Cargo Features

nrf52840-pac has no features set by default.

nrf52840-pac = { version = "0.12.2", features = ["rt"] }

Enables device of cortex-m-rt

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.

cortex-m-rt rt?