rust/src/librustc_error_codes/error_codes/E0663.md

An invalid input operand constraint was passed to the llvm_asm macro (third line).

Erroneous code example:

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

Considering that this would be a long explanation, we instead recommend you take a look at the [asm] chapter of the Unstable book: