Macro code variables shadow the retryable function parameters

[haxorcize]

I was working on this function:

[retry(DEFAULT_BACKOFF_CONFIG, logged_retry_on_error)]
async fn extract_soap_date_range(
    ctx: Arc<Mutex<StreamRunContext>>,
    soap_object_type: Arc<String>,
    properties: Arc<Vec<String>>,
    token: Arc<String>,
    subdomain: Arc<String>,
    start: DateTime<Utc>,
    end: DateTime<Utc>,
) -> Result<(

When I ran into issues with my start variable not being the type I expected.

Thankfully I am familiar with the macro code, and there are 3 variables defined by the #block is inserted into the macro:

let start = tokio::time::Instant::now();
let backoff_max = #config.backoff_max.unwrap_or(std::time::Duration::MAX);
let mut attempt = 0;

So what happens is that start from the macro is overriding my start parameter.

My solution was simple - to prefix these 3 variables with __ so the chances you'll ever have a function parameter called __start would be very low. If you want to be extra safe you might want to prefix even further, because if you do have the macro shadowing your function parameter - the compilation error is superrr weird and most folks wouldn't understand why it's happening, so if we can avoid it even further it's better.

[Brendan-Blanchard]

This works great! Since the macro only has three variables, we could parse the inner function parameters and raise a clear error if any of them match the __* ones we're using, but I think this is fine for now. Like you said, the risk of collision is fairly low, and someone prefixing with __* is likely to have an idea what's going on under the hood, much like you. Thanks for contributing back!