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

This is because it was too specific to the optimize attribute. Similar diagnostics occur for other attributes too. The example here will now emit E0539

The optimize attribute was malformed.

Erroneous code example:

#![feature(optimize_attribute)]

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

The #[optimize] attribute should be used as follows:

• #[optimize(size)] -- instructs the optimization pipeline to generate code that's smaller rather than faster

• #[optimize(speed)] -- instructs the optimization pipeline to generate code that's faster rather than smaller

For example:

#![feature(optimize_attribute)]

#[optimize(size)]
pub fn something() {}

See RFC 2412 for more details.