rustfmt breaks a /* ... */ comment into /* ... //, with no terminating */

[ssbr]

With version = "Two" and wrap_comments = true, rustfmt can break a /* ... */ comment by changing one of its lines to start with //. The error does not occur without version "Two" or without wrap_comments. Maybe worth noting, we have both of those options on at work.

(Sorry if this is a duplicate bug, couldn't find anything by searching for is:issue state:open "wrap_comments = true" "version = \"Two\"")

Reproduction steps:

cargo new test-crate
cd test-crate
echo 'version = "Two"
wrap_comments = true' > rustfmt.toml
echo 'use rustc_middle::ty::{self, Region, Ty, TyCtxt}; /* xx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                                   * xxxxxxxxxxxxx */

fn main() {}
' > src/main.rs
cargo +nightly fmt # or rustfmt +nightly src/main.rs
cat src/main.rs 

On my computer, I get:

use rustc_middle::ty::{self, Region, Ty, TyCtxt}; /* xx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
// xxxxxxxxxxxxx

fn main() {}

Which is not valid syntax.

rustfmt version:

$ rustfmt +nightly --version
rustfmt 1.7.1-nightly (506f22b 2024-09-19)

[ytmimi]

Confirming I can reproduce this using rustfmt 1.8.0-nightly (6157568 2024-09-20), style_edition=2024¹ and wrap_comments=true. There have been similar issues reported before and I'll try to dig through the backlog later to find any that seem relevant here.

Input

use rustc_middle::ty::{self, Region, Ty, TyCtxt}; /* xx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                                   * xxxxxxxxxxxxx */

Output

use rustc_middle::ty::{self, Region, Ty, TyCtxt}; /* xx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
// xxxxxxxxxxxxx

Footnotes

1. style_edition replaces version in newer rustfmt releases ↩

[hlopko]

Thanks @ytmimi!

[dqkqd]

Is it ok for me to work on this issue? @ytmimi

I think the bug occurred because we try to rewrite_comment on other_lines, which is a part of a multi line block comment.

rustfmt/src/missed_spans.rs

Lines 285 to 289 in 5f48fe9

	let other_lines = &subslice[offset + 1..];
	let comment_str =
	rewrite_comment(other_lines, false, comment_shape, self.config)
	.unwrap_or_else(|_| String::from(other_lines));
	self.push_str(&comment_str);

I suppose a correct fix would be not separating comment lines in case of multi line block comment.
However, it will wrap comment in this case below as well.

Input

use Foo; /* x x x x x x
          * x x 
          * */

Output

use Foo; /* x x x x
          * x x x x
          * */

[ytmimi]

@dqkqd if you're interested in taking a look at this that would be great!