Pattern should always be VarULE

[sffc]

Follow-up from #7411

[robertbastian]

observation: making a bound tighter based on a feature is not generally ok, but this trait is sealed

[robertbastian]

serde should only need P::Store: Serialize/Deserialize

[sffc]

For Serialize, it looks like it works to write where Pattern<P>: Serialize.

For Deserialize, on this code:

impl<'de, 'data, P: PatternBackend> serde::Deserialize<'de>
    for CompactPatterns<'data, P>
where
    'de: 'data,
    Pattern<P>: serde::Deserialize<'de>,

I'm getting this error:

error[E0275]: overflow evaluating the requirement `&icu_pattern::Pattern<_>: displaynames::provider::_::_serde::Deserialize<'_>`
  |
  = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`icu_experimental`)
  = note: required for `&icu_pattern::Pattern<_>` to implement `displaynames::provider::_::_serde::Deserialize<'_>`
  = note: 125 redundant requirements hidden
  = note: required for `&icu_pattern::Pattern<_>` to implement `displaynames::provider::_::_serde::Deserialize<'_>`

For more information about this error, try `rustc --explain E0275`.

[sffc]

It works if the bound is on Box<Pattern<P>>.

[sffc]

I could alternatively remove the requirement that Store always implement VarULE and instead ask clients to write where Pattern<P>: VarULE.

[sffc]

I could alternatively remove the requirement that Store always implement VarULE and instead ask clients to write where Pattern<P>: VarULE.

I tried doing this but then where Pattern<P>: zerovec::ule::VarULE is required everywhere that CompactPatterns is referenced, including in derives, which don't currently carry these where clauses.