Error code E0792
A type alias impl trait can only have its hidden type assigned when used fully generically (and within their defining scope). This means
#![allow(unused)] #![feature(type_alias_impl_trait)] fn main() { type Foo<T> = impl std::fmt::Debug; fn foo() -> Foo<u32> { 5u32 } }
is not accepted. If it were accepted, one could create unsound situations like
#![feature(type_alias_impl_trait)] type Foo<T> = impl Default; fn foo() -> Foo<u32> { 5u32 } fn main() { let x = Foo::<&'static mut String>::default(); }
Instead you need to make the function generic:
#![allow(unused)] #![feature(type_alias_impl_trait)] fn main() { type Foo<T> = impl std::fmt::Debug; fn foo<U>() -> Foo<U> { 5u32 } }
This means that no matter the generic parameter to foo
,
the hidden type will always be u32
.
If you want to link the generic parameter to the hidden type,
you can do that, too:
#![allow(unused)] #![feature(type_alias_impl_trait)] fn main() { use std::fmt::Debug; type Foo<T: Debug> = impl Debug; fn foo<U: Debug>() -> Foo<U> { Vec::<U>::new() } }