Cargo Features

TM4C123x has no features set by default.

[dependencies]
tm4c123x = { version = "0.9.2", features = ["rt"] }
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?