std: fix deprecated writer not handling the buffer

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/std/Io.zig b/lib/std/Io.zig
index b27217ece0..3f98ec3043 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;