Macro tool_fn_mut 

macro_rules! tool_fn_mut {
    () => { ... };
}

This is a hack that must be given as the final argument of McpServerBuilder::tool_fn when defining tools. Look away, lest ye be blinded by its vileness!

Fine, if you MUST know, it’s a horrific workaround for not having return-type notation and for this !@$#!%! bug. Trust me, the need for it hurts me more than it hurts you. –nikomatsakis