An ergonomic [Parser] for #[attributes], built on parser combinators.

let mut rename_all = None::<Casing>;
let mut untagged = false;
let mut deny_unknown_fields = false;
let mut path_to_serde: Path = parse_quote!(::serde);
let attrs: Vec<Attribute> = parse_quote! {
    #[serde(rename_all = "kebab-case", untagged)]
    #[serde(crate = "custom::path")]
};

Attrs::new()
    .once("rename_all", with::eq(set::from_str(&mut rename_all)))
    .once("untagged", set::flag(&mut untagged))
    .once("deny_unknown_fields", set::flag(&mut deny_unknown_fields))
    .once("crate", with::eq(on::parse_str(&mut path_to_serde)))
    .parse_attrs("serde", &attrs)?;

assert_eq!(rename_all, Some(Casing::Kebab));
assert!(untagged);
assert!(!deny_unknown_fields); // not encountered, so not set
assert_eq!(path_to_serde.to_token_stream().to_string(), "custom :: path");

Guide

#[attributes] as they are used in the Rust compiler and in the wild tend to look like this:

  #[repr(align(128), C)]
//  ^^^^ ^^^^^ ^^^   ^
//  path  key (val)  bare key

  #[serde(rename_all = "kebab-case", untagged)]
// ^^^^^^ ^^^^^^^^^^   ^^^^^^^^^^^^  ^^^^^^^^
//  path     key     =      val      bare key

To use this library, create an [Attrs], and register different keys, each with a parsing function.

This library provides many parsing functions, but there are four key kinds:

• lit takes a literal like true or 100 from the input.
• from_str takes a ".." string from the input, before trying to [FromStr] it into an object.
• parse_str takes a ".." string from the input, before trying to [syn::parse] it into an object.
• parse directly tries to [syn::parse] the input.

Every function takes an &mut reference to its destination, which will be filled in when the corresponding key is encountered. The [on] module acts on direct references, whereas the [set] module acts on [Option]s, filling them with [Some].

The main ways to separate a key from its value are provided as combinators in the [with] module:

• [with::eq] take an = from the input.
• [with::paren] take a group (..) from the input.

You may choose to accept a key once or many times, and you can, of course, write your own parsing functions for whatever syntax you have in mind.