src/librustc_error_codes/error_codes/E0561.md

   1 A non-ident or non-wildcard pattern has been used as a parameter of a function
   2 pointer type.
   3
   4 Erroneous code example:
   5
   6 ```compile_fail,E0561
   7 type A1 = fn(mut param: u8); // error!
   8 type A2 = fn(&param: u32); // error!
   9 ```
  10
  11 When using an alias over a function type, you cannot e.g. denote a parameter as
  12 being mutable.
  13
  14 To fix the issue, remove patterns (`_` is allowed though). Example:
  15
  16 ```
  17 type A1 = fn(param: u8); // ok!
  18 type A2 = fn(_: u32); // ok!
  19 ```
  20
  21 You can also omit the parameter name:
  22
  23 ```
  24 type A3 = fn(i16); // ok!
  25 ```