From b0c6c4f9a45cf2b965acd3d10499e9df54929b36 Mon Sep 17 00:00:00 2001
From: Brian Cully <bjc@kublai.com>
Date: Mon, 29 Jul 2019 10:48:05 -0400
Subject: WIP: Add `const-fn` feature to attempt support for stable toolchain.

To get this to work, either `FnMut` traits need to be allowed in
`const fn` in stable, or `Handler::new` needs to be non-`const`.
---
 Cargo.toml     |  1 +
 README.md      |  4 ++++
 src/handler.rs | 49 ++++++++++++++++++++++++++++++++++++++++---------
 src/lib.rs     |  2 +-
 4 files changed, 46 insertions(+), 10 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 1079e1d..6d6f192 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,6 +20,7 @@ cortex-m = "~0.6"
 
 [features]
 default = ["isr-32"]
+const-fn = []
 
 # Number of ISR closures to hold in a HandlerTable. This is pretty
 # clumsy, but doesn't require const generics.
diff --git a/README.md b/README.md
index c3c6de5..daf4e03 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,10 @@ configure the number of available slots, specify one of the following
 cargo features: `isr-8`, `isr-16`, `isr-32`, `isr-64`, `isr-128`, or
 `isr-256`. By default, 32 slots are available.
 
+If you're using a nightly toolchain, you can enable an optimization
+when calling handlers by turning on the `const-fn` feature. This will
+save one conditional branch on every call.
+
 # Example Code
 
 See the `examples` directory for some simple examples.
diff --git a/src/handler.rs b/src/handler.rs
index ec1f22e..545e622 100644
--- a/src/handler.rs
+++ b/src/handler.rs
@@ -80,20 +80,35 @@
 //!     unsafe { SYSTICK_HANDLER.call() };
 //! }
 //! ```
+#[cfg(not(feature = "const-fn"))]
+use core::ptr::NonNull;
 
+#[cfg(feature = "const-fn")]
 pub struct Handler<'a> {
     // Handler that will be executed on `call`.
     h: *const dyn FnMut(),
     lifetime: core::marker::PhantomData<&'a dyn FnMut()>,
 }
+#[cfg(not(feature = "const-fn"))]
+pub struct Handler<'a> {
+    // Handler that will be executed on `call`.
+    h: Option<NonNull<dyn FnMut() + 'a>>,
+}
 
 impl<'a> Handler<'a> {
     /// Returns a new Handler that initially does nothing when
     /// called. Override its behavior by using `replace`.
     pub const fn new() -> Self {
-        Self {
-            h: &Self::default_handler,
-            lifetime: core::marker::PhantomData,
+        #[cfg(feature = "const-fn")]
+        {
+            Self {
+                h: &Self::default_handler,
+                lifetime: core::marker::PhantomData,
+            }
+        }
+        #[cfg(not(feature = "const-fn"))]
+        {
+            Self { h: None }
         }
     }
 
@@ -105,7 +120,16 @@ impl<'a> Handler<'a> {
     /// while it is being executed. It is your responsibility to make
     /// sure that it's not being executed when you call `replace`.
     pub unsafe fn replace(&mut self, f: &(dyn FnMut() + Send + 'a)) {
-        self.h = core::mem::transmute::<_, &'a _>(f);
+        #[cfg(feature = "const-fn")]
+        {
+            self.h = core::mem::transmute::<_, &'a _>(f);
+        }
+        #[cfg(not(feature = "const-fn"))]
+        {
+            //        let ptr: *mut dyn FnMut() = core::mem::transmute::<_, &'a _>(f);
+            //        self.h = Some(NonNull::new(ptr));
+            self.h = Some(NonNull::new_unchecked(f));
+        }
     }
 
     /// Execute this handler.
@@ -116,8 +140,15 @@ impl<'a> Handler<'a> {
     /// closure is being looked up. You need to ensure that `replace`
     /// and `call` can not occur at the same time.
     pub unsafe fn call(&self) {
-        let f: &mut dyn FnMut() = &mut *(self.h as *mut dyn FnMut());
-        f();
+        #[cfg(feature = "const-fn")]
+        {
+            let f: &mut dyn FnMut() = &mut *(self.h as *mut dyn FnMut());
+            f();
+        }
+        #[cfg(not(feature = "const-fn"))]
+        {
+            self.h.map(|mut f| (f.as_mut())());
+        }
     }
 
     /// Do nothing handler. Needed by `call` until `replace` is used
@@ -152,7 +183,7 @@ mod test {
 
         let mut handler = Handler::new();
         unsafe {
-            handler.replace(&|| X += 1);
+            handler.replace(&mut || X += 1);
             assert_eq!(X, 0);
             handler.call();
             handler.call();
@@ -166,7 +197,7 @@ mod test {
         static mut X: usize = 0;
 
         unsafe {
-            HANDLER.replace(&|| X += 1);
+            HANDLER.replace(&mut || X += 1);
             assert_eq!(X, 0);
             HANDLER.call();
             HANDLER.call();
@@ -178,7 +209,7 @@ mod test {
     fn replace_with_default() {
         let mut handler = Handler::new();
         unsafe {
-            handler.replace(&Handler::default_handler);
+            handler.replace(&mut Handler::default_handler);
             handler.call()
         }
     }
diff --git a/src/lib.rs b/src/lib.rs
index 8e16b26..d0ddad5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -15,7 +15,7 @@
 //! Critical section support is supplied by the [`cs` module](cs).
 
 #![no_std]
-#![feature(const_fn)]
+#![cfg_attr(feature = "const-fn", feature(const_fn))]
 
 pub mod array;
 pub mod cs;
-- 
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