Confusing error message when rustfmt cannot format otherwise valid code

[NoraCodes]

I have Format On Save on in VSCode, and it is a reflex of mine to save a lot. Often, when I do, I get output like the following. Output from the tool automatically switches my bottom Toolbox view away from the Terminal or Problems tab, and is thus is quite annoying.

Request textDocument/formatting failed.
  Message: Reformat failed to complete successfully
  Code: -32603 

Previously reported (in error?) as a bug on rustfmt at rust-lang/rustfmt#1851

Some further poking shows that this is likely a bug in RLS, as rustfmt fails to format the code:

$ rustfmt src/lexer/mod.rs; echo $?
Rustfmt failed at /home/leo/Projects/relox/src/lexer/mod.rs:1: line exceeded maximum length (maximum: 100, found: 138) (sorry)
Rustfmt failed at /home/leo/Projects/relox/src/lexer/lexerror.rs:57: line exceeded maximum length (maximum: 100, found: 104) (sorry)
Rustfmt failed at /home/leo/Projects/relox/src/lexer/datatypes.rs:7: line exceeded maximum length (maximum: 100, found: 110) (sorry)
Rustfmt failed at /home/leo/Projects/relox/src/lexer/datatypes.rs:31: line exceeded maximum length (maximum: 100, found: 117) (sorry)
Rustfmt failed at /home/leo/Projects/relox/src/lexer/datatypes.rs:75: line exceeded maximum length (maximum: 100, found: 122) (sorry)

3  # 3 = Code is valid, but it is impossible to format it properly

Perhaps RLS could report these errors better?

[Korvox]

It really is a bug in rustfmt. Its been around over a year, but it is a blocker on 1.0 so it should eventually be fixed.

RLS reports format failed to complete successfully for all rustfmt issues I believe. Its more opinion on if it should be more verbose by default since you do the the output that shows the issue when you put it in verbose mode.

If anyone wants to actually work around it, trim your lines to <100 chars width. It is a real PITA since it prevents you from embedding long formatted text into your sources.

[Marwes]

As a start I think rls should report the warning(s) as a diagnostic. That way it is at least possible to see that the file has a problem. Took me a while to correlate these warnings with the fact that rls failed to format certain files!

[nrc]

I think this should be addressed a little now - since rustfmt is using the rustc format for errors, the error matcher in VSCode should match the diagnostic up with the source file.

We plan to address the warning-ness and exit code in Rustfmt very soon.

[MaxBittker]

Thanks @nrc !