Cargo Features

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

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

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
compat_macros

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

hashbrown compat_hash?

Enables hashbrown ^0.8.1

spin compat_sync?

Enables spin ^0.5.2