From eac2bbfec9f99c27a886f4842cb9ca42584607f3 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Mon, 8 Sep 2025 18:18:27 -0700
Subject: [PATCH] std.Io.Writer.writeSliceEndian: add compile error

check when an auto-layout struct is attempted to be memory reinterpreted
and written out. it would be writing undefined memory
---
 lib/std/Io/Writer.zig | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/std/Io/Writer.zig b/lib/std/Io/Writer.zig
index 1c34a124ae..7f1b1fd884 100644
--- a/lib/std/Io/Writer.zig
+++ b/lib/std/Io/Writer.zig
@@ -865,6 +865,11 @@ pub inline fn writeSliceEndian(
     slice: []const Elem,
     endian: std.builtin.Endian,
 ) Error!void {
+    switch (@typeInfo(Elem)) {
+        .@"struct" => |info| comptime assert(info.layout != .auto),
+        .int, .@"enum" => {},
+        else => @compileError("ill-defined memory layout"),
+    }
     if (native_endian == endian) {
         return writeAll(w, @ptrCast(slice));
     } else {