Add enum_u8 macro to simplify enum conversion

We need several enums that map to a u8 value.  This patch adds the
enum_u8 macro that automatically derives the From and TryFrom macros for
an enum to make these conversions easier.

As TryFrom is not stable yet, we add our own TryFrom trait.

3 files changed, 57 insertions, 21 deletions

diff --git a/src/hid.rs b/src/hid.rs
index 4f1d506..6274e7f 100644
--- a/src/hid.rs
+++ b/src/hid.rs
@@ -11,30 +11,20 @@ use usb_device::Result;
 
 const INTERFACE_CLASS_HID: u8 = 0x03;
 
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[repr(u8)]
-pub enum Subclass {
-    None = 0x00,
-    BootInterface = 0x01,
-}
-
-impl From<Subclass> for u8 {
-    fn from(subclass: Subclass) -> u8 {
-        subclass as u8
+enum_u8! {
+    #[derive(Clone, Copy, Debug, PartialEq)]
+    pub enum Subclass {
+        None = 0x00,
+        BootInterface = 0x01,
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[repr(u8)]
-pub enum Protocol {
-    None = 0x00,
-    Keyboard = 0x01,
-    Mouse = 0x02,
-}
-
-impl From<Protocol> for u8 {
-    fn from(protocol: Protocol) -> u8 {
-        protocol as u8
+enum_u8! {
+    #[derive(Clone, Copy, Debug, PartialEq)]
+    pub enum Protocol {
+        None = 0x00,
+        Keyboard = 0x01,
+        Mouse = 0x02,
     }
 }
 
diff --git a/src/main.rs b/src/main.rs
index b0ecafd..3857967 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -6,6 +6,9 @@
 
 extern crate panic_halt;
 
+#[macro_use]
+mod util;
+
 mod device;
 mod hid;
 
diff --git a/src/util.rs b/src/util.rs
new file mode 100644
index 0000000..4fc4448
--- /dev/null
+++ b/src/util.rs
@@ -0,0 +1,43 @@
+// Copyright 2019 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+use core::marker::Sized;
+
+macro_rules! enum_u8 {
+    (
+        $(#[$outer:meta])*
+        pub enum $name:ident {
+            $($var:ident = $num:expr),+
+            $(,)*
+        }
+    ) => {
+        $(#[$outer])*
+        #[repr(u8)]
+        pub enum $name {
+            $(
+                $var = $num,
+            )*
+        }
+
+        impl crate::util::TryFrom<u8> for $name {
+            fn try_from(val: u8) -> ::core::result::Result<Self, ()> {
+                match val {
+                    $(
+                        $num => Ok($name::$var),
+                    )*
+                    _ => Err(())
+                }
+            }
+        }
+
+        impl From<$name> for u8 {
+            fn from(val: $name) -> u8 {
+                val as u8
+            }
+        }
+    };
+}
+
+pub trait TryFrom<T>: Sized {
+    fn try_from(val: T) -> Result<Self, ()>;
+}