std.Io.Writer: add missing writeSliceSwap

lib/std/Io/Writer.zig

@@ -851,6 +851,9 @@ pub inline fn writeStruct(w: *Writer, value: anytype, endian: std.builtin.Endian
     }
 }
 
+/// If, `endian` is not native,
+/// * Asserts that the buffer storage capacity is at least enough to store `@sizeOf(Elem)`
+/// * Asserts that the buffer is aligned enough for `@alignOf(Elem)`.
 pub inline fn writeSliceEndian(
     w: *Writer,
     Elem: type,
@@ -860,7 +863,22 @@ pub inline fn writeSliceEndian(
     if (native_endian == endian) {
         return writeAll(w, @ptrCast(slice));
     } else {
-        return w.writeArraySwap(w, Elem, slice);
+        return writeSliceSwap(w, Elem, slice);
+    }
+}
+
+/// Asserts that the buffer storage capacity is at least enough to store `@sizeOf(Elem)`
+///
+/// Asserts that the buffer is aligned enough for `@alignOf(Elem)`.
+pub fn writeSliceSwap(w: *Writer, Elem: type, slice: []const Elem) Error!void {
+    var i: usize = 0;
+    while (i < slice.len) {
+        const dest_bytes = try w.writableSliceGreedy(@sizeOf(Elem));
+        const dest: []Elem = @alignCast(@ptrCast(dest_bytes[0 .. dest_bytes.len - dest_bytes.len % @sizeOf(Elem)]));
+        const copy_len = @min(dest.len, slice.len - i);
+        @memcpy(dest[0..copy_len], slice[i..][0..copy_len]);
+        i += copy_len;
+        std.mem.byteSwapAllElements(Elem, dest);
     }
 }
 
@@ -2630,3 +2648,11 @@ test writeStruct {
         }, &buffer);
     }
 }
+
+test writeSliceEndian {
+    var buffer: [4]u8 align(2) = undefined;
+    var w: Writer = .fixed(&buffer);
+    const array: [2]u16 = .{ 0x1234, 0x5678 };
+    try writeSliceEndian(&w, u16, &array, .big);
+    try testing.expectEqualSlices(u8, &.{ 0x12, 0x34, 0x56, 0x78 }, &buffer);
+}

lib/std/zig/Server.zig

@@ -118,6 +118,8 @@ pub fn init(options: Options) !Server {
         .in = options.in,
         .out = options.out,
     };
+    assert(s.out.buffer.len >= 4);
+    std.debug.assertAligned(s.out.buffer.ptr, .@"4");
     try s.serveStringMessage(.zig_version, options.zig_version);
     return s;
 }