Left behind trailing whitespace when formatting where bound with empty right hand side

With a default rustfmt gotten from rustup (version rustfmt 1.4.37-nightly (5dc8789 2021-05-21)
), the following leaves whitespace after the where bound, and gives an internal error about leaving trailing whitespace.

struct ThisIsALongStructNameToPushTheWhereToWrapLolololol;

const fn this_is_a_long_const_function_name() -> usize {
    0
}

impl ThisIsALongStructNameToPushTheWhereToWrapLolololol where
    [(); this_is_a_long_const_function_name()]: 
{
}

echo "c3RydWN0IFRoaXNJc0FMb25nU3RydWN0TmFtZVRvUHVzaFRoZVdoZXJlVG9XcmFwTG9sb2xvbG9sOwoKY29uc3QgZm4gdGhpc19pc19hX2xvbmdfY29uc3RfZnVuY3Rpb25fbmFtZSgpIC0+IHVzaXplIHsKICAgIDAKfQoKaW1wbCBUaGlzSXNBTG9uZ1N0cnVjdE5hbWVUb1B1c2hUaGVXaGVyZVRvV3JhcExvbG9sb2xvbCB3aGVyZQogICAgWygpOyB0aGlzX2lzX2FfbG9uZ19jb25zdF9mdW5jdGlvbl9uYW1lKCldOiAKewp9Cg==" | base64 -d | rustfmt is the file as base64.

Testing the code on 129c888 (Current head of master) shows the same issue. I tried getting branch v2.0.0-rc.2 working to test it there, but got a build failure.

I did see that there's been other issues opened for leaving behind trailing whitespace, but didn't see one for this specific case.

[calebcartwright]

Thanks for the report!

I did see that there's been other issues opened for leaving behind trailing whitespace, but didn't see one for this specific case.

No worries, with these particular cases it's better to open a new issue since the "error[internal]: left behind trailing whitespace" is a common symptom that can occur with a wide variety of root causes

[ytmimi]

I'm pretty sure #5019 fixes this