Allow for better optimizations of iterators for zero-sized types

src/libcollections/btree/node.rs

@@ -19,6 +19,8 @@ pub use self::TraversalItem::*;
 use core::prelude::*;
 
 use core::cmp::Ordering::{Greater, Less, Equal};
+#[cfg(not(stage0))]
+use core::intrinsics::arith_offset;
 use core::iter::Zip;
 use core::marker::PhantomData;
 use core::ops::{Deref, DerefMut, Index, IndexMut};
@@ -205,6 +207,7 @@ impl<T> RawItems<T> {
         RawItems::from_parts(slice.as_ptr(), slice.len())
     }
 
+    #[cfg(stage0)]
     unsafe fn from_parts(ptr: *const T, len: usize) -> RawItems<T> {
         if mem::size_of::<T>() == 0 {
             RawItems {
@@ -219,6 +222,22 @@ impl<T> RawItems<T> {
         }
     }
 
+    #[cfg(not(stage0))]
+    unsafe fn from_parts(ptr: *const T, len: usize) -> RawItems<T> {
+        if mem::size_of::<T>() == 0 {
+            RawItems {
+                head: ptr,
+                tail: arith_offset(ptr as *const i8, len as isize) as *const T,
+            }
+        } else {
+            RawItems {
+                head: ptr,
+                tail: ptr.offset(len as isize),
+            }
+        }
+    }
+
+    #[cfg(stage0)]
     unsafe fn push(&mut self, val: T) {
         ptr::write(self.tail as *mut T, val);
 
@@ -228,11 +247,23 @@ impl<T> RawItems<T> {
             self.tail = self.tail.offset(1);
         }
     }
+
+    #[cfg(not(stage0))]
+    unsafe fn push(&mut self, val: T) {
+        ptr::write(self.tail as *mut T, val);
+
+        if mem::size_of::<T>() == 0 {
+            self.tail = arith_offset(self.tail as *const i8, 1) as *const T;
+        } else {
+            self.tail = self.tail.offset(1);
+        }
+    }
 }
 
 impl<T> Iterator for RawItems<T> {
     type Item = T;
 
+    #[cfg(stage0)]
     fn next(&mut self) -> Option<T> {
         if self.head == self.tail {
             None
@@ -250,9 +281,29 @@ impl<T> Iterator for RawItems<T> {
             }
         }
     }
+
+    #[cfg(not(stage0))]
+    fn next(&mut self) -> Option<T> {
+        if self.head == self.tail {
+            None
+        } else {
+            unsafe {
+                let ret = Some(ptr::read(self.head));
+
+                if mem::size_of::<T>() == 0 {
+                    self.head = arith_offset(self.head as *const i8, 1) as *const T;
+                } else {
+                    self.head = self.head.offset(1);
+                }
+
+                ret
+            }
+        }
+    }
 }
 
 impl<T> DoubleEndedIterator for RawItems<T> {
+    #[cfg(stage0)]
     fn next_back(&mut self) -> Option<T> {
         if self.head == self.tail {
             None
@@ -268,6 +319,23 @@ impl<T> DoubleEndedIterator for RawItems<T> {
             }
         }
     }
+
+    #[cfg(not(stage0))]
+    fn next_back(&mut self) -> Option<T> {
+        if self.head == self.tail {
+            None
+        } else {
+            unsafe {
+                if mem::size_of::<T>() == 0 {
+                    self.tail = arith_offset(self.tail as *const i8, -1) as *const T;
+                } else {
+                    self.tail = self.tail.offset(-1);
+                }
+
+                Some(ptr::read(self.tail))
+            }
+        }
+    }
 }
 
 impl<T> Drop for RawItems<T> {

src/libcollections/vec.rs

@@ -66,6 +66,8 @@ use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{self, Hash};
 use core::intrinsics::assume;
+#[cfg(not(stage0))]
+use core::intrinsics::arith_offset;
 use core::iter::{repeat, FromIterator};
 use core::marker::PhantomData;
 use core::mem;
@@ -1527,6 +1529,7 @@ impl<T> IntoIterator for Vec<T> {
     /// }
     /// ```
     #[inline]
+    #[cfg(stage0)]
     fn into_iter(self) -> IntoIter<T> {
         unsafe {
             let ptr = *self.ptr;
@@ -1542,6 +1545,24 @@ impl<T> IntoIterator for Vec<T> {
             IntoIter { allocation: ptr, cap: cap, ptr: begin, end: end }
         }
     }
+
+    #[inline]
+    #[cfg(not(stage0))]
+    fn into_iter(self) -> IntoIter<T> {
+        unsafe {
+            let ptr = *self.ptr;
+            assume(!ptr.is_null());
+            let cap = self.cap;
+            let begin = ptr as *const T;
+            let end = if mem::size_of::<T>() == 0 {
+                arith_offset(ptr as *const i8, self.len() as isize) as *const T
+            } else {
+                ptr.offset(self.len() as isize) as *const T
+            };
+            mem::forget(self);
+            IntoIter { allocation: ptr, cap: cap, ptr: begin, end: end }
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1746,6 +1767,7 @@ impl<T> Iterator for IntoIter<T> {
     type Item = T;
 
     #[inline]
+    #[cfg(stage0)]
     fn next(&mut self) -> Option<T> {
         unsafe {
             if self.ptr == self.end {
@@ -1769,6 +1791,31 @@ impl<T> Iterator for IntoIter<T> {
         }
     }
 
+    #[inline]
+    #[cfg(not(stage0))]
+    fn next(&mut self) -> Option<T> {
+        unsafe {
+            if self.ptr == self.end {
+                None
+            } else {
+                if mem::size_of::<T>() == 0 {
+                    // purposefully don't use 'ptr.offset' because for
+                    // vectors with 0-size elements this would return the
+                    // same pointer.
+                    self.ptr = arith_offset(self.ptr as *const i8, 1) as *const T;
+
+                    // Use a non-null pointer value
+                    Some(ptr::read(EMPTY as *mut T))
+                } else {
+                    let old = self.ptr;
+                    self.ptr = self.ptr.offset(1);
+
+                    Some(ptr::read(old))
+                }
+            }
+        }
+    }
+
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let diff = (self.end as usize) - (self.ptr as usize);
@@ -1786,6 +1833,7 @@ impl<T> Iterator for IntoIter<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> DoubleEndedIterator for IntoIter<T> {
     #[inline]
+    #[cfg(stage0)]
     fn next_back(&mut self) -> Option<T> {
         unsafe {
             if self.end == self.ptr {
@@ -1805,6 +1853,28 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
             }
         }
     }
+
+    #[inline]
+    #[cfg(not(stage0))]
+    fn next_back(&mut self) -> Option<T> {
+        unsafe {
+            if self.end == self.ptr {
+                None
+            } else {
+                if mem::size_of::<T>() == 0 {
+                    // See above for why 'ptr.offset' isn't used
+                    self.end = arith_offset(self.end as *const i8, -1) as *const T;
+
+                    // Use a non-null pointer value
+                    Some(ptr::read(EMPTY as *mut T))
+                } else {
+                    self.end = self.end.offset(-1);
+
+                    Some(ptr::read(mem::transmute(self.end)))
+                }
+            }
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/intrinsics.rs

@@ -283,6 +283,20 @@ extern "rust-intrinsic" {
     /// returned value will result in undefined behavior.
     pub fn offset<T>(dst: *const T, offset: isize) -> *const T;
 
+    /// Calculates the offset from a pointer, potentially wrapping.
+    ///
+    /// This is implemented as an intrinsic to avoid converting to and from an
+    /// integer, since the conversion inhibits certain optimizations.
+    ///
+    /// # Safety
+    ///
+    /// Unlike the `offset` intrinsic, this intrinsic does not restrict the
+    /// resulting pointer to point into or one byte past the end of an allocated
+    /// object, and it wraps with two's complement arithmetic. The resulting
+    /// value is not necessarily valid to be used to actually access memory.
+    #[cfg(not(stage0))]
+    pub fn arith_offset<T>(dst: *const T, offset: isize) -> *const T;
+
     /// Copies `count * size_of<T>` bytes from `src` to `dst`. The source
     /// and destination may *not* overlap.
     ///