error[E0658]: type ascription is experimental
  --> $DIR/feature-gate-type_ascription.rs:4:13
   |
LL |     let a = 10: u8;
   |             ^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/23416
   = help: add `#![feature(type_ascription)]` to the crate attributes to enable

error: aborting due to previous error

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