Optimized representations

[larsbergstrom]

A key worry in a web browser is the size of data. We'd like to know that e.g. an enum with a single pointer constructor and 5 nullary constructors can still fit into a single machine word on 64-bit architectures. All of these optimizations need to work with custom smart pointer types, but we also need to be able to disable them for some smart pointers, e.g. pointers that are tracked by an external GC.

CC: @zwarich @pcwalton

[thestinger]

A key worry in a web browser is the size of data. We'd like to know that e.g. an enum with a single pointer constructor and 5 nullary constructors can still fit into a single machine word on 64-bit architectures.

It's not possible to do this because Rust allows taking references to the values inside an enum. It can't use tagged pointers without breaking the language semantics.

[thestinger]

Since this would be a language change rather than an optimization in the compiler, it needs to go through the RFC process. I really don't know how it would fit into the enum system though. It would need to forbid using ref / ref mut for a variant.

[pnkfelix]

Made a fork off in discuss.rust-lang.org: http://discuss.rust-lang.org/t/optimized-representations/511 to allow continued discussion that could perhaps lead to an RFC in the future.

[brson]

Since this is an issue that impacts servo I want to continue to track it.

[thestinger]

It would be backwards compatible and would need to go through an RFC so I don't really think it belongs on the issue tracker. The bug tracker is filled with non-actionable issues and that means it's a much less useful for tracking bugs and actionable feature requests. This isn't a good place to hold open discussions about stuff.

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#317