Enum rustc_feature::builtin_attrs::AttributeDuplicates

pub enum AttributeDuplicates {
    DuplicatesOk,
    WarnFollowing,
    WarnFollowingWordOnly,
    ErrorFollowing,
    ErrorPreceding,
    FutureWarnFollowing,
    FutureWarnPreceding,
}

How to handle multiple duplicate attributes on the same item.

Variants

DuplicatesOk

Duplicates of this attribute are allowed.

This should only be used with attributes where duplicates have semantic meaning, or some kind of “additive” behavior. For example, #[warn(..)] can be specified multiple times, and it combines all the entries. Or use this if there is validation done elsewhere.

WarnFollowing

Duplicates after the first attribute will be an unused_attribute warning.

This is usually used for “word” attributes, where they are used as a boolean marker, like #[used]. It is not necessarily wrong that there are duplicates, but the others should probably be removed.

WarnFollowingWordOnly

Same as WarnFollowing, but only issues warnings for word-style attributes.

This is only for special cases, for example multiple #[macro_use] can be warned, but multiple #[macro_use(...)] should not because the list form has different meaning from the word form.

ErrorFollowing

Duplicates after the first attribute will be an error.

This should be used where duplicates would be ignored, but carry extra meaning that could cause confusion. For example, #[stable(since="1.0")] #[stable(since="2.0")], which version should be used for stable?

ErrorPreceding

Duplicates preceding the last instance of the attribute will be an error.

This is the same as ErrorFollowing, except the last attribute is the one that is “used”. This is typically used in cases like codegen attributes which usually only honor the last attribute.

FutureWarnFollowing

Duplicates after the first attribute will be an unused_attribute warning with a note that this will be an error in the future.

This should be used for attributes that should be ErrorFollowing, but because older versions of rustc silently accepted (and ignored) the attributes, this is used to transition.

FutureWarnPreceding

Duplicates preceding the last instance of the attribute will be a warning, with a note that this will be an error in the future.

This is the same as FutureWarnFollowing, except the last attribute is the one that is “used”. Ideally these can eventually migrate to ErrorPreceding.

Trait Implementations

impl Clone for AttributeDuplicates

fn clone(&self) -> AttributeDuplicates

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Default for AttributeDuplicates

fn default() -> AttributeDuplicates

Returns the “default value” for a type.

impl Copy for AttributeDuplicates

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference’s “Type Layout” chapter for details on type layout guarantees.

Size: 1 byte

Size for each variant:

• DuplicatesOk: 0 bytes
• WarnFollowing: 0 bytes
• WarnFollowingWordOnly: 0 bytes
• ErrorFollowing: 0 bytes
• ErrorPreceding: 0 bytes
• FutureWarnFollowing: 0 bytes
• FutureWarnPreceding: 0 bytes