Add an option to aggressively opaque type

[upsuper]

See Gecko bug 1412077, it may be a good idea to introduce a new option to aggressively mark types opaque based solely on the whitelist. This way, we can avoid generating types and their layout tests if we don't need them at all.

We can call the new option "whitelist-only", "aggressive-opaque", or something like that. And we probably should report error when this option is specified but there is still any opaque type in the list.

[upsuper]

It would let us generate smaller binding files, and thus potentially accelerate building.

[emilio]

I doubt a lot the bottleneck of the build is that, fwiw. Since I had to investigate #1118, I have a single-file stylo input we can use to profile and figure out what is the bottleneck in bindgen.

Also, I doubt rust would struggle to handle a bunch of #[test] only free function definition.

[upsuper]

This may not be a bottleneck for bindgen itself (although outputting fewer data might help a bit), but wouldn't generating more types give rustc more pressure when resolving them, especially when there are tons of useless types?

Also smaller binding files could also help analyzing and avoiding unrelated changes / conflicts when people have them in tree.

[emilio]

stylo-input.txt

This is the input, if someone wants to profile.

[emilio]

This may not be a bottleneck for bindgen itself (although outputting fewer data might help a bit), but wouldn't generating more types give rustc more pressure when resolving them, especially when there are tons of useless types?

I... guess? I don't know, I suspect this is not a big deal, but again without numbers, it's hard to say.

[upsuper]

I... guess? I don't know, I suspect this is not a big deal, but again without numbers, it's hard to say.

I guess there is no way to verify this until we actually have such new option :)

[emilio]

Or until someone actually profiles the style crate and sees where rust spends its time :P

[upsuper]

I think I saw some bug filed against rust before about having lots of types makes rustc take lots of time. But I failed to find that specific issue. That may have been resolved, though.

[upsuper]

Oh that was about unused import. Never mind.

[tamird]

This would also be useful in cases where almost all types want to be opaque, but a select few require concrete representations.

It'd be really useful if there was a way to mark types opaque by default and then "clarify" the ones desired.

[fitzgen]

I could imagine something like

builder
    .opaque_type(".*")
    .no_opaque("SomethingWeCareAbout")
    .no_opaque("SomeOtherThing")
    // ...

which is analogous to derive_copy(true).no_copy("SomeSpecificType").