Transforming attributes into identifiers on proc macro derive - parsing

Question

I'm writing my first proc macro and despite trying to read through the source for thiserror, structopt and derive_more I can't seem to find exactly what I'm looking for. I want to transform this:
#[derive(Attach)]
#[attach(foo(SomeType, OtherType))]
#[attach(bar(OtherType))]
struct Plugin {}
into this:
impl Attach for Plugin {
fn attach(self, srv: &mut Server) {
let this = Arc::new(self);
srv.foo_order(this.clone(), &[TypeId::of::<SomeType>(), TypeId::of::<OtherType>()]);
srv.bar_order(this, &[TypeId::of::<OtherType>()]);
}
}
I've started writing a proc macro but got bungled up trying to parse the attributes:
extern crate proc_macro;
use proc_macro::{Span, TokenStream};
use quote::quote;
use std::any::TypeId;
use syn::{
parse::ParseStream, parse_macro_input, Attribute, AttributeArgs, DeriveInput, Ident, Result,
};
#[proc_macro_derive(Attach, attributes(attach))]
pub fn register_macro(input: TokenStream) -> TokenStream {
let input = parse_macro_input!(input as DeriveInput);
impl_register(input)
}
fn impl_register(input: DeriveInput) -> TokenStream {
let name = &input.ident;
let attrs = &input.attrs;
for attr in attrs {
if attr.path.is_ident("attach") {
parse_attach_attribute(&attr);
}
}
println!("{:#?}", input);
TokenStream::from(quote! {
impl ::corten::Attach for #name {
fn attach(self, srv: &mut ::corten::Server) {
}
}
})
}
fn parse_attach_attribute(attr: &Attribute) -> Result<Dependency> {
let list: syn::MetaList = attr.parse_args()?;
// println!("{:#?}", list);
let ident = list.path.get_ident().expect("expected identifier");
let method = Ident::new(&format!("{}_order", ident), ident.span());
println!("{:#?}", method);
let dependencies = list
.nested
.into_pairs()
.map(|pair| pair.into_value())
.collect::<Vec<_>>();
// How does one get the identifiers out of a NestedMeta?
println!("{:#?}", dependencies);
// attr.parse_args_with(|input: ParseStream| {
// let ident = input.p
// let method = Ident::new(&format!("{}_order", ident), Span::call_site());
// let dep = Dependency {
// method,
// }
// })
unimplemented!()
}
struct Dependency {
method: Ident,
dependencies: Vec<Ident>,
}
the difficulty I'm having is how to actually get the attributes list into a usable form? As far as I can tell, I need to get the "foo" and "bar" parsed from &[Attribute] so I can construct the method identifier, and also the "SomeType" & "OtherType" identifiers, which I'll all eventually feed into quote!. If I print the TokenStream in the console, all of the information is there:
[
Attribute {
pound_token: Pound,
style: Outer,
bracket_token: Bracket,
path: Path {
leading_colon: None,
segments: [
PathSegment {
ident: Ident {
ident: "attach",
span: #0 bytes(103..109),
},
arguments: None,
},
],
},
tokens: TokenStream [
Group {
delimiter: Parenthesis,
stream: TokenStream [
Ident {
ident: "predecode",
span: #0 bytes(110..119),
},
Group {
delimiter: Parenthesis,
stream: TokenStream [
Ident {
ident: "SomeType",
span: #0 bytes(120..128),
},
Punct {
ch: ',',
spacing: Alone,
span: #0 bytes(128..129),
},
Ident {
ident: "OtherType",
span: #0 bytes(130..139),
},
],
span: #0 bytes(119..140),
},
],
span: #0 bytes(109..141),
},
],
},
Attribute {
pound_token: Pound,
style: Outer,
bracket_token: Bracket,
path: Path {
leading_colon: None,
segments: [
PathSegment {
ident: Ident {
ident: "attach",
span: #0 bytes(145..151),
},
arguments: None,
},
],
},
tokens: TokenStream [
Group {
delimiter: Parenthesis,
stream: TokenStream [
Ident {
ident: "preresolve",
span: #0 bytes(152..162),
},
Group {
delimiter: Parenthesis,
stream: TokenStream [
Ident {
ident: "OtherType",
span: #0 bytes(163..172),
},
],
span: #0 bytes(162..173),
},
],
span: #0 bytes(151..174),
},
],
},
]
But I don't have a way to actually get at it. How do I get to tokens[0].stream.ident?

Answer 1

After much messing around I think I have something that works, although I'm happy to accept other answers that are better as I feel this is a bit messy:
extern crate proc_macro;
use proc_macro2::{Span, TokenStream};
use quote::quote;
use syn::{parse_macro_input, Attribute, DeriveInput, Ident, Result};
#[proc_macro_derive(Attach, attributes(attach))]
pub fn register_macro(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
let input = parse_macro_input!(input as DeriveInput);
proc_macro::TokenStream::from(impl_register(input))
}
fn impl_register(input: DeriveInput) -> TokenStream {
let name = &input.ident;
let attrs = &input.attrs;
// println!("{:#?}", input);
let attrs = attrs
.iter()
.filter(|attr| attr.path.is_ident("attach"))
.map(|attr| parse_attach_attribute(&attr).expect("parse failed"))
.map(|dep| {
let method: Ident = dep.method;
let dependencies = dep.dependencies.iter().map(|ident: &Ident| {
quote! {
std::any::TypeId::of::<#ident>()
}
});
quote! {
srv.#method::<#name, _>(Arc::clone(&this), &[ #(#dependencies),* ]);
}
});
quote! {
impl corten::Attach for #name {
fn attach(self, srv: &mut corten::Server) {
let this = std::sync::Arc::new(self);
#(#attrs)*
}
}
}
}
fn parse_attach_attribute(attr: &Attribute) -> Result<Dependency> {
let list: syn::MetaList = attr.parse_args()?;
// println!("{:#?}", list);
let ident = list.path.get_ident().expect("expected identifier");
let method = Ident::new(&format!("{}_order", ident), Span::call_site());
println!("{:#?}", method);
let dependencies = list
.nested
.into_pairs()
.map(|pair| pair.into_value())
.filter_map(|pair| match pair {
syn::NestedMeta::Meta(meta) => match meta {
syn::Meta::Path(path) => path.get_ident().cloned(),
_ => panic!("only path meta supported"),
},
_ => panic!("lit not supported"),
})
.collect();
println!("{:#?}", dependencies);
Ok(Dependency {
method,
dependencies,
})
}
struct Dependency {
method: Ident,
dependencies: Vec<Ident>,
}