#[main]Expand description
Marks an async fn main as the program entry point.
Rewrites the annotated function into a synchronous main that builds
an AsyncExecutor and blocks on the original body, preserving the
function’s attributes and visibility.
The attribute takes no arguments and the executor is constructed with
its defaults; a caller needing a configured executor should build one
directly and call block_on instead.
§Panics
The generated main panics if the executor cannot be created. That is
deliberate: it runs before any user code, so there is no state to
unwind and nothing useful to recover to.