`unstable_features` does absolutely nothing

rustfmt 1.4.37-stable (59eed8a2 2021-11-01)

Given a rustfmt.toml that looks like

unstable_features = <val>
imports_layout = "Vertical" # Some unstable feature

Files are formatted according to the following table

So the value of unstable_features does absolutely nothing except cause rustfmt to emit one of the following warnings when using a stable toochain

Warning: can't set `unstable_features = true`, unstable features are only available in nightly channel.
Warning: can't set `unstable_features = false`, unstable features are only available in nightly channel.

which is ridiculous.

Either this key should allow you to enable unstable formatting settings for stable toolchains, or it should allow you to disable unstable formatting settings for nightly toolchains. Preferably both. It currently has no effect at all.

There is a bug here.

Start by reproducing the rustfmt.toml configuration with unstable_features and imports_layout on the stable and nightly toolchains described in the issue. Trace how unstable_features is handled and verify the chosen behavior against both toolchain cases; done means the setting has an observable, documented effect or is removed with its misleading warnings.