Rollup merge of rust-lang#42959 - SimonSapin:nonzero-checked, r=sfackler

Make the "main" constructors of NonZero/Shared/Unique return Option

Per discussion in rust-lang#27730 (comment).

This is a breaking change to unstable APIs.

The old behavior is still available under the name `new_unchecked`. Note that only that one can be `const fn`, since `if` is currently not allowed in constant contexts.

In the case of `NonZero` this requires adding a new `is_zero` method to the `Zeroable` trait. I mildly dislike this, but it’s not much worse than having a `Zeroable` trait in the first place. `Zeroable` and `NonZero` are both unstable, this can be reworked later.

Author: Mark-Simulacrum

src/doc/nomicon

@@ -892,7 +892,7 @@ pub unsafe trait Alloc {
     {
         let k = Layout::new::<T>();
         if k.size() > 0 {
-            unsafe { self.alloc(k).map(|p| Unique::new(p as *mut T)) }
+            unsafe { self.alloc(k).map(|p| Unique::new_unchecked(p as *mut T)) }
         } else {
             Err(AllocErr::invalid_input("zero-sized type invalid for alloc_one"))
         }
@@ -963,7 +963,7 @@ pub unsafe trait Alloc {
                 unsafe {
                     self.alloc(layout.clone())
                         .map(|p| {
-                            Unique::new(p as *mut T)
+                            Unique::new_unchecked(p as *mut T)
                         })
                 }
             }
@@ -1012,7 +1012,7 @@ pub unsafe trait Alloc {
         match (Layout::array::<T>(n_old), Layout::array::<T>(n_new), ptr.as_ptr()) {
             (Some(ref k_old), Some(ref k_new), ptr) if k_old.size() > 0 && k_new.size() > 0 => {
                 self.realloc(ptr as *mut u8, k_old.clone(), k_new.clone())
-                    .map(|p|Unique::new(p as *mut T))
+                    .map(|p|Unique::new_unchecked(p as *mut T))
             }
             _ => {
                 Err(AllocErr::invalid_input("invalid layout for realloc_array"))

src/liballoc/allocator.rs

@@ -280,7 +280,7 @@ impl<T> Arc<T> {
             weak: atomic::AtomicUsize::new(1),
             data: data,
         };
-        Arc { ptr: unsafe { Shared::new(Box::into_raw(x)) } }
+        Arc { ptr: Shared::from(Box::into_unique(x)) }
     }
 
     /// Returns the contained value, if the `Arc` has exactly one strong reference.
@@ -382,7 +382,7 @@ impl<T> Arc<T> {
         // `data` field from the pointer.
         let ptr = (ptr as *const u8).offset(-offset_of!(ArcInner<T>, data));
         Arc {
-            ptr: Shared::new(ptr as *mut u8 as *mut _),
+            ptr: Shared::new_unchecked(ptr as *mut u8 as *mut _),
         }
     }
 }
@@ -842,7 +842,7 @@ impl<T> Weak<T> {
     pub fn new() -> Weak<T> {
         unsafe {
             Weak {
-                ptr: Shared::new(Box::into_raw(box ArcInner {
+                ptr: Shared::from(Box::into_unique(box ArcInner {
                     strong: atomic::AtomicUsize::new(0),
                     weak: atomic::AtomicUsize::new(1),
                     data: uninitialized(),

src/liballoc/arc.rs

@@ -297,6 +297,37 @@ impl<T: ?Sized> Box<T> {
     pub fn into_raw(b: Box<T>) -> *mut T {
         unsafe { mem::transmute(b) }
     }
+
+    /// Consumes the `Box`, returning the wrapped pointer as `Unique<T>`.
+    ///
+    /// After calling this function, the caller is responsible for the
+    /// memory previously managed by the `Box`. In particular, the
+    /// caller should properly destroy `T` and release the memory. The
+    /// proper way to do so is to convert the raw pointer back into a
+    /// `Box` with the [`Box::from_raw`] function.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Box::into_unique(b)` instead of `b.into_unique()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    ///
+    /// [`Box::from_raw`]: struct.Box.html#method.from_raw
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(unique)]
+    ///
+    /// fn main() {
+    ///     let x = Box::new(5);
+    ///     let ptr = Box::into_unique(x);
+    /// }
+    /// ```
+    #[unstable(feature = "unique", reason = "needs an RFC to flesh out design",
+               issue = "27730")]
+    #[inline]
+    pub fn into_unique(b: Box<T>) -> Unique<T> {
+        unsafe { mem::transmute(b) }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/liballoc/boxed.rs

@@ -140,24 +140,22 @@ struct BoxedNode<K, V> {
 
 impl<K, V> BoxedNode<K, V> {
     fn from_leaf(node: Box<LeafNode<K, V>>) -> Self {
-        unsafe {
-            BoxedNode { ptr: Unique::new(Box::into_raw(node)) }
-        }
+        BoxedNode { ptr: Box::into_unique(node) }
     }
 
     fn from_internal(node: Box<InternalNode<K, V>>) -> Self {
         unsafe {
-            BoxedNode { ptr: Unique::new(Box::into_raw(node) as *mut LeafNode<K, V>) }
+            BoxedNode { ptr: Unique::new_unchecked(Box::into_raw(node) as *mut LeafNode<K, V>) }
         }
     }
 
     unsafe fn from_ptr(ptr: NonZero<*const LeafNode<K, V>>) -> Self {
-        BoxedNode { ptr: Unique::new(ptr.get() as *mut LeafNode<K, V>) }
+        BoxedNode { ptr: Unique::new_unchecked(ptr.get() as *mut LeafNode<K, V>) }
     }
 
     fn as_ptr(&self) -> NonZero<*const LeafNode<K, V>> {
         unsafe {
-            NonZero::new(self.ptr.as_ptr())
+            NonZero::from(self.ptr.as_ref())
         }
     }
 }
@@ -384,21 +382,19 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
         >,
         Self
     > {
-        if self.as_leaf().parent.is_null() {
-            Err(self)
-        } else {
+        if let Some(non_zero) = NonZero::new(self.as_leaf().parent as *const LeafNode<K, V>) {
             Ok(Handle {
                 node: NodeRef {
                     height: self.height + 1,
-                    node: unsafe {
-                        NonZero::new(self.as_leaf().parent as *mut LeafNode<K, V>)
-                    },
+                    node: non_zero,
                     root: self.root,
                     _marker: PhantomData
                 },
                 idx: self.as_leaf().parent_idx as usize,
                 _marker: PhantomData
             })
+        } else {
+            Err(self)
         }
     }
 

src/liballoc/btree/node.rs

@@ -157,7 +157,7 @@ impl<T> LinkedList<T> {
         unsafe {
             node.next = self.head;
             node.prev = None;
-            let node = Some(Shared::new(Box::into_raw(node)));
+            let node = Some(Shared::from(Box::into_unique(node)));
 
             match self.head {
                 None => self.tail = node,
@@ -192,7 +192,7 @@ impl<T> LinkedList<T> {
         unsafe {
             node.next = None;
             node.prev = self.tail;
-            let node = Some(Shared::new(Box::into_raw(node)));
+            let node = Some(Shared::from(Box::into_unique(node)));
 
             match self.tail {
                 None => self.head = node,
@@ -921,7 +921,7 @@ impl<'a, T> IterMut<'a, T> {
                     Some(prev) => prev,
                 };
 
-                let node = Some(Shared::new(Box::into_raw(box Node {
+                let node = Some(Shared::from(Box::into_unique(box Node {
                     next: Some(head),
                     prev: Some(prev),
                     element: element,

src/liballoc/linked_list.rs

@@ -104,7 +104,7 @@ impl<T, A: Alloc> RawVec<T, A> {
             };
 
             RawVec {
-                ptr: Unique::new(ptr as *mut _),
+                ptr: Unique::new_unchecked(ptr as *mut _),
                 cap: cap,
                 a: a,
             }
@@ -159,7 +159,7 @@ impl<T, A: Alloc> RawVec<T, A> {
     /// If the ptr and capacity come from a RawVec created via `a`, then this is guaranteed.
     pub unsafe fn from_raw_parts_in(ptr: *mut T, cap: usize, a: A) -> Self {
         RawVec {
-            ptr: Unique::new(ptr),
+            ptr: Unique::new_unchecked(ptr),
             cap: cap,
             a: a,
         }
@@ -176,7 +176,7 @@ impl<T> RawVec<T, Heap> {
     /// If the ptr and capacity come from a RawVec, then this is guaranteed.
     pub unsafe fn from_raw_parts(ptr: *mut T, cap: usize) -> Self {
         RawVec {
-            ptr: Unique::new(ptr),
+            ptr: Unique::new_unchecked(ptr),
             cap: cap,
             a: Heap,
         }

src/liballoc/raw_vec.rs

@@ -303,18 +303,16 @@ impl<T> Rc<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new(value: T) -> Rc<T> {
-        unsafe {
-            Rc {
-                // there is an implicit weak pointer owned by all the strong
-                // pointers, which ensures that the weak destructor never frees
-                // the allocation while the strong destructor is running, even
-                // if the weak pointer is stored inside the strong one.
-                ptr: Shared::new(Box::into_raw(box RcBox {
-                    strong: Cell::new(1),
-                    weak: Cell::new(1),
-                    value: value,
-                })),
-            }
+        Rc {
+            // there is an implicit weak pointer owned by all the strong
+            // pointers, which ensures that the weak destructor never frees
+            // the allocation while the strong destructor is running, even
+            // if the weak pointer is stored inside the strong one.
+            ptr: Shared::from(Box::into_unique(box RcBox {
+                strong: Cell::new(1),
+                weak: Cell::new(1),
+                value: value,
+            })),
         }
     }
 
@@ -418,7 +416,7 @@ impl<T> Rc<T> {
 
         let ptr = (ptr as *const u8).offset(-offset_of!(RcBox<T>, value));
         Rc {
-            ptr: Shared::new(ptr as *mut u8 as *mut _)
+            ptr: Shared::new_unchecked(ptr as *mut u8 as *mut _)
         }
     }
 }
@@ -443,7 +441,7 @@ impl Rc<str> {
             // Combine the allocation address and the string length into a fat pointer to `RcBox`.
             let rcbox_ptr: *mut RcBox<str> = mem::transmute([ptr as usize, value.len()]);
             assert!(aligned_len * size_of::<usize>() == size_of_val(&*rcbox_ptr));
-            Rc { ptr: Shared::new(rcbox_ptr) }
+            Rc { ptr: Shared::new_unchecked(rcbox_ptr) }
         }
     }
 }
@@ -476,7 +474,7 @@ impl<T> Rc<[T]> {
             // Free the original allocation without freeing its (moved) contents.
             box_free(Box::into_raw(value));
 
-            Rc { ptr: Shared::new(ptr as *mut _) }
+            Rc { ptr: Shared::new_unchecked(ptr as *mut _) }
         }
     }
 }
@@ -1016,7 +1014,7 @@ impl<T> Weak<T> {
     pub fn new() -> Weak<T> {
         unsafe {
             Weak {
-                ptr: Shared::new(Box::into_raw(box RcBox {
+                ptr: Shared::from(Box::into_unique(box RcBox {
                     strong: Cell::new(0),
                     weak: Cell::new(1),
                     value: uninitialized(),

src/liballoc/rc.rs

@@ -1126,7 +1126,7 @@ impl<T> Vec<T> {
                 tail_start: end,
                 tail_len: len - end,
                 iter: range_slice.iter(),
-                vec: Shared::new(self as *mut _),
+                vec: Shared::from(self),
             }
         }
     }
@@ -1727,7 +1727,7 @@ impl<T> IntoIterator for Vec<T> {
             let cap = self.buf.cap();
             mem::forget(self);
             IntoIter {
-                buf: Shared::new(begin),
+                buf: Shared::new_unchecked(begin),
                 cap: cap,
                 ptr: begin,
                 end: end,

src/liballoc/vec.rs

@@ -893,7 +893,7 @@ impl<T> VecDeque<T> {
         self.head = drain_tail;
 
         Drain {
-            deque: unsafe { Shared::new(self as *mut _) },
+            deque: Shared::from(&mut *self),
             after_tail: drain_head,
             after_head: head,
             iter: Iter {