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.

3 files changed, 28 insertions, 6 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());
diff --git a/src/handler.rs b/src/handler.rs
index 1087b21..ca8aea6 100644
--- a/src/handler.rs
+++ b/src/handler.rs
@@ -85,7 +85,9 @@
 //!     unsafe { SYSTICK_HANDLER.call() };
 //! }
 //! ```
-#[cfg(not(feature = "const-fn"))]
+#[cfg(feature = "const-fn")]
+use crate::fnnop::NOP;
+
 use core::cell::UnsafeCell;
 #[cfg(not(feature = "const-fn"))]
 use core::ptr::NonNull;
@@ -93,7 +95,7 @@ use core::ptr::NonNull;
 #[cfg(feature = "const-fn")]
 pub struct Handler<'a> {
     // Handler that will be executed on `call`.
-    h: *mut dyn FnMut(),
+    h: UnsafeCell<*mut dyn FnMut()>,
     lifetime: core::marker::PhantomData<&'a dyn FnMut()>,
 }
 #[cfg(not(feature = "const-fn"))]
@@ -108,7 +110,7 @@ impl<'a> Handler<'a> {
     #[cfg(feature = "const-fn")]
     pub const fn new() -> Self {
         Self {
-            h: &Self::default_handler,
+            h: UnsafeCell::new(unsafe { NOP.get() }),
             lifetime: core::marker::PhantomData,
         }
     }
@@ -130,7 +132,7 @@ impl<'a> Handler<'a> {
     pub unsafe fn replace(&self, f: &mut (dyn FnMut() + Send + 'a)) {
         #[cfg(feature = "const-fn")]
         {
-            self.h = core::mem::transmute::<_, &'a _>(f);
+            *self.h.get() = core::mem::transmute::<_, &'a mut _>(f);
         }
         #[cfg(not(feature = "const-fn"))]
         {
@@ -150,7 +152,7 @@ impl<'a> Handler<'a> {
     pub unsafe fn call(&self) {
         #[cfg(feature = "const-fn")]
         {
-            let f: &mut dyn FnMut() = &mut *(self.h as *mut dyn FnMut());
+            let f: &mut dyn FnMut() = &mut **self.h.get();
             f();
         }
         #[cfg(not(feature = "const-fn"))]
@@ -167,6 +169,13 @@ impl<'a> Handler<'a> {
 }
 
 impl<'a> core::fmt::Debug for Handler<'a> {
+    #[cfg(feature = "const-fn")]
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        let (f0, f1) = unsafe { core::mem::transmute::<_, (usize, usize)>(*self.h.get()) };
+        write!(f, "Handler{{ h: (0x{:x}, 0x{:x}) }}", f0, f1)
+    }
+
+    #[cfg(not(feature = "const-fn"))]
     fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         let (f0, f1) = unsafe { core::mem::transmute::<_, (usize, usize)>(*self.h.get()) };
         write!(f, "Handler{{ h: (0x{:x}, 0x{:x}) }}", f0, f1)
@@ -174,6 +183,14 @@ impl<'a> core::fmt::Debug for Handler<'a> {
 }
 
 unsafe impl Sync for Handler<'_> {}
+#[cfg(feature = "const-fn")]
+impl Copy for Handler<'_> {}
+#[cfg(feature = "const-fn")]
+impl Clone for Handler<'_> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
 
 #[cfg(test)]
 mod test {
diff --git a/src/lib.rs b/src/lib.rs
index d0ddad5..4ae3fd1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -16,9 +16,13 @@
 
 #![no_std]
 #![cfg_attr(feature = "const-fn", feature(const_fn))]
+#![cfg_attr(feature = "const-fn", feature(fn_traits))]
+#![cfg_attr(feature = "const-fn", feature(unboxed_closures))]
 
 pub mod array;
 pub mod cs;
+#[cfg(feature = "const-fn")]
+mod fnnop;
 pub mod handler;
 
 pub use array::HandlerArray;