WIP: stalled out trying to implement const-fn feature.stable-support

Unfortunately, to maintain API compatibility, `replace` needs to be a
regular borrow, so we need interior mutability with
`UnsafeCell`. Simultaneously, `HandlerArray` needs to be `const fn` so
it can be used where `lazy_static` isn't available, which means it
needs to initialize its array in one go without the help of
`MaybeUninit`. To do that, `Handler` must be `Copy`, but since it
contains an `UnsafeCell` now, it cannot be copy.

And thus we are boned. There are times when I think the premise of
rust is admirable, but implementation is impossible.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/fnnop.rs b/src/fnnop.rs
index 871a6ab..13b9379 100644
--- a/src/fnnop.rs
+++ b/src/fnnop.rs
@@ -2,6 +2,7 @@
 //
 //#![feature(unboxed_closures)]
 //#![feature(fn_traits)]
+use core::cell::UnsafeCell;
 
 pub struct FnNOP();
 
@@ -16,4 +17,4 @@ impl FnOnce<()> for FnNOP {
     extern "rust-call" fn call_once(self, _args: ()) {}
 }
 
-static mut NOP: FnNOP = FnNOP();
+pub static mut NOP: UnsafeCell<FnNOP> = UnsafeCell::new(FnNOP());