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Rollup merge of #110577 - compiler-errors:drop-impl-fulfill, r=lcnr
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Use fulfillment to check `Drop` impl compatibility

Use an `ObligationCtxt` to ensure that a `Drop` impl does not have stricter requirements than the ADT that it's implemented for, rather than using a `SimpleEqRelation` to (more or less) syntactically equate predicates on an ADT with predicates on an impl.

r? types

### Some background

The old code reads:

```rust
// An earlier version of this code attempted to do this checking
// via the traits::fulfill machinery. However, it ran into trouble
// since the fulfill machinery merely turns outlives-predicates
// 'a:'b and T:'b into region inference constraints. It is simpler
// just to look for all the predicates directly.
```

I'm not sure what this means, but perhaps in the 8 years since that this comment was written (cc #23638) it's gotten easier to process region constraints after doing fulfillment? I don't know how this logic differs from anything we do in the `compare_impl_item` module. Ironically, later on it says:

```rust
// However, it may be more efficient in the future to batch
// the analysis together via the fulfill (see comment above regarding
// the usage of the fulfill machinery), rather than the
// repeated `.iter().any(..)` calls.
```

Also:
* Removes `SimpleEqRelation` which was far too syntactical in its relation.
* Fixes #110557
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@matthiaskrgr
matthiaskrgr authored May 6, 2023
2 parents 151a070 + 2e346b6 commit bcc9aa0
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324 changes: 91 additions & 233 deletions compiler/rustc_hir_analysis/src/check/dropck.rs
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Original file line number Diff line number Diff line change
@@ -1,12 +1,14 @@
// FIXME(@lcnr): Move this module out of `rustc_hir_analysis`.
//
// We don't do any drop checking during hir typeck.
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::{struct_span_err, ErrorGuaranteed};
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
use rustc_infer::infer::outlives::env::OutlivesEnvironment;
use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::util::IgnoreRegions;
use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
use rustc_middle::ty::{self, TyCtxt};
use rustc_trait_selection::traits::{self, ObligationCtxt};

use crate::errors;
use crate::hir::def_id::{DefId, LocalDefId};
Expand Down Expand Up @@ -43,21 +45,20 @@ pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), Erro
}
}
let dtor_self_type = tcx.type_of(drop_impl_did).subst_identity();
let dtor_predicates = tcx.predicates_of(drop_impl_did);
match dtor_self_type.kind() {
ty::Adt(adt_def, self_to_impl_substs) => {
ty::Adt(adt_def, adt_to_impl_substs) => {
ensure_drop_params_and_item_params_correspond(
tcx,
drop_impl_did.expect_local(),
adt_def.did(),
self_to_impl_substs,
adt_to_impl_substs,
)?;

ensure_drop_predicates_are_implied_by_item_defn(
tcx,
dtor_predicates,
drop_impl_did.expect_local(),
adt_def.did().expect_local(),
self_to_impl_substs,
adt_to_impl_substs,
)
}
_ => {
Expand All @@ -78,9 +79,9 @@ fn ensure_drop_params_and_item_params_correspond<'tcx>(
tcx: TyCtxt<'tcx>,
drop_impl_did: LocalDefId,
self_type_did: DefId,
drop_impl_substs: SubstsRef<'tcx>,
adt_to_impl_substs: SubstsRef<'tcx>,
) -> Result<(), ErrorGuaranteed> {
let Err(arg) = tcx.uses_unique_generic_params(drop_impl_substs, IgnoreRegions::No) else {
let Err(arg) = tcx.uses_unique_generic_params(adt_to_impl_substs, IgnoreRegions::No) else {
return Ok(())
};

Expand Down Expand Up @@ -111,237 +112,94 @@ fn ensure_drop_params_and_item_params_correspond<'tcx>(
/// implied by assuming the predicates attached to self_type_did.
fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
tcx: TyCtxt<'tcx>,
dtor_predicates: ty::GenericPredicates<'tcx>,
self_type_did: LocalDefId,
self_to_impl_substs: SubstsRef<'tcx>,
drop_impl_def_id: LocalDefId,
adt_def_id: LocalDefId,
adt_to_impl_substs: SubstsRef<'tcx>,
) -> Result<(), ErrorGuaranteed> {
let mut result = Ok(());

// Here is an example, analogous to that from
// `compare_impl_method`.
//
// Consider a struct type:
//
// struct Type<'c, 'b:'c, 'a> {
// x: &'a Contents // (contents are irrelevant;
// y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
// }
//
// and a Drop impl:
//
// impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
// fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
// }
//
// We start out with self_to_impl_substs, that maps the generic
// parameters of Type to that of the Drop impl.
let infcx = tcx.infer_ctxt().build();
let ocx = ObligationCtxt::new(&infcx);

// Take the param-env of the adt and substitute the substs that show up in
// the implementation's self type. This gives us the assumptions that the
// self ty of the implementation is allowed to know just from it being a
// well-formed adt, since that's all we're allowed to assume while proving
// the Drop implementation is not specialized.
//
// self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
//
// Applying this to the predicates (i.e., assumptions) provided by the item
// definition yields the instantiated assumptions:
//
// ['y : 'z]
//
// We then check all of the predicates of the Drop impl:
//
// ['y:'z, 'x:'y]
//
// and ensure each is in the list of instantiated
// assumptions. Here, `'y:'z` is present, but `'x:'y` is
// absent. So we report an error that the Drop impl injected a
// predicate that is not present on the struct definition.

// We can assume the predicates attached to struct/enum definition
// hold.
let generic_assumptions = tcx.predicates_of(self_type_did);

let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
let assumptions_in_impl_context = assumptions_in_impl_context.predicates;

debug!(?assumptions_in_impl_context, ?dtor_predicates.predicates);

let self_param_env = tcx.param_env(self_type_did);

// An earlier version of this code attempted to do this checking
// via the traits::fulfill machinery. However, it ran into trouble
// since the fulfill machinery merely turns outlives-predicates
// 'a:'b and T:'b into region inference constraints. It is simpler
// just to look for all the predicates directly.

assert_eq!(dtor_predicates.parent, None);
for &(predicate, predicate_sp) in dtor_predicates.predicates {
// (We do not need to worry about deep analysis of type
// expressions etc because the Drop impls are already forced
// to take on a structure that is roughly an alpha-renaming of
// the generic parameters of the item definition.)

// This path now just checks *all* predicates via an instantiation of
// the `SimpleEqRelation`, which simply forwards to the `relate` machinery
// after taking care of anonymizing late bound regions.
//
// However, it may be more efficient in the future to batch
// the analysis together via the fulfill (see comment above regarding
// the usage of the fulfill machinery), rather than the
// repeated `.iter().any(..)` calls.
// We don't need to normalize this param-env or anything, since we're only
// substituting it with free params, so no additional param-env normalization
// can occur on top of what has been done in the param_env query itself.
let param_env = ty::EarlyBinder(tcx.param_env(adt_def_id))
.subst(tcx, adt_to_impl_substs)
.with_constness(tcx.constness(drop_impl_def_id));

for (pred, span) in tcx.predicates_of(drop_impl_def_id).instantiate_identity(tcx) {
let normalize_cause = traits::ObligationCause::misc(span, adt_def_id);
let pred = ocx.normalize(&normalize_cause, param_env, pred);
let cause = traits::ObligationCause::new(span, adt_def_id, traits::DropImpl);
ocx.register_obligation(traits::Obligation::new(tcx, cause, param_env, pred));
}

// This closure is a more robust way to check `Predicate` equality
// than simple `==` checks (which were the previous implementation).
// It relies on `ty::relate` for `TraitPredicate`, `ProjectionPredicate`,
// `ConstEvaluatable` and `TypeOutlives` (which implement the Relate trait),
// while delegating on simple equality for the other `Predicate`.
// This implementation solves (Issue #59497) and (Issue #58311).
// It is unclear to me at the moment whether the approach based on `relate`
// could be extended easily also to the other `Predicate`.
let predicate_matches_closure = |p: Predicate<'tcx>| {
let mut relator: SimpleEqRelation<'tcx> = SimpleEqRelation::new(tcx, self_param_env);
let predicate = predicate.kind();
let p = p.kind();
match (predicate.skip_binder(), p.skip_binder()) {
(
ty::PredicateKind::Clause(ty::Clause::Trait(a)),
ty::PredicateKind::Clause(ty::Clause::Trait(b)),
) => relator.relate(predicate.rebind(a), p.rebind(b)).is_ok(),
(
ty::PredicateKind::Clause(ty::Clause::Projection(a)),
ty::PredicateKind::Clause(ty::Clause::Projection(b)),
) => relator.relate(predicate.rebind(a), p.rebind(b)).is_ok(),
(
ty::PredicateKind::ConstEvaluatable(a),
ty::PredicateKind::ConstEvaluatable(b),
) => relator.relate(predicate.rebind(a), predicate.rebind(b)).is_ok(),
(
ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
ty_a,
lt_a,
))),
ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
ty_b,
lt_b,
))),
) => {
relator.relate(predicate.rebind(ty_a), p.rebind(ty_b)).is_ok()
&& relator.relate(predicate.rebind(lt_a), p.rebind(lt_b)).is_ok()
}
(ty::PredicateKind::WellFormed(arg_a), ty::PredicateKind::WellFormed(arg_b)) => {
relator.relate(predicate.rebind(arg_a), p.rebind(arg_b)).is_ok()
}
_ => predicate == p,
// All of the custom error reporting logic is to preserve parity with the old
// error messages.
//
// They can probably get removed with better treatment of the new `DropImpl`
// obligation cause code, and perhaps some custom logic in `report_region_errors`.

let errors = ocx.select_all_or_error();
if !errors.is_empty() {
let mut guar = None;
let mut root_predicates = FxHashSet::default();
for error in errors {
let root_predicate = error.root_obligation.predicate;
if root_predicates.insert(root_predicate) {
let item_span = tcx.def_span(adt_def_id);
let self_descr = tcx.def_descr(adt_def_id.to_def_id());
guar = Some(
struct_span_err!(
tcx.sess,
error.root_obligation.cause.span,
E0367,
"`Drop` impl requires `{root_predicate}` \
but the {self_descr} it is implemented for does not",
)
.span_note(item_span, "the implementor must specify the same requirement")
.emit(),
);
}
};

if !assumptions_in_impl_context.iter().copied().any(predicate_matches_closure) {
let item_span = tcx.def_span(self_type_did);
let self_descr = tcx.def_descr(self_type_did.to_def_id());
let reported = struct_span_err!(
tcx.sess,
predicate_sp,
E0367,
"`Drop` impl requires `{predicate}` but the {self_descr} it is implemented for does not",
)
.span_note(item_span, "the implementor must specify the same requirement")
.emit();
result = Err(reported);
}
return Err(guar.unwrap());
}

result
}

/// This is an implementation of the [`TypeRelation`] trait with the
/// aim of simply comparing for equality (without side-effects).
///
/// It is not intended to be used anywhere else other than here.
pub(crate) struct SimpleEqRelation<'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
}

impl<'tcx> SimpleEqRelation<'tcx> {
fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> SimpleEqRelation<'tcx> {
SimpleEqRelation { tcx, param_env }
}
}

impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}

fn param_env(&self) -> ty::ParamEnv<'tcx> {
self.param_env
}

fn tag(&self) -> &'static str {
"dropck::SimpleEqRelation"
}

fn a_is_expected(&self) -> bool {
true
}

fn relate_with_variance<T: Relate<'tcx>>(
&mut self,
_: ty::Variance,
_info: ty::VarianceDiagInfo<'tcx>,
a: T,
b: T,
) -> RelateResult<'tcx, T> {
// Here we ignore variance because we require drop impl's types
// to be *exactly* the same as to the ones in the struct definition.
self.relate(a, b)
}

fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
debug!("SimpleEqRelation::tys(a={:?}, b={:?})", a, b);
ty::relate::super_relate_tys(self, a, b)
}

fn regions(
&mut self,
a: ty::Region<'tcx>,
b: ty::Region<'tcx>,
) -> RelateResult<'tcx, ty::Region<'tcx>> {
debug!("SimpleEqRelation::regions(a={:?}, b={:?})", a, b);

// We can just equate the regions because LBRs have been
// already anonymized.
if a == b {
Ok(a)
} else {
// I'm not sure is this `TypeError` is the right one, but
// it should not matter as it won't be checked (the dropck
// will emit its own, more informative and higher-level errors
// in case anything goes wrong).
Err(TypeError::RegionsPlaceholderMismatch)
let errors = ocx.infcx.resolve_regions(&OutlivesEnvironment::new(param_env));
if !errors.is_empty() {
let mut guar = None;
for error in errors {
let item_span = tcx.def_span(adt_def_id);
let self_descr = tcx.def_descr(adt_def_id.to_def_id());
let outlives = match error {
RegionResolutionError::ConcreteFailure(_, a, b) => format!("{b}: {a}"),
RegionResolutionError::GenericBoundFailure(_, generic, r) => {
format!("{generic}: {r}")
}
RegionResolutionError::SubSupConflict(_, _, _, a, _, b, _) => format!("{b}: {a}"),
RegionResolutionError::UpperBoundUniverseConflict(a, _, _, _, b) => {
format!("{b}: {a}", a = tcx.mk_re_var(a))
}
};
guar = Some(
struct_span_err!(
tcx.sess,
error.origin().span(),
E0367,
"`Drop` impl requires `{outlives}` \
but the {self_descr} it is implemented for does not",
)
.span_note(item_span, "the implementor must specify the same requirement")
.emit(),
);
}
return Err(guar.unwrap());
}

fn consts(
&mut self,
a: ty::Const<'tcx>,
b: ty::Const<'tcx>,
) -> RelateResult<'tcx, ty::Const<'tcx>> {
debug!("SimpleEqRelation::consts(a={:?}, b={:?})", a, b);
ty::relate::super_relate_consts(self, a, b)
}

fn binders<T>(
&mut self,
a: ty::Binder<'tcx, T>,
b: ty::Binder<'tcx, T>,
) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
where
T: Relate<'tcx>,
{
debug!("SimpleEqRelation::binders({:?}: {:?}", a, b);

// Anonymizing the LBRs is necessary to solve (Issue #59497).
// After we do so, it should be totally fine to skip the binders.
let anon_a = self.tcx.anonymize_bound_vars(a);
let anon_b = self.tcx.anonymize_bound_vars(b);
self.relate(anon_a.skip_binder(), anon_b.skip_binder())?;

Ok(a)
}
Ok(())
}
11 changes: 11 additions & 0 deletions compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,17 @@ pub enum RegionResolutionError<'tcx> {
),
}

impl<'tcx> RegionResolutionError<'tcx> {
pub fn origin(&self) -> &SubregionOrigin<'tcx> {
match self {
RegionResolutionError::ConcreteFailure(origin, _, _)
| RegionResolutionError::GenericBoundFailure(origin, _, _)
| RegionResolutionError::SubSupConflict(_, _, origin, _, _, _, _)
| RegionResolutionError::UpperBoundUniverseConflict(_, _, _, origin, _) => origin,
}
}
}

struct RegionAndOrigin<'tcx> {
region: Region<'tcx>,
origin: SubregionOrigin<'tcx>,
Expand Down
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