Add code comment to explain discard condition

askama_derive/src/generator.rs

@@ -989,6 +989,16 @@ impl<'a> Generator<'a> {
         if block_fragment_write {
             self.buf_writable.discard = true;
         }
+        // If previous discard value (ie, from the parent node) is different than the current one,
+        // then it can be only because we are only rendering a given block and the parent was not
+        // this block.
+        //
+        // It can only be this case because if we enter the block we want to render, its children
+        // will be rendered as well, meaning that this condition will never be false.
+        //
+        // Now why not call `self.write_buf_writable`? It's because of the recursion: the parent
+        // nodes will be stored but may not have been written into the buffer yet. So if we write
+        // everything right away, the rendering will be reverted: children will be generated first.
         if buf.discard != prev_buf_discard {
             self.write_buf_writable(buf)?;
         }