error[E0658]: box pattern syntax is experimental
  --> $DIR/feature-gate-box_patterns.rs:2:9
   |
LL |     let box x = Box::new('c');
   |         ^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29641
   = help: add #![feature(box_patterns)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.