error[E0658]: trait aliases are experimental --> $DIR/feature-gate-trait-alias.rs:1:1 | LL | trait Foo = Default; | ^^^^^^^^^^^^^^^^^^^^ | = note: see issue #41517 for more information = help: add `#![feature(trait_alias)]` to the crate attributes to enable = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0658`.