Implement upper filter

[LucasGrugru]

Hello,

I find the project really interesting. I propose a very small contribution to add the uppercase filter.

I'm not a rust expert so feel free to tell me if anything needs to be changed.

[LilyAcorn]

This is a really good start thanks!

Unfortunately, upper is a bit weird so this will need some updating to handle that.

[LilyAcorn]

upper is a bit unusual because it's not considered HTML-safe upstream in Django.

To handle this correctly here, we'll need to replace .map_content with something like:

Suggested change

	.map_content(|content| Cow::Owned(content.to_uppercase())),
	.map(|content| content.content().to_uppercase())
	.map(|content| Cow::Owned(Content::String(content))),

We'll also need a test to demonstrate we follow Django's behaviour here. (I'd add it in the Python tests.) This test should include a html tag which will become html-escaped as well as uppercased in the expected output.

For more information about this quirk, see my blog post or the ticket and Django PR.

[LucasGrugru]

This Django case is really interesting.

I've been trying to use upper filter for HTML text in Django, and I'm not sure about the proposed solution. Django uses uppercase to encode text before encoding HTML characters. In this case, the resolve_string() method will encode HTML characters; I always have to call resolve_string() before calling to_uppercase() in the code. So the encoded HTML characters will be capitalized, which Django doesn't do.

I added test cases, especially the last one which I feel is not solvable without change resolve_string for allowing to return a HtmlUnsafe ?

[LilyAcorn]

That's a very good point - I think this is essentially a bug in the existing Rust implementation that's been exposed by your work here.

I've had a play around with your idea of introducing a HtmlUnsafe variant to the Content and ContentString enums and I've got the tests passing. 🚀

[codecov]

Codecov Report

Attention: Patch coverage is 97.44681% with 6 lines in your changes missing coverage. Please review.

Project coverage is 98.78%. Comparing base (0986a06) to head (3b89e91).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/render/types.rs	89.18%	0 Missing and 4 partials ⚠️
src/render/filters.rs	97.93%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   98.73%   98.78%   +0.04%     
==========================================
  Files          47       48       +1     
  Lines        7832     7958     +126     
  Branches     2811     2819       +8     
==========================================
+ Hits         7733     7861     +128     
  Misses         61       61              
+ Partials       38       36       -2     

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[LilyAcorn]

Thanks for making such a great start with this @LucasGrugru! I expect some of the other filters will be more straightforward to implement.