attribute-derive

Basically clap for attribute macros:

use attribute_derive::Attribute;use syn::Type;#[derive(Attribute)]#[attribute(ident = collection)]#[attribute(error(missing_field = "`{field}` was not specified"))]structCollectionAttribute{// Options are optional by default (will be set to None if not specified)authority:Option<String>,name:String,// Any type implementing default can be flagged as default// This will be set to Vec::default() when not specified#[attribute(optional)]views:Vec<Type>,// Booleans can be used without assigning a value, i.e., as a flag.// If omitted they are set to falsesome_flag:bool,}

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).