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Use the normalizing param-env always in check_type_bounds
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@compiler-errors
compiler-errors committed Oct 25, 2023
1 parent df871fb commit 69563f0
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Showing 4 changed files with 164 additions and 133 deletions.
266 changes: 143 additions & 123 deletions compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2162,128 +2162,10 @@ pub(super) fn check_type_bounds<'tcx>(
impl_ty: ty::AssocItem,
impl_trait_ref: ty::TraitRef<'tcx>,
) -> Result<(), ErrorGuaranteed> {
let param_env = tcx.param_env(impl_ty.def_id);
let container_id = impl_ty.container_id(tcx);
// Given
//
// impl<A, B> Foo<u32> for (A, B) {
// type Bar<C> = Wrapper<A, B, C>
// }
//
// - `impl_trait_ref` would be `<(A, B) as Foo<u32>>`
// - `normalize_impl_ty_args` would be `[A, B, ^0.0]` (`^0.0` here is the bound var with db 0 and index 0)
// - `normalize_impl_ty` would be `Wrapper<A, B, ^0.0>`
// - `rebased_args` would be `[(A, B), u32, ^0.0]`, combining the args from
// the *trait* with the generic associated type parameters (as bound vars).
//
// A note regarding the use of bound vars here:
// Imagine as an example
// ```
// trait Family {
// type Member<C: Eq>;
// }
//
// impl Family for VecFamily {
// type Member<C: Eq> = i32;
// }
// ```
// Here, we would generate
// ```notrust
// forall<C> { Normalize(<VecFamily as Family>::Member<C> => i32) }
// ```
// when we really would like to generate
// ```notrust
// forall<C> { Normalize(<VecFamily as Family>::Member<C> => i32) :- Implemented(C: Eq) }
// ```
// But, this is probably fine, because although the first clause can be used with types C that
// do not implement Eq, for it to cause some kind of problem, there would have to be a
// VecFamily::Member<X> for some type X where !(X: Eq), that appears in the value of type
// Member<C: Eq> = .... That type would fail a well-formedness check that we ought to be doing
// elsewhere, which would check that any <T as Family>::Member<X> meets the bounds declared in
// the trait (notably, that X: Eq and T: Family).
let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
smallvec::SmallVec::with_capacity(tcx.generics_of(impl_ty.def_id).params.len());
// Extend the impl's identity args with late-bound GAT vars
let normalize_impl_ty_args = ty::GenericArgs::identity_for_item(tcx, container_id).extend_to(
tcx,
impl_ty.def_id,
|param, _| match param.kind {
GenericParamDefKind::Type { .. } => {
let kind = ty::BoundTyKind::Param(param.def_id, param.name);
let bound_var = ty::BoundVariableKind::Ty(kind);
bound_vars.push(bound_var);
Ty::new_bound(
tcx,
ty::INNERMOST,
ty::BoundTy { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
)
.into()
}
GenericParamDefKind::Lifetime => {
let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
let bound_var = ty::BoundVariableKind::Region(kind);
bound_vars.push(bound_var);
ty::Region::new_late_bound(
tcx,
ty::INNERMOST,
ty::BoundRegion { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
)
.into()
}
GenericParamDefKind::Const { .. } => {
let bound_var = ty::BoundVariableKind::Const;
bound_vars.push(bound_var);
ty::Const::new_bound(
tcx,
ty::INNERMOST,
ty::BoundVar::from_usize(bound_vars.len() - 1),
tcx.type_of(param.def_id)
.no_bound_vars()
.expect("const parameter types cannot be generic"),
)
.into()
}
},
);
// When checking something like
//
// trait X { type Y: PartialEq<<Self as X>::Y> }
// impl X for T { default type Y = S; }
//
// We will have to prove the bound S: PartialEq<<T as X>::Y>. In this case
// we want <T as X>::Y to normalize to S. This is valid because we are
// checking the default value specifically here. Add this equality to the
// ParamEnv for normalization specifically.
let normalize_impl_ty = tcx.type_of(impl_ty.def_id).instantiate(tcx, normalize_impl_ty_args);
let rebased_args = normalize_impl_ty_args.rebase_onto(tcx, container_id, impl_trait_ref.args);
let bound_vars = tcx.mk_bound_variable_kinds(&bound_vars);
let normalize_param_env = {
let mut predicates = param_env.caller_bounds().iter().collect::<Vec<_>>();
match normalize_impl_ty.kind() {
ty::Alias(ty::Projection, proj)
if proj.def_id == trait_ty.def_id && proj.args == rebased_args =>
{
// Don't include this predicate if the projected type is
// exactly the same as the projection. This can occur in
// (somewhat dubious) code like this:
//
// impl<T> X for T where T: X { type Y = <T as X>::Y; }
}
_ => predicates.push(
ty::Binder::bind_with_vars(
ty::ProjectionPredicate {
projection_ty: ty::AliasTy::new(tcx, trait_ty.def_id, rebased_args),
term: normalize_impl_ty.into(),
},
bound_vars,
)
.to_predicate(tcx),
),
};
ty::ParamEnv::new(tcx.mk_clauses(&predicates), Reveal::UserFacing)
};
debug!(?normalize_param_env);
let param_env = param_env_with_gat_bounds(tcx, trait_ty, impl_ty, impl_trait_ref);
debug!(?param_env);

let container_id = impl_ty.container_id(tcx);
let impl_ty_def_id = impl_ty.def_id.expect_local();
let impl_ty_args = GenericArgs::identity_for_item(tcx, impl_ty.def_id);
let rebased_args = impl_ty_args.rebase_onto(tcx, container_id, impl_trait_ref.args);
Expand Down Expand Up @@ -2336,8 +2218,7 @@ pub(super) fn check_type_bounds<'tcx>(
debug!("check_type_bounds: item_bounds={:?}", obligations);

for mut obligation in util::elaborate(tcx, obligations) {
let normalized_predicate =
ocx.normalize(&normalize_cause, normalize_param_env, obligation.predicate);
let normalized_predicate = ocx.normalize(&normalize_cause, param_env, obligation.predicate);
debug!("compare_projection_bounds: normalized predicate = {:?}", normalized_predicate);
obligation.predicate = normalized_predicate;

Expand All @@ -2358,6 +2239,145 @@ pub(super) fn check_type_bounds<'tcx>(
ocx.resolve_regions_and_report_errors(impl_ty_def_id, &outlives_env)
}

/// Install projection predicates that allow GATs to project to their own
/// definition types. This is not allowed in general in cases of default
/// associated types in trait definitions, or when specialization is involved,
/// but is needed when checking these definition types actually satisfy the
/// trait bounds of the GAT.
///
/// # How it works
///
/// ```ignore (example)
/// impl<A, B> Foo<u32> for (A, B) {
/// type Bar<C> = Wrapper<A, B, C>
/// }
/// ```
///
/// - `impl_trait_ref` would be `<(A, B) as Foo<u32>>`
/// - `normalize_impl_ty_args` would be `[A, B, ^0.0]` (`^0.0` here is the bound var with db 0 and index 0)
/// - `normalize_impl_ty` would be `Wrapper<A, B, ^0.0>`
/// - `rebased_args` would be `[(A, B), u32, ^0.0]`, combining the args from
/// the *trait* with the generic associated type parameters (as bound vars).
///
/// A note regarding the use of bound vars here:
/// Imagine as an example
/// ```
/// trait Family {
/// type Member<C: Eq>;
/// }
///
/// impl Family for VecFamily {
/// type Member<C: Eq> = i32;
/// }
/// ```
/// Here, we would generate
/// ```ignore (pseudo-rust)
/// forall<C> { Normalize(<VecFamily as Family>::Member<C> => i32) }
/// ```
///
/// when we really would like to generate
/// ```ignore (pseudo-rust)
/// forall<C> { Normalize(<VecFamily as Family>::Member<C> => i32) :- Implemented(C: Eq) }
/// ```
///
/// But, this is probably fine, because although the first clause can be used with types `C` that
/// do not implement `Eq`, for it to cause some kind of problem, there would have to be a
/// `VecFamily::Member<X>` for some type `X` where `!(X: Eq)`, that appears in the value of type
/// `Member<C: Eq> = ....` That type would fail a well-formedness check that we ought to be doing
/// elsewhere, which would check that any `<T as Family>::Member<X>` meets the bounds declared in
/// the trait (notably, that `X: Eq` and `T: Family`).
fn param_env_with_gat_bounds<'tcx>(
tcx: TyCtxt<'tcx>,
trait_ty: ty::AssocItem,
impl_ty: ty::AssocItem,
impl_trait_ref: ty::TraitRef<'tcx>,
) -> ty::ParamEnv<'tcx> {
let param_env = tcx.param_env(impl_ty.def_id);
let container_id = impl_ty.container_id(tcx);
let mut predicates = param_env.caller_bounds().to_vec();

let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
smallvec::SmallVec::with_capacity(tcx.generics_of(impl_ty.def_id).params.len());
// Extend the impl's identity args with late-bound GAT vars
let normalize_impl_ty_args = ty::GenericArgs::identity_for_item(tcx, container_id).extend_to(
tcx,
impl_ty.def_id,
|param, _| match param.kind {
GenericParamDefKind::Type { .. } => {
let kind = ty::BoundTyKind::Param(param.def_id, param.name);
let bound_var = ty::BoundVariableKind::Ty(kind);
bound_vars.push(bound_var);
Ty::new_bound(
tcx,
ty::INNERMOST,
ty::BoundTy { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
)
.into()
}
GenericParamDefKind::Lifetime => {
let kind = ty::BoundRegionKind::BrNamed(param.def_id, param.name);
let bound_var = ty::BoundVariableKind::Region(kind);
bound_vars.push(bound_var);
ty::Region::new_late_bound(
tcx,
ty::INNERMOST,
ty::BoundRegion { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
)
.into()
}
GenericParamDefKind::Const { .. } => {
let bound_var = ty::BoundVariableKind::Const;
bound_vars.push(bound_var);
ty::Const::new_bound(
tcx,
ty::INNERMOST,
ty::BoundVar::from_usize(bound_vars.len() - 1),
tcx.type_of(param.def_id)
.no_bound_vars()
.expect("const parameter types cannot be generic"),
)
.into()
}
},
);
// When checking something like
//
// trait X { type Y: PartialEq<<Self as X>::Y> }
// impl X for T { default type Y = S; }
//
// We will have to prove the bound S: PartialEq<<T as X>::Y>. In this case
// we want <T as X>::Y to normalize to S. This is valid because we are
// checking the default value specifically here. Add this equality to the
// ParamEnv for normalization specifically.
let normalize_impl_ty = tcx.type_of(impl_ty.def_id).instantiate(tcx, normalize_impl_ty_args);
let rebased_args = normalize_impl_ty_args.rebase_onto(tcx, container_id, impl_trait_ref.args);
let bound_vars = tcx.mk_bound_variable_kinds(&bound_vars);

match normalize_impl_ty.kind() {
ty::Alias(ty::Projection, proj)
if proj.def_id == trait_ty.def_id && proj.args == rebased_args =>
{
// Don't include this predicate if the projected type is
// exactly the same as the projection. This can occur in
// (somewhat dubious) code like this:
//
// impl<T> X for T where T: X { type Y = <T as X>::Y; }
}
_ => predicates.push(
ty::Binder::bind_with_vars(
ty::ProjectionPredicate {
projection_ty: ty::AliasTy::new(tcx, trait_ty.def_id, rebased_args),
term: normalize_impl_ty.into(),
},
bound_vars,
)
.to_predicate(tcx),
),
};

ty::ParamEnv::new(tcx.mk_clauses(&predicates), Reveal::UserFacing)
}

fn assoc_item_kind_str(impl_item: &ty::AssocItem) -> &'static str {
match impl_item.kind {
ty::AssocKind::Const => "const",
Expand Down
18 changes: 18 additions & 0 deletions tests/ui/generic-associated-types/assume-gat-normalization-for-nested-goals.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
// check-pass

#![feature(associated_type_defaults)]

trait Foo {
type Bar<T>: Baz<Self> = i32;
// We should be able to prove that `i32: Baz<Self>` because of
// the impl below, which requires that `Self::Bar<()>: Eq<i32>`
// which is true, because we assume `for<T> Self::Bar<T> = i32`.
}

trait Baz<T: ?Sized> {}
impl<T: Foo + ?Sized> Baz<T> for i32 where T::Bar<()>: Eq<i32> {}

trait Eq<T> {}
impl<T> Eq<T> for T {}

fn main() {}
2 changes: 1 addition & 1 deletion tests/ui/traits/new-solver/specialization-unconstrained.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ trait Default {
}

impl<T> Default for T {
default type Id = T; //~ ERROR type annotations needed
default type Id = T;
}

fn test<T: Default<Id = U>, U>() {}
Expand Down
11 changes: 2 additions & 9 deletions tests/ui/traits/new-solver/specialization-unconstrained.stderr
View file
Original file line number Diff line number Diff line change
Expand Up @@ -20,13 +20,6 @@ note: required by a bound in `test`
LL | fn test<T: Default<Id = U>, U>() {}
| ^^^^^^ required by this bound in `test`

error[E0282]: type annotations needed
--> $DIR/specialization-unconstrained.rs:14:22
|
LL | default type Id = T;
| ^ cannot infer type for associated type `<T as Default>::Id`

error: aborting due to 2 previous errors; 1 warning emitted
error: aborting due to previous error; 1 warning emitted

Some errors have detailed explanations: E0282, E0284.
For more information about an error, try `rustc --explain E0282`.
For more information about this error, try `rustc --explain E0284`.

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