NOTE: this currently includes the tools-performance change as it depends on that change. Ignore those files here.

Description

This adds a tools project for dealing with babel with regards to react-native transformations. This builds upon some of the work from the metro-transformer-oxc package as well as some of the work I was looking at for using native tools for transformation.

This package has tools that deal with babel configuration and parsing.

Babel Configuration

Several utilities are provided here:

• A central implementation of getting the babel configuration in a way compatible with what is in @react-native/metro-babel-transformer, which is the standard upstream babel transformer for react-native.
• Helpers for parsing babel's plugin and preset types.
• An implementation of the plugin visitor factory for instrumenting plugins.
• A routine to update babel configs with a visitor pattern that is will only make changes if something is updated. It can be used to update plugins or remove them entirely.
• A filter function for filtering plugins by key.

Babel Parsing

Parsing is one of the most time consuming and memory intensive parts of the transformation process. There are faster parsers but they generally produce estree ASTs rather than babel-ASTs. The current metro-transformer-oxc uses a utility estree-to-babel to convert the AST, which mostly works, but when timing the performance it ends up being almost as slow as babel parsing.

The parsing in here:

• can parse with oxc, hermes-parser, or babel
• has a reworked and heavily tuned AST converted that mutates in place, greatly reducing memory churn and making this drastically faster than hermes-parser end-to-end, which is itself substantially faster than babel.
• uses a huge set of test fixtures (now in the @rnx-kit/test-fixtures package) to validate and test AST compatibility both at a raw level, as well as with generation and transformation.

The tests are fairly wordy for output but this is useful for allowing tools like Claude to gain information about what is happening and what the differences are.