Rollup merge of #72431 - RalfJung:ub-warning, r=shepmaster

add warning sign to UB examples

Just to make it less likely that people miss the fact that these are examples for how to *not* do it.

src/libcore/mem/maybe_uninit.rs

@@ -20,9 +20,9 @@ use crate::mem::ManuallyDrop;
 /// # #![allow(invalid_value)]
 /// use std::mem::{self, MaybeUninit};
 ///
-/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior!
+/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! ⚠️
 /// // The equivalent code with `MaybeUninit<&i32>`:
-/// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior!
+/// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! ⚠️
 /// ```
 ///
 /// This is exploited by the compiler for various optimizations, such as eliding
@@ -35,9 +35,9 @@ use crate::mem::ManuallyDrop;
 /// # #![allow(invalid_value)]
 /// use std::mem::{self, MaybeUninit};
 ///
-/// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior!
+/// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
 /// // The equivalent code with `MaybeUninit<bool>`:
-/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
+/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
 /// ```
 ///
 /// Moreover, uninitialized memory is special in that the compiler knows that
@@ -49,9 +49,9 @@ use crate::mem::ManuallyDrop;
 /// # #![allow(invalid_value)]
 /// use std::mem::{self, MaybeUninit};
 ///
-/// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior!
+/// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
 /// // The equivalent code with `MaybeUninit<i32>`:
-/// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
+/// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
 /// ```
 /// (Notice that the rules around uninitialized integers are not finalized yet, but
 /// until they are, it is advisable to avoid them.)
@@ -348,7 +348,7 @@ impl<T> MaybeUninit<T> {
     /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
     /// let x = unsafe { x.assume_init() };
     /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
-    /// // This is undefined behavior.
+    /// // This is undefined behavior. ⚠️
     /// ```
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
     #[inline]
@@ -400,7 +400,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// let x = MaybeUninit::<Vec<u32>>::uninit();
     /// let x_vec = unsafe { &*x.as_ptr() };
-    /// // We have created a reference to an uninitialized vector! This is undefined behavior.
+    /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
     /// ```
     ///
     /// (Notice that the rules around references to uninitialized data are not finalized yet, but
@@ -437,7 +437,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
     /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
-    /// // We have created a reference to an uninitialized vector! This is undefined behavior.
+    /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
     /// ```
     ///
     /// (Notice that the rules around references to uninitialized data are not finalized yet, but
@@ -489,7 +489,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// let x = MaybeUninit::<Vec<u32>>::uninit();
     /// let x_init = unsafe { x.assume_init() };
-    /// // `x` had not been initialized yet, so this last line caused undefined behavior.
+    /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
     /// ```
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
     #[inline(always)]
@@ -553,7 +553,7 @@ impl<T> MaybeUninit<T> {
     /// x.write(Some(vec![0,1,2]));
     /// let x1 = unsafe { x.read() };
     /// let x2 = unsafe { x.read() };
-    /// // We now created two copies of the same vector, leading to a double-free when
+    /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
     /// // they both get dropped!
     /// ```
     #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
@@ -603,7 +603,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// let x = MaybeUninit::<Vec<u32>>::uninit();
     /// let x_vec: &Vec<u32> = unsafe { x.get_ref() };
-    /// // We have created a reference to an uninitialized vector! This is undefined behavior.
+    /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
     /// ```
     ///
     /// ```rust,no_run
@@ -686,7 +686,7 @@ impl<T> MaybeUninit<T> {
     /// unsafe {
     ///     *b.get_mut() = true;
     ///     // We have created a (mutable) reference to an uninitialized `bool`!
-    ///     // This is undefined behavior.
+    ///     // This is undefined behavior. ⚠️
     /// }
     /// ```
     ///