error[E0658]: specialization is unstable
  --> $DIR/track6.rs:LL:CC
   |
LL |     default fn bar() {}
   |     ^^^^^^^^^^^^^^^^^^^
-Ztrack-diagnostics: created at compiler/rustc_ast_passes/src/feature_gate.rs:LL:CC
   |
   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
   = help: add `#![feature(specialization)]` 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   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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`.