parse generics for implement macro

Signed-off-by: Jason Volk <jason@zemos.net>

src/macros/implement.rs

-use proc_macro::TokenStream;
+use proc_macro::{Span, TokenStream};
 use quote::quote;
-use syn::{ItemFn, Meta, MetaList};
+use syn::{Error, ItemFn, Meta, Path};
+use utils::get_named_generics;
 
-use crate::Result;
+use crate::{utils, Result};
 
 pub(super) fn implement(item: ItemFn, args: &[Meta]) -> Result<TokenStream> {
-	let Meta::List(MetaList {
-		path,
-		..
-	}) = &args
-		.first()
-		.expect("missing path to trait or item to implement")
-	else {
-		panic!("invalid path to item for implement");
-	};
-
+	let generics = get_named_generics(args, "generics")?;
+	let receiver = get_receiver(args)?;
+	let params = get_named_generics(args, "params")?;
 	let input = item;
 	let out = quote! {
-		impl #path {
+		impl #generics #receiver #params {
 			#input
 		}
 	};
 
 	Ok(out.into())
 }
+
+fn get_receiver(args: &[Meta]) -> Result<Path> {
+	let receiver = &args
+		.first()
+		.ok_or_else(|| Error::new(Span::call_site().into(), "Missing required argument to receiver"))?;
+
+	let Meta::Path(receiver) = receiver else {
+		return Err(Error::new(Span::call_site().into(), "First argument is not path to receiver"));
+	};
+
+	Ok(receiver.clone())
+}

src/macros/utils.rs

-use syn::{Expr, Lit, Meta};
+use syn::{parse_str, Expr, Generics, Lit, Meta};
+
+use crate::Result;
+
+pub(crate) fn get_named_generics(args: &[Meta], name: &str) -> Result<Generics> {
+	const DEFAULT: &str = "<>";
+
+	parse_str::<Generics>(&get_named_string(args, name).unwrap_or_else(|| DEFAULT.to_owned()))
+}
 
 pub(crate) fn get_named_string(args: &[Meta], name: &str) -> Option<String> {
 	args.iter().find_map(|arg| {