Fixed unsoundness of using &mut MaybeUninit<T> as the out type for …

.cargo/config

@@ -0,0 +1,2 @@
+[build]
+rustdocflags = ["-C", "opt-level=2"]

Cargo.toml

@@ -1,3 +1,6 @@
+[lib]
+crate-type = ["rlib", "cdylib"]
+
 [package]
 name = "uninit"
 version = "0.1.0"
@@ -18,10 +21,10 @@ license = "MIT"
 require_unsafe_in_body = "0.2.0"
 
 [features]
-chain = []
-downcast_as_ReadIntoUninit = []
 nightly = []
-polonius-check = ["nightly"]
+specialization = ["nightly"]
+const_generics = ["nightly"]
+chain = []
 
 default = []
 

README.md

@@ -51,15 +51,28 @@ Note that there are other ways to trigger this UB without explicitely using
     ```
 
       - this is exactly equivalent to calling `mem::uninitialized::<T>()`,
+        which breaks the _validity_ invariant of `T` and thus causes
+        "instant UB".
+
+        There currently only two exceptions / _valid_ use cases:
+
+          - either [`type T = [MaybeUninit<U>; N]`][`uninit_array`],
+
+          - or `T` is an inhabited ZST (this may, however, break safety
+            invariants associated with the properties of the type, causing UB
+            once such broken invariant is witnessed).
 
       - yes, using [`MaybeUninit`] is more subtle than just changing a function
         call.
 
   - ```rust
     let mut vec: Vec<u8> = Vec::with_capacity(100); // Fine
     unsafe {
-        vec.set_len(100); // UB: we have an uninitialized [u8; 100] in the heap
+        vec.set_len(100); // we have an uninitialized [u8; 100] in the heap
+        // This has broken the _safety_ invariant of `Vec`, but is not yet UB
+        // since no code has witnessed the broken state
     }
+    let heap_bytes: &[u8] = &*vec; // Witness the broken safety invariant: UB!
     ```
 
 ## Instead, (you can) use [`MaybeUninit`]
@@ -98,6 +111,7 @@ It is all about the _**delayed** initialization pattern_:
         let mut x = MaybeUninit::<i32>::uninit();
 
         x = MaybeUninit::new(42);
+        assert_eq!(42, unsafe { x.assume_init() });
         ```
 
       - or through a raw `*mut T` pointer (contrary to Rust references,
@@ -110,9 +124,24 @@ It is all about the _**delayed** initialization pattern_:
 
         unsafe {
             x.as_mut_ptr().write(42);
+            assert_eq!(x.assume_init(), 42);
         }
         ```
 
+      - or, if you use the tools of this crate, by upgrading the
+        `&mut MaybeUninit<T>` into a "`&out T`" type called
+        [`Out<T>`][`crate::prelude::Out`]:
+
+        ```rust
+        #![forbid(unsafe_code)] // no unsafe!
+        use ::core::mem::MaybeUninit;
+        use ::uninit::prelude::*;
+
+        let mut x = MaybeUninit::uninit();
+        let at_init_x: &i32 = x.as_out().write(42);
+        assert_eq!(at_init_x, &42);
+        ```
+
  3. **Type-level upgrade**
 
     Once we know, for sure, that the memory has been initialized, we can
@@ -151,9 +180,9 @@ pub trait Read {
 }
 ```
 
-that is, there is no way to `.read()` into an unitialized buffer (it would
+that is, there is no way to `.read()` into an uninitialized buffer (it would
 require an api taking either a `(*mut u8, usize)` pair, or, equivalently and
-by the way more ergonomically, a `&mut [MaybeUninit<u8>]`).
+by the way more ergonomically, a [`&out [u8]`][`crate::prelude::Out`]).
 
 # Enter `::uninit`
 
@@ -163,7 +192,7 @@ So, the objective of this crate is double:
 
     For instance:
 
-      - [`uninit_byte_array!`]
+      - [`uninit_array!`]
 
       - [`Vec::reserve_uninit`]
 
@@ -174,19 +203,14 @@ So, the objective of this crate is double:
 
       - [`ReadIntoUninit`]
 
-      - [`.init_with_copy_from_slice()`]
-
-## Status
-
-This is currently at an realy stage, so it "only" includes
-utilities to work with **uninitialized bytes** or integers.
+      - [Initialize an uninitialized buffer with `.copy_from_slice()`]
 
 [`Read`]: https://doc.rust-lang.org/1.36.0/std/io/trait.Read.html
 [`mem::uninitialized`]: https://doc.rust-lang.org/core/mem/fn.uninitialized.html
 [`MaybeUninit`]: https://doc.rust-lang.org/core/mem/union.MaybeUninit.html
-[`.assume_init_by_ref()`]: https://docs.rs/uninit/0.1.0/uninit/trait.MaybeUninitExt.html#method.assume_init_by_ref
-[`.assume_init_by_mut()`]: https://docs.rs/uninit/0.1.0/uninit/trait.MaybeUninitExt.html#method.assume_init_by_mut
-[`uninit_byte_array!`]: https://docs.rs/uninit/0.1.0/uninit/macro.uninit_byte_array.html
-[`Vec::reserve_uninit`]: https://docs.rs/uninit/0.1.0/uninit/trait.VecReserveUninit.html#tymethod.reserve_uninit
-[`.init_with_copy_from_slice()`]: https://docs.rs/uninit/0.1.0/uninit/trait.InitWithCopyFromSlice.html#tymethod.init_with_copy_from_slice
-[`ReadIntoUninit`]: https://docs.rs/uninit/0.1.0/uninit/trait.ReadIntoUninit.html
+[`.assume_init_by_ref()`]: `crate::extension_traits::MaybeUninitExt::assume_init_by_ref`
+[`.assume_init_by_mut()`]: `crate::extension_traits::MaybeUninitExt::assume_init_by_mut`
+[`uninit_array!`]: `uninit_array`
+[`Vec::reserve_uninit`]: `crate::extension_traits::VecCapacity::reserve_uninit`
+[Initialize an uninitialized buffer with `.copy_from_slice()`]: `crate::out_ref::Out::copy_from_slice`
+[`ReadIntoUninit`]: `crate::read::ReadIntoUninit`

src/extension_traits/as_out.rs

@@ -0,0 +1,173 @@
+use_prelude!();
+
+use ::core::mem::ManuallyDrop;
+
+#[cfg(doc)]
+use crate::extension_traits::ManuallyDropMut;
+
+/// Extension trait to convert a `&mut _` into a `&out ` by calling
+/// `.as_out()` on it.
+///
+/// By autoref, this means that you can even just extract a `&out T` reference
+/// out of a `mut` element simply by calling `.as_out()` on it.
+///
+/// There is, however, one restriction: to be able to call `.as_out()` on
+/// something, it needs to be either `Copy`, or a value wrapped in a
+/// [`MaybeUninit`] / a [`ManuallyDrop`].
+///
+/// This is by design. Indeed, [`Out`] references do not call the destructor
+/// of the overwritten element (since it may not be initialized).
+/// This could cause memory leaks when there is an initialized element with
+/// [drop glue][`core::mem::needs_drop`].
+///
+/// To solve this limitation, one must explicitly call
+/// [`.manually_drop_mut()`][`ManuallyDropMut::manually_drop_mut`]
+/// to automagically transmute the `&mut _` reference into a
+/// `&mut ManuallyDrop<_>`.
+///
+/// # Examples
+///
+/// ```rust
+/// use ::uninit::prelude::*;
+///
+/// let mut x = 0;
+/// x.as_out().write(42);
+///
+/// let mut y = ::core::mem::MaybeUninit::uninit();
+/// y.as_out().write(42);
+/// let y = unsafe { y.assume_init() };
+///
+/// assert_eq!(x, y);
+/// ```
+pub
+trait AsOut<Pointee : ?Sized> {
+    fn as_out<'out> (self: &'out mut Self)
+      -> Out<'out, Pointee>
+    ;
+}
+
+impl<T> AsOut<T> for MaybeUninit<T> {
+    #[inline]
+    fn as_out<'out> (self: &'out mut MaybeUninit<T>)
+      -> Out<'out, T>
+    {
+        self.into()
+    }
+}
+
+impl<T> AsOut<T> for T
+where
+    T : Copy,
+{
+    #[inline]
+    fn as_out<'out> (self: &'out mut T)
+      -> Out<'out, T>
+    {
+        self.into()
+    }
+}
+
+impl<T> AsOut<[T]> for [MaybeUninit<T>] {
+    #[inline]
+    fn as_out<'out> (self: &'out mut [MaybeUninit<T>])
+      -> Out<'out, [T]>
+    {
+        self.into()
+    }
+}
+
+impl<T> AsOut<[T]> for [T]
+where
+    T : Copy,
+{
+    #[inline]
+    fn as_out<'out> (self: &'out mut [T])
+      -> Out<'out, [T]>
+    {
+        self.into()
+    }
+}
+
+impl<T> AsOut<T> for ManuallyDrop<T> {
+    #[inline]
+    fn as_out<'out> (self: &'out mut ManuallyDrop<T>)
+      -> Out<'out, T>
+    {
+        self.into()
+    }
+}
+
+impl<T> AsOut<[T]> for [ManuallyDrop<T>] {
+    #[inline]
+    fn as_out<'out> (self: &'out mut [ManuallyDrop<T>])
+      -> Out<'out, [T]>
+    {
+        self.into()
+    }
+}
+
+#[cfg(not(feature = "const_generics"))]
+const _: () = {
+    macro_rules! impl_arrays {( $($N:tt)* ) => ($(
+        impl<T> AsOut<[T]> for [MaybeUninit<T>; $N] {
+            #[inline]
+            fn as_out<'out> (self: &'out mut Self)
+              -> Out<'out, [T]>
+            {
+                From::from(&mut self[..])
+            }
+        }
+        impl<T> AsOut<[T]> for [T; $N]
+        where
+            T : Copy,
+        {
+            #[inline]
+            fn as_out<'out> (self: &'out mut Self)
+              -> Out<'out, [T]>
+            {
+                From::from(&mut self[..])
+            }
+        }
+    )*)}
+
+    impl_arrays! {
+           0  1  2  3  4  5  6  7
+           8  9 10 11 12 13 14 15
+          16 17 18 19 20 21 22 23
+          24 25 26 27 28 29 30 31
+          32 33 34 35 36 37 38 39
+          40 41 42 43 44 45 46 47
+          48 49 50 51 52 53 54 55
+          56 57 58 59 60 61 62 63
+          64
+         128
+         256
+         512
+        1024
+    }
+};
+
+#[cfg(feature = "const_generics")]
+const _: () = {
+    #[doc(cfg(feature = "const_generics"))]
+    impl<T, const N: usize> AsOut<[T]> for [MaybeUninit<T>; N] {
+        #[inline]
+        fn as_out<'out> (self: &'out mut Self)
+          -> Out<'out, [T]>
+        {
+            From::from(&mut self[..])
+        }
+    }
+    #[doc(cfg(feature = "const_generics"))]
+    impl<T, const N: usize> AsOut<[T]> for [T; N]
+    where
+        T : Copy,
+    {
+        #[inline]
+        fn as_out<'out> (self: &'out mut Self)
+          -> Out<'out, [T]>
+        {
+            From::from(&mut self[..])
+        }
+    }
+};