rust/compiler/rustc_error_codes/src/error_codes/E0661.md

An invalid syntax was passed to the second argument of an llvm_asm macro line.

Erroneous code example:

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

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