F523: Single variable leads to literal rendering of newline

[addisoncrump]

Reproduction

The following snippet is a minimal reproduction:

x = 5
"\n".format(x)

The fix attempted:

x = 5
"
".format(x)

Command line

ruff check format2.py --fix --isolated

Version information

Built on e82160a (latest, bleeding).

Misc

Discovered by #4822.

This is potentially a duplicate of #4823, but I couldn't diagnose a root cause. Notably, I chose to open a separate issue because, where #4823 causes an incorrect replacement of single quotes with double quotes, this issue literally renders the content of a string (!) and may have a separate root cause.

[charliermarsh]

I think we should rewrite the F523 fix to use LibCST.

[dhruvmanila]

This got fixed in d8a6109:

--- /Users/dhruv/playground/ruff/src/F523.py
+++ /Users/dhruv/playground/ruff/src/F523.py
@@ -1,2 +1,2 @@
 x = 5
-"\n".format(x)
+"\n".format()

Would fix 1 error.

[charliermarsh]

I think it will still do the wrong thing in some cases, like if we need to rewrite the string to remove positionals, in which case we end up reconstructing from the AST.

[charliermarsh]

Actually, I don't think we can use LibCST for this. It doesn't include a format-string parser. Instead, we probably need to change the format-string parser to include ranges on the various pieces, so that we can update the string in-place rather than recreate it.