sdk: add macro for unchecked div with const denominator

sdk/program/src/lib.rs

@@ -72,3 +72,127 @@ extern crate serde_derive;
 
 #[macro_use]
 extern crate solana_frozen_abi_macro;
+
+/// Convenience macro for doing integer division where the opersation's safety
+/// can be checked at compile-time
+///
+/// Since `unchecked_div_by_const!()` is supposed to fail at compile-time, abuse
+/// doctests to cover failure modes
+/// Literal denominator div-by-zero fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # let _ = unchecked_div_by_const!(10, 0);
+/// # }
+/// ```
+/// #
+/// # Const denominator div-by-zero fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # const D: u64 = 0;
+/// # let _ = unchecked_div_by_const!(10, D);
+/// # }
+/// ```
+/// #
+/// # Non-const denominator fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # let d = 0;
+/// # let _ = unchecked_div_by_const!(10, d);
+/// # }
+/// ```
+/// #
+/// Literal denominator div-by-zero fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # const N: u64 = 10;
+/// # let _ = unchecked_div_by_const!(N, 0);
+/// # }
+/// ```
+/// #
+/// # Const denominator div-by-zero fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # const N: u64 = 10;
+/// # const D: u64 = 0;
+/// # let _ = unchecked_div_by_const!(N, D);
+/// # }
+/// ```
+/// #
+/// # Non-const denominator fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # const N: u64 = 10;
+/// # let d = 0;
+/// # let _ = unchecked_div_by_const!(N, d);
+/// # }
+/// ```
+/// #
+/// Literal denominator div-by-zero fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # let n = 10;
+/// # let _ = unchecked_div_by_const!(n, 0);
+/// # }
+/// ```
+/// #
+/// # Const denominator div-by-zero fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # let n = 10;
+/// # const D: u64 = 0;
+/// # let _ = unchecked_div_by_const!(n, D);
+/// # }
+/// ```
+/// #
+/// # Non-const denominator fails
+/// ```compile_fail
+/// # use solana_program::unchecked_div_by_const;
+/// # fn main() {
+/// # let n = 10;
+/// # let d = 0;
+/// # let _ = unchecked_div_by_const!(n, d);
+/// # }
+/// ```
+/// #
+#[macro_export]
+macro_rules! unchecked_div_by_const {
+    ($num:expr, $den:expr) => {{
+        // Ensure the denominator is compile-time constant
+        let _ = [(); ($den - $den) as usize];
+        // Compile-time constant integer div-by-zero passes for some reason
+        // when invoked from a compilation unit other than that where this
+        // macro is defined. Do an explicit zero-check for now. Sorry about the
+        // ugly error messages!
+        // https://users.rust-lang.org/t/unexpected-behavior-of-compile-time-integer-div-by-zero-check-in-declarative-macro/56718
+        let _ = [(); ($den as usize) - 1];
+        #[allow(clippy::integer_arithmetic)]
+        let quotient = $num / $den;
+        quotient
+    }};
+}
+
+#[cfg(test)]
+mod tests {
+    use super::unchecked_div_by_const;
+
+    #[test]
+    fn test_unchecked_div_by_const() {
+        const D: u64 = 2;
+        const N: u64 = 10;
+        let n = 10;
+        assert_eq!(unchecked_div_by_const!(10, 2), 5);
+        assert_eq!(unchecked_div_by_const!(N, 2), 5);
+        assert_eq!(unchecked_div_by_const!(n, 2), 5);
+        assert_eq!(unchecked_div_by_const!(10, D), 5);
+        assert_eq!(unchecked_div_by_const!(N, D), 5);
+        assert_eq!(unchecked_div_by_const!(n, D), 5);
+    }
+}