# Error code E0539

An invalid meta-item was used inside an attribute.

Erroneous code example:

ⓘ

    #![feature(staged_api)]
    #![stable(since = "1.0.0", feature = "test")]

    #[deprecated(note)] // error!
    #[unstable(feature = "deprecated_fn", issue = "123")]
    fn deprecated() {}

    #[unstable(feature = "unstable_struct", issue)] // error!
    struct Unstable;

    #[rustc_const_unstable(feature)] // error!
    const fn unstable_fn() {}

    #[stable(feature = "stable_struct", since)] // error!
    struct Stable;

    #[rustc_const_stable(feature)] // error!
    const fn stable_fn() {}

Run

Meta items are the key-value pairs inside of an attribute. To fix these issues you need to give required key-value pairs.

    #![feature(staged_api)]
    #![stable(since = "1.0.0", feature = "test")]

    #[deprecated(since = "1.39.0", note = "reason")] // ok!
    #[unstable(feature = "deprecated_fn", issue = "123")]
    fn deprecated() {}

    #[unstable(feature = "unstable_struct", issue = "123")] // ok!
    struct Unstable;

    #[rustc_const_unstable(feature = "unstable_fn", issue = "124")] // ok!
    const fn unstable_fn() {}

    #[stable(feature = "stable_struct", since = "1.39.0")] // ok!
    struct Stable;

    #[rustc_const_stable(feature = "stable_fn", since = "1.39.0")] // ok!
    const fn stable_fn() {}

Run

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