The unwind attribute was malformed.

Erroneous code example:

#![feature(unwind_attributes)]

#[unwind()] // error: expected one argument
pub extern fn something() {}

fn main() {}

The #[unwind] attribute should be used as follows:

• #[unwind(aborts)] -- specifies that if a non-Rust ABI function should abort the process if it attempts to unwind. This is the safer and preferred option.

• #[unwind(allowed)] -- specifies that a non-Rust ABI function should be allowed to unwind. This can easily result in Undefined Behavior (UB), so be careful.

NB. The default behavior here is “allowed”, but this is unspecified and likely to change in the future.