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.

1 files changed, 43 insertions, 0 deletions

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, ()>;
+}