Improve ergonomics of icu4x-datagen

CONTRIBUTING.md

@@ -44,6 +44,43 @@ To build all code paths, improve build times in VSCode, and prevent locking the
 
 Note: the path in `ICU4X_DATA_DIR` is relative to `provider/data/*/src/lib.rs` and it causes VSCode to build ICU4X with only the `und` locale. This reduces build times but also makes some tests fail; to run them normally, run `cargo test --all-features` on the command line.
 
+### Building and Rebuilding Repo Data
+
+In the ICU4X repository, there are a few types of locale data:
+
+1. Test data: used for internal ICU4X development purposes only
+    - Downloaded data sources: `provider/source/tests/data`
+        - Regen: `cargo make download-repo-sources`
+    - Generated JSON data: `provider/source/data/debug`
+        - Regen: `cargo make testdata`
+2. Hard-coded source data: source of truth is this repo; used by icu4x-datagen
+    - Segmenter TOML files: `provider/source/data/segmenter`
+3. Runtime default compiled data: the `icu_*_data` crates
+    - Crate roots: `provider/data`
+        - Regen: `cargo make bakeddata`
+        - Regen a specific component: `cargo make bakeddata <component>`
+
+During development, it is often convenient to generate only a single data marker as JSON. To do this (fully offline), you can run, for example:
+
+```bash
+$ cargo run -p icu4x-datagen \
+    --no-default-features --features provider,fs_exporter \
+    -- --format fs --pretty -o _debug/data \
+	--cldr-root provider/source/tests/data/cldr \
+	--icuexport-root provider/source/tests/data/icuexport \
+	--segmenter-lstm-root provider/source/tests/data/lstm \
+	--tzdb-root provider/source/tests/data/tzdb \
+	--deduplication none \
+	--locales ru th \
+	--markers DatetimePatternsDateGregorianV1 DatetimePatternsDateBuddhistV1
+```
+
+Tips:
+
+- Set your desired locales and data markers on the bottom two lines.
+- To overwrite the directly, add: `-W`
+- To print verbose logs, add: `-v`
+
 ## Contributing a Pull Request
 
 The first step is to fork the repository to your namespace and create a branch off of the `main` branch to work with.
@@ -79,10 +116,12 @@ There are various files that auto-generated across the ICU4X repository.  Here a
 need to run in order to recreate them.  These files may be run in more comprehensive tests such as those included in `cargo make ci-job-test` or `cargo make ci-all`.
 
 - `cargo make testdata` - regenerates all test data in the `provider/source/debug` directory.
-	- `cargo make bakeddata` - regenerates baked data in the `provider/data` directory.
-		- `cargo make bakeddata foo` can be used to generate data in `provider/data/foo` only.
+    - Tip: See [Building and Rebuilding Repo Data](#building-and-rebuilding-repo-data) for additional shortcuts.
+- `cargo make bakeddata` - regenerates baked data in the `provider/data` directory.
+    - `cargo make bakeddata foo` can be used to generate data in `provider/data/foo` only.
 - `cargo make generate-readmes` - generates README files according to Rust docs. Output files must be committed in git for check to pass.
 - `cargo make diplomat-gen` - recreates the Diplomat generated files in the `ffi/capi` directory.
+- `cargo make codegen` - recreates certain Askama generated files in the `ffi/capi/src` directory based on templates in `tools/make/codegen/templates`.
 
 ### Testing
 

provider/icu4x-datagen/Cargo.toml

@@ -19,7 +19,7 @@ version.workspace = true
 [dependencies]
 icu_provider = { workspace = true }
 icu = { workspace = true, features = ["datagen"] }
-icu_provider_export = { workspace = true, features = ["rayon"] }
+icu_provider_export = { workspace = true }
 icu_provider_source = { workspace = true, optional = true }
 icu_provider_registry = { workspace = true }
 
@@ -31,7 +31,7 @@ log = { workspace = true }
 simple_logger = { workspace = true }
 
 [features]
-default = ["use_wasm", "networking", "fs_exporter", "blob_exporter", "baked_exporter", "provider"]
+default = ["use_wasm", "networking", "fs_exporter", "blob_exporter", "baked_exporter", "provider", "rayon"]
 provider = ["dep:icu_provider_source"]
 baked_exporter = ["icu_provider_export/baked_exporter"]
 blob_exporter = ["icu_provider_export/blob_exporter"]
@@ -46,6 +46,7 @@ use_wasm = ["icu_provider_source?/use_wasm"]
 use_icu4c = ["icu_provider_source?/use_icu4c"]
 networking = ["icu_provider_source?/networking"]
 experimental = ["icu_provider_source?/experimental", "icu/experimental"]
+rayon = ["icu_provider_export/rayon"]
 
 [package.metadata.cargo-all-features]
 # We don't need working CPT builders for check

provider/icu4x-datagen/src/main.rs

@@ -380,7 +380,7 @@ fn main() -> eyre::Result<()> {
                 #[cfg(not(feature = "networking"))]
                 (None, _) => {
                     eyre::bail!(
-                        "Downloading data from tags requires the `networking` Cargo feature"
+                        "Please set --cldr-root or enable the `networking` Cargo feature"
                     )
                 }
             };
@@ -396,7 +396,7 @@ fn main() -> eyre::Result<()> {
                 #[cfg(not(feature = "networking"))]
                 (None, _) => {
                     eyre::bail!(
-                        "Downloading data from tags requires the `networking` Cargo feature"
+                        "Please set --icuexport-root or enable the `networking` Cargo feature"
                     )
                 }
             };
@@ -412,7 +412,7 @@ fn main() -> eyre::Result<()> {
                 #[cfg(not(feature = "networking"))]
                 (None, _) => {
                     eyre::bail!(
-                        "Downloading data from tags requires the `networking` Cargo feature"
+                        "Please set --segmenter-lstm-root or enable the `networking` Cargo feature"
                     )
                 }
             };
@@ -428,7 +428,7 @@ fn main() -> eyre::Result<()> {
                 #[cfg(not(feature = "networking"))]
                 (None, _) => {
                     eyre::bail!(
-                        "Downloading data from tags requires the `networking` Cargo feature"
+                        "Please set --tzdb-root or enable the `networking` Cargo feature"
                     )
                 }
             };