error[E0658]: use of unstable library feature 'unstable_default' --> $DIR/generics-default-stability-where.rs:7:45 | LL | impl Trait3 for T where T: Trait2 { | ^^^^^ | = help: add `#![feature(unstable_default)]` to the crate attributes to enable error: aborting due to previous error For more information about this error, try `rustc --explain E0658`.