Understanding and Fixing Rust Error E0668

Understanding Rust Error E0668

Rust Error E0668 occurs when inline assembly, written within your code, is rejected by the LLVM backend for being malformed. The error message may look like the following:

error[E0668]: malformed inline assembly
  --> src/main.rs:5:9
   |
5  |         llvm_asm!("" :"={rax"(rax));
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in the macro `$crate::llvm_asm` (in Nightly builds, run with -Z macro-backtrace for more info)

Root Cause Analysis

One of the most common reasons for Rust Error E0668 is improperly formed register constraints, such as leaving out a closing brace. The following erroneous example demonstrates this:

#![feature(llvm_asm)]

fn main() {
    let rax: u64;
    unsafe {
        llvm_asm!("" :"={rax"(rax));
        println!("Accumulator is: {}", rax);
    }
}

How to Fix Rust Error E0668

To fix Rust Error E0668, it's essential to examine the inline assembly and correct any malformations present. Most often, this will involve adding missing closing braces or adjusting the constraint layout. Here's the corrected code from the previous example:

#![feature(llvm_asm)]

fn main() {
    let rax: u64;
    unsafe {
        llvm_asm!("" :"={rax}"(rax));
        println!("Accumulator is: {}", rax);
    }
}

In this corrected version, the closing brace is added to the constraint, resolving the malformed assembly error.

Tips for Avoiding Rust Error E0668

To avoid Rust Error E0668, follow these best practices when working with inline assembly:

1. Double-check constraints: Ensure that the constraints are properly formed with correct syntax, including braces and colons.

2. Verify assembly strings: Verify that your assembly strings are well-formed, with the correct instructions and formats.

3. Test inline assembly independently: Before integrating inline assembly into your Rust code, validate the assembly using other tools to ensure it's correctly formed.