use std::env;
use std::fs::File;
use std::io::Write;
use std::iter;
use std::path::Path;

/*
#[doc(hidden)]
#[macro_export]
macro_rules! count {
    () => { proc_macro_call_0!() };
    (!) => { proc_macro_call_1!() };
    (!!) => { proc_macro_call_2!() };
    ...
}
*/

fn main() {
    let out_dir = env::var("OUT_DIR").unwrap();
    let dest_path = Path::new(&out_dir).join("count.rs");
    let mut f = File::create(&dest_path).unwrap();

    let mut content = String::new();
    content += "
        #[doc(hidden)]
        #[macro_export]
        macro_rules! count {
    ";
    for i in 0..=64 {
        let bangs = iter::repeat("!").take(i).collect::<String>();
        content += &format!("({}) => {{ proc_macro_call_{}!() }};", bangs, i);
    }
    content += "
            ($(!)+) => {
                compile_error!(\"this macro does not support >64 nested macro invocations\")
            };
        }
    ";

    f.write_all(content.as_bytes()).unwrap();
}

use std::env;
use std::fs::File;
use std::io::Write;
use std::iter;
use std::path::Path;

/*
#[doc(hidden)]
#[macro_export]
macro_rules! count {
    () => { proc_macro_call_0!() };
    (!) => { proc_macro_call_1!() };
    (!!) => { proc_macro_call_2!() };
    ...
}
*/

fn main() {
    let out_dir = env::var("OUT_DIR").unwrap();
    let dest_path = Path::new(&out_dir).join("count.rs");
    let mut f = File::create(&dest_path).unwrap();

    let mut content = String::new();
    content += "
        #[doc(hidden)]
        #[macro_export]
        macro_rules! count {
    ";
    for i in 0..=64 {
        let bangs = iter::repeat("!").take(i).collect::<String>();
        content += &format!("({}) => {{ proc_macro_call_{}!() }};", bangs, i);
    }
    content += "
        }
    ";

    f.write_all(content.as_bytes()).unwrap();
}

--- 
+++ 
@@ -31,6 +31,9 @@
         content += &format!("({}) => {{ proc_macro_call_{}!() }};", bangs, i);
     }
     content += "
+            ($(!)+) => {
+                compile_error!(\"this macro does not support >64 nested macro invocations\")
+            };
         }
     ";
 
Add error message for nested macros above the limit

    error: this macro does not support >64 nested macro invocations
      --> src/main.rs:13:9
       |
    13 | /         select! {
    14 | |             _ = next_msg => {
    15 | |                 println!("1");
    16 | |                 println!("2");
    ...  |
    80 | |             }
    81 | |         }
       | |_________^
