RUF072: skip formfeeds on dedent

[second-ed]

Summary

fixes: #24246

Skips the formfeed on the dedent if the line starts with the formfeed and then readds it back to the start of the line.

Test Plan

Added bug report example to snapshot test cases

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[MichaReiser]

I'll do a full review later. Can you add the new test at the end? It reduces the snapshot diff size, which makes the changes easier to review

[second-ed]

Yeah sure, I'll make those changes and push up, I was trying to keep the error cases and ok cases together but agree it makes the diff horrible to read

[MichaReiser]

Let's update the documentation to explain how form feeds are handled

[MichaReiser]

Here's another test case that currently panics:

    #[test]
    #[rustfmt::skip]
    fn dedent_to_ignores_leading_form_feeds_when_checking_indentation() {
        let x = [
            "    1",
            "\x0C\x0C  2",
        ].join("\n");
        let y = [
            "1",
            "\x0C\x0C  2",
        ].join("\n");
        assert_eq!(dedent_to(&x, ""), Some(y));
    }

[second-ed]

Cheers, I'll have a look at this after work and see if I can find more test cases to sure it up

[MichaReiser]

I found two more failing test cases

#[test]
    #[rustfmt::skip]
    fn dedent_to_preserves_less_indented_later_line() {
        let x = [
            "   foo\n",
            "  bar\n",
        ].join("");
        let y = [
            "foo\n",
            "  bar\n",
        ].join("");
        assert_eq!(dedent_to(&x, ""), Some(y));
    }

    #[test]
    #[rustfmt::skip]
    fn dedent_to_preserves_less_indented_later_line_with_crlf() {
        let x = [
            "   foo\r\n",
            "  bar\r\n",
        ].join("");
        let y = [
            "foo\r\n",
            "  bar\r\n",
        ].join("");
        assert_eq!(dedent_to(&x, ""), Some(y));
    }

[MichaReiser]

Thank you. This turned out to be trickier than I thought.

[second-ed]

cheers for your help it was a fun one 😄