From 9129656082890abcef660768e5133cbb0dfe74a7 Mon Sep 17 00:00:00 2001
From: Oren Leiman <oren.leiman@redpanda.com>
Date: Wed, 7 Aug 2024 11:21:21 -0700
Subject: [PATCH] serde: Require T::serde_fields OR arity <= 1 for all
 envelopes

Applies only if a field tuple is actually needed by a particular
envelope. For example, if a struct T implements both serde_read
and serde_write, the field tuple is not needed, so T::serde_fields
is not required.
---
 src/v/serde/envelope_for_each_field.h | 159 +-------------------------
 1 file changed, 4 insertions(+), 155 deletions(-)

diff --git a/src/v/serde/envelope_for_each_field.h b/src/v/serde/envelope_for_each_field.h
index 935a2d8ab7907..57d6e288da0dd 100644
--- a/src/v/serde/envelope_for_each_field.h
+++ b/src/v/serde/envelope_for_each_field.h
@@ -36,167 +36,16 @@ constexpr inline auto envelope_to_tuple(T& t) {
     static_assert(std::is_standard_layout_v<T>);
     static_assert(!std::is_polymorphic_v<T>);
 
+    // TODO(oren): template error here is not very clear. an assert is probably
+    // the best we can do, but in practice the error is not much easier to
+    // understand
+
     constexpr auto const a = reflection::arity<T>() - 1;
     if constexpr (a == 0) {
         return std::tie();
     } else if constexpr (a == 1) {
         auto& [p1] = t;
         return std::tie(p1);
-    } else if constexpr (a == 2) {
-        auto& [p1, p2] = t;
-        return std::tie(p1, p2);
-    } else if constexpr (a == 3) {
-        auto& [p1, p2, p3] = t;
-        return std::tie(p1, p2, p3);
-    } else if constexpr (a == 4) {
-        auto& [p1, p2, p3, p4] = t;
-        return std::tie(p1, p2, p3, p4);
-    } else if constexpr (a == 5) {
-        auto& [p1, p2, p3, p4, p5] = t;
-        return std::tie(p1, p2, p3, p4, p5);
-    } else if constexpr (a == 6) {
-        auto& [p1, p2, p3, p4, p5, p6] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6);
-    } else if constexpr (a == 7) {
-        auto& [p1, p2, p3, p4, p5, p6, p7] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6, p7);
-    } else if constexpr (a == 8) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6, p7, p8);
-    } else if constexpr (a == 9) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6, p7, p8, p9);
-    } else if constexpr (a == 10) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6, p7, p8, p9, p10);
-    } else if constexpr (a == 11) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11);
-    } else if constexpr (a == 12) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12);
-    } else if constexpr (a == 13) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13] = t;
-        return std::tie(p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13);
-    } else if constexpr (a == 14) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14] = t;
-        return std::tie(
-          p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14);
-    } else if constexpr (a == 15) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15]
-          = t;
-        return std::tie(
-          p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15);
-    } else if constexpr (a == 16) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15, p16]
-          = t;
-        return std::tie(
-          p1,
-          p2,
-          p3,
-          p4,
-          p5,
-          p6,
-          p7,
-          p8,
-          p9,
-          p10,
-          p11,
-          p12,
-          p13,
-          p14,
-          p15,
-          p16);
-    } else if constexpr (a == 17) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15, p16, p17]
-          = t;
-        return std::tie(
-          p1,
-          p2,
-          p3,
-          p4,
-          p5,
-          p6,
-          p7,
-          p8,
-          p9,
-          p10,
-          p11,
-          p12,
-          p13,
-          p14,
-          p15,
-          p16,
-          p17);
-    } else if constexpr (a == 18) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15, p16, p17, p18]
-          = t;
-        return std::tie(
-          p1,
-          p2,
-          p3,
-          p4,
-          p5,
-          p6,
-          p7,
-          p8,
-          p9,
-          p10,
-          p11,
-          p12,
-          p13,
-          p14,
-          p15,
-          p16,
-          p17,
-          p18);
-    } else if constexpr (a == 19) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15, p16, p17, p18, p19]
-          = t;
-        return std::tie(
-          p1,
-          p2,
-          p3,
-          p4,
-          p5,
-          p6,
-          p7,
-          p8,
-          p9,
-          p10,
-          p11,
-          p12,
-          p13,
-          p14,
-          p15,
-          p16,
-          p17,
-          p18,
-          p19);
-    } else if constexpr (a == 20) {
-        auto& [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15, p16, p17, p18, p19, p20]
-          = t;
-        return std::tie(
-          p1,
-          p2,
-          p3,
-          p4,
-          p5,
-          p6,
-          p7,
-          p8,
-          p9,
-          p10,
-          p11,
-          p12,
-          p13,
-          p14,
-          p15,
-          p16,
-          p17,
-          p18,
-          p19,
-          p20);
     }
 }