Error code E0778

The instruction_set attribute was malformed.

Erroneous code example:

#![feature(isa_attribute)]

#[instruction_set()] // error: expected one argument
pub fn something() {}
fn main() {}

The parenthesized instruction_set attribute requires the parameter to be specified:

#![feature(isa_attribute)]

#[cfg_attr(target_arch="arm", instruction_set(arm::a32))]
fn something() {}

or:

#![feature(isa_attribute)]

#[cfg_attr(target_arch="arm", instruction_set(arm::t32))]
fn something() {}

For more information see the instruction_set attribute section of the Reference.