These two attributes are used to change the entry point into a Rust program, but
for now they're being put behind feature gates until we have a chance to think
about them a little more. The #[start] attribute specifically may have its
signature changed.
This is a breaking change to due the usage of these attributes generating errors
by default now. If your crate is using these attributes, add this to your crate
root:
#![feature(start)] // if you're using the #[start] attribute
#![feature(main)] // if you're using the #[main] attribute
cc #20064
cannot use an `as` expression to coerce, so I used a one-off function
instead (this is a no-op in stage0, but in stage1+ it triggers
coercion from the fn pointer to the fn item type).
