foo.rs

use proc_macro::TokenStream;

use quote::quote;

use syn::{Data, DeriveInput, Fields, Lit, Meta, MetaNameValue, parse_macro_input};

#[proc_macro_derive(SimplePrometheus, attributes(prefix))]
pub fn simple_prometheus_derive(input: TokenStream) -> TokenStream {
    let ast = parse_macro_input!(input as DeriveInput);
    let fields = match &ast.data {
        Data::Struct(s) => s.fields.clone(),
        _ => unimplemented!(),
    };

    let prefix = get_prefix(&ast.attrs)
        .map(|prefix| {
            if !prefix.starts_with("_") {
                format!("{}_", prefix)
            } else {
                prefix
            }
        })
        .unwrap_or("".to_string());

    let name = &ast.ident;

    let field_names = match fields {
        Fields::Named(field_named) => field_named.named.into_iter().map(|f| f.ident.unwrap()),
        _ => unimplemented!(),
    };

    let write_statements = field_names.map(|field_name| {
        let prefix = format!("{}{}", prefix, field_name);
        let format_label = format!("{}{} {}", prefix, "{{server={}}}", "{}");
        let format = format!("{} {}", prefix, "{}");
        quote! {
             if let Some(ref server) = server {
         writeln!(out, #format_label, server, self.#field_name)?;
            } else {
         writeln!(out, #format, self.#field_name)?;
            }
        }
    });

    let code = quote! {
    impl simple_prometheus::SimplePrometheus for #name {
        fn to_prometheus_metrics(&self, server: Option<String>) -> Result<String, core::fmt::Error> {
        use std::fmt::Write;
        let mut out = String::new();
        #( #write_statements )*

        Ok(out)
        }
    }
    };
    //println!("{}", code.to_string());
    code.into()
}

fn get_prefix(attrs: &[syn::Attribute]) -> Option<String> {
    attrs
        .iter()
        .filter(|attr| attr.path().is_ident("simple_prometheus"))
        .filter_map(|attr| {
            if let Meta::NameValue(MetaNameValue {
                path,
                value: syn::Expr::Lit(expr_lit),
                ..
            }) = attr.meta.clone()
            {
                Some((path, expr_lit))
            } else {
                None
            }
        })
        .filter_map(|(path, attr)| {
            if path.is_ident("prefix") {
                if let Lit::Str(lit_str) = &attr.lit {
                    Some(lit_str.value())
                } else {
                    None
                }
            } else {
                None
            }
        })
        .next()
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_simple() {}
}

test.txt

[2025-10-21T10:19:43Z INFO  statsbot] prometheus metrics: users{server=lechuck.hackint.dn42} 10218
    users_memory{server=lechuck.hackint.dn42} 1226160
    users_invited{server=lechuck.hackint.dn42} 0
    users_invited_memory{server=lechuck.hackint.dn42} 0
    user_channels{server=lechuck.hackint.dn42} 30523
    user_channels_memory{server=lechuck.hackint.dn42} 732552
    users_away{server=lechuck.hackint.dn42} 496
    users_away_memory{server=lechuck.hackint.dn42} 14377
    local_client_conf_count{server=lechuck.hackint.dn42} 24
    local_client_conf_memory{server=lechuck.hackint.dn42} 576
    conf_count{server=lechuck.hackint.dn42} 0
    conf_memory{server=lechuck.hackint.dn42} 0
    classes_count{server=lechuck.hackint.dn42} 12
    classes_memory{server=lechuck.hackint.dn42} 960
    channels_count{server=lechuck.hackint.dn42} 1980
    channels_memory{server=lechuck.hackint.dn42} 813460
    channel_ban_count{server=lechuck.hackint.dn42} 826
    channel_ban_memory{server=lechuck.hackint.dn42} 66080
    channel_exceptions_count{server=lechuck.hackint.dn42} 31
    channel_exceptions_memory{server=lechuck.hackint.dn42} 2480
    channel_invex_count{server=lechuck.hackint.dn42} 553
    channel_invex_memory{server=lechuck.hackint.dn42} 44240
    channel_quiets_count{server=lechuck.hackint.dn42} 131
    channel_quiets_memory{server=lechuck.hackint.dn42} 10480
    channel_members{server=lechuck.hackint.dn42} 30523
    channel_members_memory{server=lechuck.hackint.dn42} 732552
    channel_invites_count{server=lechuck.hackint.dn42} 0
    channel_invites_memory{server=lechuck.hackint.dn42} 0
    whowas_count{server=lechuck.hackint.dn42} 15000
    whowas_memory{server=lechuck.hackint.dn42} 5756896
    hash_client_count{server=lechuck.hackint.dn42} 131072
    hash_client_memory{server=lechuck.hackint.dn42} 3145728
    hash_channel_count{server=lechuck.hackint.dn42} 65536
    hash_channel_memory{server=lechuck.hackint.dn42} 1572864
    linebuf_count{server=lechuck.hackint.dn42} 0
    linebuf_memory{server=lechuck.hackint.dn42} 0
    servers_cached_number{server=lechuck.hackint.dn42} 8
    servers_cached_memory{server=lechuck.hackint.dn42} 1152
    hostname_count{server=lechuck.hackint.dn42} 131072
    hostname_memory{server=lechuck.hackint.dn42} 3145728
    total_channel_memory{server=lechuck.hackint.dn42} 1612092
    local_client_count{server=lechuck.hackint.dn42} 0
    local_client_memory{server=lechuck.hackint.dn42} 0
    remote_client_count{server=lechuck.hackint.dn42} 0
    remote_client_memory{server=lechuck.hackint.dn42} 0
    total{server=lechuck.hackint.dn42} 7371100