代码如下所示，RefInner有两个mut引用Inner和Inner.a，使用From特征代码没有错误，但cp()将显示cannot borrow内部as mutable more than once at a time。错误详细信息为：

error[E0499]: cannot borrow `inner` as mutable more than once at a time
  --> src/main.rs:40:12
   |
37 |       let b: RefInner = RefInner {
   |  _______________________-
38 | |         // b: a.geta().into(), //no error
39 | |         b: cp(inner.geta()),
   | |               ------------ first mutable borrow occurs here
40 | |         a: &mut inner,
   | |            ^^^^^^^^^^ second mutable borrow occurs here
41 | |     };
   | |_____- first borrow later used here

For more information about this error, try `rustc --explain E0499`.
warning: `hello` (bin "hello") generated 1 warning
error: could not compile `hello` due to previous error; 1 warning emitted

#[derive(Debug)]
pub struct Inner {
    pub a: u64,
    pub b: u64,
}

pub struct RefInner<'a> {
    pub a: &'a mut Inner,
    pub b: &'a mut ResponseContext,
}

impl Inner {
    fn geta(&mut self) -> &mut u64 {
        &mut self.a
    }
}

#[repr(C)]
#[derive(Debug)]
pub struct ResponseContext {
    seq_id: u8,
    _ignore: [u8; 7],
}

impl From<&mut u64> for &mut ResponseContext {
    fn from(value: &mut u64) -> Self {
        unsafe { std::mem::transmute(value) }
    }
}

fn cp(n: &mut u64) -> &mut ResponseContext {
    unsafe { std::mem::transmute(n) }
}

fn main() {
    let mut inner: Inner = Inner { a: 1, b: 2 };
    let b: RefInner = RefInner {
        // b: a.geta().into(), //no error
        b: cp(inner.geta()), //cannot borrow `inner` as mutable more than once at a time
        a: &mut inner,
    };
}

• • 有人能解释其中的区别吗**