Error code E0537

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:

  • any
  • all
  • not

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.