Understanding Rust Error E0778: Malformed instruction_set Attribute

Introduction to Rust Error E0778

Rust Error E0778 occurs when the instruction_set attribute is improperly formed. In Rust, the instruction_set attribute is utilized to specify the instruction set for a particular function on a target architecture, such as ARM or x86. This error indicates that the attribute is missing the required parameters or has otherwise incorrect syntax.

Erroneous code example

#![feature(isa_attribute)]

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

In the example above, the instruction_set attribute is provided without the required parameter. This will result in Rust Error E0778.

Correcting Rust Error E0778

To fix Rust Error E0778, you need to specify the required parameter for the instruction_set attribute. The parameter should be formatted as follows: arm::a32 or arm::t32, depending on the target architecture.

Corrected code example 1: Using arm::a32

#![allow(unused)]
#![feature(isa_attribute)]

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

In this corrected example, the instruction_set attribute is specified with the arm::a32 parameter for the ARM target architecture.

Corrected code example 2: Using arm::t32

#![allow(unused)]
#![feature(isa_attribute)]

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

In this second corrected example, the instruction_set attribute is specified with the arm::t32 parameter instead of arm::a32.

Additional Considerations

Keep in mind that the instruction_set attribute is only available when the isa_attribute feature flag is enabled, as it is an unstable feature in Rust. Be cautious when using this attribute, as misconfiguration can lead to unexpected results or undefined behavior.

Recommended Reading

• instruction_set attribute in Rust Reference