Parses (a very specific subset) of syn::ConstExpr for use with procedural macros

syn_evaluate_binary_sum.rs

fn evaluate_bin_sum(expr: &syn::ConstExpr) -> u64 {
    match expr {
        // TODO: Fix the pattern (first match is left == literal, right == binary op, second match should be both are literals)
        &syn::ConstExpr::Binary(syn::BinOp::Add, ref left, ref right) => {
            extract_int_value(left) + evaluate_bin_sum(right)
        },
        &syn::ConstExpr::Binary(syn::BinOp::Add, ref left, ref right) => {
            extract_int_value(left) + extract_int_value(right)
        },
        _ => {
            panic!("Expected a binary expression such as 0 + 1 + 1 + 1")
        }
    }
}

fn extract_int_value(lit: &syn::ConstExpr) -> u64 {
    match lit {
        &syn::ConstExpr::Lit(syn::Lit::Int(val, _)) => {
            val
        },
        _ => panic!("Expected a literal")
    }
}

I had written this when I thought I needed to parse a 0 + 1 + 1 ... const expr (which was generated by a macro...) in a proc macro, but ended up not needing this... should take 60 seconds to fix the matching bug!