Need external ICU callbacks for Chromium integration

[drott]

Chromium updates ICU project wide and existing C-HarfBuzz is configured with HAVE_ICU and depends on "//third_party/icu:icuuc".

This enables us to control ICU behavior in shaping in sync with other ICU behavior of Chromium.

If we only rely on built-in ICU and the scripts/update-unicode-tables.mk script, we would have overhead for keeping HarfRust and rest of Chromium in sync on ICU version. This becomes particularly tricky, should we need to roll back ICU when upgrading to a new version.

[behdad]

While custom unicode-funcs, if possible (ala the custom font-funcs @dfrg is working on), would be closer to HB design, for this feature per se, we can also make a crate-wide icu4x feature that would use ICU instead of the internal ucd.

[behdad]

Wait, can we rely on icu4x, or need to ffi icu4c in Chrome?

[drott]

Depends on which icu4x part you need: https://source.chromium.org/chromium/chromium/src/+/main:third_party/rust/, look for icu_*. We don't have everything there in my understanding. In which icu4x bit are the things we need?

[behdad]

Depends on which icu4x part you need: https://source.chromium.org/chromium/chromium/src/+/main:third_party/rust/, look for icu_*. We don't have everything there in my understanding. In which icu4x bit are the things we need?

It's probably icu_properties that is listed there.

[drott]

If we can get it to work with that, this might be an option. It looks like icu_properties is a 2.2.0 which matches at least ICU 78.1rc - which should be where the rest of Chromium and its ICU4C is.