Macro rustfmt_nightly::is_nightly_channel

macro_rules! is_nightly_channel {
    () => { ... };
}

Checks if we’re in a nightly build.

The environment variable CFG_RELEASE_CHANNEL is set during the rustc bootstrap to “stable”, “beta”, or “nightly” depending on what toolchain is being built. If we are being built as part of the stable or beta toolchains, we want to disable unstable configuration options.

If we’re being built by cargo (e.g., cargo +nightly install rustfmt-nightly), CFG_RELEASE_CHANNEL is not set. As we only support being built against the nightly compiler when installed from crates.io, default to nightly mode.