Understanding Rust Error E0661: Invalid Syntax in llvm_asm Macro

Introduction to Rust Error E0661

Rust Error E0661 occurs when an invalid syntax is passed to the second argument of an llvm_asm macro line. This error is no longer emitted by the compiler, but it can still be encountered in older projects that used this functionality. It is essential to understand the cause and potential solutions to address this issue.

Erroneous Code Example

The following code example demonstrates the Rust Error E0661 when invalid syntax is provided in an llvm_asm macro:

let a;
llvm_asm!("nop" : "r"(a));

Understanding the Error

In the example above, the llvm_asm macro is used for inline assembly code. The second argument, the output constraint, should contain a valid constraint, so the error is raised due to the incorrect use of 'r'(a).

Addressing the Error

As Rust Error E0661 is no longer emitted by the compiler, it is recommended to use the newer asm macro available in Rust (already stable in nightly builds) for inline assembly code. The new asm macro has improved syntax and prevents issues associated with incorrect constraints. An equivalent code example using the asm macro would be:

let a: i32;
unsafe { asm!("nop", out("reg") a); }


The Rust Error E0661 is an obsolete error emitted when invalid syntax is passed as the second argument to an llvm_asm macro line. Since this error is no longer raised by the compiler, it is advised to use the newer asm macro for inline assembly in Rust. The above examples provide insights into the possible cause of this error and its alternative solution.