[ExpressionLanguage] Add comment support to expression language

[valtzu]

Q	A
Branch?	7.2
Bug fix?	no
New feature?	yes
Deprecations?	no
License	MIT

When expression language expressions are provided by the user, it would be really helpful if we could include comments in the expression. Many times the user will write something like

customer.group == 'good_customer' or customer.id == 123

which in my opinion, would be much better with a comment

customer.group == 'good_customer'
or customer.id == 123 /* 123 is the test customer */

so that the next person reading the expression knows what the magical 123 is.

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The implementation is quite simple, given it's ok to strip the comments from AST & the compiled version.

Why /* & */

This seems to be the most common way among different programming languages, including JS, C#, Go, C, PHP, Java, Rust – also in business scenarios you often times want multi-line comments, for which this syntax usually stands for. Also, this makes it possible to use comments in the middle of single-line expressions.

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At the moment, unlike multi-line comments in PHP, we always require closing */.

[carsonbot]

Hey!

Thanks for your PR. You are targeting branch "7.1" but it seems your PR description refers to branch "7.2".
Could you update the PR description or change target branch? This helps core maintainers a lot.

Cheers!

Carsonbot

[GromNaN]

This feature is useful when used with user input where it is not otherwise possible to add comments in the code.

I think you could add tests on expressions like /* /* comment */ and /* comment */ */ to ensure the regex catch the right comment separators.

[valtzu]

Added some extra tests + changelog (too late for 7.1 as I understand)

[fabpot]

Thank you @valtzu.