Adjust own-line comment placement between branches (#21185)
This PR attempts to improve the placement of own-line comments between
branches in the setting where the comment is more indented than the
preceding node.
There are two main changes.
### First change: Preceding node has leading content
If the preceding node has leading content, we now regard the comment as
automatically _less_ indented than the preceding node, and format
accordingly.
For example,
```python
if True: preceding_node
# leading on `else`, not trailing on `preceding_node`
else: ...
```
This is more compatible with `black`, although there is a (presumably
very uncommon) edge case:
```python
if True:
this;that
# leading on `else`, but trailing in `black`
else: ...
```
I'm sort of okay with this - presumably if one wanted a comment for
those semi-colon separated statements, one should have put it _above_
them, and one wanted a comment only for `that` then it ought to have
been on the same line?
### Second change: searching for last child in body
While searching for the (recursively) last child in the body of the
preceding _branch_, we implicitly assumed that the preceding node had to
have a body to begin the recursion. But actually, in the base case, the
preceding node _is_ the last child in the body of the preceding branch.
So, for example:
```python
if True:
something
last_child_but_no_body
# leading on else for `main` but trailing in this PR
else: ...
```
### More examples
The table below is an attempt to summarize the changes in behavior. The
rows alternate between an example snippet with `while` and the same
example with `if` - in the former case we do _not_ have an `else` node
and in the latter we do.
Notice that:
1. On `main` our handling of `if` vs. `while` is not consistent, whereas
it is consistent in the present PR
2. We disagree with `black` in all cases except that last example on
`main`, but agree in all cases for the present PR (though see above for
a wonky edge case where we disagree).
<table>
<tr>
<th>Original
</th>
<th><code>main</code> </th>
<th>This
PR </th>
<th><code>black</code> </th>
</tr>
<tr>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
else:
# comment
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
</tr>
<tr>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
</tr>
<tr>
<td>
<pre lang="python">
while True: pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
</tr>
<tr>
<td>
<pre lang="python">
if True: pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
</tr>
<tr>
<td>
<pre lang="python">
while True: pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
else:
# comment
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
while True:
pass
# comment
else:
pass
</pre>
</td>
</tr>
<tr>
<td>
<pre lang="python">
if True: pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
<td>
<pre lang="python">
if True:
pass
# comment
else:
pass
</pre>
</td>
</tr>
</table>
