Add a project .editorconfig file

[mhucka]

This adds settings based on some common-sense values and the Cirq project's current conventions, such as line length and use of spaces instead of tabs.

This resolves #6998.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.17%. Comparing base (931bc66) to head (2531c2f).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7081      +/-   ##
==========================================
- Coverage   98.17%   98.17%   -0.01%     
==========================================
  Files        1089     1089              
  Lines       95217    95217              
==========================================
- Hits        93481    93475       -6     
- Misses       1736     1742       +6     

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[pavoljuhas]

Can you please comment when is this needed?

My impression was that trailing spaces in md were basically a noise that would be better removed.

[mhucka]

It's a little bit bonkers, but the use of 2 trailing spaces is one of the ways that md lets you indicate an explicit line breeak. Here are a couple of places where this is described:

• https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#line-breaks
• https://www.markdownguide.org/basic-syntax/#:~:text=Line%20Breaks,spaces%2C%20and%20then%20type%20return.

I'll try to find a public spec somewhere and add a comment to the .editorconfig file to point to it.

The Google md style recommentation (which I can't find atm) recommends avoiding this construct. However, because it's technically allowed, and because other style guides and formatters pay attention to it, and because there doesn't seem to be a setting in editorconfig for "allow exactly 2 spaces but remove 1 or 3+ spaces", the only safe thing to do for md files seems to be to disable the setting to remove trailing whitespace. (If there's a better way, I'm completely happy to do it!)

[mhucka]

Actually, I didn't test this, but I bet 3 or more spaces would also produce a line break.

[mhucka]

Ah, markdownlint has a setting to control this: https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md#md009---trailing-spaces

The default is 2 spaces, meaning it will flag single spaces at the ends of lines.

[pavoljuhas]

There are only 3 places in our markdown files that have 2 trailing blanks. None of those is necessary; they don't really seem intentional.

$ git grep -n "\s{2,}$" "*.md" "*.mark*"
.github/ISSUE_TEMPLATE/project_health.md:3:about: Issues related to CI, automation infra, clean code, depreca>
docs/dev/nomenclature.md:14:`wave_function`, or `state` for this object (`state` is too overloaded).
docs/dev/style.md:13:with the following guidance.

I think the Google style guide recommendation against trailing double-spaces is reasonable.
They make text behave differently without visible markup, which makes it harder to judge if those spaces are intentional or left over from text editing. It is IMO better to stick to the explicit \ or <br> syntax for creating manual linebreaks.

Can we disallow trailing blanks in editorconfig and also flag them as errors in markdownlint?

[mhucka]

Yes; I'll make the change and push an update. (Actually, I just looked at our .markdownlintrc again, and it turns out it already disallows trailing spaces.)

[pavoljuhas]

Thank you!