Understanding Rust Error E0663

Introduction to Rust Error E0663

Rust Error E0663 occurs when an invalid input operand constraint is passed to the llvm_asm macro. Although the documentation states that the compiler no longer emits this error code, it's important to understand the code's root cause in order to be prepared and knowledgeable for any possible future instances.

Examining the Erroneous Code Example

The erroneous code example provided in the official Rust documentation for Error E0663 is as follows:

llvm_asm!("xor %eax, %eax"
          : "+test"("a")

In this code, an invalid input operand constraint "+test" is passed to the llvm_asm macro.

llvm_asm Syntax and Constraints

The llvm_asm macro in Rust allows inline assembly code to be inserted in your Rust program. The syntax for llvm_asm macro is:

llvm_asm!("assembly code": "outputs"("constraints") : "inputs"("constraints"), additionalOptions);

Assembly instructions and constraints are part of the macro's parameters. Constraints are used to specify the relationship between the assembly code and operands. The input and output constraints play an essential role in ensuring correct data flow between Rust code and assembly code.

Understanding Input and Output Constraints

Constraints are classified into two types: input constraints and output constraints. Input constraints define the variables used by the assembly code, while output constraints define the variables that receive the results of assembly operations. In the erroneous code example, the input constraint is the problem (+test).

Fixing the Error

To fix the Rust Error E0663, it's crucial to correct the input constraint by either removing it or modifying it to a valid type. Here's an example of correcting the input constraint using the llvm_asm macro:

llvm_asm!("xor %eax, %eax"
          : "r"("a")
          : {"eax"}

In this example, the invalid input constraint "+test" has been replaced with a valid one - "r". The "r" constraint indicates that an input register should be allocated, and the variable "a" should be made available in the register.


Rust Error E0663 is related to the use of an invalid input operand constraint in the llvm_asm macro. Although this error code is no longer emitted by the compiler, understanding its cause helps developers identify and correct similar issues. By ensuring proper use of input and output constraints in the llvm_asm macro, developers can prevent similar errors in the future.

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