Make GlobalAllocator allocation calls uncollectable

The GlobalAllocator is responsible for allocating all objects which are
*not* managed by the GC, so we need to ensure that it doesn't try and
allocate them in the managed pool.

This is not currently possible for gc_malloc_precise due to limitations
in Boehm, so for now, it needs to be prevented.

Author: jacob-hughes

allocator/src/boehm.rs

@@ -70,6 +70,9 @@ extern "C" {
     #[cfg(feature = "rustgc")]
     pub(crate) fn GC_malloc_atomic(nbytes: usize) -> *mut u8;
 
+    #[cfg(feature = "rustgc")]
+    pub(crate) fn GC_malloc_atomic_uncollectable(nbytes: usize) -> *mut u8;
+
     pub(crate) fn GC_thread_is_registered() -> u32;
 
     pub(crate) fn GC_register_my_thread(stack_base: *mut u8) -> i32;

allocator/src/lib.rs

@@ -13,9 +13,6 @@ pub struct GcAllocator;
 
 unsafe impl GlobalAlloc for GcAllocator {
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
-        #[cfg(feature = "rustgc")]
-        return boehm::GC_malloc(layout.size()) as *mut u8;
-        #[cfg(not(feature = "rustgc"))]
         return boehm::GC_malloc_uncollectable(layout.size()) as *mut u8;
     }
 
@@ -30,20 +27,19 @@ unsafe impl GlobalAlloc for GcAllocator {
     #[cfg(feature = "rustgc")]
     #[inline]
     unsafe fn alloc_precise(&self, layout: Layout, bitmap: usize, bitmap_size: usize) -> *mut u8 {
-        let gc_descr = boehm::GC_make_descriptor(&bitmap, bitmap_size);
-        boehm::GC_malloc_explicitly_typed(layout.size(), gc_descr) as *mut u8
+        unimplemented!("Boehm does not provide an uncollectable version of this call")
     }
 
     #[cfg(feature = "rustgc")]
     #[inline]
     fn alloc_conservative(&self, layout: Layout) -> *mut u8 {
-        unsafe { boehm::GC_malloc(layout.size()) as *mut u8 }
+        unsafe { boehm::GC_malloc_uncollectable(layout.size()) as *mut u8 }
     }
 
     #[cfg(feature = "rustgc")]
     #[inline]
     unsafe fn alloc_untraceable(&self, layout: Layout) -> *mut u8 {
-        boehm::GC_malloc_atomic(layout.size()) as *mut u8
+        boehm::GC_malloc_atomic_uncollectable(layout.size()) as *mut u8
     }
 }