Setup snapshot testing for editoast_derive using insta

[leovalais]

closes #7467

Setup snapshot testing library insta in order to unit-test our derive macros. This approach has several benefits:

• Tests are easy to write, so we can have many
• When the snapshot changes, insta shows a nice diff, making debugging our changes to the macro easier
• Facilitates the review: snapshots are committed, so their diff appear in the PR description

Since we may soon have some extensive changes coming to our macros with the new error system, having tests and ease the review process are essential.

A few drawbacks I can think of:

• The macro expansion need to be determinist: no more HashMaps are allowed
• We need to refrain as much as possible from using macros in our code gen, otherwise it hinders the readability of the snapshots
• We have to "accept" diffs manually (either by renaming the file or by using cargo insta, their official interactive snapshot management CLI tool)

Important

Review this PR commit by commit to see how the snapshots evolve when the macro output changes.

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Attention: Patch coverage is 96.37681% with 10 lines in your changes missing coverage. Please review.

Project coverage is 79.88%. Comparing base (02ccfc8) to head (8023bd3).
Report is 107 commits behind head on dev.

Files with missing lines	Patch %	Lines
editoast/editoast_derive/src/model/identifier.rs	50.00%	6 Missing ⚠️
editoast/editoast_derive/src/model.rs	82.60%	4 Missing ⚠️

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@@             Coverage Diff             @@
##              dev    #9938       +/-   ##
===========================================
+ Coverage   38.21%   79.88%   +41.66%     
===========================================
  Files         992     1054       +62     
  Lines       92158   105597    +13439     
  Branches     1186      759      -427     
===========================================
+ Hits        35221    84353    +49132     
+ Misses      56486    21202    -35284     
+ Partials      451       42      -409     

Flag	Coverage Δ
editoast	73.72% <96.37%> (+0.32%)	⬆️
front	89.24% <ø> (+69.05%)	⬆️
gateway	2.18% <ø> (+<0.01%)	⬆️
osrdyne	3.28% <ø> (ø)
railjson_generator	87.50% <ø> (+0.01%)	⬆️
tests	87.00% <ø> (ø)

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[woshilapin]

We have to "accept" diffs manually (either by renaming the file or by using cargo insta, their official interactive snapshot management CLI tool)

I actually don't see this as a drawback. Sure, it's more work for the developer, but we actually always do that even for "normal" unit tests: a expected output changed, we need to review it manually and validate that the new output is valid. So it's actually a good thing. Maybe you meant it's more cumbersome to do that operation of validating a different test output?

In general, this PR is going to bring much more to the experience of dealing with macros...

[flomonster]

Great idea! Thanks for the PR.

[leovalais]

Thanks for the feedback! I've added *.snap.new to our .gitignore to avoid accidentally commit snapshots that still require some review (missing cargo insta review).

[Khoyo]

We should probably add some documentation about working with insta, either in editoast README or in a new one for editoast_derive

[woshilapin]

Thanks for digging up about the "macro into function" proposition, even if it ended up not satisfying. And thanks for the additional documentation. Great improvement for editoast_derive in general! ❤️