cell/

cell.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Modified work Copyright 2018-2019 Daniel Mueller (deso@posteo.net).
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::cell::Cell;
use std::cell::UnsafeCell;
use std::cmp::Ordering;
use std::fmt;
use std::fmt::Debug;
use std::fmt::Display;
use std::mem;
use std::ops::Deref;
use std::ops::DerefMut;


/// A mutable memory location with dynamically checked borrow rules
///
/// See the [module-level documentation](index.html) for more.
pub struct RefCell<T: ?Sized> {
    borrow: Cell<BorrowFlag>,
    value: UnsafeCell<T>,
}

/// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
pub struct BorrowError {
    _private: (),
}

impl Debug for BorrowError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("BorrowError").finish()
    }
}

impl Display for BorrowError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        Display::fmt("already mutably borrowed", f)
    }
}

/// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
pub struct BorrowMutError {
    _private: (),
}

impl Debug for BorrowMutError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("BorrowMutError").finish()
    }
}

impl Display for BorrowMutError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        Display::fmt("already borrowed", f)
    }
}

// Positive values represent the number of `Ref` active. Negative values
// represent the number of `RefMut` active. Multiple `RefMut`s can only be
// active at a time if they refer to distinct, nonoverlapping components of a
// `RefCell` (e.g., different ranges of a slice).
//
// `Ref` and `RefMut` are both two words in size, and so there will likely never
// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
// range. Thus, a `BorrowFlag` will probably never overflow or underflow.
// However, this is not a guarantee, as a pathological program could repeatedly
// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
// explicitly check for overflow and underflow in order to avoid unsafety, or at
// least behave correctly in the event that overflow or underflow happens (e.g.,
// see BorrowRef::new).
type BorrowFlag = isize;
const UNUSED: BorrowFlag = 0;

#[inline(always)]
fn is_writing(x: BorrowFlag) -> bool {
    x < UNUSED
}

#[inline(always)]
fn is_reading(x: BorrowFlag) -> bool {
    x > UNUSED
}

impl<T> RefCell<T> {
    /// Creates a new `RefCell` containing `value`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    /// ```
    #[inline]
    pub const fn new(value: T) -> RefCell<T> {
        RefCell {
            value: UnsafeCell::new(value),
            borrow: Cell::new(UNUSED),
        }
    }

    /// Consumes the `RefCell`, returning the wrapped value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let five = c.into_inner();
    /// ```
    #[inline]
    pub fn into_inner(self) -> T {
        // Since this function takes `self` (the `RefCell`) by value, the
        // compiler statically verifies that it is not currently borrowed.
        // Therefore the following assertion is just a `debug_assert!`.
        debug_assert!(self.borrow.get() == UNUSED);
        self.value.into_inner()
    }

    /// Replaces the wrapped value with a new one, returning the old value,
    /// without deinitializing either one.
    ///
    /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let cell = RefCell::new(5);
    /// let old_value = cell.replace(6);
    /// assert_eq!(old_value, 5);
    /// assert_eq!(cell, RefCell::new(6));
    /// ```
    #[inline]
    pub fn replace(&self, t: T) -> T {
        mem::replace(&mut *self.borrow_mut(), t)
    }

    /// Replaces the wrapped value with a new one computed from `f`, returning
    /// the old value, without deinitializing either one.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let cell = RefCell::new(5);
    /// let old_value = cell.replace_with(|&mut old| old + 1);
    /// assert_eq!(old_value, 5);
    /// assert_eq!(cell, RefCell::new(6));
    /// ```
    #[inline]
    pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
        let mut_borrow = &mut *self.borrow_mut();
        let replacement = f(mut_borrow);
        mem::replace(mut_borrow, replacement)
    }

    /// Swaps the wrapped value of `self` with the wrapped value of `other`,
    /// without deinitializing either one.
    ///
    /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
    ///
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let c = RefCell::new(5);
    /// let d = RefCell::new(6);
    /// c.swap(&d);
    /// assert_eq!(c, RefCell::new(6));
    /// assert_eq!(d, RefCell::new(5));
    /// ```
    #[inline]
    pub fn swap(&self, other: &Self) {
        mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
    }
}

impl<T: ?Sized> RefCell<T> {
    /// Immutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `Ref` exits scope. Multiple
    /// immutable borrows can be taken out at the same time.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
    /// [`try_borrow`](#method.try_borrow).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let borrowed_five = c.borrow();
    /// let borrowed_five2 = c.borrow();
    /// ```
    ///
    /// An example of panic:
    ///
    /// ```
    /// use std::cell::RefCell;
    /// use std::thread;
    ///
    /// let result = thread::spawn(move || {
    ///    let c = RefCell::new(5);
    ///    let m = c.borrow_mut();
    ///
    ///    let b = c.borrow(); // this causes a panic
    /// }).join();
    ///
    /// assert!(result.is_err());
    /// ```
    #[inline]
    pub fn borrow(&self) -> Ref<T> {
        self.try_borrow().expect("already mutably borrowed")
    }

    /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
    /// borrowed.
    ///
    /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
    /// taken out at the same time.
    ///
    /// This is the non-panicking variant of [`borrow`](#method.borrow).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow_mut();
    ///     assert!(c.try_borrow().is_err());
    /// }
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(c.try_borrow().is_ok());
    /// }
    /// ```
    #[inline]
    pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
        match BorrowRef::new(&self.borrow) {
            Some(b) => Ok(Ref {
                value: unsafe { &*self.value.get() },
                borrow: b,
            }),
            None => Err(BorrowError { _private: () }),
        }
    }

    /// Mutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
    /// from it exit scope. The value cannot be borrowed while this borrow is
    /// active.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed. For a non-panicking variant, use
    /// [`try_borrow_mut`](#method.try_borrow_mut).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// *c.borrow_mut() = 7;
    ///
    /// assert_eq!(*c.borrow(), 7);
    /// ```
    ///
    /// An example of panic:
    ///
    /// ```
    /// use std::cell::RefCell;
    /// use std::thread;
    ///
    /// let result = thread::spawn(move || {
    ///    let c = RefCell::new(5);
    ///    let m = c.borrow();
    ///
    ///    let b = c.borrow_mut(); // this causes a panic
    /// }).join();
    ///
    /// assert!(result.is_err());
    /// ```
    #[inline]
    pub fn borrow_mut(&self) -> RefMut<T> {
        self.try_borrow_mut().expect("already borrowed")
    }

    /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
    ///
    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
    /// from it exit scope. The value cannot be borrowed while this borrow is
    /// active.
    ///
    /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(c.try_borrow_mut().is_err());
    /// }
    ///
    /// assert!(c.try_borrow_mut().is_ok());
    /// ```
    #[inline]
    pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
        match BorrowRefMut::new(&self.borrow) {
            Some(b) => Ok(RefMut {
                value: unsafe { &mut *self.value.get() },
                borrow: b,
            }),
            None => Err(BorrowMutError { _private: () }),
        }
    }

    /// Returns a raw pointer to the underlying data in this cell.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let ptr = c.as_ptr();
    /// ```
    #[inline]
    pub fn as_ptr(&self) -> *mut T {
        self.value.get()
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows `RefCell` mutably (at compile-time) so there is no
    /// need for dynamic checks.
    ///
    /// However be cautious: this method expects `self` to be mutable, which is
    /// generally not the case when using a `RefCell`. Take a look at the
    /// [`borrow_mut`] method instead if `self` isn't mutable.
    ///
    /// Also, please be aware that this method is only for special circumstances and is usually
    /// not what you want. In case of doubt, use [`borrow_mut`] instead.
    ///
    /// [`borrow_mut`]: #method.borrow_mut
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let mut c = RefCell::new(5);
    /// *c.get_mut() += 1;
    ///
    /// assert_eq!(c, RefCell::new(6));
    /// ```
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        unsafe {
            &mut *self.value.get()
        }
    }
}

unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}

// Note that `RefCell` does not explicitly have a negative trait bound
// for `Sync` as negative trait bounds are only available on nightly but
// we want to be available on stable. However, `RefCell` still won't
// implement `Sync` as it contains an `std::cell::UnsafeCell` which has
// such a negative trait bound for `Sync`.

impl<T: Clone> Clone for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed.
    #[inline]
    fn clone(&self) -> RefCell<T> {
        RefCell::new(self.borrow().clone())
    }
}

impl<T:Default> Default for RefCell<T> {
    /// Creates a `RefCell<T>`, with the `Default` value for T.
    #[inline]
    fn default() -> RefCell<T> {
        RefCell::new(Default::default())
    }
}

impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn eq(&self, other: &RefCell<T>) -> bool {
        *self.borrow() == *other.borrow()
    }
}

impl<T: ?Sized + Eq> Eq for RefCell<T> {}

impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
        self.borrow().partial_cmp(&*other.borrow())
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn lt(&self, other: &RefCell<T>) -> bool {
        *self.borrow() < *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn le(&self, other: &RefCell<T>) -> bool {
        *self.borrow() <= *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn gt(&self, other: &RefCell<T>) -> bool {
        *self.borrow() > *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn ge(&self, other: &RefCell<T>) -> bool {
        *self.borrow() >= *other.borrow()
    }
}

impl<T: ?Sized + Ord> Ord for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn cmp(&self, other: &RefCell<T>) -> Ordering {
        self.borrow().cmp(&*other.borrow())
    }
}

impl<T> From<T> for RefCell<T> {
    fn from(t: T) -> RefCell<T> {
        RefCell::new(t)
    }
}

struct BorrowRef<'b> {
    borrow: &'b Cell<BorrowFlag>,
}

impl<'b> BorrowRef<'b> {
    #[inline]
    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
        let b = borrow.get();
        if is_writing(b) || b == isize::max_value() {
            // If there's currently a writing borrow, or if incrementing the
            // refcount would overflow into a writing borrow.
            None
        } else {
            borrow.set(b + 1);
            Some(BorrowRef { borrow })
        }
    }
}

impl Drop for BorrowRef<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_reading(borrow));
        self.borrow.set(borrow - 1);
    }
}

impl Clone for BorrowRef<'_> {
    #[inline]
    fn clone(&self) -> Self {
        // Since this Ref exists, we know the borrow flag
        // is a reading borrow.
        let borrow = self.borrow.get();
        debug_assert!(is_reading(borrow));
        // Prevent the borrow counter from overflowing into
        // a writing borrow.
        assert!(borrow != isize::max_value());
        self.borrow.set(borrow + 1);
        BorrowRef { borrow: self.borrow }
    }
}

/// Wraps a borrowed reference to a value in a `RefCell` box.
/// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](index.html) for more.
pub struct Ref<'b, T: ?Sized + 'b> {
    value: &'b T,
    borrow: BorrowRef<'b>,
}

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

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

impl<'b, T: ?Sized> Ref<'b, T> {
    /// Copies a `Ref`.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
    /// with the widespread use of `r.borrow().clone()` to clone the contents of
    /// a `RefCell`.
    #[inline]
    pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
        Ref {
            value: orig.value,
            borrow: orig.borrow.clone(),
        }
    }

    /// Makes a new `Ref` for a component of the borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as `Ref::map(...)`.
    /// A method would interfere with methods of the same name on the contents
    /// of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, Ref};
    ///
    /// let c = RefCell::new((5, 'b'));
    /// let b1: Ref<(u32, char)> = c.borrow();
    /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
    /// assert_eq!(*b2, 5)
    /// ```
    #[inline]
    pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
        where F: FnOnce(&T) -> &U
    {
        Ref {
            value: f(orig.value),
            borrow: orig.borrow,
        }
    }

    /// Splits a `Ref` into multiple `Ref`s for different components of the
    /// borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::map_split(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{Ref, RefCell};
    ///
    /// let cell = RefCell::new([1, 2, 3, 4]);
    /// let borrow = cell.borrow();
    /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
    /// assert_eq!(*begin, [1, 2]);
    /// assert_eq!(*end, [3, 4]);
    /// ```
    #[inline]
    pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
        where F: FnOnce(&T) -> (&U, &V)
    {
        let (a, b) = f(orig.value);
        let borrow = orig.borrow.clone();
        (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
    }

    /// Make a new `RefVal` from the borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this operation
    /// cannot fail.
    #[inline]
    pub fn map_val<U: Sized, F>(orig: Ref<'b, T>, f: F) -> RefVal<'b, U>
        where F: FnOnce(&'b T) -> U
    {
        RefVal {
            value: f(orig.value),
            borrow: orig.borrow,
        }
    }
}

impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.value.fmt(f)
    }
}

impl<'b, T: ?Sized> RefMut<'b, T> {
    /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
    /// variant.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::map(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let c = RefCell::new((5, 'b'));
    /// {
    ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
    ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
    ///     assert_eq!(*b2, 5);
    ///     *b2 = 42;
    /// }
    /// assert_eq!(*c.borrow(), (42, 'b'));
    /// ```
    #[inline]
    pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
        where F: FnOnce(&mut T) -> &mut U
    {
        // FIXME(nll-rfc#40): fix borrow-check
        let RefMut { value, borrow } = orig;
        RefMut {
            value: f(value),
            borrow,
        }
    }

    /// Splits a `RefMut` into multiple `RefMut`s for different components of the
    /// borrowed data.
    ///
    /// The underlying `RefCell` will remain mutably borrowed until both
    /// returned `RefMut`s go out of scope.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::map_split(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let cell = RefCell::new([1, 2, 3, 4]);
    /// let borrow = cell.borrow_mut();
    /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
    /// assert_eq!(*begin, [1, 2]);
    /// assert_eq!(*end, [3, 4]);
    /// begin.copy_from_slice(&[4, 3]);
    /// end.copy_from_slice(&[2, 1]);
    /// ```
    #[inline]
    pub fn map_split<U: ?Sized, V: ?Sized, F>(
        orig: RefMut<'b, T>, f: F
    ) -> (RefMut<'b, U>, RefMut<'b, V>)
        where F: FnOnce(&mut T) -> (&mut U, &mut V)
    {
        let (a, b) = f(orig.value);
        let borrow = orig.borrow.clone();
        (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
    }

    /// Make a new `RefValMut` from the borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this operation
    /// cannot fail.
    #[inline]
    pub fn map_val<U: Sized, F>(orig: RefMut<'b, T>, f: F) -> RefValMut<'b, U>
        where F: FnOnce(&'b mut T) -> U
    {
        RefValMut {
            value: f(orig.value),
            borrow: orig.borrow,
        }
    }
}

struct BorrowRefMut<'b> {
    borrow: &'b Cell<BorrowFlag>,
}

impl Drop for BorrowRefMut<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_writing(borrow));
        self.borrow.set(borrow + 1);
    }
}

impl<'b> BorrowRefMut<'b> {
    #[inline]
    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
        // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
        // mutable reference, and so there must currently be no existing
        // references. Thus, while clone increments the mutable refcount, here
        // we explicitly only allow going from UNUSED to UNUSED - 1.
        match borrow.get() {
            UNUSED => {
                borrow.set(UNUSED - 1);
                Some(BorrowRefMut { borrow })
            },
            _ => None,
        }
    }

    // Clone a `BorrowRefMut`.
    //
    // This is only valid if each `BorrowRefMut` is used to track a mutable
    // reference to a distinct, nonoverlapping range of the original object.
    // This isn't in a Clone impl so that code doesn't call this implicitly.
    #[inline]
    fn clone(&self) -> BorrowRefMut<'b> {
        let borrow = self.borrow.get();
        debug_assert!(is_writing(borrow));
        // Prevent the borrow counter from underflowing.
        assert!(borrow != isize::min_value());
        self.borrow.set(borrow - 1);
        BorrowRefMut { borrow: self.borrow }
    }
}

/// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](index.html) for more.
pub struct RefMut<'b, T: ?Sized + 'b> {
    value: &'b mut T,
    borrow: BorrowRefMut<'b>,
}

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

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

impl<T: ?Sized> DerefMut for RefMut<'_, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        self.value
    }
}

impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.value.fmt(f)
    }
}


/// A type containing a value that contains a borrowed reference to a
/// value from a `RefCell<T>`.
///
/// See the [module-level documentation](index.html) for more.
pub struct RefVal<'b, T> {
    value: T,
    borrow: BorrowRef<'b>,
}

impl<'b, T> RefVal<'b, T> {
    /// Copies a `RefVal`.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::clone(...)`.  A `Clone` implementation or a method would interfere
    /// with the widespread use of `r.borrow().clone()` to clone the contents of
    /// a `RefCell`.
    #[inline]
    pub fn clone(orig: &RefVal<'b, T>) -> RefVal<'b, T>
        where T: Clone
    {
        RefVal {
            value: orig.value.clone(),
            borrow: orig.borrow.clone(),
        }
    }

    /// Make a new `RefVal` from the another `RefVal`.
    ///
    /// The `RefCell` is already immutably borrowed, so this operation
    /// cannot fail.
    #[inline]
    pub fn map<U: Sized, F>(orig: RefVal<'b, T>, f: F) -> RefVal<'b, U>
        where F: FnOnce(T) -> U
    {
        RefVal {
            value: f(orig.value),
            borrow: orig.borrow,
        }
    }
}


impl<T> Deref for RefVal<'_, T> {
    type Target = T;

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

impl<T> DerefMut for RefVal<'_, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        &mut self.value
    }
}

impl<T: fmt::Display> fmt::Display for RefVal<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.value.fmt(f)
    }
}


/// A type containing a value that contains a borrowed mutable reference
/// to a value from a `RefCell<T>`.
///
/// See the [module-level documentation](index.html) for more.
pub struct RefValMut<'b, T> {
    value: T,
    borrow: BorrowRefMut<'b>,
}

impl<'b, T> RefValMut<'b, T> {
    /// Make a new `RefValMut` from the another `RefValMut`.
    ///
    /// The `RefCell` is already mutably borrowed, so this operation
    /// cannot fail.
    #[inline]
    pub fn map<U: Sized, F>(orig: RefValMut<'b, T>, f: F) -> RefValMut<'b, U>
        where F: FnOnce(T) -> U
    {
        RefValMut {
            value: f(orig.value),
            borrow: orig.borrow,
        }
    }
}

impl<T> Deref for RefValMut<'_, T> {
    type Target = T;

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

impl<T> DerefMut for RefValMut<'_, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        &mut self.value
    }
}

impl<T: fmt::Display> fmt::Display for RefValMut<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.value.fmt(f)
    }
}