Cargo Features

regmaps has no features set by default.

regmaps = { version = "0.0.0", features = ["rustfmt"] }
rustfmt = rustfmt-nightly

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.

rustfmt-nightly rustfmt?