What is a crate attribute and where do I add it?

A crate attribute is an attribute (#[...]) that applies to the enclosing context (#![...]). This attribute must be added to the top of your crate root, thus the context is the crate itself:

#![attribute_name]
#![attribute_name(arg1, ...)]

If you are creating

• a library — the crate root will be a file called lib.rs
• an application — the crate root would be the primary .rs file you build. In many cases, this will be called main.rs
• an integration test - the crate root is each file in tests/
• an example - the crate root is each file in examples/

The Rust Programming Language and the Rust Reference talk a bit about attributes in general. The Unstable Book contains a list of feature flags and brief documentation on what they do.

There are many different crate attributes, but the feature crate attribute (#![feature(feature1, feature2)]) may only be used in a nightly version of the compiler. Unstable features are not allowed to be used in stable Rust versions.