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.