Plaster [[nodiscard]] on some RAII classes

The idea is to prevent silly mistakes such as

  QMutexLocker(mutex);
  doSomething();

where the locker is constructed and destroyed immediately. Compilers
don't normally warn in these cases (as the constructor/destructor
pairs involved do have side effects), but we can mark the type as
[[nodiscard]] to encourage warnings.

There is another couple of classes for which this would make sense
(notably, the R/W lockers), but unfortunately those are exported
classes, and GCC has a bug where one can't mix two different attribute
syntaxes on the same entity [1], so I'm skipping those.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102399

Change-Id: I75a2443dc71e6b80613b8edd52a04d3379355728
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

0 files changed, 0 insertions, 0 deletions