auto merge of #13290 : alexcrichton/rust/rollup, r=alexcrichton

Closes #13285 (rustc: Stop using LLVMGetSectionName)
Closes #13280 (std: override clone_from for Vec.)
Closes #13277 (serialize: add a few missing pubs to base64)
Closes #13275 (Add and remove some ignore-win32 flags)
Closes #13273 (Removed managed boxes from libarena.)
Closes #13270 (Minor copy-editing for the tutorial)
Closes #13267 (fix Option<~ZeroSizeType>)
Closes #13265 (Update emacs mode to support new `#![inner(attribute)]` syntax.)
Closes #13263 (syntax: Remove AbiSet, use one Abi)

1 files changed, 11 insertions, 2 deletions

diff --git a/src/libstd/rt/global_heap.rs b/src/libstd/rt/global_heap.rs
index 23b23cf8af0..5c1b6cd4791 100644
--- a/src/libstd/rt/global_heap.rs
+++ b/src/libstd/rt/global_heap.rs
@@ -68,8 +68,17 @@ pub unsafe fn realloc_raw(ptr: *mut u8, size: uint) -> *mut u8 {
 #[cfg(not(test))]
 #[lang="exchange_malloc"]
 #[inline]
-pub unsafe fn exchange_malloc(size: uint) -> *u8 {
-    malloc_raw(size) as *u8
+pub unsafe fn exchange_malloc(size: uint) -> *mut u8 {
+    // The compiler never calls `exchange_free` on ~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.
+    static EMPTY: () = ();
+
+    if size == 0 {
+        &EMPTY as *() as *mut u8
+    } else {
+        malloc_raw(size)
+    }
 }
 
 // FIXME: #7496