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Deny return-position impl Trait in traits for object safety #101681

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Sep 12, 2022
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9 changes: 9 additions & 0 deletions compiler/rustc_middle/src/traits/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -908,6 +908,12 @@ impl ObjectSafetyViolation {
ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
format!("method `{}` references the `Self` type in its return type", name).into()
}
ObjectSafetyViolation::Method(
name,
MethodViolationCode::ReferencesImplTraitInTrait,
_,
) => format!("method `{}` references an `impl Trait` type in its return type", name)
.into(),
ObjectSafetyViolation::Method(
name,
MethodViolationCode::WhereClauseReferencesSelf,
Expand Down Expand Up @@ -1014,6 +1020,9 @@ pub enum MethodViolationCode {
/// e.g., `fn foo(&self) -> Self`
ReferencesSelfOutput,

/// e.g., `fn foo(&self) -> impl Sized`
ReferencesImplTraitInTrait,

/// e.g., `fn foo(&self) where Self: Clone`
WhereClauseReferencesSelf,

Expand Down
26 changes: 26 additions & 0 deletions compiler/rustc_trait_selection/src/traits/object_safety.rs
Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ use super::elaborate_predicates;
use crate::infer::TyCtxtInferExt;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::{self, Obligation, ObligationCause};
use hir::def::DefKind;
use rustc_errors::{FatalError, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
Expand Down Expand Up @@ -431,6 +432,9 @@ fn virtual_call_violation_for_method<'tcx>(
if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
return Some(MethodViolationCode::ReferencesSelfOutput);
}
if contains_illegal_impl_trait_in_trait(tcx, sig.output()) {
return Some(MethodViolationCode::ReferencesImplTraitInTrait);
}

// We can't monomorphize things like `fn foo<A>(...)`.
let own_counts = tcx.generics_of(method.def_id).own_counts();
Expand Down Expand Up @@ -793,6 +797,12 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
ControlFlow::CONTINUE
}
}
ty::Projection(ref data)
if self.tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder =>
{
// We'll deny these later in their own pass
ControlFlow::CONTINUE
}
ty::Projection(ref data) => {
// This is a projected type `<Foo as SomeTrait>::X`.

Expand Down Expand Up @@ -861,6 +871,22 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
.is_break()
}

pub fn contains_illegal_impl_trait_in_trait<'tcx>(
tcx: TyCtxt<'tcx>,
ty: ty::Binder<'tcx, Ty<'tcx>>,
) -> bool {
// FIXME(RPITIT): Perhaps we should use a visitor here?
ty.skip_binder().walk().any(|arg| {
if let ty::GenericArgKind::Type(ty) = arg.unpack()
&& let ty::Projection(proj) = ty.kind()
{
tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
} else {
false
}
})
}

pub fn provide(providers: &mut ty::query::Providers) {
*providers = ty::query::Providers { object_safety_violations, ..*providers };
}
22 changes: 22 additions & 0 deletions src/test/ui/impl-trait/in-trait/object-safety.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,22 @@
#![feature(return_position_impl_trait_in_trait)]
#![allow(incomplete_features)]

use std::fmt::Debug;

trait Foo {
fn baz(&self) -> impl Debug;
}

impl Foo for u32 {
fn baz(&self) -> u32 {
32
}
}

fn main() {
let i = Box::new(42_u32) as Box<dyn Foo>;
//~^ ERROR the trait `Foo` cannot be made into an object
//~| ERROR the trait `Foo` cannot be made into an object
let s = i.baz();
//~^ ERROR the trait `Foo` cannot be made into an object
}
50 changes: 50 additions & 0 deletions src/test/ui/impl-trait/in-trait/object-safety.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,50 @@
error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/object-safety.rs:17:33
|
LL | let i = Box::new(42_u32) as Box<dyn Foo>;
| ^^^^^^^^^^^^ `Foo` cannot be made into an object
|
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> $DIR/object-safety.rs:7:8
|
LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn baz(&self) -> impl Debug;
| ^^^ ...because method `baz` references an `impl Trait` type in its return type
= help: consider moving `baz` to another trait

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/object-safety.rs:20:13
|
LL | let s = i.baz();
| ^^^^^^^ `Foo` cannot be made into an object
|
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> $DIR/object-safety.rs:7:8
|
LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn baz(&self) -> impl Debug;
| ^^^ ...because method `baz` references an `impl Trait` type in its return type
= help: consider moving `baz` to another trait

error[E0038]: the trait `Foo` cannot be made into an object
--> $DIR/object-safety.rs:17:13
|
LL | let i = Box::new(42_u32) as Box<dyn Foo>;
| ^^^^^^^^^^^^^^^^ `Foo` cannot be made into an object
|
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> $DIR/object-safety.rs:7:8
|
LL | trait Foo {
| --- this trait cannot be made into an object...
LL | fn baz(&self) -> impl Debug;
| ^^^ ...because method `baz` references an `impl Trait` type in its return type
= help: consider moving `baz` to another trait
= note: required for `Box<u32>` to implement `CoerceUnsized<Box<dyn Foo>>`
= note: required by cast to type `Box<dyn Foo>`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0038`.