Cargo Features

[dependencies]
sum = { version = "0.1.7", default-features = false, features = ["10", "19", "28", "32", "futures", "serde"] }
default = 0, 1, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 2, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 3, 30, 31, 32, 4, 5, 6, 7, 8, 9

These default features are set whenever sum is added without default-features = false somewhere in the dependency tree.

0 1 2 3 4 5 6 7 8 9 10 default
11 12 13 14 15 16 17 18 19 default
20 21 22 23 24 25 26 27 28 default
29 30 31 32 default
futures = futures-core

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.

futures-core futures?
serde implicit feature

Enables serde

serde:

A generic serialization/deserialization framework