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Fix replacing supertrait aliases in ReplaceProjectionWith

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This commit is contained in:
Michael Goulet 2025-04-16 19:47:56 +00:00
parent c6aad02ddb
commit 3863018d96
10 changed files with 238 additions and 68 deletions
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24
compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
View file

@ -92,16 +92,20 @@ where
let ty::Dynamic(bounds, _, _) = goal.predicate.self_ty().kind() else {
panic!("expected object type in `probe_and_consider_object_bound_candidate`");
};
ecx.add_goals(
GoalSource::ImplWhereBound,
structural_traits::predicates_for_object_candidate(
ecx,
goal.param_env,
goal.predicate.trait_ref(cx),
bounds,
),
);
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
match structural_traits::predicates_for_object_candidate(
ecx,
goal.param_env,
goal.predicate.trait_ref(cx),
bounds,
) {
Ok(requirements) => {
ecx.add_goals(GoalSource::ImplWhereBound, requirements);
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
Err(_) => {
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
}
}
})
}

157
compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
View file

@ -5,9 +5,10 @@ use derive_where::derive_where;
use rustc_type_ir::data_structures::HashMap;
use rustc_type_ir::inherent::*;
use rustc_type_ir::lang_items::TraitSolverLangItem;
use rustc_type_ir::solve::inspect::ProbeKind;
use rustc_type_ir::{
self as ty, Interner, Movability, Mutability, TypeFoldable, TypeFolder, TypeSuperFoldable,
Upcast as _, elaborate,
self as ty, FallibleTypeFolder, Interner, Movability, Mutability, TypeFoldable,
TypeSuperFoldable, Upcast as _, elaborate,
};
use rustc_type_ir_macros::{TypeFoldable_Generic, TypeVisitable_Generic};
use tracing::instrument;
@ -822,7 +823,7 @@ pub(in crate::solve) fn const_conditions_for_destruct<I: Interner>(
/// impl Baz for dyn Foo<Item = Ty> {}
/// ```
///
/// However, in order to make such impls well-formed, we need to do an
/// However, in order to make such impls non-cyclical, we need to do an
/// additional step of eagerly folding the associated types in the where
/// clauses of the impl. In this example, that means replacing
/// `<Self as Foo>::Bar` with `Ty` in the first impl.
@ -833,11 +834,11 @@ pub(in crate::solve) fn const_conditions_for_destruct<I: Interner>(
// normalize eagerly here. See https://github.com/lcnr/solver-woes/issues/9
// for more details.
pub(in crate::solve) fn predicates_for_object_candidate<D, I>(
ecx: &EvalCtxt<'_, D>,
ecx: &mut EvalCtxt<'_, D>,
param_env: I::ParamEnv,
trait_ref: ty::TraitRef<I>,
object_bounds: I::BoundExistentialPredicates,
) -> Vec<Goal<I, I::Predicate>>
) -> Result<Vec<Goal<I, I::Predicate>>, Ambiguous>
where
D: SolverDelegate<Interner = I>,
I: Interner,
@ -871,72 +872,130 @@ where
.extend(cx.item_bounds(associated_type_def_id).iter_instantiated(cx, trait_ref.args));
}
let mut replace_projection_with = HashMap::default();
let mut replace_projection_with: HashMap<_, Vec<_>> = HashMap::default();
for bound in object_bounds.iter() {
if let ty::ExistentialPredicate::Projection(proj) = bound.skip_binder() {
// FIXME: We *probably* should replace this with a dummy placeholder,
// b/c don't want to replace literal instances of this dyn type that
// show up in the bounds, but just ones that come from substituting
// `Self` with the dyn type.
let proj = proj.with_self_ty(cx, trait_ref.self_ty());
let old_ty = replace_projection_with.insert(proj.def_id(), bound.rebind(proj));
assert_eq!(
old_ty,
None,
"{:?} has two generic parameters: {:?} and {:?}",
proj.projection_term,
proj.term,
old_ty.unwrap()
);
replace_projection_with.entry(proj.def_id()).or_default().push(bound.rebind(proj));
}
}
let mut folder =
ReplaceProjectionWith { ecx, param_env, mapping: replace_projection_with, nested: vec![] };
let folded_requirements = requirements.fold_with(&mut folder);
let mut folder = ReplaceProjectionWith {
ecx,
param_env,
self_ty: trait_ref.self_ty(),
mapping: &replace_projection_with,
nested: vec![],
};
folder
let requirements = requirements.try_fold_with(&mut folder)?;
Ok(folder
.nested
.into_iter()
.chain(folded_requirements.into_iter().map(|clause| Goal::new(cx, param_env, clause)))
.collect()
.chain(requirements.into_iter().map(|clause| Goal::new(cx, param_env, clause)))
.collect())
}
struct ReplaceProjectionWith<'a, D: SolverDelegate<Interner = I>, I: Interner> {
ecx: &'a EvalCtxt<'a, D>,
struct ReplaceProjectionWith<'a, 'b, I: Interner, D: SolverDelegate<Interner = I>> {
ecx: &'a mut EvalCtxt<'b, D>,
param_env: I::ParamEnv,
mapping: HashMap<I::DefId, ty::Binder<I, ty::ProjectionPredicate<I>>>,
self_ty: I::Ty,
mapping: &'a HashMap<I::DefId, Vec<ty::Binder<I, ty::ProjectionPredicate<I>>>>,
nested: Vec<Goal<I, I::Predicate>>,
}
impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I>
for ReplaceProjectionWith<'_, D, I>
impl<D, I> ReplaceProjectionWith<'_, '_, I, D>
where
D: SolverDelegate<Interner = I>,
I: Interner,
{
fn projection_may_match(
&mut self,
source_projection: ty::Binder<I, ty::ProjectionPredicate<I>>,
target_projection: ty::AliasTerm<I>,
) -> bool {
source_projection.item_def_id() == target_projection.def_id
&& self
.ecx
.probe(|_| ProbeKind::ProjectionCompatibility)
.enter(|ecx| -> Result<_, NoSolution> {
let source_projection = ecx.instantiate_binder_with_infer(source_projection);
ecx.eq(self.param_env, source_projection.projection_term, target_projection)?;
ecx.try_evaluate_added_goals()
})
.is_ok()
}
/// Try to replace an alias with the term present in the projection bounds of the self type.
/// Returns `Ok<None>` if this alias is not eligible to be replaced, or bail with
/// `Err(Ambiguous)` if it's uncertain which projection bound to replace the term with due
/// to multiple bounds applying.
fn try_eagerly_replace_alias(
&mut self,
alias_term: ty::AliasTerm<I>,
) -> Result<Option<I::Term>, Ambiguous> {
if alias_term.self_ty() != self.self_ty {
return Ok(None);
}
let Some(replacements) = self.mapping.get(&alias_term.def_id) else {
return Ok(None);
};
// This is quite similar to the `projection_may_match` we use in unsizing,
// but here we want to unify a projection predicate against an alias term
// so we can replace it with the the projection predicate's term.
let mut matching_projections = replacements
.iter()
.filter(|source_projection| self.projection_may_match(**source_projection, alias_term));
let Some(replacement) = matching_projections.next() else {
// This shouldn't happen.
panic!("could not replace {alias_term:?} with term from from {:?}", self.self_ty);
};
// FIXME: This *may* have issues with duplicated projections.
if matching_projections.next().is_some() {
// If there's more than one projection that we can unify here, then we
// need to stall until inference constrains things so that there's only
// one choice.
return Err(Ambiguous);
}
let replacement = self.ecx.instantiate_binder_with_infer(*replacement);
self.nested.extend(
self.ecx
.eq_and_get_goals(self.param_env, alias_term, replacement.projection_term)
.expect("expected to be able to unify goal projection with dyn's projection"),
);
Ok(Some(replacement.term))
}
}
/// Marker for bailing with ambiguity.
pub(crate) struct Ambiguous;
impl<D, I> FallibleTypeFolder<I> for ReplaceProjectionWith<'_, '_, I, D>
where
D: SolverDelegate<Interner = I>,
I: Interner,
{
type Error = Ambiguous;
fn cx(&self) -> I {
self.ecx.cx()
}
fn fold_ty(&mut self, ty: I::Ty) -> I::Ty {
fn try_fold_ty(&mut self, ty: I::Ty) -> Result<I::Ty, Ambiguous> {
if let ty::Alias(ty::Projection, alias_ty) = ty.kind() {
if let Some(replacement) = self.mapping.get(&alias_ty.def_id) {
// We may have a case where our object type's projection bound is higher-ranked,
// but the where clauses we instantiated are not. We can solve this by instantiating
// the binder at the usage site.
let proj = self.ecx.instantiate_binder_with_infer(*replacement);
// FIXME: Technically this equate could be fallible...
self.nested.extend(
self.ecx
.eq_and_get_goals(
self.param_env,
alias_ty,
proj.projection_term.expect_ty(self.ecx.cx()),
)
.expect(
"expected to be able to unify goal projection with dyn's projection",
),
);
proj.term.expect_ty()
} else {
ty.super_fold_with(self)
if let Some(term) = self.try_eagerly_replace_alias(alias_ty.into())? {
return Ok(term.expect_ty());
}
} else {
ty.super_fold_with(self)
}
ty.try_super_fold_with(self)
}
}

2
compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
View file

@ -944,7 +944,7 @@ where
target_projection: ty::Binder<I, ty::ExistentialProjection<I>>| {
source_projection.item_def_id() == target_projection.item_def_id()
&& ecx
.probe(|_| ProbeKind::UpcastProjectionCompatibility)
.probe(|_| ProbeKind::ProjectionCompatibility)
.enter(|ecx| -> Result<_, NoSolution> {
ecx.enter_forall(target_projection, |ecx, target_projection| {
let source_projection =

8
compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
View file

@ -292,7 +292,7 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
inspect::ProbeStep::NestedProbe(ref probe) => {
match probe.kind {
// These never assemble candidates for the goal we're trying to solve.
inspect::ProbeKind::UpcastProjectionCompatibility
inspect::ProbeKind::ProjectionCompatibility
| inspect::ProbeKind::ShadowedEnvProbing => continue,
inspect::ProbeKind::NormalizedSelfTyAssembly
@ -314,8 +314,10 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
}
match probe.kind {
inspect::ProbeKind::UpcastProjectionCompatibility
| inspect::ProbeKind::ShadowedEnvProbing => bug!(),
inspect::ProbeKind::ProjectionCompatibility
| inspect::ProbeKind::ShadowedEnvProbing => {
bug!()
}
inspect::ProbeKind::NormalizedSelfTyAssembly | inspect::ProbeKind::UnsizeAssembly => {}

2
compiler/rustc_trait_selection/src/solve/select.rs
View file

@ -177,7 +177,7 @@ fn to_selection<'tcx>(
},
ProbeKind::NormalizedSelfTyAssembly
| ProbeKind::UnsizeAssembly
| ProbeKind::UpcastProjectionCompatibility
| ProbeKind::ProjectionCompatibility
| ProbeKind::OpaqueTypeStorageLookup { result: _ }
| ProbeKind::Root { result: _ }
| ProbeKind::ShadowedEnvProbing

10
compiler/rustc_type_ir/src/solve/inspect.rs
View file

@ -118,10 +118,12 @@ pub enum ProbeKind<I: Interner> {
/// Used in the probe that wraps normalizing the non-self type for the unsize
/// trait, which is also structurally matched on.
UnsizeAssembly,
/// During upcasting from some source object to target object type, used to
/// do a probe to find out what projection type(s) may be used to prove that
/// the source type upholds all of the target type's object bounds.
UpcastProjectionCompatibility,
/// Used to do a probe to find out what projection type(s) match a given
/// alias bound or projection predicate. For trait upcasting, this is used
/// to prove that the source type upholds all of the target type's object
/// bounds. For object type bounds, this is used when eagerly replacing
/// supertrait aliases.
ProjectionCompatibility,
/// Looking for param-env candidates that satisfy the trait ref for a projection.
ShadowedEnvProbing,
/// Try to unify an opaque type with an existing key in the storage.

22
tests/ui/traits/next-solver/supertrait-alias-1.rs Normal file
View file

@ -0,0 +1,22 @@
//@ compile-flags: -Znext-solver
//@ check-pass
// Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/171>.
// Tests that we don't try to replace `<V as Super>::Output` when replacing projections in the
// required bounds for `dyn Trait`, b/c `V` is not relevant to the dyn type, which we were
// previously encountering b/c we were walking into the existential projection bounds of the dyn
// type itself.
pub trait Trait: Super {}
pub trait Super {
type Output;
}
fn bound<T: Trait + ?Sized>() {}
fn visit_simd_operator<V: Super + ?Sized>() {
bound::<dyn Trait<Output = <V as Super>::Output>>();
}
fn main() {}

25
tests/ui/traits/next-solver/supertrait-alias-2.rs Normal file
View file

@ -0,0 +1,25 @@
//@ compile-flags: -Znext-solver
//@ check-pass
// Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/171>.
// Tests that we don't try to replace `<T as Other>::Assoc` when replacing projections in the
// required bounds for `dyn Foo`, b/c `T` is not relevant to the dyn type, which we were
// encountering when walking through the elaborated supertraits of `dyn Foo`.
trait Other<X> {}
trait Foo<T: Foo<T>>: Other<<T as Foo<T>>::Assoc> {
type Assoc;
}
impl<T> Foo<T> for T {
type Assoc = ();
}
impl<T: ?Sized> Other<()> for T {}
fn is_foo<T: Foo<()> + ?Sized>() {}
fn main() {
is_foo::<dyn Foo<(), Assoc = ()>>();
}

32
tests/ui/traits/next-solver/supertrait-alias-3.rs Normal file
View file

@ -0,0 +1,32 @@
//@ compile-flags: -Znext-solver
//@ check-pass
// Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/171>.
// Exercises a case where structural equality is insufficient when replacing projections in a dyn's
// bounds. In this case, the bound will contain `<Self as Super<<i32 as Mirror>:Assoc>::Assoc`, but
// the existential projections from the dyn will have `<Self as Super<i32>>::Assoc` because as an
// optimization we eagerly normalize aliases in goals.
trait Other<T> {}
impl<T> Other<T> for T {}
trait Super<T> {
type Assoc;
}
trait Mirror {
type Assoc;
}
impl<T> Mirror for T {
type Assoc = T;
}
trait Foo<A, B>: Super<<A as Mirror>::Assoc, Assoc = A> {
type FooAssoc: Other<<Self as Super<<A as Mirror>::Assoc>>::Assoc>;
}
fn is_foo<F: Foo<T, U> + ?Sized, T, U>() {}
fn main() {
is_foo::<dyn Foo<i32, u32, FooAssoc = i32>, _, _>();
}

24
tests/ui/traits/next-solver/supertrait-alias-4.rs Normal file
View file

@ -0,0 +1,24 @@
//@ compile-flags: -Znext-solver
//@ check-pass
// Exercises the ambiguity that comes from replacing the associated types within the bounds
// that are required for a `impl Trait for dyn Trait` built-in object impl to hold.
trait Sup<T> {
type Assoc;
}
trait Foo<A, B>: Sup<A, Assoc = A> + Sup<B, Assoc = B> {
type Other: Bar<<Self as Sup<A>>::Assoc>;
}
trait Bar<T> {}
impl Bar<i32> for () {}
fn foo<A, B>(x: &(impl Foo<A, B> + ?Sized)) {}
fn main() {
let x: &dyn Foo<_, _, Other = ()> = todo!();
foo(x);
let y: &dyn Foo<i32, u32, Other = ()> = x;
}