This change fixes an unsoundness issue discovered by YruamaLairba.
The problem is that the external methods of an extension need to cast the LV2_Handle they receive from the host to a Rust type in order to use them. Currently, the plugin instance the host deals with has the type PluginInstance<P>, where P is the type of the plugin. Therefore, an external extension method would need to cast the LV2_Handle to *mut PluginInstance<P>.
However, I didn't think about that when I wrote the State extension and casted the LV2_Handle to *mut P, which may be unsound since PluginInstance is repr(Rust) and therefore a pointer to an instance of this struct may or may not be a pointer to P too.
I could fix that in this instance, but others might forget this problem too. Therefore, I decided to make PluginInstance<P>repr(C) and assured that P is the first field of the struct. This means that a valid *mut PluginInstance<P> is also a valid *mut P and the entire source of the problem is solved.
This change fixes an unsoundness issue discovered by YruamaLairba.
The problem is that the external methods of an extension need to cast the
LV2_Handlethey receive from the host to a Rust type in order to use them. Currently, the plugin instance the host deals with has the typePluginInstance<P>, wherePis the type of the plugin. Therefore, an external extension method would need to cast theLV2_Handleto*mut PluginInstance<P>.However, I didn't think about that when I wrote the
Stateextension and casted theLV2_Handleto*mut P, which may be unsound sincePluginInstanceisrepr(Rust)and therefore a pointer to an instance of this struct may or may not be a pointer toPtoo.I could fix that in this instance, but others might forget this problem too. Therefore, I decided to make
PluginInstance<P>repr(C)and assured thatPis the first field of the struct. This means that a valid*mut PluginInstance<P>is also a valid*mut Pand the entire source of the problem is solved.