rust/src/librustc_error_codes/error_codes/E0535.md

An unknown argument was given to the inline attribute.

Erroneous code example:

#[inline(unknown)] // error: invalid argument
pub fn something() {}

fn main() {}

The inline attribute only supports two arguments:

• always
• never

All other arguments given to the inline attribute will return this error. Example:

#[inline(never)] // ok!
pub fn something() {}

fn main() {}

For more information about the inline attribute, https: read://doc.rust-lang.org/reference.html#inline-attributes