The tracking issue for this feature is: #34981

At present, literals are only accepted as the value of a key-value pair in attributes. What's more, only string literals are accepted. This means that literals can only appear in forms of #[attr(name = "value")] or #[attr = "value"].

The attr_literals unstable feature allows other types of literals to be used in attributes. Here are some examples of attributes that can now be used with this feature enabled:

#[attr]
#[attr(true)]
#[attr(ident)]
#[attr(ident, 100, true, "true", ident = 100, ident = "hello", ident(100))]
#[attr(100)]
#[attr(enabled = true)]
#[enabled(true)]
#[attr("hello")]
#[repr(C, align = 4)]
#[repr(C, align(4))]