Cargo Features

serde_macros has no features set by default.

[dependencies]
serde_macros = { version = "0.8.9", features = ["unstable-testing"] }
unstable-testing = clippy

Enables unstable-testing of serde ^0.8.9 and serde_codegen =0.8.9

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.

clippy unstable-testing?