error[E0277]: the size for values of type `T` cannot be known at compilation time --> $DIR/unsized-enum.rs:6:36 | LL | fn foo2() { not_sized::>() } | - ^^^^^^ 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 { 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` --> $DIR/unsized-enum.rs:4:10 | LL | enum Foo { FooSome(U), FooNone } | ^ - ...if indirection were used here: `Box` | | | this could be changed to `U: ?Sized`... help: consider removing the `?Sized` bound to make the type parameter `Sized` | LL - fn foo2() { not_sized::>() } LL + fn foo2() { not_sized::>() } | error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.