Rust error E0669: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

Note: this error code is no longer emitted by the compiler.

Cannot convert inline assembly operand to a single LLVM value.

Erroneous code example:

#![feature(llvm_asm)]

fn main() {
    unsafe {
        llvm_asm!("" :: "r"("")); // error!
    }
}

This error usually happens when trying to pass in a value to an input inline assembly operand that is actually a pair of values. In particular, this can happen when trying to pass in a slice, for instance a &str. In Rust, these values are represented internally as a pair of values, the pointer and its length. When passed as an input operand, this pair of values can not be coerced into a register and thus we must fail with an error.

From rustc --explain E0669 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.