FFI and union

[sanxiyn]

How would one call C functions involving union with Rust FFI?

SpiderMonkey's jsval is one example.

[thestinger]

There could be unsafe enum with the layout defined to be the same as C for interoperability. The only other way to deal with it would be finding the alignof and sizeof of the union in C for each platform and then translating that to Rust.

[sanxiyn]

Referencing Aatch/rust-xcb#2.

[yichoi]

referencing servo/servo#398

referencing servo/rust-mozjs#9

[Aatch]

The unsafe enum idea appeals to me, since I thought about it as an option when trying to solve the union issue in rust-xcb, but decided that relying on the representation of enums was too "hacky" and fragile.

[pnkfelix]

brson mentions in the description for #6346 that a "macro based solution" would be appropriate here, though I do not current know what that would entail. (It sounds to me like a potential alternative to the changes to the grammar to add unsafe enum that have been discussed here.)

[pnkfelix]

Nominating for milestone 3, feature complete.

[emberian]

I don't think a "macro-based solution" would be appropriate, as you need to restrict the valid range of values at the site of usage, which macros cannot do.

[graydon]

An attribute on an enum that makes it have no discriminant and makes any match on the variant-part succeed, should be sufficient. Not pretty but neither are C union semantics.

[graydon]

accepted for feature-complete milestone

[Skrylar]

I ran in to this problem recently as well; Allegro makes use of Unions for passing events around in C, which turns out to be a pain to deal with in Rust.

[pnkfelix]

We do want to solve this problem eventually, but it need not block 1.0. Assigning P-low.