- [x] Have you followed the [guidelines for contributing](https://github.com/snapcore/snapcraft/blob/master/CONTRIBUTING.md)? - [x] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [x] If this is a bugfix. Have you checked that there is a bug report open for the issue you are trying to fix on [bug reports](https://bugs.launchpad.net/snapcraft)? - [x] If this is a new feature. Have you discussed the design on the [forum](https://forum.snapcraft.io)? - [x] Have you successfully run `./runtests.sh static`? - [x] Have you successfully run `./runtests.sh unit`? ----- Currently Snapcraft supports a flat list of architecture overrides in the snapcraft.yaml that look something like this: architectures: [amd64, i386] If this property is specified, Snapcraft passes it on verbatim to the snap.yaml, which then assumes the same generated snap supports both of those architectures (called a "fat" snap). This PR resolves #1681 by expanding Snapcraft's `architectures` property to support the advanced grammar, which allows the property to be dual-purposed into not only run-time architecture support, but build-time (i.e. specify the architecture on which it should be built, as well as the architectures on which the resulting snap can be run). That looks something like this: architectures: - on i386: [amd64, i386] - on armhf: [arm64, armhf] The final snap.yaml will still only contain a flat list of architectures.