Avoid multiline expression if format specifier is present

[dhruvmanila]

Summary

This PR fixes the bug where the formatter would format an f-string and could potentially change the AST.

For a triple-quoted f-string, the element can't be formatted into multiline if it has a format specifier because otherwise the newline would be treated as part of the format specifier.

Given the following f-string:

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f} ddddddddddddddd eeeeeeee"""

The formatter sees that the f-string is already multiline so it assumes that it can contain line breaks i.e., broken into multiple lines. But, in this specific case we can't format it as:

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f
} ddddddddddddddd eeeeeeee"""

Because the format specifier string would become ".3f\n", which is not the original string (.3f).

If the original source code already contained a newline, they'll be preserved. For example:

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f
} ddddddddddddddd eeeeeeee"""

The above will be formatted as:

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {variable:.3f
} ddddddddddddddd eeeeeeee"""

Note that the newline after .3f is part of the format specifier which needs to be preserved.

The Python version is irrelevant in this case.

fixes: #10040

Test Plan

Add some test cases to verify this behavior.

[github-actions]

ruff-ecosystem results

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[dhruvmanila]

This is an additional change which isn't relevant to this PR but should be a follow-up. I've included it in here for simplicity. For example,

x = f"aaaaaaaaaaaaaaa {
    {'aaaaaaaaaaaaaaaaaa', 'dddddddddddddddddd'}!s
}"

This should be formatted as:

x = f"aaaaaaaaaaaaaaa { {'aaaaaaaaaaaaaaaaaa', 'dddddddddddddddddd'}!s}"

Instead of:

x = f"aaaaaaaaaaaaaaa { {'aaaaaaaaaaaaaaaaaa', 'dddddddddddddddddd'}!s }"
#                                                                     ^ this isn't required if there's format spec or conversion spec

[MichaReiser]

I'm confused, you say:

Note that the newline after .3f is part of the format specifier which needs to be preserved.

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {variable:.3f
} ddddddddddddddd eeeeeeee"""

But the original example had no newline after 3f.

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f} ddddddddddddddd eeeeeeee"""

[MichaReiser]

Lol what so # comment here is not a comment 🤣

[dhruvmanila]

Yeah, it isn't but only if it's a triple-quoted f-string

https://play.ruff.rs/b1d0326b-f8dc-4724-870f-db3770fb09c5

[dhruvmanila]

I'm confused, you say:

Note that the newline after .3f is part of the format specifier which needs to be preserved.

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {variable:.3f
} ddddddddddddddd eeeeeeee"""

But the original example had no newline after 3f.

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f} ddddddddddddddd eeeeeeee"""

The note is for the two snippets above it. The two code snippets you've provided aren't connected. I meant that the following code

f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
    variable:.3f
} ddddddddddddddd eeeeeeee"""

will be formatted as

-f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
-    variable:.3f
+f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {variable:.3f
 } ddddddddddddddd eeeeeeee"""

The formatter collapsed the f-string expression element but there's still a newline in the format spec which is why you see that the f-string is still multiline.