Merge #225

225: A fix that stops panics from the old MaybeUninit impl r=korken89 a=korken89

Sometimes when running in debug mode the current implementation of `MaybeUninit` would panic, an example can be found here: 2bb89427fa/examples/rtfm-uarte-interrupts/src/main.rs (L106)
On debug this line will always panic, on release all is fine.

The issue seemed to be the manual `MaybeUninit` implementation, so I have merged the two implementations together.
Please have a look and see if my fix is unsound (it does however work, but not sure on UB).

cc @japaric @TeXitoi 

Co-authored-by: Emil Fresk <emil.fresk@gmail.com>

CHANGELOG.md

@@ -5,6 +5,11 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Fixed
+
+- Soundness issue in the `MaybeUninit` implementation
+- Warning for deprecated `mem::uninitialized`
+
 ## [v0.4.3] - 2019-04-21
 
 ### Changed

README.md

@@ -41,7 +41,7 @@ A concurrency framework for building real time systems.
 
 ## Requirements
 
-- Rust 1.31.0+
+- Rust 1.36.0+
 
 - Applications must be written using the 2018 edition.
 

book/ru/src/README_RU.md

@@ -39,7 +39,7 @@
 
 ## Требования
 
-- Rust 1.31.0+
+- Rust 1.36.0+
 
 - Программы нужно писать используя 2018 edition.
 

macros/src/codegen.rs

@@ -1948,13 +1948,13 @@ fn pre_init(ctxt: &Context, app: &App, analysis: &Analysis) -> proc_macro2::Toke
         // these are `MaybeUninit` arrays
         for task in ctxt.tasks.values() {
             let inputs = &task.inputs;
-            exprs.push(quote!(#inputs.write(core::mem::uninitialized());))
+            exprs.push(quote!(#inputs.write(core::mem::MaybeUninit::uninit());))
         }
 
         #[cfg(feature = "timer-queue")]
         for task in ctxt.tasks.values() {
             let scheduleds = &task.scheduleds;
-            exprs.push(quote!(#scheduleds.write(core::mem::uninitialized());))
+            exprs.push(quote!(#scheduleds.write(core::mem::MaybeUninit::uninit());))
         }
 
         // these are `MaybeUninit` `ReadyQueue`s

src/export.rs

@@ -1,7 +1,5 @@
 //! IMPLEMENTATION DETAILS. DO NOT USE ANYTHING IN THIS MODULE
 
-#[cfg(not(feature = "nightly"))]
-use core::ptr;
 use core::{cell::Cell, u8};
 
 #[cfg(armv7m)]
@@ -64,13 +62,11 @@ impl Priority {
     }
 }
 
-#[cfg(feature = "nightly")]
 pub struct MaybeUninit<T> {
     // we newtype so the end-user doesn't need `#![feature(maybe_uninit)]` in their code
     inner: core::mem::MaybeUninit<T>,
 }
 
-#[cfg(feature = "nightly")]
 impl<T> MaybeUninit<T> {
     pub const fn uninit() -> Self {
         MaybeUninit {
@@ -86,61 +82,23 @@ impl<T> MaybeUninit<T> {
         self.inner.as_mut_ptr()
     }
 
-    pub fn write(&mut self, value: T) -> &mut T {
-        self.inner.write(value)
-    }
-}
-
-#[cfg(not(feature = "nightly"))]
-pub struct MaybeUninit<T> {
-    value: Option<T>,
-}
-
-#[cfg(not(feature = "nightly"))]
-const MSG: &str =
-    "you have hit a bug (UB) in RTFM implementation; try enabling this crate 'nightly' feature";
-
-#[cfg(not(feature = "nightly"))]
-impl<T> MaybeUninit<T> {
-    pub const fn uninit() -> Self {
-        MaybeUninit { value: None }
-    }
-
-    pub fn as_ptr(&self) -> *const T {
-        if let Some(x) = self.value.as_ref() {
-            x
-        } else {
-            unreachable!(MSG)
-        }
-    }
-
-    pub fn as_mut_ptr(&mut self) -> *mut T {
-        if let Some(x) = self.value.as_mut() {
-            x
-        } else {
-            unreachable!(MSG)
-        }
-    }
-
     pub unsafe fn get_ref(&self) -> &T {
-        if let Some(x) = self.value.as_ref() {
-            x
-        } else {
-            unreachable!(MSG)
-        }
+        &*self.inner.as_ptr()
     }
 
     pub unsafe fn get_mut(&mut self) -> &mut T {
-        if let Some(x) = self.value.as_mut() {
-            x
-        } else {
-            unreachable!(MSG)
-        }
+        &mut *self.inner.as_mut_ptr()
     }
 
-    pub fn write(&mut self, val: T) {
-        // NOTE(volatile) we have observed UB when this uses a plain `ptr::write`
-        unsafe { ptr::write_volatile(&mut self.value, Some(val)) }
+    #[cfg(feature = "nightly")]
+    pub fn write(&mut self, value: T) -> &mut T {
+        self.inner.write(value)
+    }
+
+    #[cfg(not(feature = "nightly"))]
+    pub fn write(&mut self, value: T) -> &mut T {
+        self.inner = core::mem::MaybeUninit::new(value);
+        unsafe { self.get_mut() }
     }
 }
 

src/lib.rs

@@ -16,7 +16,7 @@
 //!
 //! # Minimum Supported Rust Version (MSRV)
 //!
-//! This crate is guaranteed to compile on stable Rust 1.31 (2018 edition) and up. It *might*
+//! This crate is guaranteed to compile on stable Rust 1.36 (2018 edition) and up. It *might*
 //! compile on older versions but that may change in any new patch release.
 //!
 //! # Semantic Versioning
@@ -41,7 +41,7 @@
 //! usage, runtime overhead and initialization overhead. This feature requires a nightly compiler
 //! and may stop working at any time!
 
-#![cfg_attr(feature = "nightly", feature(maybe_uninit))]
+#![cfg_attr(feature = "nightly", feature(maybe_uninit_extra))]
 #![deny(missing_docs)]
 #![deny(warnings)]
 #![no_std]