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Auto merge of #113704 - compiler-errors:rpitit-assumed-wf-inherit, r=…
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…spastorino

Make RPITITs inherit the `assumed_wf_types` of their parent method

... and then move the RPITIT well-formedness check to just use the regular logic of wfchecking an associated type.

---

We need to inherit the `assumed_wf_types` of the RPITIT's parent function in order for the given code to be considered well-formed:

```rust
trait Foo {
  fn bar<'a, T>(_: &'a T) -> impl Iterator<Output = &'a T>;
}
```

Since for `&'a T` to be WF, we need `T: 'a`.

In order for this to work for late-bound lifetimes, we need to do some additional mapping of any late-bound lifetimes captured by these assumed wf types. This is because within the body of the function (and thus in the `assumed_wf_types`), they're represented as `ReFree` variants of the original late-bound lifetimes declared in the function's generics, but in the RPITIT's GAT, they're represented as "reified" `ReEarlyBound` vars (duplicated during opaque type lowering). Luckily, the mapping between these two is already [stored in the opaque](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.OpaqueTy.html#structfield.lifetime_mapping).

Fixes #113796
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bors committed Jul 29, 2023
2 parents 32303b2 + 744e770 commit b969b83
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Showing 13 changed files with 151 additions and 117 deletions.
99 changes: 11 additions & 88 deletions compiler/rustc_hir_analysis/src/check/wfcheck.rs
Original file line number Diff line number Diff line change
Expand Up @@ -284,6 +284,17 @@ fn check_trait_item(tcx: TyCtxt<'_>, trait_item: &hir::TraitItem<'_>) {
};
check_object_unsafe_self_trait_by_name(tcx, trait_item);
check_associated_item(tcx, def_id, span, method_sig);

if matches!(trait_item.kind, hir::TraitItemKind::Fn(..)) {
for &assoc_ty_def_id in tcx.associated_types_for_impl_traits_in_associated_fn(def_id) {
check_associated_item(
tcx,
assoc_ty_def_id.expect_local(),
tcx.def_span(assoc_ty_def_id),
None,
);
}
}
}

/// Require that the user writes where clauses on GATs for the implicit
Expand Down Expand Up @@ -1466,13 +1477,6 @@ fn check_fn_or_method<'tcx>(

check_where_clauses(wfcx, span, def_id);

check_return_position_impl_trait_in_trait_bounds(
wfcx,
def_id,
sig.output(),
hir_decl.output.span(),
);

if sig.abi == Abi::RustCall {
let span = tcx.def_span(def_id);
let has_implicit_self = hir_decl.implicit_self != hir::ImplicitSelfKind::None;
Expand Down Expand Up @@ -1507,87 +1511,6 @@ fn check_fn_or_method<'tcx>(
}
}

/// Basically `check_associated_type_bounds`, but separated for now and should be
/// deduplicated when RPITITs get lowered into real associated items.
#[tracing::instrument(level = "trace", skip(wfcx))]
fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
wfcx: &WfCheckingCtxt<'_, 'tcx>,
fn_def_id: LocalDefId,
fn_output: Ty<'tcx>,
span: Span,
) {
let tcx = wfcx.tcx();
let Some(assoc_item) = tcx.opt_associated_item(fn_def_id.to_def_id()) else {
return;
};
if assoc_item.container != ty::AssocItemContainer::TraitContainer {
return;
}
fn_output.visit_with(&mut ImplTraitInTraitFinder {
wfcx,
fn_def_id,
depth: ty::INNERMOST,
seen: FxHashSet::default(),
});
}

// FIXME(-Zlower-impl-trait-in-trait-to-assoc-ty): Even with the new lowering
// strategy, we can't just call `check_associated_item` on the new RPITITs,
// because tests like `tests/ui/async-await/in-trait/implied-bounds.rs` will fail.
// That's because we need to check that the bounds of the RPITIT hold using
// the special args that we create during opaque type lowering, otherwise we're
// getting a bunch of early bound and free regions mixed up... Haven't looked too
// deep into this, though.
struct ImplTraitInTraitFinder<'a, 'tcx> {
wfcx: &'a WfCheckingCtxt<'a, 'tcx>,
fn_def_id: LocalDefId,
depth: ty::DebruijnIndex,
seen: FxHashSet<DefId>,
}
impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ImplTraitInTraitFinder<'_, 'tcx> {
type BreakTy = !;

fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<!> {
let tcx = self.wfcx.tcx();
if let ty::Alias(ty::Opaque, unshifted_opaque_ty) = *ty.kind()
&& self.seen.insert(unshifted_opaque_ty.def_id)
&& let Some(opaque_def_id) = unshifted_opaque_ty.def_id.as_local()
&& let origin = tcx.opaque_type_origin(opaque_def_id)
&& let hir::OpaqueTyOrigin::FnReturn(source) | hir::OpaqueTyOrigin::AsyncFn(source) = origin
&& source == self.fn_def_id
{
let opaque_ty = tcx.fold_regions(unshifted_opaque_ty, |re, _depth| {
match re.kind() {
ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReError(_) | ty::ReStatic => re,
r => bug!("unexpected region: {r:?}"),
}
});
for (bound, bound_span) in tcx
.explicit_item_bounds(opaque_ty.def_id)
.iter_instantiated_copied(tcx, opaque_ty.args)
{
let bound = self.wfcx.normalize(bound_span, None, bound);
self.wfcx.register_obligations(traits::wf::predicate_obligations(
self.wfcx.infcx,
self.wfcx.param_env,
self.wfcx.body_def_id,
bound.as_predicate(),
bound_span,
));
// Set the debruijn index back to innermost here, since we already eagerly
// shifted the args that we use to generate these bounds. This is unfortunately
// subtly different behavior than the `ImplTraitInTraitFinder` we use in `param_env`,
// but that function doesn't actually need to normalize the bound it's visiting
// (whereas we have to do so here)...
let old_depth = std::mem::replace(&mut self.depth, ty::INNERMOST);
bound.visit_with(self);
self.depth = old_depth;
}
}
ty.super_visit_with(self)
}
}

const HELP_FOR_SELF_TYPE: &str = "consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, \
`self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one \
of the previous types except `Self`)";
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
Original file line number Diff line number Diff line change
Expand Up @@ -246,6 +246,7 @@ provide! { tcx, def_id, other, cdata,
debug_assert_eq!(tcx.def_kind(def_id), DefKind::OpaqueTy);
cdata.root.tables.is_type_alias_impl_trait.get(cdata, def_id.index)
}
assumed_wf_types_for_rpitit => { table }
collect_return_position_impl_trait_in_trait_tys => {
Ok(cdata
.root
Expand Down
6 changes: 6 additions & 0 deletions compiler/rustc_metadata/src/rmeta/encoder.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1560,6 +1560,12 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
}
if let Some(rpitit_info) = item.opt_rpitit_info {
record!(self.tables.opt_rpitit_info[def_id] <- rpitit_info);
if matches!(rpitit_info, ty::ImplTraitInTraitData::Trait { .. }) {
record_array!(
self.tables.assumed_wf_types_for_rpitit[def_id]
<- self.tcx.assumed_wf_types_for_rpitit(def_id)
);
}
}
}

Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_metadata/src/rmeta/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -457,6 +457,7 @@ define_tables! {
trait_impl_trait_tys: Table<DefIndex, LazyValue<FxHashMap<DefId, ty::EarlyBinder<Ty<'static>>>>>,
doc_link_resolutions: Table<DefIndex, LazyValue<DocLinkResMap>>,
doc_link_traits_in_scope: Table<DefIndex, LazyArray<DefId>>,
assumed_wf_types_for_rpitit: Table<DefIndex, LazyArray<(Ty<'static>, Span)>>,
}

#[derive(TyEncodable, TyDecodable)]
Expand Down
7 changes: 7 additions & 0 deletions compiler/rustc_middle/src/query/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -885,6 +885,13 @@ rustc_queries! {
desc { |tcx| "computing the implied bounds of `{}`", tcx.def_path_str(key) }
}

/// We need to store the assumed_wf_types for an RPITIT so that impls of foreign
/// traits with return-position impl trait in traits can inherit the right wf types.
query assumed_wf_types_for_rpitit(key: DefId) -> &'tcx [(Ty<'tcx>, Span)] {
desc { |tcx| "computing the implied bounds of `{}`", tcx.def_path_str(key) }
separate_provide_extern
}

/// Computes the signature of the function.
query fn_sig(key: DefId) -> ty::EarlyBinder<ty::PolyFnSig<'tcx>> {
desc { |tcx| "computing function signature of `{}`", tcx.def_path_str(key) }
Expand Down
86 changes: 85 additions & 1 deletion compiler/rustc_ty_utils/src/implied_bounds.rs
Original file line number Diff line number Diff line change
@@ -1,13 +1,22 @@
use rustc_data_structures::fx::FxHashMap;
use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::LocalDefId;
use rustc_middle::middle::resolve_bound_vars as rbv;
use rustc_middle::query::Providers;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_span::Span;
use std::iter;

pub fn provide(providers: &mut Providers) {
*providers = Providers { assumed_wf_types, ..*providers };
*providers = Providers {
assumed_wf_types,
assumed_wf_types_for_rpitit: |tcx, def_id| {
assert!(tcx.is_impl_trait_in_trait(def_id.to_def_id()));
tcx.assumed_wf_types(def_id)
},
..*providers
};
}

fn assumed_wf_types<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &'tcx [(Ty<'tcx>, Span)] {
Expand Down Expand Up @@ -42,6 +51,81 @@ fn assumed_wf_types<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &'tcx [(Ty<'
let mut impl_spans = impl_spans(tcx, def_id);
tcx.arena.alloc_from_iter(tys.into_iter().map(|ty| (ty, impl_spans.next().unwrap())))
}
DefKind::AssocTy if let Some(data) = tcx.opt_rpitit_info(def_id.to_def_id()) => match data {
ty::ImplTraitInTraitData::Trait { fn_def_id, opaque_def_id } => {
let hir::OpaqueTy { lifetime_mapping, .. } =
*tcx.hir().expect_item(opaque_def_id.expect_local()).expect_opaque_ty();
// We need to remap all of the late-bound lifetimes in theassumed wf types
// of the fn (which are represented as ReFree) to the early-bound lifetimes
// of the RPITIT (which are represented by ReEarlyBound owned by the opaque).
// Luckily, this is very easy to do because we already have that mapping
// stored in the HIR of this RPITIT.
//
// Side-note: We don't really need to do this remapping for early-bound
// lifetimes because they're already "linked" by the bidirectional outlives
// predicates we insert in the `explicit_predicates_of` query for RPITITs.
let mut mapping = FxHashMap::default();
let generics = tcx.generics_of(def_id);
for &(lifetime, new_early_bound_def_id) in
lifetime_mapping.expect("expected lifetime mapping for RPITIT")
{
if let Some(rbv::ResolvedArg::LateBound(_, _, def_id)) =
tcx.named_bound_var(lifetime.hir_id)
{
let name = tcx.hir().name(lifetime.hir_id);
let index = generics
.param_def_id_to_index(tcx, new_early_bound_def_id.to_def_id())
.unwrap();
mapping.insert(
ty::Region::new_free(
tcx,
fn_def_id,
ty::BoundRegionKind::BrNamed(def_id, name),
),
ty::Region::new_early_bound(
tcx,
ty::EarlyBoundRegion {
def_id: new_early_bound_def_id.to_def_id(),
index,
name,
},
),
);
}
}
// FIXME: This could use a real folder, I guess.
let remapped_wf_tys = tcx.fold_regions(
tcx.assumed_wf_types(fn_def_id.expect_local()).to_vec(),
|region, _| {
// If `region` is a `ReFree` that is captured by the
// opaque, remap it to its corresponding the early-
// bound region.
if let Some(remapped_region) = mapping.get(&region) {
*remapped_region
} else {
region
}
},
);
tcx.arena.alloc_from_iter(remapped_wf_tys)
}
// Assumed wf types for RPITITs in an impl just inherit (and instantiate)
// the assumed wf types of the trait's RPITIT GAT.
ty::ImplTraitInTraitData::Impl { .. } => {
let impl_def_id = tcx.local_parent(def_id);
let rpitit_def_id = tcx.associated_item(def_id).trait_item_def_id.unwrap();
let args = ty::GenericArgs::identity_for_item(tcx, def_id).rebase_onto(
tcx,
impl_def_id.to_def_id(),
tcx.impl_trait_ref(impl_def_id).unwrap().instantiate_identity().args,
);
tcx.arena.alloc_from_iter(
ty::EarlyBinder::bind(tcx.assumed_wf_types_for_rpitit(rpitit_def_id))
.iter_instantiated_copied(tcx, args)
.chain(tcx.assumed_wf_types(impl_def_id).into_iter().copied()),
)
}
},
DefKind::AssocConst | DefKind::AssocTy => tcx.assumed_wf_types(tcx.local_parent(def_id)),
DefKind::OpaqueTy => match tcx.def_kind(tcx.local_parent(def_id)) {
DefKind::TyAlias => ty::List::empty(),
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_ty_utils/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@
#![feature(assert_matches)]
#![feature(iterator_try_collect)]
#![feature(let_chains)]
#![feature(if_let_guard)]
#![feature(never_type)]
#![feature(box_patterns)]
#![recursion_limit = "256"]
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -4,11 +4,11 @@ error[E0311]: the parameter type `U` may not live long enough
LL | async fn foo(&self) -> &(T, U) where T: Debug + Sized, U: Hash;
| ^^^^^^^
|
note: the parameter type `U` must be valid for the anonymous lifetime defined here...
note: the parameter type `U` must be valid for the anonymous lifetime as defined here...
--> $DIR/async-generics-and-bounds.rs:12:18
|
LL | async fn foo(&self) -> &(T, U) where T: Debug + Sized, U: Hash;
| ^^^^^
| ^
note: ...so that the reference type `&(T, U)` does not outlive the data it points at
--> $DIR/async-generics-and-bounds.rs:12:28
|
Expand All @@ -21,11 +21,11 @@ error[E0311]: the parameter type `T` may not live long enough
LL | async fn foo(&self) -> &(T, U) where T: Debug + Sized, U: Hash;
| ^^^^^^^
|
note: the parameter type `T` must be valid for the anonymous lifetime defined here...
note: the parameter type `T` must be valid for the anonymous lifetime as defined here...
--> $DIR/async-generics-and-bounds.rs:12:18
|
LL | async fn foo(&self) -> &(T, U) where T: Debug + Sized, U: Hash;
| ^^^^^
| ^
note: ...so that the reference type `&(T, U)` does not outlive the data it points at
--> $DIR/async-generics-and-bounds.rs:12:28
|
Expand Down
8 changes: 4 additions & 4 deletions tests/ui/async-await/in-trait/async-generics.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -4,11 +4,11 @@ error[E0311]: the parameter type `U` may not live long enough
LL | async fn foo(&self) -> &(T, U);
| ^^^^^^^
|
note: the parameter type `U` must be valid for the anonymous lifetime defined here...
note: the parameter type `U` must be valid for the anonymous lifetime as defined here...
--> $DIR/async-generics.rs:9:18
|
LL | async fn foo(&self) -> &(T, U);
| ^^^^^
| ^
note: ...so that the reference type `&(T, U)` does not outlive the data it points at
--> $DIR/async-generics.rs:9:28
|
Expand All @@ -21,11 +21,11 @@ error[E0311]: the parameter type `T` may not live long enough
LL | async fn foo(&self) -> &(T, U);
| ^^^^^^^
|
note: the parameter type `T` must be valid for the anonymous lifetime defined here...
note: the parameter type `T` must be valid for the anonymous lifetime as defined here...
--> $DIR/async-generics.rs:9:18
|
LL | async fn foo(&self) -> &(T, U);
| ^^^^^
| ^
note: ...so that the reference type `&(T, U)` does not outlive the data it points at
--> $DIR/async-generics.rs:9:28
|
Expand Down
29 changes: 29 additions & 0 deletions tests/ui/impl-trait/in-trait/assumed-wf-bounds-in-impl.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,29 @@
// check-pass
// edition: 2021
// issue: 113796

#![feature(async_fn_in_trait)]

trait AsyncLendingIterator {
type Item<'a>
where
Self: 'a;

async fn next(&mut self) -> Option<Self::Item<'_>>;
}

struct Lend<I>(I);
impl<I> AsyncLendingIterator for Lend<I> {
type Item<'a> = &'a I
where
Self: 'a;

// Checking that the synthetic `<Self as AsyncLendingIterator>::next()` GAT
// is well-formed requires being able to assume the WF types of `next`.

async fn next(&mut self) -> Option<Self::Item<'_>> {
todo!()
}
}

fn main() {}
Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,6 @@ impl<'a, I: 'a + Iterable> Iterable for &'a I {
//~^ ERROR impl has stricter requirements than trait

fn iter(&self) -> impl 'a + Iterator<Item = I::Item<'a>> {
//~^ ERROR the type `&'a I` does not fulfill the required lifetime
(*self).iter()
}
}
Expand Down
17 changes: 2 additions & 15 deletions tests/ui/impl-trait/in-trait/bad-item-bound-within-rpitit.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -12,19 +12,6 @@ help: copy the `where` clause predicates from the trait
LL | where Self: 'b;
| ~~~~~~~~~~~~~~

error[E0477]: the type `&'a I` does not fulfill the required lifetime
--> $DIR/bad-item-bound-within-rpitit.rs:19:23
|
LL | fn iter(&self) -> impl 'a + Iterator<Item = I::Item<'a>> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: type must outlive the anonymous lifetime as defined here
--> $DIR/bad-item-bound-within-rpitit.rs:10:28
|
LL | fn iter(&self) -> impl '_ + Iterator<Item = Self::Item<'_>>;
| ^^

error: aborting due to 2 previous errors
error: aborting due to previous error

Some errors have detailed explanations: E0276, E0477.
For more information about an error, try `rustc --explain E0276`.
For more information about this error, try `rustc --explain E0276`.
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