Abseil Team ae1b7ad430 Mark ACTION_Pn()-generated functions as must-use-result, adding non-compilation tests.
This catches when a client creates an action and discards it, thinking that the action has actually been applied to something.

This will help people who make the mistake of defining, for example, both `void Use(Foo*, Bar)` and `ACTION_P(Use, bar) { Use(arg, bar); }` for later application to a Foo.  With such an overload, a client may then write `Use(bar);`, selecting the Action<> overload and being confused why nothing happens.

This also catches when a client defines their own action in terms of an ACTION_P()-generated one, invokes the Action<>'s builder, and then fails to invoke the resulting action, thinking it's operating on the outer action's parameters.

PiperOrigin-RevId: 433197479
Change-Id: I98e4389150d01a5e753230113016d9fc38b1d260
2022-03-08 06:42:11 -08:00
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