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

Question

In order to get a feel for how Rust works, I decided to look at a little terminal-based text editor called Iota. I cloned the repository and ran cargo build only to be told:

error: *if let* syntax is experimental  help: add #![feature(if_let)] to the crate attributes to enable 

Where am I supposed to add #![feature(if_let)] to the crate attributes?

Answer 1

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/

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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.