I'm not usually in the "defending C++" camp, but when I see this:

  pub const File = struct {
  
    pub fn writer(self: *File, buffer: []u8) Writer{
      return .{
        .file = self,
        .interface = std.Io.Writer{
          .buffer = buffer,
          .vtable = .{.drain = Writer.drain},
        }
      };
    }
  
    pub const Writer = struct {
      file: *File,
      interface: std.Io.Writer,
      // this has a bunch of other fields
  
      fn drain(io_w: *Writer, data: []const []const u8, splat: usize) !usize {
        const self: *Writer = @fieldParentPtr("interface", io_w);
        // ....
      }
    }
  }

...I can't help but think of this:

  struct FileWriter: public Writer {
      File *file;
      // this has a bunch of other fields
  
      FileWriter(File *self, span<char> buffer)
          : Writer(buffer), file(self) {}
  
      size_t drain(span<span<char const> const> data, size_t splat) override {
        // ....
      }
  };

Writing code to build a vtable and having it implicitly run at compile time is pretty neat, though!