Re: [PATCH 10/10] rust: bindings for MemoryRegionOps

[Zhao Liu]

> > > +pub struct MemoryRegionOps<T>(
> > > +    bindings::MemoryRegionOps,
> > > +    // Note: quite often you'll see PhantomData<fn(&T)> mentioned when 
> > > discussing
> > > +    // covariance and contravariance; you don't need any of those to 
> > > understand
> > > +    // this usage of PhantomData.  Quite simply, MemoryRegionOps<T> 
> > > *logically*
> > > +    // holds callbacks that take an argument of type &T, except the type 
> > > is erased
> > > +    // before the callback is stored in the bindings::MemoryRegionOps 
> > > field.
> > > +    // The argument of PhantomData is a function pointer in order to 
> > > represent
> > > +    // that relationship; while that will also provide desirable and 
> > > safe variance
> > > +    // for T, variance is not the point but just a consequence.
> > > +    PhantomData<fn(&T)>,
> > > +);
> >
> > Wow, it can be wrapped like this!
> 
> I like your enthusiasm but I'm not sure what you refer to. ;) Maybe
> it's worth documenting this pattern, so please tell me more (after
> your holidays).

Throughout the entire holiday, I couldn't think of a better way to
express this. I find it particularly useful when wrapping multiple
callbacks. In the future, I want to explore more use cases where this
pattern can be applied.

> > > +impl MemoryRegion {
> > > +    // inline to ensure that it is not included in tests, which only
> > > +    // link to hwcore and qom.  FIXME: inlining is actually the opposite
> > > +    // of what we want, since this is the type-erased version of the
> > > +    // init_io function below.  Look into splitting the qemu_api crate.
> >
> > Ah, I didn't understand the issue described in this comment. Why would
> > inlining affect the linking of tests?
> 
> If you don't inline it, do_init_io will always be linked into the
> tests because it is a non-generic function. The tests then fail to
> link, because memory_region_init_io is undefined.

I find even if I drop the `inline` attribution, the test.rs can still be
compiled (by `make check`), I think it's because test.rs hasn't involved
memory related tests so that do_init_io isn't linked into test.rs.

> This is ugly because do_init_io exists *exactly* to extract the part
> that is not generic. (See
> https://users.rust-lang.org/t/soft-question-significantly-improve-rust-compile-time-via-minimizing-generics/103632/8
> for an example of this; I think there's even a procedural macro crate
> that does that for you, but I can't find it right now).

Thanks! I see. I agree to keep `inline` anyway.