attribute-derive

Basically clap for attribute macros:

#[derive(Attribute)]
#[attribute(ident = "collection")]
#[attribute(invalid_field = "Error when an unsupported value is set (e.g. meaning=42")]
struct CollectionAttribute {
    // Options are optional by default (will be set to None if not specified)
    authority: Option<String>,
    #[attribute(missing = "Error when the value is not set")]
    name: String,
    // Any type implementing default can be flagged as default
    // This will be set to Vec::default() when not specified
    #[attribute(default)]
    #[attribute(expected = "Error when an error occured while parsing")]
    views: Vec<Type>,
    // Booleans can be used without assiging a value. as a flag.
    // If omitted they are set to false
    some_flag
}

Will be able to parse an attribute like this:

#[collection(authority="Some String", name = r#"Another string"#, views = [Option, ()])]

Limitations

There are some limitations in syntax parsing that will be lifted future releases.

• literals in top level (meaning something like #[attr(42, 3.14, "hi")]
• function like arguments (something like #[attr(view(a = "test"))]
• other syntaxes, maybe something like key: value

Parse methods

There are multiple ways of parsing a struct deriving Attribute.

For helper attributes there is:

• Attribute::from_attributes which takes in an IntoIterator<Item = &'a syn::Attribute (e.g. a &Vec<syn::Attribute>). Most useful for derive macros.
• Attribute::remove_attributes which takes an &mut Vec<syn::Attribute> and does not only parse the Attribute but also removes those matching. Useful for helper attributes for proc macros, where the helper attributes need to be removed.

For parsing a single TokenStream e.g. for parsing the proc macro input there a two ways:

• Attribute::from_args taking in a TokenStream
• As derive(Attribute) also derives Parse so you can use the parse API, e.g. with parse_macro_input!(tokens as Attribute).