Understanding Rust Error E0537: Unknown Predicate in cfg Attribute

Introduction to Rust Error E0537

Rust Error E0537 occurs when an unknown predicate is used inside the cfg attribute. The cfg attribute is designed for conditional compilation and is used to conditionally include or exclude code blocks based on certain settings or system configurations.

Causes of Error E0537

Error E0537 is triggered by the usage of an invalid predicate inside the cfg attribute. The official Rust documentation provides an example of this error:

#[cfg(unknown())] // error: invalid predicate `unknown`
pub fn something() {}

pub fn main() {}

In this example, the `unknown()` predicate is not supported by the cfg attribute, causing Rust Error E0537.

Supported Predicates in cfg Attribute

The cfg attribute supports only three kinds of predicates: any, all, and not. Below are examples of correct usage for each of the supported predicates:

any:

#[cfg(any(target_os = "windows", target_os = "macos"))]
pub fn something() {}

pub fn main() {}

This example will include the `something` function when compiling for either Windows or macOS.

all:

#[cfg(all(target_os = "linux", target_arch = "x86"))]
pub fn something() {}

pub fn main() {}

This example will include the `something` function only when compiling for Linux on an x86 architecture.

not:

#[cfg(not(target_os = "linux"))]
pub fn something() {}

pub fn main() {}

This example will include the `something` function when compiling for any operating system except Linux.

Fixing Rust Error E0537

To fix Rust Error E0537, ensure the predicate used in the cfg attribute is one of the three supported predicates: any, all, or not. Check the predicate syntax and replace it with a valid one if necessary.

For more information about the cfg attribute and conditional compilation in Rust, refer to the Rust Reference on Conditional Compilation.

Recommended Reading

• Conditional Compilation in The Rust Reference