GenericWriter

pub fn GenericWriter(
    comptime Context: type,
    comptime WriteError: type,
    comptime writeFn: fn (context: Context, bytes: []const u8) WriteError!usize,
) type {
    return struct {
        context: Context,

        const Self = @This();
        pub const Error = WriteError;

        pub inline fn write(self: Self, bytes: []const u8) Error!usize {
            return writeFn(self.context, bytes);
        }

        pub inline fn writeAll(self: Self, bytes: []const u8) Error!void {
            return @errorCast(self.any().writeAll(bytes));
        }

        pub inline fn print(self: Self, comptime format: []const u8, args: anytype) Error!void {
            return @errorCast(self.any().print(format, args));
        }

        pub inline fn writeByte(self: Self, byte: u8) Error!void {
            return @errorCast(self.any().writeByte(byte));
        }

        pub inline fn writeByteNTimes(self: Self, byte: u8, n: usize) Error!void {
            return @errorCast(self.any().writeByteNTimes(byte, n));
        }

        pub inline fn writeBytesNTimes(self: Self, bytes: []const u8, n: usize) Error!void {
            return @errorCast(self.any().writeBytesNTimes(bytes, n));
        }

        pub inline fn writeInt(self: Self, comptime T: type, value: T, endian: std.builtin.Endian) Error!void {
            return @errorCast(self.any().writeInt(T, value, endian));
        }

        pub inline fn writeStruct(self: Self, value: anytype) Error!void {
            return @errorCast(self.any().writeStruct(value));
        }

        pub inline fn writeStructEndian(self: Self, value: anytype, endian: std.builtin.Endian) Error!void {
            return @errorCast(self.any().writeStructEndian(value, endian));
        }

        pub inline fn any(self: *const Self) AnyWriter {
            return .{
                .context = @ptrCast(&self.context),
                .writeFn = typeErasedWriteFn,
            };
        }

        fn typeErasedWriteFn(context: *const anyopaque, bytes: []const u8) anyerror!usize {
            const ptr: *const Context = @ptrCast(@alignCast(context));
            return writeFn(ptr.*, bytes);
        }

        /// Helper for bridging to the new `Writer` API while upgrading.
        pub fn adaptToNewApi(self: *const Self, buffer: []u8) Adapter {
            return .{
                .derp_writer = self.*,
                .new_interface = .{
                    .buffer = buffer,
                    .vtable = &.{ .drain = Adapter.drain },
                },
            };
        }

        pub const Adapter = struct {
            derp_writer: Self,
            new_interface: Writer,
            err: ?Error = null,

            fn drain(w: *std.io.Writer, data: []const []const u8, splat: usize) std.io.Writer.Error!usize {
                _ = splat;
                const a: *@This() = @alignCast(@fieldParentPtr("new_interface", w));
                const buffered = w.buffered();
                if (buffered.len != 0) return w.consume(a.derp_writer.write(buffered) catch |err| {
                    a.err = err;
                    return error.WriteFailed;
                });
                return a.derp_writer.write(data[0]) catch |err| {
                    a.err = err;
                    return error.WriteFailed;
                };
            }
        };
    };
}