error[E0277]: the size for values of type `T` cannot be known at compilation time --> $DIR/unsized-enum.rs:6:36 | LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() } | - ^^^^^^ doesn't have a size known at compile-time | | | this type parameter needs to be `Sized` | note: required by a bound in `Foo` --> $DIR/unsized-enum.rs:4:10 | LL | enum Foo<U> { FooSome(U), FooNone } | ^ required by this bound in `Foo` help: you could relax the implicit `Sized` bound on `U` if it were used through indirection like `&U` or `Box<U>` --> $DIR/unsized-enum.rs:4:10 | LL | enum Foo<U> { FooSome(U), FooNone } | ^ - ...if indirection were used here: `Box<U>` | | | this could be changed to `U: ?Sized`... help: consider removing the `?Sized` bound to make the type parameter `Sized` | LL - fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() } LL + fn foo2<T>() { not_sized::<Foo<T>>() } | error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.