extract index and bit-set related types into their own crate

[nikomatsakis]

Right now they live in rustc_data_structures, but we can rely on crates now. It'd be nicer if they were over in a rust-lang crate, which would let us optimize, test, and otherwise develop them independently from rustc. (It'd also be useful in projects like borrow-check). As you can see in this comment, the current plan is to begin doing this "in tree" and only later decide whether to move out of tree.

[saleemjaffer]

@nikomatsakis I would like to work on this. I'm a newbie to rust but would like to give this a shot!

[saleemjaffer]

@spastorino Should we use an existing crate in rust-lang-nursery for this purpose? Or should we create a new github project for this purpose similar to https://crates.io/crates/sparse-bitfield

[eddyb]

cc @nnethercote

[spastorino]

@rustbot release-assignment

[flavius]

I'm new to the rust compiler, having played on-off with rust for the past two years, and I'd like to get my feet wet with this. Also, what's the go-to communication channel in case I have questions?

cc @nikomatsakis
cc @eddyb

Just added the stage1 toolchain to rustup.

[nnethercote]

@flavius: Some people are on Discord, see here. Unfortunately, some other people are on Zulip, which isn't mentioned on that page. But Discord is a decent place to start.

[nikomatsakis]

We discussed this a bit on Zulip, and we had the idea that we should start by just creating the crate in tree before we talk about moving it out of tree.

Here are some mentoring instructions for how we would go about it:

• Create a directory src/librustc_index or whatever we decide to call it.
• Copy src/librustc_data_structures/Cargo.toml into that directory and adapt it -- probably deleting most all the dependencies and just tweaking the name.
• To start, I would make add rustc_index as a dep of rustc_data_structures and then pub use its API, so that downstream code doesn't have to change.
• Then move over the types one by one -- I'd probably start with the "index" code as all the other types use this Idx trait as a kind of common abstraction.
• Once this works, I personally would prefer to remove the pub use from librustc_data_structures bit by bit and update the downstream crates. I don't like having multiple paths that lead to the same thing, it's confusing.

I think that should be it!