turso/macros/src/lib.rs

extern crate proc_macro;
use proc_macro::{token_stream::IntoIter, Group, TokenStream, TokenTree};
use std::collections::HashMap;

/// A procedural macro that derives a `Description` trait for enums.
/// This macro extracts documentation comments (specified with `/// Description...`) for enum variants
/// and generates an implementation for `get_description`, which returns the associated description.
#[proc_macro_derive(Description, attributes(desc))]
pub fn derive_description_from_doc(item: TokenStream) -> TokenStream {
    // Convert the TokenStream into an iterator of TokenTree
    let mut tokens = item.into_iter();

    let mut enum_name = String::new();

    // Vector to store enum variants and their associated payloads (if any)
    let mut enum_variants: Vec<(String, Option<String>)> = Vec::<(String, Option<String>)>::new();

    // HashMap to store descriptions associated with each enum variant
    let mut variant_description_map: HashMap<String, String> = HashMap::new();

    // Parses the token stream to extract the enum name and its variants
    while let Some(token) = tokens.next() {
        match token {
            TokenTree::Ident(ident) if ident.to_string() == "enum" => {
                // Get the enum name
                if let Some(TokenTree::Ident(name)) = tokens.next() {
                    enum_name = name.to_string();
                }
            }
            TokenTree::Group(group) => {
                let mut group_tokens_iter: IntoIter = group.stream().into_iter();

                let mut last_seen_desc: Option<String> = None;
                while let Some(token) = group_tokens_iter.next() {
                    match token {
                        TokenTree::Punct(punct) => {
                            if punct.to_string() == "#" {
                                last_seen_desc = process_description(&mut group_tokens_iter);
                            }
                        }
                        TokenTree::Ident(ident) => {
                            // Capture the enum variant name and associate it with its description
                            let ident_str = ident.to_string();
                            if let Some(desc) = &last_seen_desc {
                                variant_description_map.insert(ident_str.clone(), desc.clone());
                            }
                            enum_variants.push((ident_str, None));
                            last_seen_desc = None;
                        }
                        TokenTree::Group(group) => {
                            // Capture payload information for the current enum variant
                            if let Some(last_variant) = enum_variants.last_mut() {
                                last_variant.1 = Some(process_payload(group));
                            }
                        }
                        _ => {}
                    }
                }
            }
            _ => {}
        }
    }
    generate_get_description(enum_name, &variant_description_map, enum_variants)
}

/// Processes a Rust docs to extract the description string.
fn process_description(token_iter: &mut IntoIter) -> Option<String> {
    if let Some(doc_token_tree) = token_iter.next() {
        if let TokenTree::Group(doc_group) = doc_token_tree {
            let mut doc_group_iter = doc_group.stream().into_iter();
            // Skip the `desc` and `(` tokens to reach the actual description
            doc_group_iter.next();
            doc_group_iter.next();
            if let Some(TokenTree::Literal(description)) = doc_group_iter.next() {
                return Some(description.to_string());
            }
        }
    }
    None
}

/// Processes the payload of an enum variant to extract variable names (ignoring types).
fn process_payload(payload_group: Group) -> String {
    let mut payload_group_iter = payload_group.stream().into_iter();
    let mut variable_name_list = String::from("");
    let mut is_variable_name = true;
    while let Some(token) = payload_group_iter.next() {
        match token {
            TokenTree::Ident(ident) => {
                if is_variable_name {
                    variable_name_list.push_str(&format!("{},", ident.to_string()));
                }
                is_variable_name = false;
            }
            TokenTree::Punct(punct) => {
                if punct.to_string() == "," {
                    is_variable_name = true;
                }
            }
            _ => {}
        }
    }
    format!("{{ {} }}", variable_name_list).to_string()
}
/// Generates the `get_description` implementation for the processed enum.
fn generate_get_description(
    enum_name: String,
    variant_description_map: &HashMap<String, String>,
    enum_variants: Vec<(String, Option<String>)>,
) -> TokenStream {
    let mut all_enum_arms = String::from("");
    for (variant, payload) in enum_variants {
        let payload = payload.unwrap_or("".to_string());
        let desc;
        if let Some(description) = variant_description_map.get(&variant) {
            desc = format!("Some({})", description);
        } else {
            desc = "None".to_string();
        }
        all_enum_arms.push_str(&format!(
            "{}::{} {} => {},\n",
            enum_name, variant, payload, desc
        ));
    }

    let enum_impl = format!(
        "impl {}  {{ 
     pub fn get_description(&self) -> Option<&str> {{
     match self {{
     {}
     }}
     }}
     }}",
        enum_name, all_enum_arms
    );
    enum_impl.parse().unwrap()
}