RFC: remove virtual structs

[aturon]

Summary

Removes the "virtual struct" (aka struct inheritance) feature, which is currently feature gated.

Motivation

Virtual structs were added experimentally prior to the RFC process as a way of inheriting fields from one struct when defining a new struct.

The feature was introduced and remains behind a feature gate.

The motivations for removing this feature altogether are:

1. The feature is likely to be replaced by a more general mechanism, as part of the need to address hierarchies such as the DOM, ASTs, and so on. See this post for some recent discussion.
2. The implementation is somewhat buggy and incomplete, and the feature is not well-documented.
3. Although it's behind a feature gate, keeping the feature around is still a maintenance burden.

Detailed design

Remove the implementation and feature gate for virtual structs.

Retain the virtual keyword as reserved for possible future use.

Drawbacks

The language will no longer offer any built-in mechanism for avoiding repetition of struct fields. Macros offer a reasonable workaround until a more general mechanism is added.

Unresolved questions

None known.

[sfackler]

As one data point, there are no uses of virtual struct anywhere on Github except for the tests within rust-lang/rust itself: https://github.com/search?l=rust&q=%22virtual+struct%22&type=Code&utf8=%E2%9C%93

[nrc]

+1. We should keep virtual as a reserved keyword though (I think this is the only use of virtual at the moment).

[sinistersnare]

isn't struct inheritance hidden behind an feature gate? If so, why does it require a RFC to be removed? thats the point of feature gates. they may be removed at any time....

[aturon]

@sinistersnare A few months ago we made a change in policy to require RFCs when removing any language features, even if feature-gated.

See this policy.

In general, we want to go through the formal feedback and decision process to change the direction on major features, whether adding or removing them.

[reem]

+1. This seems like a generally unused feature that will probably be made unneeded by one of the many inheritance proposals.

[brson]

Merged as RFC 71. Discussion. Tracking.