Firefox Nightly Fluent core moved to Rust

[Zibi Braniecki (Gandalf)]

Hi all,

This is a PSA about the change that landed last night in bug 1560038 [0].

We migrated the core of Fluent Localization System in Gecko from JavaScript
[1] to Rust [2].

The expected benefits are:
- Lays groundwork for shared memory for Fluent (Fission) (next steps in
bug 1613705 [3])
- Removal of JS/XPCOM engine bootstrap from startup path for Fluent/DOM
- Slight performance improvement with hopes for greater wins in bug
1613705 [3].
- Full AST parser allows for better diagnostics and error reporting

This is a change that should result in no observable difference in behavior
but since it is a significant change in the core of the system I'd like to
ask all users of Nightly to watch for any unexpected behavior related to
localization and report it on Bugzilla (Core::Internationalization) or on
Matrix (#l10n-dev:mozilla.org).

Thank you!
zb.

[0] https://bugzilla.mozilla.org/show_bug.cgi?id=1560038
[1] https://github.com/projectfluent/fluent.js/tree/master/fluent-bundle
[2] https://crates.io/crates/fluent
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=1613705