Auto merge of #54924 - RalfJung:use-maybe-uninit2, r=cramertj

Use MaybeUninit in liballoc

All code by @japaric. This is a re-submission of a part of #53508 that hopefully does not regress performance.

src/etc/gdb_rust_pretty_printing.py

@@ -322,8 +322,11 @@ def to_string(self):
  def children(self):
  (length, data_ptr) = \
  rustpp.extract_length_and_ptr_from_std_btreeset(self.__val)
- val = GdbValue(data_ptr.get_wrapped_value().dereference()).get_child_at_index(3)
- gdb_ptr = val.get_wrapped_value()
+ leaf_node = GdbValue(data_ptr.get_wrapped_value().dereference())
+ maybe_uninit_keys = leaf_node.get_child_at_index(3)
+ manually_drop_keys = maybe_uninit_keys.get_child_at_index(1)
+ keys = manually_drop_keys.get_child_at_index(0)
+ gdb_ptr = keys.get_wrapped_value()
  for index in xrange(length):
  yield (str(index), gdb_ptr[index])
 
@@ -345,9 +348,14 @@ def to_string(self):
  def children(self):
  (length, data_ptr) = \
  rustpp.extract_length_and_ptr_from_std_btreemap(self.__val)
- keys = GdbValue(data_ptr.get_wrapped_value().dereference()).get_child_at_index(3)
+ leaf_node = GdbValue(data_ptr.get_wrapped_value().dereference())
+ maybe_uninit_keys = leaf_node.get_child_at_index(3)
+ manually_drop_keys = maybe_uninit_keys.get_child_at_index(1)
+ keys = manually_drop_keys.get_child_at_index(0)
  keys_ptr = keys.get_wrapped_value()
- vals = GdbValue(data_ptr.get_wrapped_value().dereference()).get_child_at_index(4)
+ maybe_uninit_vals = leaf_node.get_child_at_index(4)
+ manually_drop_vals = maybe_uninit_vals.get_child_at_index(1)
+ vals = manually_drop_vals.get_child_at_index(0)
  vals_ptr = vals.get_wrapped_value()
  for index in xrange(length):
  yield (str(index), keys_ptr[index])

src/liballoc/collections/btree/node.rs

@@ -42,7 +42,7 @@
 // This implies that even an empty internal node has at least one edge.
 
 use core::marker::PhantomData;
-use core::mem;
+use core::mem::{self, MaybeUninit};
 use core::ptr::{self, Unique, NonNull};
 use core::slice;
 
@@ -58,9 +58,6 @@ pub const CAPACITY: usize = 2 * B - 1;
 /// these should always be put behind pointers, and specifically behind `BoxedNode` in the owned
 /// case.
 ///
-/// See also rust-lang/rfcs#197, which would make this structure significantly more safe by
-/// avoiding accidentally dropping unused and uninitialized keys and values.
-///
 /// We put the metadata first so that its position is the same for every `K` and `V`, in order
 /// to statically allocate a single dummy node to avoid allocations. This struct is `repr(C)` to
 /// prevent them from being reordered.
@@ -73,7 +70,7 @@ struct LeafNode<K, V> {
  /// This node's index into the parent node's `edges` array.
  /// `*node.parent.edges[node.parent_idx]` should be the same thing as `node`.
  /// This is only guaranteed to be initialized when `parent` is nonnull.
- parent_idx: u16,
+ parent_idx: MaybeUninit<u16>,
 
  /// The number of keys and values this node stores.
  ///
@@ -83,8 +80,8 @@ struct LeafNode<K, V> {
 
  /// The arrays storing the actual data of the node. Only the first `len` elements of each
  /// array are initialized and valid.
- keys: [K; CAPACITY],
- vals: [V; CAPACITY],
+ keys: MaybeUninit<[K; CAPACITY]>,
+ vals: MaybeUninit<[V; CAPACITY]>,
 }
 
 impl<K, V> LeafNode<K, V> {
@@ -94,10 +91,10 @@ impl<K, V> LeafNode<K, V> {
  LeafNode {
  // As a general policy, we leave fields uninitialized if they can be, as this should
  // be both slightly faster and easier to track in Valgrind.
- keys: mem::uninitialized(),
- vals: mem::uninitialized(),
+ keys: MaybeUninit::uninitialized(),
+ vals: MaybeUninit::uninitialized(),
  parent: ptr::null(),
- parent_idx: mem::uninitialized(),
+ parent_idx: MaybeUninit::uninitialized(),
  len: 0
  }
  }
@@ -115,10 +112,10 @@ unsafe impl Sync for LeafNode<(), ()> {}
 // ever take a pointer past the first key.
 static EMPTY_ROOT_NODE: LeafNode<(), ()> = LeafNode {
  parent: ptr::null(),
- parent_idx: 0,
+ parent_idx: MaybeUninit::uninitialized(),
  len: 0,
- keys: [(); CAPACITY],
- vals: [(); CAPACITY],
+ keys: MaybeUninit::uninitialized(),
+ vals: MaybeUninit::uninitialized(),
 };
 
 /// The underlying representation of internal nodes. As with `LeafNode`s, these should be hidden
@@ -430,7 +427,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
  root: self.root,
  _marker: PhantomData
  },
- idx: self.as_leaf().parent_idx as usize,
+ idx: unsafe { usize::from(*self.as_leaf().parent_idx.get_ref()) },
  _marker: PhantomData
  })
  } else {
@@ -567,7 +564,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
  // the node, which is allowed by LLVM.
  unsafe {
  slice::from_raw_parts(
- self.as_leaf().keys.as_ptr(),
+ self.as_leaf().keys.as_ptr() as *const K,
  self.len()
  )
  }
@@ -578,7 +575,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
  debug_assert!(!self.is_shared_root());
  unsafe {
  slice::from_raw_parts(
- self.as_leaf().vals.as_ptr(),
+ self.as_leaf().vals.as_ptr() as *const V,
  self.len()
  )
  }
@@ -605,7 +602,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
  } else {
  unsafe {
  slice::from_raw_parts_mut(
- &mut self.as_leaf_mut().keys as *mut [K] as *mut K,
+ self.as_leaf_mut().keys.get_mut() as *mut [K] as *mut K,
  self.len()
  )
  }
@@ -616,7 +613,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
  debug_assert!(!self.is_shared_root());
  unsafe {
  slice::from_raw_parts_mut(
- &mut self.as_leaf_mut().vals as *mut [V] as *mut V,
+ self.as_leaf_mut().vals.get_mut() as *mut [V] as *mut V,
  self.len()
  )
  }
@@ -1013,7 +1010,7 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
  let ptr = self.node.as_internal_mut() as *mut _;
  let mut child = self.descend();
  child.as_leaf_mut().parent = ptr;
- child.as_leaf_mut().parent_idx = idx;
+ child.as_leaf_mut().parent_idx.set(idx);
  }
 
  /// Unsafely asserts to the compiler some static information about whether the underlying
@@ -1152,12 +1149,12 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV>
 
  ptr::copy_nonoverlapping(
  self.node.keys().as_ptr().add(self.idx + 1),
- new_node.keys.as_mut_ptr(),
+ new_node.keys.as_mut_ptr() as *mut K,
  new_len
  );
  ptr::copy_nonoverlapping(
  self.node.vals().as_ptr().add(self.idx + 1),
- new_node.vals.as_mut_ptr(),
+ new_node.vals.as_mut_ptr() as *mut V,
  new_len
  );
 
@@ -1210,12 +1207,12 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
 
  ptr::copy_nonoverlapping(
  self.node.keys().as_ptr().add(self.idx + 1),
- new_node.data.keys.as_mut_ptr(),
+ new_node.data.keys.as_mut_ptr() as *mut K,
  new_len
  );
  ptr::copy_nonoverlapping(
  self.node.vals().as_ptr().add(self.idx + 1),
- new_node.data.vals.as_mut_ptr(),
+ new_node.data.vals.as_mut_ptr() as *mut V,
  new_len
  );
  ptr::copy_nonoverlapping(

src/liballoc/lib.rs

@@ -119,6 +119,7 @@
 #![feature(rustc_const_unstable)]
 #![feature(const_vec_new)]
 #![feature(slice_partition_dedup)]
+#![feature(maybe_uninit)]
 
 // Allow testing this library