Replace runtime size check with compile time check

Add a macro `const_assert` that uses some const declaration trickery to
trigger a compile time error if a boolean expression is false.

Replace runtime checks using `debug_assert_eq!` with the newly defined
`const_asert!` macro.

src/consensus/encode.rs

@@ -241,7 +241,7 @@ macro_rules! decoder_fn {
     ($name:ident, $val_type:ty, $readfn:ident, $byte_len: expr) => {
         #[inline]
         fn $name(&mut self) -> Result<$val_type, Error> {
-            debug_assert_eq!(::core::mem::size_of::<$val_type>(), $byte_len); // size_of isn't a constfn in 1.22
+            const_assert!(::core::mem::size_of::<$val_type>() == $byte_len);
             let mut val = [0; $byte_len];
             self.read_exact(&mut val[..]).map_err(Error::Io)?;
             Ok(endian::$readfn(&val))

src/internal_macros.rs

@@ -564,3 +564,12 @@ macro_rules! user_enum {
         }
     );
 }
+
+/// Asserts a boolean expression at compile time.
+macro_rules! const_assert {
+    ($x:expr) => {
+        {
+            const _: [(); 0 - !$x as usize] = [];
+        }
+    };
+}

src/util/endian.rs

@@ -28,7 +28,7 @@ macro_rules! define_be_to_array {
     ($name: ident, $type: ty, $byte_len: expr) => {
         #[inline]
         pub fn $name(val: $type) -> [u8; $byte_len] {
-            debug_assert_eq!(::core::mem::size_of::<$type>(), $byte_len); // size_of isn't a constfn in 1.22
+            const_assert!(::core::mem::size_of::<$type>() == $byte_len);
             let mut res = [0; $byte_len];
             for i in 0..$byte_len {
                 res[i] = ((val >> ($byte_len - i - 1)*8) & 0xff) as u8;
@@ -41,7 +41,7 @@ macro_rules! define_le_to_array {
     ($name: ident, $type: ty, $byte_len: expr) => {
         #[inline]
         pub fn $name(val: $type) -> [u8; $byte_len] {
-            debug_assert_eq!(::core::mem::size_of::<$type>(), $byte_len); // size_of isn't a constfn in 1.22
+            const_assert!(::core::mem::size_of::<$type>() == $byte_len);
             let mut res = [0; $byte_len];
             for i in 0..$byte_len {
                 res[i] = ((val >> i*8) & 0xff) as u8;