Understanding Rust Error E0662

Introduction to Rust Error E0662

Rust Error E0662 occurs when an invalid input operand constraint is passed to the llvm_asm macro. Although this error code is no longer emitted by the compiler, it's essential to understand the context behind this error.

Error E0662 Example

The erroneous code example below demonstrates how Rust Error E0662 could have been triggered:

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

Background on LLVM Assembly and Rust

Low-Level Virtual Machine (LLVM) is a compiler infrastructure that provides a collection of libraries and tools for various programming languages, including Rust. The llvm_asm macro allows Rust developers to include inline assembly code to customize and optimize their programs. However, when an invalid constraint is given, the Rust compiler would have emitted Error E0662.

Understanding Input Operand Constraints

Input operand constraints are used to specify the inputs required for inline assembly and how they should be used. These constraints should follow specific rules and conventions. When constraints are incorrect or mismatched, errors like E0662 used to occur.

Fixing Error E0662

To fix Rust Error E0662, ensure the input operand constraints are compatible and follow the rules and conventions for inline assembly. It is essential to check the official LLVM documentation and follow the respective conventions for the target architecture.


Although Rust Error E0662 is no longer emitted by the compiler, understanding the context behind this error and its associations with LLVM assembly and input constraints is beneficial. Keep this knowledge in mind when using inline assembly in Rust to avoid potential pitfalls.