Error code E0561

A non-ident or non-wildcard pattern has been used as a parameter of a function pointer type.

Erroneous code example:

#![allow(unused)] fn main() { type A1 = fn(mut param: u8); // error! type A2 = fn(&param: u32); // error! }

When using an alias over a function type, you cannot e.g. denote a parameter as being mutable.

To fix the issue, remove patterns (_ is allowed though). Example:

#![allow(unused)] fn main() { type A1 = fn(param: u8); // ok! type A2 = fn(_: u32); // ok! }

You can also omit the parameter name:

#![allow(unused)] fn main() { type A3 = fn(i16); // ok! }