commit ba53cc7ea9e26e1553b9f54feb5de5420301fee8
author Alexander Timin <altimin@chromium.org> Fri Feb 05 15:46:57 2021 +0000
committer Alexander Timin <altimin@chromium.org> Fri Feb 05 15:46:57 2021 +0000
tree 4f7c69c53936d650bb01239f66dfe1692188c9b8
parent 4e669c89cf41ef7cd55124aa67335c0b8f461f5f

TracedValue: template-based resolution

Add more complex rules for determining whether a class can be written
into a trace.

The following rules are checked in the given order:
- If T::WriteIntoTracedValue(perfetto::TracedValue) exists, use it.
  (allows for simpler code in most of the cases, but does not work when you can't modify the class).
- If perfetto::TraceFormatTraits<T>::WriteIntoTracedValue(
  perfetto::TracedValue, T) exists, use it.
- If T has operator (), taking perfetto::TracedValue, invoke it
  (useful for lambdas)
- If T is iterable and its nested type supports being written into a
  trace, iterate over the elements of T and write them into an array.
  (classes having std::begin(T) defined are considered to be iterables)

This patch also adds the following helpers:
- check_write_support<T>::{value,type} - defined when type T can be
  written in a trace. Useful for enabling support for a complex type
  only when a nested type also can be written into a trace.
- has_write_support<T>::value - boolean, whether T can be written into a
  trace or not (useful when this value has to be or'ed or negated, in
  other cases check_write_support produces more helpful error messages
  with the entire chain of template deduction attempts).

Some real-world examples (removing blink::TracedValue) can be found here: crrev.com/c/2484654
(Note: it is based on the earlier naming iteration, using OutputContext and ConvertToTraceFormat
instead of TracedValue and WriteIntoTracedValue).

R=primiano@google.com,eseckler@google.com,skyostil@google.com

Change-Id: Ic5067fc904c2f78be8c285bdd4b5d6d897485751