Auto merge of #27488 - Gankro:uninit-docs, r=bluss

Inspired by https://github.com/rust-lang/rust/issues/27484

src/libcore/mem.rs

@@ -253,21 +253,89 @@ pub unsafe fn dropped<T>() -> T {
     dropped_impl()
 }
 
-/// Creates an uninitialized value.
+/// Bypasses Rust's normal memory-initialization checks by pretending to
+/// produce a value of type T, while doing nothing at all.
 ///
-/// Care must be taken when using this function, if the type `T` has a destructor and the value
+/// **This is incredibly dangerous, and should not be done lightly. Deeply
-/// falls out of scope (due to unwinding or returning) before being initialized, then the
+/// consider initializing your memory with a default value instead.**
-/// destructor will run on uninitialized data, likely leading to crashes.
 ///
-/// This is useful for FFI functions sometimes, but should generally be avoided.
+/// This is useful for FFI functions and initializing arrays sometimes,
+/// but should generally be avoided.
+///
+/// # Undefined Behaviour
+///
+/// It is Undefined Behaviour to read uninitialized memory. Even just an
+/// uninitialized boolean. For instance, if you branch on the value of such
+/// a boolean your program may take one, both, or neither of the branches.
+///
+/// Note that this often also includes *writing* to the uninitialized value.
+/// Rust believes the value is initialized, and will therefore try to Drop
+/// the uninitialized value and its fields if you try to overwrite the memory
+/// in a normal manner. The only way to safely initialize an arbitrary
+/// uninitialized value is with one of the `ptr` functions: `write`, `copy`, or
+/// `copy_nonoverlapping`. This isn't necessary if `T` is a primitive
+/// or otherwise only contains types that don't implement Drop.
+///
+/// If this value *does* need some kind of Drop, it must be initialized before
+/// it goes out of scope (and therefore would be dropped). Note that this
+/// includes a `panic` occurring and unwinding the stack suddenly.
 ///
 /// # Examples
 ///
+/// Here's how to safely initialize an array of `Vec`s.
+///
 /// ```
 /// use std::mem;
+/// use std::ptr;
 ///
-/// let x: i32 = unsafe { mem::uninitialized() };
+/// // Only declare the array. This safely leaves it
+/// // uninitialized in a way that Rust will track for us.
+/// // However we can't initialize it element-by-element
+/// // safely, and we can't use the `[value; 1000]`
+/// // constructor because it only works with `Copy` data.
+/// let mut data: [Vec<u32>; 1000];
+///
+/// unsafe {
+///     // So we need to do this to initialize it.
+///     data = mem::uninitialized();
+///
+///     // DANGER ZONE: if anything panics or otherwise
+///     // incorrectly reads the array here, we will have
+///     // Undefined Behaviour.
+///
+///     // It's ok to mutably iterate the data, since this
+///     // doesn't involve reading it at all.
+///     // (ptr and len are statically known for arrays)
+///     for elem in &mut data[..] {
+///         // *elem = Vec::new() would try to drop the
+///         // uninitialized memory at `elem` -- bad!
+///         //
+///         // Vec::new doesn't allocate or do really
+///         // anything. It's only safe to call here
+///         // because we know it won't panic.
+///         ptr::write(elem, Vec::new());
+///     }
+///
+///     // SAFE ZONE: everything is initialized.
+/// }
+///
+/// println!("{:?}", &data[0]);
 /// ```
+///
+/// Hopefully this example emphasizes to you exactly how delicate
+/// and dangerous doing this is. Note that the `vec!` macro
+/// *does* let you initialize every element with a value that
+/// is only `Clone`, so the following is equivalent and vastly
+/// less dangerous, as long as you can live with an extra heap
+/// allocation:
+///
+/// ```
+/// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
+/// println!("{:?}", &data[0]);
+/// ```
+///
+/// For large arrays this is probably advisable
+/// anyway to avoid blowing the stack.
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn uninitialized<T>() -> T {