COM object implementations are agile and indicate this by implementing both IAgileObject and IMarshal. This is the default and suitable for the vast majority of implementations, whether or not you need support for COM apartments.

For C++/WinRT I provided a non_agile marker type that indicated that the object would not provide an implementation of IAgileObject or IMarshal directly. You are then free to implement them yourself or not at all. This supports the few cases where you need more control over the apartment model of the caller and callee.

The implement macro now provides an equivalent option to control the agility directly. Here is an example from the accompanying test:

// IAgileObject and IMarshal are implemented
#[implement(ITest)]
struct DefaultAgile;

// IAgileObject and IMarshal are *not* implemented
#[implement(ITest, Agile = false)]
struct AgileFalse;

// IAgileObject and IMarshal are implemented
#[implement(ITest, Agile = true)]
struct AgileTrue;

// IAgileObject is implemented while IMarshal is not
#[implement(ITest, Agile = false, IAgileObject)]
struct ExplicitAgile;

Fixes: #3768