Use zeroed allocations in the mir interpreter instead eagerly touching the memory

compiler/rustc_middle/src/lib.rs

@@ -23,6 +23,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(allocator_api)]
 #![feature(array_windows)]
 #![feature(assert_matches)]
 #![feature(backtrace)]
@@ -33,6 +34,7 @@
 #![feature(discriminant_kind)]
 #![feature(never_type)]
 #![feature(extern_types)]
+#![feature(new_uninit)]
 #![feature(nll)]
 #![feature(once_cell)]
 #![feature(min_specialization)]

compiler/rustc_middle/src/mir/interpret/allocation.rs

@@ -131,8 +131,7 @@ impl<Tag> Allocation<Tag> {
     /// Try to create an Allocation of `size` bytes, failing if there is not enough memory
     /// available to the compiler to do so.
     pub fn uninit(size: Size, align: Align, panic_on_fail: bool) -> InterpResult<'static, Self> {
-        let mut bytes = Vec::new();
-        bytes.try_reserve(size.bytes_usize()).map_err(|_| {
+        let bytes = Box::<[u8]>::try_new_zeroed_slice(size.bytes_usize()).map_err(|_| {
             // This results in an error that can happen non-deterministically, since the memory
             // available to the compiler can change between runs. Normally queries are always
             // deterministic. However, we can be non-determinstic here because all uses of const
@@ -146,7 +145,9 @@ impl<Tag> Allocation<Tag> {
             });
             InterpError::ResourceExhaustion(ResourceExhaustionInfo::MemoryExhausted)
         })?;
-        bytes.resize(size.bytes_usize(), 0);
+        // SAFETY: This turns a Box<[MaybeUninit<u8>]> into a Vec<u8>. This is safe since the box
+        // was zero-allocated which is a valid value for u8.
+        let bytes = unsafe { bytes.assume_init().into_vec() };
         Ok(Allocation {
             bytes,
             relocations: Relocations::new(),

library/alloc/src/boxed.rs

@@ -589,6 +589,38 @@ impl<T> Box<[T]> {
     pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
         unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
     }
+
+    /// Constructs a new boxed slice with uninitialized contents, with the memory
+    /// being filled with `0` bytes. Returns an error if the allocation fails
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
+    /// let values = unsafe { values.assume_init() };
+    ///
+    /// assert_eq!(*values, [0, 0, 0]);
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    ///
+    /// [zeroed]: mem::MaybeUninit::zeroed
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[inline]
+    pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
+        unsafe {
+            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
+                Ok(l) => l,
+                Err(_) => return Err(AllocError),
+            };
+            let ptr = Global.allocate_zeroed(layout)?;
+            Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
+        }
+    }
 }
 
 impl<T, A: Allocator> Box<[T], A> {