Rust error E0537: #### Note: this error code is no longer emitted by the compiler An unknown predicate was used inside the `cfg` attribute

The compiler team's explanation

Note: this error code is no longer emitted by the compiler

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.

From rustc --explain E0537 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.