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Add examples to show when {Arc,Rc}::get_mut_unchecked is disallowed.

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This commit is contained in:
Zachary S 2022-09-02 00:47:15 -05:00
parent 96650fc714
commit 8c38cb7709
2 changed files with 64 additions and 0 deletions
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32
library/alloc/src/rc.rs
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@ -1110,6 +1110,38 @@ impl<T: ?Sized> Rc<T> {
/// }
/// assert_eq!(*x, "foo");
/// ```
/// Other `Rc` pointers to the same allocation must be to the same type.
/// ```ignore
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let x: Rc<str> = Rc::from("Hello, world!");
/// let mut y: Rc<[u8]> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type is str, not [u8]
/// Rc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
/// }
/// println!("{}", &*x); // Invliad UTF-8 in a str
/// ```
/// Other `Rc` pointers to the same allocation must be to the exact same type, including lifetimes.
/// ```ignore
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let x: Rc<&str> = Rc::new("Hello, world!");
/// {
/// let s = String::from("Oh, no!");
/// let mut y: Rc<&str> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type
/// // is &'long str, not &'short str
/// *Rc::get_mut_unchecked(&mut y) = &s;
/// }
/// }
/// println!("{}", &*x); // Use-after-free
/// ```
#[inline]
#[unstable(feature = "get_mut_unchecked", issue = "63292")]
pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {

32
library/alloc/src/sync.rs
View File

@ -1649,6 +1649,38 @@ impl<T: ?Sized> Arc<T> {
/// }
/// assert_eq!(*x, "foo");
/// ```
/// Other `Arc` pointers to the same allocation must be to the same type.
/// ```ignore
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let x: Arc<str> = Arc::from("Hello, world!");
/// let mut y: Arc<[u8]> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type is str, not [u8]
/// Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
/// }
/// println!("{}", &*x); // Invliad UTF-8 in a str
/// ```
/// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
/// ```ignore
/// #![feature(get_mut_unchecked)]
///
/// use std::sync::Arc;
///
/// let x: Arc<&str> = Arc::new("Hello, world!");
/// {
/// let s = String::from("Oh, no!");
/// let mut y: Arc<&str> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type
/// // is &'long str, not &'short str
/// *Arc::get_mut_unchecked(&mut y) = &s;
/// }
/// }
/// println!("{}", &*x); // Use-after-free
/// ```
#[inline]
#[unstable(feature = "get_mut_unchecked", issue = "63292")]
pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {