Attributes

An attribute is written @name above a declaration, optionally with a string argument. Attributes are a closed set: the compiler knows every one it accepts and rejects anything else with an error. A misspelled attribute can therefore never be silently ignored, which is the failure mode that makes attributes untrustworthy in other languages — @tset is a compile error, not a test that quietly never runs.

use system.io

// @non_exhaustive: this enum may gain variants later, so a match on it
// outside the defining module must carry a default arm.
@non_exhaustive
public enum ConnectionError
    Refused(String)
    TimedOut(int)

// @must_use: the value may not be silently discarded at a call site.
@must_use
public enum Receipt
    Written(int)

// @deprecated carries the replacement, and callers get a warning.
@deprecated("acquire a Clock and use clock.now()")
fn legacy_timestamp() int
    0

fn main()
    println("attributes are a closed set — an unknown one is a compile error")

Attribute	Applies to	Argument	Effect
`@non_exhaustive`	Enum	—	The enum may gain variants, so a `match` on it outside the defining module needs a `default` arm.
`@must_use`	Enum	—	The value may not be silently discarded at a call site.
`@deprecated`	Function, class, enum	Required	Callers get a warning carrying the message — normally the replacement to use.
`@test`	Function	—	Marks the function for discovery by `miri test`.
`@ignore`	Function	Required	Skips the test and reports the reason. Requires `@test`.
`@xfail`	Function	Required	The test is expected to fail. Requires `@test`.

Applying an attribute to the wrong kind of declaration, omitting a required argument, or passing one where none is accepted are all errors as well — as is using @ignore or @xfail without the @test they depend on.