Cargo Features

fftw-sys = { version = "0.8.0", default-features = false, features = ["system", "source", "intel-mkl"] }
default = source

The source feature is set by default whenever fftw-sys is added without default-features = false somewhere in the dependency tree.

source default = fftw-src
intel-mkl = intel-mkl-src

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features.

fftw-src source
intel-mkl-src intel-mkl?