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The book was located under 'src/doc/trpl' because originally, it was going to be hosted under that URL. Late in the game, before 1.0, we decided that /book was a better one, so we changed the output, but not the input. This causes confusion for no good reason. So we'll change the source directory to look like the output directory, like for every other thing in src/doc.
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% Attributes
Declarations can be annotated with ‘attributes’ in Rust. They look like this:
#[test]
# fn foo() {}
or like this:
# mod foo {
#![test]
# }
The difference between the two is the !, which changes what the attribute
applies to:
#[foo]
struct Foo;
mod bar {
#![bar]
}
The #[foo] attribute applies to the next item, which is the struct
declaration. The #![bar] attribute applies to the item enclosing it, which is
the mod declaration. Otherwise, they’re the same. Both change the meaning of
the item they’re attached to somehow.
For example, consider a function like this:
#[test]
fn check() {
assert_eq!(2, 1 + 1);
}
It is marked with #[test]. This means it’s special: when you run
tests, this function will execute. When you compile as usual, it won’t
even be included. This function is now a test function.
Attributes may also have additional data:
#[inline(always)]
fn super_fast_fn() {
# }
Or even keys and values:
#[cfg(target_os = "macos")]
mod macos_only {
# }
Rust attributes are used for a number of different things. There is a full list of attributes in the reference. Currently, you are not allowed to create your own attributes, the Rust compiler defines them.