diff --git a/lib/std/Io.zig b/lib/std/Io.zig
index b27217ece03fd99f0238f7c446b945aca0665d36..3f98ec3043246c65b9ed68e4276aa23877493d69 100644
--- a/lib/std/Io.zig
+++ b/lib/std/Io.zig
@@ -419,9 +419,14 @@ pub fn GenericWriter(
             new_interface: Writer,
             err: ?Error = null,
 
-            fn drain(w: *Writer, data: []const []const u8, splat: usize) Writer.Error!usize {
+            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;
diff --git a/lib/std/Io/DeprecatedWriter.zig b/lib/std/Io/DeprecatedWriter.zig
index 391b9853570100e5adecc1e2b0d562372f911aa7..81774b357c76d6933b6d5379e2b176d88078f7ac 100644
--- a/lib/std/Io/DeprecatedWriter.zig
+++ b/lib/std/Io/DeprecatedWriter.zig
@@ -100,7 +100,12 @@ pub const Adapter = struct {
 
     fn drain(w: *std.io.Writer, data: []const []const u8, splat: usize) std.io.Writer.Error!usize {
         _ = splat;
-        const a: *@This() = @fieldParentPtr("new_interface", w);
+        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;