Do the opaque check earlier in `CompInfo::codegen`

[fitzgen]

Right now we generate all of a compound type's fields and methods eagerly, and then throw away the result of that computation for opaque types here:

https://github.com/rust-lang-nursery/rust-bindgen/blob/8aa41d195b278d36b9cde9d17c01f5651d9b79b3/src/codegen/mod.rs#L1657-L1675

Instead, we should only codegen the fields and methods if the type isn't opaque in the first place.

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

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[fitzgen]

@MaloJaffre, were you interested in picking this issue up as well? I saw you left a @highfive: assign me comment, but then deleted it. It seems like our highfive bot integrations is half broken right now (probably since the move to rust-lang-nursery)

[MaloJaffre]

@fitzgen, sorry, I've mixed up the two issues.

[jhod0]

@fitzgen, I have a fix which is passing tests, but I am not completely sure of its correctness - it doesn't call struct_layout.saw_vtable() when item is opaque.

I pretty much just added some if !is_opaque {...} wrappers around the code paths that add to fields and methods

[fitzgen]

@jhod0 can you make a WIP pull request? That's the easiest way to have discussion about the code. Thanks!

[jhod0]

@fitzgen I'll make one later, and ping you when I do

[jhod0]

@fitzgen there is now a PR!