An unknown predicate was used inside the cfg
attribute.
Erroneous code example:
#[cfg(unknown())] // error: invalid predicate `unknown` pub fn something() {} pub fn main() {}
The cfg
attribute supports only three kinds of predicates:
Example:
#[cfg(not(target_os = "linux"))] // ok! pub fn something() {} pub fn main() {}
For more information about the cfg
attribute, read the section on Conditional Compilation in the Reference.