macro-string

This crate is a helper library for procedural macros to perform eager evaluation of standard library string macros like concat! and env! in macro input.

For example, to implement a macro such as the following:

// Parses JSON at compile time and expands to a serde_json::Value.
let j = include_json!(concat!(env!("CARGO_MANIFEST_DIR"), "/manifest.json"));

the implementation of include_json! will need to parse and eagerly evaluate the two macro calls within its input tokens.

use macro_string::MacroString;
use proc_macro::TokenStream;
use std::fs;
use syn::parse_macro_input;

#[proc_macro]
pub fn include_json(input: TokenStream) -> TokenStream {
    let macro_string = parse_macro_input!(input as MacroString);
    let path = match macro_string.eval() {
        Ok(path) => path,
        Err(err) => return TokenStream::from(err.to_compile_error()),
    };

    let content = match fs::read(&path) {
        Ok(content) => content,
        Err(err) => return TokenStream::from(macro_string.error(err).to_compile_error()),
    };

    let json: serde_json::Value = match serde_json::from_slice(&content) {
        Ok(json) => json,
        Err(err) => return TokenStream::from(macro_string.error(err).to_compile_error()),
    };

    /*TODO: print serde_json::Value to TokenStream*/
}

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