Non-idempotent rustfmt run, adding commas to comments

[jaybosamiya]

Consider this (minimized) code:

struct Tau {}
impl Tau where (): Clone //,
{
}

Every time you run rustfmt foo.rs, it adds another comma to that comment.

$ cat foo.rs
struct Tau {}
impl Tau where (): Clone //,
{
}

$ for i in $(seq 1 10); do rustfmt foo.rs; wc -c foo.rs; done
48 foo.rs
49 foo.rs
50 foo.rs
51 foo.rs
52 foo.rs
53 foo.rs
54 foo.rs
55 foo.rs
56 foo.rs
57 foo.rs

$ cat foo.rs
struct Tau {}
impl Tau where (): Clone //,,,,,,,,,,,
{
}

Tested on multiple versions:

• rustfmt 1.5.2-stable (eb26296 2023-08-03)
• rustfmt 1.5.2-stable (90c54180 2023-05-31)

[ytmimi]

Thanks for the report!

Confirming I can reproduce the issue using rustfmt 1.6.0-nightly (547577fa 2023-10-10).
I thought one solution would be to place the , before the comment, but rustfmt will turn

struct Tau {}
impl Tau where (): Clone, // comment
{
}

into:

struct Tau {}
impl Tau where (): Clone // comment,
{
}

kicking off the idempotent cycle 🙃

[ytmimi]

It seems you can work around this right now using block comments:

Running rustfmt on this:

struct Tau {}
impl Tau where (): Clone, /* comment */
{
}

produces this output, which is idempotent:

struct Tau {}
impl Tau where (): Clone /* comment */ {}

Curiously, we don't add the trailing comma in this case.