error[E0277]: the size for values of type `X` cannot be known at compilation time --> $DIR/unsized7.rs:12:21 | LL | impl T1 for S3 { | - ^^^^^ doesn't have a size known at compile-time | | | this type parameter needs to be `Sized` | note: required by a bound in `T1` --> $DIR/unsized7.rs:7:10 | LL | trait T1 { | ^ required by this bound in `T1` help: consider removing the `?Sized` bound to make the type parameter `Sized` | LL - impl T1 for S3 { LL + impl T1 for S3 { | help: consider relaxing the implicit `Sized` restriction | LL | trait T1 { | ++++++++ error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.