How does Figment Env provider handle fields with underscores?

[nrdxp]

I'm trying to figure out how figment handles fields with underscores. For example:

struct Foo {
  bar: Bar
}
sturct Bar {
  baz_buz: bool
}

I have fig.admerge(Env::prefixed("APP_").split("_")), but I can't seem to figure out how to access this. APP_BAR_BAZ_BUZ doesn't hit, neither does APP_BAR='{"baz_buz":true}'. In general I'm just confused how figment handles these as nothing I've tried worked (i.e. removing the underscore, mapping it to a nested object, trying a . or -, etc, etc)