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use proc_macro::{TokenStream, TokenTree};
use proc_macro2::TokenStream as TokenStream2;
use quote::{quote, ToTokens};
use syn::{parse_macro_input, FnArg, ItemFn};
enum AttrParam {
Name,
Type,
}
struct VarargConfig {
macro_name: TokenStream2,
prefix: TokenStream2,
}
#[proc_macro_attribute]
pub fn vararg(attrs: TokenStream, input: TokenStream) -> TokenStream {
let func = parse_macro_input!(input as ItemFn);
let name = func.sig.ident.clone();
let args = func.sig.inputs.clone();
let (first, last) = args.into_iter().fold((vec![], None), |mut t, e| match t.1 {
None => (t.0, Some(e)),
Some(c) => {
t.0.push(c);
(t.0, Some(e))
}
});
let last: FnArg = last.expect("Function with 'vararg' attribute has no args");
if let FnArg::Receiver(_) = last {
panic!("Function with 'vararg' attribute has only self arg")
}
let (first_args, first_uses): (String, String) = (0..first.len())
.map(|i| (format!("$arg{}:expr, ", i), format!("$arg{}, ", i)))
.unzip();
let mut need_comma: Option<TokenStream2> = Some(",".parse().unwrap());
let first_args = first_args.strip_suffix(", ").unwrap_or_else(|| {
need_comma = None;
""
});
let first_args: TokenStream2 = first_args.parse().unwrap();
let first_uses: TokenStream2 = first_uses.parse().unwrap();
let VarargConfig { macro_name, prefix } = attrs_parse(attrs, name.to_token_stream());
let result = quote! {
#func
macro_rules! #macro_name {
(#first_args) => {#name(#first_uses #prefix [])};
(#first_args #need_comma $($lasts:expr),*) => {#name(#first_uses #prefix [$($lasts),*])};
}
};
result.to_token_stream().into()
}
fn attrs_parse(attrs: TokenStream, default_name: TokenStream2) -> VarargConfig {
let attr_format_panic_msg = "vararg attribute params list must be comma-separated list of \
params in 'ident = ident' format. \n\
For example: `#[vararg(name = macro_name, type = vec)]`";
if attrs.is_empty() {
return VarargConfig {
macro_name: default_name,
prefix: "".parse().unwrap(),
};
}
let mut macro_name = default_name;
let mut prefix: TokenStream2 = "".parse().unwrap();
let v = attrs.into_iter().collect::<Vec<_>>();
for chunk in v.chunks(4) {
let p = match &chunk[0] {
TokenTree::Ident(i) => match i.to_string().as_str() {
"name" => AttrParam::Name,
"type" => AttrParam::Type,
_ => panic!("vararg attribute params currently can be 'name' or 'type' only"),
},
_ => panic!("{}", attr_format_panic_msg),
};
match &chunk[1] {
TokenTree::Punct(p) => {
if p.as_char() != '=' {
panic!("{}", attr_format_panic_msg)
}
}
_ => panic!("{}", attr_format_panic_msg),
}
match &chunk[2] {
TokenTree::Ident(i) => match (p, i.to_string().as_str()) {
(AttrParam::Name, i) => macro_name = i.parse().unwrap(),
(AttrParam::Type, "vec") => prefix = "vec!".parse().unwrap(),
(AttrParam::Type, "array") => prefix = "".parse().unwrap(),
(AttrParam::Type, "slice") => prefix = "&".parse().unwrap(),
(AttrParam::Type, _) => panic!("type param must be 'vec', 'array' or 'slice'"),
},
_ => panic!("{}", attr_format_panic_msg),
}
}
VarargConfig { macro_name, prefix }
}