Strange formatting with nested macro_rules! and long comment

[Aaron1011]

Input

macro_rules! bad {
    () => {
        macro_rules! inner {
            () => {
                // This needs to have a width of over 100 characters to trigger the issue 12345678901
                ("a", "B")
            };
        }
    };
}

Output

macro_rules! bad {
    () => {
        macro_rules! inner {
                    () => {
                        // This needs to have a width of over 100 characters to trigger the issue 12345678901
                        ("a", "B")
                    };
                }
    };
}

Expected output

macro_rules! bad {
    () => {
        macro_rules! inner {
            () => {
                // This needs to have a width of over 100 characters to trigger the issue 12345678901
                ("a", "B")
            };
        }
    };
}

Meta

• rustfmt version: rustfmt 1.4.16-stable (939e164 2020-06-11)
• From where did you install rustfmt?: rustup

Modifying the comment to be less than 100 characters causes rustfmt to leave the code unchanged.

[ayazhafiz]

The problem is that we bail on trying to format the inner code block if any formatted line comes out to >100 chars:

rustfmt/src/formatting/util.rs

Lines 101 to 106 in ff57cc6

	} else if line.len() > config.max_width() {
	// If there are lines that are larger than max width, we cannot tell
	// whether we have succeeded but have some comments or strings that
	// are too long, or we have failed to format code block. We will be
	// conservative and just return `None` in this case.
	return None;

I'm not convinced this check is needed since we jump out early if retrieving the formatted snippet fails:

rustfmt/src/formatting/util.rs

Line 81 in ff57cc6

let mut formatted = format_snippet(&snippet, &config_with_unix_newline)?;

And no system/idempotency tests are broken by removing this. In the most conservative case I think it is okay to make an exemption for macros here, since we can't do better anyway.