macro fails to be wrapped if it is exactly one character beyond max_width

[AaronKutch]

macro_rules! test {
    ($asdfghj:expr, $qwertyuiop:expr, $zxcvbnmasdfghjkl:expr, $aeiouaeiouaeiou:expr, $add:expr) => {{
        return;
    }};
}

Using rustfmt 1.4.8 nightly with a .rustfmt.toml with the following:

unstable_features = true
error_on_line_overflow = true
error_on_unformatted = true
#format_macro_matchers = true

If this is manually formatted, overflow will not happen. However, if the format_macro_matchers = true line is uncommented, the line will always be reformatted to overflow until another character is added.

[ytmimi]

Confirming I can reproduce the issue with rustfmt 1.5.1-nightly (f2c31ba0 2022-07-19). rustfmt does not format the given input when format_macro_matchers is true even though it exceeds the max_width of 100.

input (length of the macro matcher line including indentation is 101)

macro_rules! test {
    ($asdfghj:expr, $qwertyuiop:expr, $zxcvbnmasdfghjkl:expr, $aeiouaeiouaeiou:expr, $add:expr) => {{
        return;
    }};
}

output format_macro_matchers=true (lunchanged)

macro_rules! test {  
    ($asdfghj:expr, $qwertyuiop:expr, $zxcvbnmasdfghjkl:expr, $aeiouaeiouaeiou:expr, $add:expr) => {{
        return;
    }};
}

---

When the macro_matcher is 102 characters long, then it wraps. I assume we're not taking the length of the 2nd { into account.

input (length of the macro matcher line including indentation is 102)

macro_rules! test {
    ($aasdfghj:expr, $qwertyuiop:expr, $zxcvbnmasdfghjkl:expr, $aeiouaeiouaeiou:expr, $add:expr) => {{
        return;
    }};
}

output format_macro_matchers=true (wrapped)

macro_rules! test {
    (
        $aasdfghj:expr, $qwertyuiop:expr, $zxcvbnmasdfghjkl:expr, $aeiouaeiouaeiou:expr, $add:expr
    ) => {{
        return;
    }};
}

For anyone who wants to take a look at this I'd start in format_macro_args, and then move on to wrap_macro_args

rustfmt/src/macros.rs

Lines 966 to 987 in f2c31ba

	// This is a bit sketchy. The token rules probably need tweaking, but it works
	// for some common cases. I hope the basic logic is sufficient. Note that the
	// meaning of some tokens is a bit different here from usual Rust, e.g., `*`
	// and `(`/`)` have special meaning.
	//
	// We always try and format on one line.
	// FIXME: Use multi-line when every thing does not fit on one line.
	fn format_macro_args(
	context: &RewriteContext<'_>,
	token_stream: TokenStream,
	shape: Shape,
	) -> Option<String> {
	if !context.config.format_macro_matchers() {
	let span = span_for_token_stream(&token_stream);
	return Some(match span {
	Some(span) => context.snippet(span).to_owned(),
	None => String::new(),
	});
	}
	let parsed_args = MacroArgParser::new().parse(token_stream)?;
	wrap_macro_args(context, &parsed_args, shape)
	}

Linking the format_macro_matchers tracking issue #3354

[davidBar-On]

Submitted PR #5582 with a suggested fix.