Macro __internal_declare_basic_blocks 

#[doc(hidden)]
pub macro __internal_declare_basic_blocks {
    () => { ... },
    ($name:ident (cleanup) $($rest:tt)*) => { ... },
    ($name:ident $($rest:tt)*) => { ... },
}

🔬This is a nightly-only experimental API. (custom_mir)

Helper macro that declares the basic blocks.