warning: the feature `let_chains` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/let-chains-attr.rs:3:12
   |
LL | #![feature(let_chains)]
   |            ^^^^^^^^^^
   |
   = note: `#[warn(incomplete_features)]` on by default
   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

warning: 1 warning emitted