Cargo Features

swc_ecma_transforms_optimization has no features set by default.

[dependencies]
swc_ecma_transforms_optimization = { version = "4.0.0", features = ["concurrent", "debug"] }
concurrent = rayon

Enables concurrent of swc_common and swc_ecma_transforms_base

debug

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?