Fix handling of curly braces in FormattableStrings

[abonie]

Fixes #11782

The problem was, that in interpolated strings we turn {{ into { (and }} into }) during lexing.

This fixes it by converting it back to {{ (and }}) before calling FormattableStringFactory.Create (after interpolation expressions ({expr}) have already been processed).

[psfinaki]

Thanks for fixing the bug, nevertheless I'd appreciate some meaningful examples in tests, like more or less real usecases for all this quite specific topic.

Also, since you've become a guru in the F# interpolated strings, consider checking correctness and completeness of our docs. Also, I guess it's worth adding a note about F# to the docs here and here.

[abonie]

Thanks for fixing the bug, nevertheless I'd appreciate some meaningful examples in tests, like more or less real usecases for all this quite specific topic.

Also, since you've become a guru in the F# interpolated strings, consider checking correctness and completeness of our docs. Also, I guess it's worth adding a note about F# to the docs here and here.

Thanks for the comment, somehow it made me take another look at this change and realize it will be broken again if we implement this extended interpolated strings RFC. I'll turn this PR back to draft and give this some more thought, because it doesn't make sense to introduce a hacky fix if we already see how it will break again.

Regarding request for more tests, I actually don't think we should extensively test FormattableStrings in F# outside of some cases that may interact weirdly with F# idiosyncrasies (like this bug).

[abonie]

Actually, I might have too hastily proclaimed that this will be broken by extended interpolation syntax - I think it happens to work fine, because whatever { and } are there after turning interpolated expressions into %P()s should be escaped before being passed to FormattableStringFactory.Create.

But it is now more apparent to me, that this is a hacky fix, because it implicitly couples this part of checking to some behavior of the lexer. On the other hand, it seems to me that they are already somewhat coupled together...

edit: Did some thinking and some testing - it will be fine after all, even with extended string interpolation syntax. Marking as "ready for review" :)

[psfinaki]

/azp run

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[psfinaki]

Same feedback - thanks for the fix, some clearer examples in tests and maybe some docs would be also helpful :)