Rust build error on M1 Mac

[mdbergmann]

Environment

• Millet version: git HEAD
• VS Code version: N/A
• OS version: macOS 13.0.1

Steps to reproduce

Run cargo xtask ci

Expected behavior

Task runs through and I get a binary to try in Emacs.

Actual behavior

   Compiling xshell v0.2.2
   Compiling xtask v0.1.0 (/Users/mbergmann/Development/MySources/millet/xtask)
error[E0658]: use of unstable library feature 'bool_to_option'
   --> xtask/src/main.rs:106:29
    |
106 |   let release_arg = release.then_some("--release");
    |                             ^^^^^^^^^
    |
    = note: see issue #80967 <https://github.com/rust-lang/rust/issues/80967> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `xtask` due to previous error

Thanks.

[mdbergmann]

OK. Updated rustc and now it works.