heap: optimize EMPTY to avoid relocations

Sized deallocation makes it pointless to provide an address that never
overlaps with pointers returned by an allocator. Code can branch on the
capacity of the allocation instead of a comparison with this sentinel.

This improves the situation in #8859, and the remaining issues are only
from the logging API, which should be disabled by default in optimized
release builds anyway along with debug assertions. The remaining issues
are part of #17081.

Closes #8859

src/liballoc/heap.rs

@@ -86,10 +86,11 @@ pub fn stats_print() {
     imp::stats_print();
 }
 
-// The compiler never calls `exchange_free` on Box<ZeroSizeType>, so zero-size
-// allocations can point to this `static`. It would be incorrect to use a null
-// pointer, due to enums assuming types like unique pointers are never null.
-pub static mut EMPTY: uint = 12345;
+/// An arbitrary non-null address to represent zero-size allocations.
+///
+/// This preserves the non-null invariant for types like `Box<T>`. The address may overlap with
+/// non-zero-size memory allocations.
+pub static EMPTY: *mut () = 0x1 as *mut ();
 
 /// The allocator for unique pointers.
 #[cfg(not(test))]
@@ -97,7 +98,7 @@ pub static mut EMPTY: uint = 12345;
 #[inline]
 unsafe fn exchange_malloc(size: uint, align: uint) -> *mut u8 {
     if size == 0 {
-        &EMPTY as *const uint as *mut u8
+        EMPTY as *mut u8
     } else {
         allocate(size, align)
     }