error[E0658]: `let...else` statements are unstable
  --> /w/foo.rs:13:5
   |
13 |     let [mut n] = a[..] else {unreachable!()};
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #87335 <https://github.com/rust-lang/rust/issues/87335> for more information

error: aborting due to previous error

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

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