Fix Live Debugger returned arrow instrumentation

[watson]

What and why?

Fix a Live Debugger transform bug that could emit invalid JavaScript when instrumenting a block-bodied function that returns an anonymous expression-bodied arrow.

For example, this input:

export const make = (cond) => {
    return () => cond ? a() : b();
};

Could generate output shaped like this before the fix, formatted here for readability:

return (
    $dd_rv1 = () => {
        const $dd_p0 = $dd_probes('src/utils.ts;<anonymous>');
        try {
            if ($dd_p0) $dd_entry($dd_p0, this);
            const $dd_rv0 =
                cond ? a() : b(),
                $dd_p1 ? $dd_return($dd_p1, $dd_rv1, this, {cond}) : $dd_rv1
            );

            if ($dd_p0) $dd_return($dd_p0, $dd_rv0, this);
            return $dd_rv0;
        } catch(e) {
            if ($dd_p0) $dd_throw($dd_p0, e, this);
            throw e;
        }
    }
);

In all-functions mode, both the inner arrow suffix and the outer return wrapper could be inserted at the same byte offset. The outer return-capture tail landed before the inner arrow suffix closed, leaving invalid JavaScript. Named-only mode avoided the issue by skipping the anonymous inner arrow.

How?

Change the outer return EXPR wrapper tail from appendLeft to appendRight at ret.argEnd, so an inner expression-body arrow suffix already queued at that offset is emitted first and closes the inner instrumentation before the outer return capture tail.

Add regression cases for block-bodied arrows and function declarations returning expression-bodied arrows, including a JSX mapping case, plus a block-bodied inner arrow control. The table continues to parse transformed output and now also asserts return instrumentation is still present.

[watson]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add decorator support to Live Debugger instrumentation #448
• Fix Live Debugger parenthesized arrow body instrumentation #447
• Fix Live Debugger returned arrow instrumentation #442 👈 (View in Graphite)
• Fix live debugger curried function instrumentation #441
• Fix Safari dormant overhead in Live Debugger instrumentation #438
• Add Live Debugger runtime benchmark coverage #429 : 1 other dependent PR (#440 )
• master

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[datadog-datadog-prod-us1-2]

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