Add const_assert_eq! and const_assert_ne! macros (#293)

Leverages some idiosyncrasies of the Rust type system to define macros
which can be used to make compile-time assertions about const generic
parameters, working around the "can't use generic parameters from outer
item" error which would occur if one simply used `assert_eq`/`assert_ne`

src/lib.rs

@@ -155,7 +155,7 @@
 extern crate alloc;
 
 #[macro_use]
-mod nlimbs;
+mod macros;
 
 #[cfg(feature = "generic-array")]
 mod array;

src/macros.rs

@@ -0,0 +1,79 @@
+//! Macro definitions which are a part of the public API.
+
+/// Internal implementation detail of [`const_assert_eq`] and [`const_assert_ne`].
+#[doc(hidden)]
+#[macro_export]
+macro_rules! const_assert_n {
+    ($n:expr, $($arg:tt)*) => {{
+        // TODO(tarcieri): gensym a name so it's unique per invocation of the macro?
+        mod __const_assert {
+            pub(super) struct Assert<const N: usize>;
+
+            impl<const N: usize> Assert<N> {
+                pub(super) const ASSERT: () = assert!($($arg)*);
+            }
+        }
+
+        __const_assert::Assert::<$n>::ASSERT
+    }};
+}
+
+/// Const-friendly assertion that two values are equal.
+///
+/// ```
+/// const _: () = crypto_bigint::const_assert_eq!(0, 0, "zero equals zero");
+/// ```
+#[macro_export]
+macro_rules! const_assert_eq {
+    ($left:expr, $right:expr $(,)?) => (
+        $crate::const_assert_n!($left, $left == $right)
+    );
+    ($left:expr, $right:expr, $($arg:tt)+) => (
+        $crate::const_assert_n!($left, $left == $right, $($arg)+)
+    );
+}
+
+/// Const-friendly assertion that two values are NOT equal.
+///
+/// ```
+/// const _: () = crypto_bigint::const_assert_ne!(0, 1, "zero is NOT equal to one");
+/// ```
+#[macro_export]
+macro_rules! const_assert_ne {
+    ($left:expr, $right:expr $(,)?) => (
+        $crate::const_assert_n!($left, $left != $right)
+    );
+    ($left:expr, $right:expr, $($arg:tt)+) => (
+        $crate::const_assert_n!($left, $left != $right, $($arg)+)
+    );
+}
+
+/// Calculate the number of limbs required to represent the given number of bits.
+// TODO(tarcieri): replace with `generic_const_exprs` (rust-lang/rust#76560) when stable
+#[macro_export]
+macro_rules! nlimbs {
+    ($bits:expr) => {
+        $bits / $crate::Limb::BITS
+    };
+}
+
+#[cfg(test)]
+mod tests {
+    #[cfg(target_pointer_width = "32")]
+    #[test]
+    fn nlimbs_for_bits_macro() {
+        assert_eq!(nlimbs!(64), 2);
+        assert_eq!(nlimbs!(128), 4);
+        assert_eq!(nlimbs!(192), 6);
+        assert_eq!(nlimbs!(256), 8);
+    }
+
+    #[cfg(target_pointer_width = "64")]
+    #[test]
+    fn nlimbs_for_bits_macro() {
+        assert_eq!(nlimbs!(64), 1);
+        assert_eq!(nlimbs!(128), 2);
+        assert_eq!(nlimbs!(192), 3);
+        assert_eq!(nlimbs!(256), 4);
+    }
+}

src/uint/modular/runtime_mod.rs

@@ -269,7 +269,6 @@ impl<const LIMBS: usize> zeroize::Zeroize for DynResidue<LIMBS> {
 #[cfg(test)]
 mod test {
     use super::*;
-    use crate::nlimbs;
 
     const LIMBS: usize = nlimbs!(64);