Confusing error when trying to create Collator for en-US

[autarch]

I'm encountering a surprising error when trying to create a Collator for the en-US locale.

Here's the entire code example:

extern crate alloc;

use icu::{
    collator::{Collator, CollatorOptions},
    locid::Locale,
};

struct UnstableProvider;
include!("./icu-data/mod.rs");
impl_data_provider!(UnstableProvider);

fn main() {
    let locale = Locale::try_from_bytes("en-US".as_bytes()).unwrap();
    let provider = UnstableProvider;
    let opts = CollatorOptions::new();
    Collator::try_new_unstable(&provider, &locale.into(), opts).unwrap();
}

The final unwrap fails with:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Data(DataError { kind: MissingLocale, key: Some(DataKey{collator/meta@1}), str_context: None, silent: false })', src/main.rs:16:65

The data was generated like this:

$> icu4x-datagen --locales full --format mod --keys all --out ./src/icu-data --overwrite

I've created a test repo for this issue at https://github.com/autarch/datagen-issue

If I use en-US-POSIX I don't get an error. But surely en-US is a valid locale, right?

When I look at the generated data for collation, I see that it has a relatively small number of locale ids in it. It seems like maybe this isn't using the parent locales appropriately. Shouldn't en-US try en and then root?

[sffc]

Please wrap the UnstableProvider in a LocaleFallbackProvider.

[autarch]

Thanks! I'll close this. I would've suggested a doc update but you're already ahead of me.