Cargo Features

no-std-compat has no features set by default.

no-std-compat = { version = "0.4.1", features = ["std", "unstable", "compat_hash", "compat_sync", "compat_macros"] }

Affects no-std-compat::prelude

alloc unstable
compat_hash = hashbrown

This will sadly also add hashbrown even if overriden by std. Cargo does not seem to provide any cfg(compat_hash && !std) functionality.
Luckily, hashbrown is really small.

compat_sync = spin

Features from optional dependencies

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

hashbrown compat_hash?

Enables default features of hashbrown ^0.8.1

spin compat_sync?

Enables spin ^0.5.2