darling 0.24.0

A proc-macro library for reading attributes into structs when implementing custom derives.
Documentation 
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//! This example shows how to parse completely arbitrary input (a self-closing HTML tag) -
//! via implementing [`FromMeta::from_invalid_expr`]

use darling::ast::MetaNameValueInvalidExpr;
use syn::{Ident, Token};

fn main() {
    let input = quote::quote! {
        #[args(tag = <br />, foo = 10)]
        struct Input;
    };
    let input = syn::parse2::<syn::DeriveInput>(input).unwrap();
    let input = <Input as darling::FromDeriveInput>::from_derive_input(&input).unwrap();

    assert_eq!(
        input.tag,
        TagName(Ident::new("br", proc_macro2::Span::call_site()))
    );
    assert_eq!(input.foo, 10);
}

#[derive(Debug, PartialEq, Eq)]
struct TagName(Ident);

impl syn::parse::Parse for TagName {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        input.parse::<Token![<]>()?;
        let tag_name = input.parse::<Ident>()?;
        input.parse::<Token![/]>()?;
        input.parse::<Token![>]>()?;

        Ok(Self(tag_name))
    }
}

impl darling::FromMeta for TagName {
    fn from_invalid_expr(value: &MetaNameValueInvalidExpr) -> darling::Result<Self> {
        syn::parse2(value.value.clone()).map_err(Into::into)
    }
}

#[derive(darling::FromDeriveInput)]
#[darling(attributes(args))]
struct Input {
    tag: TagName,
    foo: usize,
}