Error code E0536

The not cfg-predicate was malformed.

Erroneous code example:

pub fn main() {
    if cfg!(not()) { }
}

The not predicate expects one cfg-pattern. Example:

pub fn main() {
    if cfg!(not(target_os = "linux")) { } // ok!
}

For more information about the cfg macro, read the section on Conditional Compilation in the Reference.