Attributes
Attributes, indicated by the [] symbols, allows attachment of metadata, annotations, or special instructions to various language constructs, such as types, fields, and functions. Currently, only the built-in ffi attribute is supported.
Example
[foo("arg1", "arg2")]
pub type Foo {
    [bar("arg1", "arg2")]
    pub field Bar: i32;
    [baz("arg1", "arg2")]
    pub fn Baz() {
    }
}