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Rollup merge of rust-lang#106038 - aliemjay:opaque-implied, r=lcnr
use implied bounds when checking opaque types During opaque type inference, we check for the well-formedness of the hidden type in the opaque type's own environment, not the one of the defining site, which are different in the case of TAIT. However in the case of associated-type-impl-trait, we don't use implied bounds from the impl header. This caused us to reject the following: ```rust trait Service<Req> { type Output; fn call(req: Req) -> Self::Output; } impl<'a, Req> Service<&'a Req> for u8 { type Output= impl Sized; // we can't prove WF of hidden type `WF(&'a Req)` although it's implied by the impl //~^ ERROR type parameter Req doesn't live long enough fn call(req: &'a Req) -> Self::Output { req } } ``` although adding an explicit bound would make it pass: ```diff - impl<'a, Req> Service<&'a Req> for u8 { + impl<'a, Req> Service<&'a Req> for u8 where Req: 'a, { ``` I believe it should pass as we already allow the concrete type to be used: ```diff impl<'a, Req> Service<&'a Req> for u8 { - type Output= impl Sized; + type Output= &'a Req; ``` Fixes rust-lang#95922 Builds on rust-lang#105982 cc `@lcnr` (because implied bounds) r? `@oli-obk`
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25 changes: 25 additions & 0 deletions
25
tests/ui/type-alias-impl-trait/wf-in-associated-type.fail.stderr
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error[E0309]: the parameter type `T` may not live long enough | ||
--> $DIR/wf-in-associated-type.rs:36:23 | ||
| | ||
LL | type Opaque = impl Sized + 'a; | ||
| ^^^^^^^^^^^^^^^ ...so that the type `&'a T` will meet its required lifetime bounds | ||
| | ||
help: consider adding an explicit lifetime bound... | ||
| | ||
LL | impl<'a, T: 'a> Trait<'a, T> for () { | ||
| ++++ | ||
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error[E0309]: the parameter type `T` may not live long enough | ||
--> $DIR/wf-in-associated-type.rs:36:23 | ||
| | ||
LL | type Opaque = impl Sized + 'a; | ||
| ^^^^^^^^^^^^^^^ ...so that the reference type `&'a T` does not outlive the data it points at | ||
| | ||
help: consider adding an explicit lifetime bound... | ||
| | ||
LL | impl<'a, T: 'a> Trait<'a, T> for () { | ||
| ++++ | ||
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error: aborting due to 2 previous errors | ||
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For more information about this error, try `rustc --explain E0309`. |
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// WF check for impl Trait in associated type position. | ||
// | ||
// revisions: pass fail | ||
// [pass] check-pass | ||
// [fail] check-fail | ||
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#![feature(impl_trait_in_assoc_type)] | ||
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// The hidden type here (`&'a T`) requires proving `T: 'a`. | ||
// We know it holds because of implied bounds from the impl header. | ||
#[cfg(pass)] | ||
mod pass { | ||
trait Trait<Req> { | ||
type Opaque1; | ||
fn constrain_opaque1(req: Req) -> Self::Opaque1; | ||
} | ||
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impl<'a, T> Trait<&'a T> for () { | ||
type Opaque1 = impl IntoIterator<Item = impl Sized + 'a>; | ||
fn constrain_opaque1(req: &'a T) -> Self::Opaque1 { | ||
[req] | ||
} | ||
} | ||
} | ||
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// The hidden type here (`&'a T`) requires proving `T: 'a`, | ||
// but that is not known to hold in the impl. | ||
#[cfg(fail)] | ||
mod fail { | ||
trait Trait<'a, T> { | ||
type Opaque; | ||
fn constrain_opaque(req: &'a T) -> Self::Opaque; | ||
} | ||
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impl<'a, T> Trait<'a, T> for () { | ||
type Opaque = impl Sized + 'a; | ||
//[fail]~^ ERROR the parameter type `T` may not live long enough | ||
//[fail]~| ERROR the parameter type `T` may not live long enough | ||
fn constrain_opaque(req: &'a T) -> Self::Opaque { | ||
req | ||
} | ||
} | ||
} | ||
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fn main() {} |
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error[E0310]: the parameter type `T` may not live long enough | ||
--> $DIR/wf-nested.rs:55:27 | ||
| | ||
LL | type InnerOpaque<T> = impl Sized; | ||
| ^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds... | ||
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note: ...that is required by this bound | ||
--> $DIR/wf-nested.rs:12:20 | ||
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LL | struct IsStatic<T: 'static>(T); | ||
| ^^^^^^^ | ||
help: consider adding an explicit lifetime bound... | ||
| | ||
LL | type InnerOpaque<T: 'static> = impl Sized; | ||
| +++++++++ | ||
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error: aborting due to previous error | ||
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For more information about this error, try `rustc --explain E0310`. |
14 changes: 14 additions & 0 deletions
14
tests/ui/type-alias-impl-trait/wf-nested.pass_sound.stderr
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error[E0310]: the parameter type `T` may not live long enough | ||
--> $DIR/wf-nested.rs:46:17 | ||
| | ||
LL | let _ = outer.get(); | ||
| ^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds | ||
| | ||
help: consider adding an explicit lifetime bound... | ||
| | ||
LL | fn test<T: 'static>() { | ||
| +++++++++ | ||
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error: aborting due to previous error | ||
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For more information about this error, try `rustc --explain E0310`. |
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// Well-formedness of nested opaque types, i.e. `impl Sized` in | ||
// `type Outer = impl Trait<Assoc = impl Sized>`. | ||
// See the comments below. | ||
// | ||
// revisions: pass pass_sound fail | ||
// [pass] check-pass | ||
// [pass_sound] check-fail | ||
// [fail] check-fail | ||
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#![feature(type_alias_impl_trait)] | ||
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struct IsStatic<T: 'static>(T); | ||
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trait Trait<In> { | ||
type Out; | ||
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fn get(&self) -> Result<Self::Out, ()> { | ||
Err(()) | ||
} | ||
} | ||
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impl<T> Trait<&'static T> for () { | ||
type Out = IsStatic<T>; | ||
} | ||
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// The hidden type for `impl Sized` is `IsStatic<T>`, which requires `T: 'static`. | ||
// We know it is well-formed because it can *only* be referenced as a projection: | ||
// <OuterOpaque<T> as Trait<&'static T>>::Out`. | ||
// So any instantiation of the type already requires proving `T: 'static`. | ||
#[cfg(pass)] | ||
mod pass { | ||
use super::*; | ||
type OuterOpaque<T> = impl Trait<&'static T, Out = impl Sized>; | ||
fn define<T>() -> OuterOpaque<T> {} | ||
} | ||
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// Test the soundness of `pass` - We should require `T: 'static` at the use site. | ||
#[cfg(pass_sound)] | ||
mod pass_sound { | ||
use super::*; | ||
type OuterOpaque<T> = impl Trait<&'static T, Out = impl Sized>; | ||
fn define<T>() -> OuterOpaque<T> {} | ||
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fn test<T>() { | ||
let outer = define::<T>(); | ||
let _ = outer.get(); //[pass_sound]~ ERROR `T` may not live long enough | ||
} | ||
} | ||
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// Similar to `pass` but here `impl Sized` can be referenced directly as | ||
// InnerOpaque<T>, so we require an explicit bound `T: 'static`. | ||
#[cfg(fail)] | ||
mod fail { | ||
use super::*; | ||
type InnerOpaque<T> = impl Sized; //[fail]~ ERROR `T` may not live long enough | ||
type OuterOpaque<T> = impl Trait<&'static T, Out = InnerOpaque<T>>; | ||
fn define<T>() -> OuterOpaque<T> {} | ||
} | ||
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fn main() {} |