error[E0277]: the size for values of type `X` cannot be known at compilation time --> $DIR/unsized7.rs:22:21 | LL | impl T1 for S3 { | ^^^^^ doesn't have a size known at compile-time | = help: the trait `std::marker::Sized` is not implemented for `X` = note: to learn more, visit = help: consider adding a `where X: std::marker::Sized` bound error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.