Error on nightly: Unions in const fn are unstable

[MajorBreakfast]

I'm getting this error on recent nightlies:

error[E0658]: unions in const fn are unstable (see issue #51909)
  --> /Users/josef/.cargo/registry/src/github.com-1ecc6299db9ec823/heapless-0.3.6/src/__core.rs:43:9
   |
43 |         U { none: () }.some
   |         ^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(const_fn_union)] to the crate attributes to enable

error: aborting due to previous error

[MajorBreakfast]

#![cfg_attr(feature = "const-fn", feature(const_fn_union))] can be easily added to lib.rs, but I get another error after that. One of them is:

error[E0080]: this static likely exhibits undefined behavior
   --> src/linear_map.rs:456:9
    |
456 |         static mut _L: LinearMap<i32, i32, U8>= LinearMap::new();
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered undefined bytes at .buffer.buffer.value.data.parent1.parent1.parent1.data.0
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior

[MajorBreakfast]

I'm now using the master branch in my project which allows disabling the const-fn crate feature.

[MajorBreakfast]

@japaric This is awesome! Thanks!