Pretty print the flow tree to json (Fix #12675)

[yoyo930021]

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #12675 (github issue number if applicable).

• There are tests for these changes OR
• These changes do not require tests because is debug use no test

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This change is

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @emilio (or someone else) soon.

[highfive]

Warning

• These commits modify layout code, but no tests are modified. Please consider adding a test!

[yoyo930021]

doesn't make for print_extra_flow_children function ->
self.print_extra_flow_children(print_tree);

        print_tree.new_level(format!("{:?}", self));
        self.print_extra_flow_children(print_tree);
        for kid in child_iter(self) {
            kid.print_with_tree(print_tree);
        }
        print_tree.end_level();

I can't handle this part,
so I want to open issues for it.

[izgzhen]

Hmm ... the issue you are trying to solve doesn't look easy to me actually. And I suppose that we can try to add another function emitting JSON object while retaining the old print function. But I am not very sure about the plan. @shinglyu idea?

[shinglyu]

This looks OK, but in order not to surprise people who uses this feature, we should wait until we have another tool that can print in the old format. (Maybe a python script that takes the JSON and print as a tree) Then we can merge all the patches at once.

Also I forgot to mention, we should check if bit of information in the old format is in the new JSON format, if something is missing, we should definitely add that to the JSON output.

[shinglyu]

Or we could create a new option in components/util/opts.rs called dump-flow-tree-json, and don't touch the original dump-flow-tree, this will keep the backward compatibility so we can experiment more freely.

[yoyo930021]

@shinglyu
I try!

[bors-servo]

☔ The latest upstream changes (presumably #13346) made this pull request unmergeable. Please resolve the merge conflicts.

[nox]

@yoyo930021 Any news on this?

[yoyo930021]

Sorry , no time to do it.
And I meet a few problem.

[shinglyu]

This feature can now use the JOSN serialization from #13740. If someone is interested I can point them to the related code.

[wafflespeanut]

Closing based on inactivity :/