Cargo Features

[dependencies]
json_macros = { version = "0.3.2", default-features = false, features = ["with-serde", "with-rustc-serialize"] }
default = with-rustc-serialize

The with-rustc-serialize feature is set by default whenever json_macros is added without default-features = false somewhere in the dependency tree.

with-serde = serde_json
with-rustc-serialize default = rustc-serialize

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.

rustc-serialize with-rustc-serialize
serde_json with-serde?

Enables serde_json ^0.6