How to pass in &str when using rust asm! inline assembly

I want this function to operate the register with the specified name, but it seems that the asm! macro does not support it?

pub fn read_cpuid(reg: &str) -> u64 {
    let val: u64;
    unsafe {
        asm!("mrs {}, {}", 
            out(reg) val,
            reg
        );
    }
    val
}

The error message is as follows：

error: expected one of `clobber_abi`, `const`, `in`, `inlateout`, `inout`, `lateout`, `options`, `out`, or `sym`, found `r`

If I change r to pass it in using reg, it still doesn’t work.

use core::arch::asm;
pub fn read_cpuid(r: &str) -> u64 {
    let val: u64;
    unsafe {
        asm!("mrs {}, {}", 
            out(reg) val,
            in(reg) r
        );
    }
    val
}

error: cannot use value of type `*const str` for inline assembly
note: only integers, floats, SIMD vectors, pointers and function pointers can be used as arguments for inline assembly

I can only use enumeration to solve it now, but the scalability of this approach is too poor. Is there a better way?

>Solution :

*const str is a "fat" pointer, meaning it stores the length of the str alongside the pointer to the beginning of the str. Therefore, it can’t fit within a single register.

Use str::as_ptr so get a *const u8 which is just a pointer.

use core::arch::asm;

pub fn read_cpuid(r: &str) -> u64 {
    let val: u64;
    unsafe {
        asm!("mrs {}, {}", 
            out(reg) val,
            in(reg) r.as_ptr()
        );
    }
    val
}

This error message is not great. I recommend opening an issue suggesting that the diagnostic be improved.