Prevent insta-stable no alloc shim support

You will need to add the following as replacement for the old __rust_*
definitions when not using the alloc shim.

    #[no_mangle]
    static __rust_no_alloc_shim_is_unstable: u8 = 0;

Author: bjorn3

alloc/src/alloc.rs

@@ -37,6 +37,9 @@ extern "Rust" {
     #[rustc_allocator_zeroed]
     #[rustc_nounwind]
     fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
+
+    #[cfg(not(bootstrap))]
+    static __rust_no_alloc_shim_is_unstable: u8;
 }
 
 /// The global memory allocator.
@@ -90,7 +93,14 @@ pub use std::alloc::Global;
 #[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn alloc(layout: Layout) -> *mut u8 {
-    unsafe { __rust_alloc(layout.size(), layout.align()) }
+    unsafe {
+        // Make sure we don't accidentally allow omitting the allocator shim in
+        // stable code until it is actually stabilized.
+        #[cfg(not(bootstrap))]
+        core::ptr::read_volatile(&__rust_no_alloc_shim_is_unstable);
+
+        __rust_alloc(layout.size(), layout.align())
+    }
 }
 
 /// Deallocate memory with the global allocator.