 +109,31 @@ impl<'tcx> InferCtxt<'tcx> {
self.tcx.replace_bound_vars_uncached(binder, delegate)
}

/// Replaces all bound variables (lifetimes, types, and constants) bound by
/// `binder` with placeholder variables in a new universe and then calls the
/// closure `f` with the instantiated value. The new placeholders can only be
/// named by inference variables created inside of the closure `f` or afterwards.
///
/// This is the first step of checking subtyping when higher-ranked things are involved.
/// For more details visit the relevant sections of the [rustc dev guide].
///
/// This method should be preferred over `fn enter_forall_and_leak_universe`.
///
/// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
#[instrument(level = "debug", skip(self, f))]
pub fn enter_forall<T, U>(&self, forall: ty::Binder<'tcx, T>, f: impl FnOnce(T) -> U) -> U
where
T: TypeFoldable<TyCtxt<'tcx>> + Copy,
{
// FIXME: currently we do nothing to prevent placeholders with the new universe being
// used after exiting `f`. For example region subtyping can result in outlives constraints
// that name placeholders created in this function. Nested goals from type relations can
// also contain placeholders created by this function.
let value = self.enter_forall_and_leak_universe(forall);
debug!("?value");
f(value)
}

/// See [RegionConstraintCollector::leak_check][1]. We only check placeholder
/// leaking into `outer_universe`, i.e. placeholders which cannot be named by that
/// universe.
Expand Down
97 changes: 50 additions & 47 deletions compiler/rustc_infer/src/infer/relate/nll.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -261,52 +261,55 @@ where
Ok(a)
}

#[instrument(skip(self), level = "debug")]
fn instantiate_binder_with_placeholders<T>(&mut self, binder: ty::Binder<'tcx, T>) -> T
fn enter_forall<T, U>(
&mut self,
binder: ty::Binder<'tcx, T>,
f: impl FnOnce(&mut Self, T) -> U,
) -> U
where
T: ty::TypeFoldable<TyCtxt<'tcx>> + Copy,
{
if let Some(inner) = binder.no_bound_vars() {
return inner;
}

let mut next_region = {
let nll_delegate = &mut self.delegate;
let mut lazy_universe = None;
let value = if let Some(inner) = binder.no_bound_vars() {
inner
} else {
let mut next_region = {
let nll_delegate = &mut self.delegate;
let mut lazy_universe = None;

move |br: ty::BoundRegion| {
// The first time this closure is called, create a
// new universe for the placeholders we will make
// from here out.
let universe = lazy_universe.unwrap_or_else(|| {
let universe = nll_delegate.create_next_universe();
lazy_universe = Some(universe);
universe
});

let placeholder = ty::PlaceholderRegion { universe, bound: br };
debug!(?placeholder);
let placeholder_reg = nll_delegate.next_placeholder_region(placeholder);
debug!(?placeholder_reg);

placeholder_reg
}
};

move |br: ty::BoundRegion| {
// The first time this closure is called, create a
// new universe for the placeholders we will make
// from here out.
let universe = lazy_universe.unwrap_or_else(|| {
let universe = nll_delegate.create_next_universe();
lazy_universe = Some(universe);
universe
});

let placeholder = ty::PlaceholderRegion { universe, bound: br };
debug!(?placeholder);
let placeholder_reg = nll_delegate.next_placeholder_region(placeholder);
debug!(?placeholder_reg);

placeholder_reg
}
};
let delegate = FnMutDelegate {
regions: &mut next_region,
types: &mut |_bound_ty: ty::BoundTy| {
unreachable!("we only replace regions in nll_relate, not types")
},
consts: &mut |_bound_var: ty::BoundVar, _ty| {
unreachable!("we only replace regions in nll_relate, not consts")
},
};

let delegate = FnMutDelegate {
regions: &mut next_region,
types: &mut |_bound_ty: ty::BoundTy| {
unreachable!("we only replace regions in nll_relate, not types")
},
consts: &mut |_bound_var: ty::BoundVar, _ty| {
unreachable!("we only replace regions in nll_relate, not consts")
},
self.infcx.tcx.replace_bound_vars_uncached(binder, delegate)
};

let replaced = self.infcx.tcx.replace_bound_vars_uncached(binder, delegate);
debug!(?replaced);

replaced
debug!(?value);
f(self, value)
}

#[instrument(skip(self), level = "debug")]
Expand Down Expand Up @@ -630,10 +633,10 @@ where

// Note: the order here is important. Create the placeholders first, otherwise
// we assign the wrong universe to the existential!
let b_replaced = self.instantiate_binder_with_placeholders(b);
let a_replaced = self.instantiate_binder_with_existentials(a);

self.relate(a_replaced, b_replaced)?;
self.enter_forall(b, |this, b| {
let a = this.instantiate_binder_with_existentials(a);
this.relate(a, b)
})?;

self.ambient_variance = variance;
}
Expand All @@ -650,10 +653,10 @@ where
let variance =
std::mem::replace(&mut self.ambient_variance, ty::Variance::Contravariant);

let a_replaced = self.instantiate_binder_with_placeholders(a);
let b_replaced = self.instantiate_binder_with_existentials(b);

self.relate(a_replaced, b_replaced)?;
self.enter_forall(a, |this, a| {
let b = this.instantiate_binder_with_existentials(b);
this.relate(a, b)
})?;

self.ambient_variance = variance;
}
Expand Down
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