Internal error when trying to format code with inline block comment

[Sorseg]

When formatting this file
cargo +nightly fmt does not modify the file and reports this error:

error[internal]: left behind trailing whitespace
   --> <...>/riano/src/main.rs:353:353:58
    |
353 |                                     piano.sustain = true; 
    |                                                          ^
    |

warning: rustfmt has failed to format. See previous 1 errors.

$ cargo +nightly fmt --version
rustfmt 1.7.0-nightly (59e2c01 2024-06-17)

Removing the block comment on this line allows rustfmt to format the file again.

Thank you for maintaining this! 🧡

[ytmimi]

@Sorseg linking to the file is definitely useful. When you get a chance could you try to create a minimal test case that someone could use to reproduce the issue.

[Sorseg]

I was able to boil it down to this

struct S {
    f: u32,
}

fn main() {
    let s = Some(S { f: 42 });
    match s {
        Some( S{f: /* some special value */3}) => { }
        _ => {}
    }
}

cargo fmt doesn't error, just silently does nothing

Is this workable?

Thank you for taking a look

[ytmimi]

If you set error_on_unformatted = true you'll get some additional information about why this failed to format. The issue here is that rustfmt isn't expecting to find a comment between f: 3 in the pattern. To prevent dropping the comment rustfmt doesn't rewrite the code:

You should see an error similar to this:

error[internal]: not formatted because a comment would be lost
 --> <stdin>:7
  |
7 |     match s {
  |
  = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

warning: rustfmt has failed to format. See previous 1 errors.

[ytmimi]

I believe we'd need to make adjustments in the Rewrite for PatField impl to address this, though my current recommendation would be to not write comments within struct fields:

rustfmt/src/patterns.rs

Lines 385 to 433 in e494418

	impl Rewrite for PatField {
	fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
	let hi_pos = if let Some(last) = self.attrs.last() {
	last.span.hi()
	} else {
	self.pat.span.lo()
	};
	let attrs_str = if self.attrs.is_empty() {
	String::from("")
	} else {
	self.attrs.rewrite(context, shape)?
	};
	let pat_str = self.pat.rewrite(context, shape)?;
	if self.is_shorthand {
	combine_strs_with_missing_comments(
	context,
	&attrs_str,
	&pat_str,
	mk_sp(hi_pos, self.pat.span.lo()),
	shape,
	false,
	)
	} else {
	let nested_shape = shape.block_indent(context.config.tab_spaces());
	let id_str = rewrite_ident(context, self.ident);
	let one_line_width = id_str.len() + 2 + pat_str.len();
	let pat_and_id_str = if one_line_width <= shape.width {
	format!("{id_str}: {pat_str}")
	} else {
	format!(
	"{}:\n{}{}",
	id_str,
	nested_shape.indent.to_string(context.config),
	self.pat.rewrite(context, nested_shape)?
	)
	};
	combine_strs_with_missing_comments(
	context,
	&attrs_str,
	&pat_and_id_str,
	mk_sp(hi_pos, self.pat.span.lo()),
	nested_shape,
	false,
	)
	}
	}
	}