Cargo Features

swc_ecma_transforms_compat has no features set by default.

[dependencies]
swc_ecma_transforms_compat = { version = "6.0.1", features = ["concurrent"] }
concurrent = rayon

Enables concurrent of swc_ecma_transforms_base and swc_ecma_utils

swc_ecma_utils:

Process in parallel.

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

rayon concurrent?