关于我的类似RefCell的结构上的rust：borrow_mut()不起作用

borrow_mut() on my RefCell-like structure doesn't work

我尝试编写自己的类似于RefCell的可变内存位置，但没有运行时借用检查(无开销)。我采用了RefCell(和Ref和RefMut)的代码体系结构。我可以毫无问题地调用.borrow()，但是如果我调用.borrow_mut()，那么rust编译器会显示cannot borrow as mutable。我没有看到问题，我的.borrow_mut() impl看起来不错吗？

失败的代码：

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let real_refcell= Rc::from(RefCell::from(MyStruct::new()));
let nooverhead_refcell = Rc::from(NORefCell::from(MyStruct::new()));

// works
let refmut_refcell = real_refcell.borrow_mut();

// cannot borrow as mutable
let refmut_norefcell = nooverhead_refcell.borrow_mut();

norc.rs(无开销RefCell)

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use crate::norc_ref::{NORefMut, NORef};
use std::cell::UnsafeCell;
use std::borrow::Borrow;

#[derive(Debug)]
pub struct NORefCell<T: ?Sized> {
value: UnsafeCell< T >
}

impl< T > NORefCell< T > {

pub fn from(t: T) -> NORefCell< T > {
NORefCell {
value: UnsafeCell::from(t)
}
}

pub fn borrow(&self) -> NORef<'_, T> {
NORef {
value: unsafe { &*self.value.get() }
}
}

pub fn borrow_mut(&mut self) -> NORefMut<'_, T> {
NORefMut {
value: unsafe { &mut *self.value.get() }
}
}

}

norc_ref.rs(NORefCell.borrow[_mut]()返回的数据结构

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use std::ops::{Deref, DerefMut};

#[derive(Debug)]
pub struct NORef<'b, T: ?Sized + 'b> {
pub value: &'b T,
}

impl<T: ?Sized> Deref for NORef<'_, T> {
type Target = T;

#[inline]
fn deref(&self) -> &T {
self.value
}
}

/// No Overhead Ref Cell: Mutable Reference
#[derive(Debug)]
pub struct NORefMut<'b, T: ?Sized + 'b> {
pub value: &'b mut T,
}

impl<T: ?Sized> Deref for NORefMut<'_, T> {
type Target = T;

#[inline]
fn deref(&self) -> &T {
self.value
}
}

impl<T: ?Sized> DerefMut for NORefMut<'_, T> {

#[inline]
fn deref_mut(&mut self) -> &mut T {
self.value
}
}