Never use fallback for baked segmentation data (#4510)

Fixes #4489

CHANGELOG.md

@@ -15,6 +15,7 @@
  - `icu_datagen`
  - Datagen shows elapsed time for keys that are slow to generate (https://github.com/unicode-org/icu4x/pull/4469)
  - Datagen performance improvement by caching supported locales (https://github.com/unicode-org/icu4x/pull/4470)
+ - Never use fallback for baked segmentation data (https://github.com/unicode-org/icu4x/pull/4510)
  - `icu_provider`
  - (Small breakage) `DataPayload::new_owned()` is no longer `const`, this was a mistake (https://github.com/unicode-org/icu4x/pull/4456)
  - `icu_provider_blob`
@@ -28,7 +29,7 @@
  - Changed file extensions for better compatibility with nodejs modules (https://github.com/rust-diplomat/diplomat/pull/387)
  - C++
  - Fixed a bug where a result header defines a struct that shadows the class' name (https://github.com/rust-diplomat/diplomat/pull/394)
- - Add `expclicit` keyword to internal constructors (https://github.com/rust-diplomat/diplomat/pull/386)
+ - Add `explicit` keyword to internal constructors (https://github.com/rust-diplomat/diplomat/pull/386)
 - Utilities
  - `calendrical_calculations`:
  - Add Keviyah/Four Gates based optimized calculations module for the Hebrew calendar.

provider/datagen/src/driver.rs

@@ -438,7 +438,9 @@ impl DatagenDriver {
 
  let transform_duration = instant1.elapsed();
 
- if fallback == FallbackMode::Runtime {
+ // segmenter uses hardcoded locales internally, so fallback is not necessary.
+ // TODO(#4511): Use auxiliary keys for segmenter
+ if fallback == FallbackMode::Runtime && !key.path().get().starts_with("segmenter") {
  sink.flush_with_built_in_fallback(key, BuiltInFallbackMode::Standard)
  } else {
  sink.flush(key)

tools/bakeddata-scripts/src/main.rs

@@ -131,18 +131,14 @@ fn main() {
  .unwrap();
  }
 
- if component == "segmenter" {
- // segmenter uses hardcoded locales internally, so fallback is not necessary.
- driver.clone().with_fallback_mode(FallbackMode::Hybrid)
- } else {
- driver.clone()
- }
- .with_keys(keys.iter().copied())
- .export(
- &source,
- BakedExporter::new(path.join("data"), options).unwrap(),
- )
- .unwrap();
+ driver
+ .clone()
+ .with_keys(keys.iter().copied())
+ .export(
+ &source,
+ BakedExporter::new(path.join("data"), options).unwrap(),
+ )
+ .unwrap();
 
  for file in ["data/any.rs", "data/mod.rs"] {
  std::fs::remove_file(path.join(file)).unwrap();