Accept multiple values on proc macro attribute - rust

Question

I wanted to be able to retrieve the content from an attribute like this:
#[foreign_key(table = "some_table", column = "some_column")]
This is how I am trying:
impl TryFrom<&&Attribute> for EntityFieldAnnotation {
type Error = syn::Error;
fn try_from(attribute: &&Attribute) -> Result<Self, Self::Error> {
if attribute.path.is_ident("foreign_key") {
match attribute.parse_args()? {
syn::Meta::NameValue(nv) =>
println!("NAME VALUE: {:?}, {:?}, {:?}",
nv.path.get_ident(),
nv.eq_token.to_token_stream(),
nv.lit.to_token_stream(),
),
_ => println!("Not interesting")
}
} else {
println!("No foreign key")
}
// ... More Rust code
}
Everything works fine if I just put in there only one NameValue. When I add the comma,
everything brokes.
The only error:
error: unexpected token
How can I fix my logic to enable the possibility of have more than just one NameValue?
Thanks

Answer 1

UPDATE: While writing this answer, I had forgotten that Meta has List variant as well which gives you NestedMeta. I would generally prefer doing that instead of what I did in the answer below for more flexibility.
Although, for your particular case, using Punctuated still seems simpler and cleaner to me.
MetaNameValue represents only a single name-value pair. In your case it is delimited by ,, so, you need to parse all of those delimited values as MetaNameValue instead.
Instead of calling parse_args, you can use parse_args_with along with Punctuated::parse_terminated:
use syn::{punctuated::Punctuated, MetaNameValue, Token};
let name_values: Punctuated<MetaNameValue, Token![,]> = attribute.parse_args_with(Punctuated::parse_terminated).unwrap(); // handle error instead of unwrap
Above name_values has type Punctuated which is an iterator. You can iterate over it to get various MetaNameValue in your attribute.
Updates based on comments:
Getting value out as String from MetaNameValue:
let name_values: Result<Punctuated<MetaNameValue, Token![,]>, _> = attr.parse_args_with(Punctuated::parse_terminated);
match name_values {
Ok(name_value) => {
for nv in name_value {
println!("Meta NV: {:?}", nv.path.get_ident());
let value = match nv.lit {
syn::Lit::Str(v) => v.value(),
_ => panic!("expeced a string value"), // handle this err and don't panic
};
println!( "Meta VALUE: {:?}", value )
}
},
Err(_) => todo!(),
};