return RewriteResult for rewrite_block and rewrite_closure

[ding-young]

Tracked by #6206

Description

modify the signature of rewrite_block and rewrite_closure to return RewriteResult

Side notes

• I did not change the return type of rewrite_empty_block(it returns None when given block is not empty) since every caller tries rewrite_empty_block first and then applies formatting logic for non empty block if it returns None. It seems that there's no need to propagate RewriteError upward from this function currently.

[ytmimi]

Mostly looks like a simple translation from Option<String> -> RewriteResult, but I not 100% sure about one of the calls to rewrite_block_with_visitor. Could you double check that one.

[ytmimi]

This doesn't look like a one-to-one translations from Option<String> -> RewriteResult. Also, doesn't rewrite_block_with_visitor now return RewriteResult? I would have expected to see something closer this:

    let result = rewrite_block_with_visitor(context, &prefix, block, attrs, label, shape, true);
    if let Ok(ref result_str) = result {
        if allow_single_line && result_str.lines().count() <= 3 {
            if let rw @ Ok(_) = rewrite_single_line_block(context, &prefix, block, attrs, label, shape)
            {
                return rw;
            }
        }
    }

[ding-young]

Yes, it isn't a one-to-one translation but I made this change intentionally.

From what I understood of the original code:

(1) If rewrite_block_with_visitor fails, return None.
(2) If rewrite_block_with_visitor succeeds, check whether we should try to format it into a single-line block.
      a. If rewrite_single_line_block succeeds, return that result.
      b. If rewrite_single_line_block fails, just return the result from rewrite_block_with_visitor.

My intention was to simplify if let Ok(ref result_str) = result {...} with using ? operator on calling rewrite_block_with_visitor. I thought it won't make a difference in control flow or formatting logic described above.
Still, if you think this change does not worth it or it actually breaks the original control flow, please tell me.

[ytmimi]

After taking another look at this the change seems to preserve the original behavior so we're good here!