How to setup doc_cfg for Rust to document your cfg features (credit to benesch and mcarton on stackoverflow)

_INSTRUCTIONS.md

1. Setup the nightly feature in your lib.rs (lib.rs)
2. Add a copy of your feature toggle (other_file.rs)
3. Setup your Cargo.toml to have docs.rs enable the conditional compilation from the first 2 steps (Cargo.toml)

Note: step #2 is now only necessary in situations in which rustdoc currently cannot tell cfg() applies to an item (for example a public item inside a private module which is itself cfg'd which then is re-exported). See this issue for more info.

Cargo.toml

[package.metadata.docs.rs]
all-features = true
rustdoc-args = ["--cfg", "docsrs"]

lib.rs

#![cfg_attr(docsrs, feature(doc_cfg))]

other_file.rs

#[cfg(feature = "my-feature")]
#[cfg_attr(docsrs, doc(cfg(feature = "my-feature")))]
fn my_func_behind_feature() {
    // ...
}