From 7a3bb7d3ecaa0ba1ef425a2bb9669b60b8f48059 Mon Sep 17 00:00:00 2001
From: "Tobin C. Harding" <me@tobin.cc>
Date: Wed, 20 Apr 2022 09:17:46 +1000
Subject: [PATCH] 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_assert!` macro.
---
 src/consensus/encode.rs | 2 +-
 src/internal_macros.rs  | 9 +++++++++
 src/util/endian.rs      | 4 ++--
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/consensus/encode.rs b/src/consensus/encode.rs
index 603d47bf..9e3b2804 100644
--- a/src/consensus/encode.rs
+++ b/src/consensus/encode.rs
@@ -242,7 +242,7 @@ 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))
diff --git a/src/internal_macros.rs b/src/internal_macros.rs
index c0581130..f8e6b160 100644
--- a/src/internal_macros.rs
+++ b/src/internal_macros.rs
@@ -593,3 +593,12 @@ macro_rules! write_err {
         }
     }
 }
+
+/// Asserts a boolean expression at compile time.
+macro_rules! const_assert {
+    ($x:expr) => {
+        {
+            const _: [(); 0 - !$x as usize] = [];
+        }
+    };
+}
diff --git a/src/util/endian.rs b/src/util/endian.rs
index f09a1237..c63a906b 100644
--- a/src/util/endian.rs
+++ b/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;