Scattered collection PoC

[mmastrac]

This is the PoC for a new crate built on top of ctor and link-section: scattered-collect. This won't land directly -- it will get carved up into smaller PRs and polished.

The idea is:

#[gather]
pub static COLLECTION: ScatteredSlice<T>;

#[scatter(COLLECTION)]
pub static ITEM: T = { ... };

Why?

Distributed slice is awesome and nearly perfect but it's a static snapshot out of the linker and downstream code occasionally needs more functionality on top of it, ie:

• Sorting/ordering
• Lookups by something other than ordinal index

Also, it doesn't support WASM which is a tougher to fix.

Inventory supports pretty much everything, but it's just a #[ctor] with a linked list and doesn't offer easy random access.

TODO

I'll spin some PRs off of this one and slowly land some pieces:

• PR for placeholder crate (& publish a 0.0.0)
• Land the top-level scatter/gather macro implementations
• Implement and land the slice/referenced slice trivial types
• Implement and land sorted versions of slices
• Implement and land map
• Implement and land iterable

Data types

• (TODO) Iterable: collected items, only available via iterator (analogous to inventory crate)
• Slice: collected items, not available at original definition site
• Referenced slice: collection items, available at original definition site (analogous to distributed slice crate)
• Sorted slice: collected and sorted items, not available at original definition site
• Sorted referenced slice: collected and sorted items, available at original definition site
• Map: similar to hashmap's swisstable implementation, but space is allocated at link time and hashes are precomputed to make construction much cheaper

There are "referenced" and "unreferenced" versions of the slices - the unreferenced versions may be faster as the code is free to re-organize the data more easily.

Const hashing

• New trait: ConstHasher (should be able to derive this for custom structs as well)

Platform challenge notes

• WASM: link sections are pretty much useless unless we do some sort of post-processing on the binary, which might be an option. We can, however, at least count the number of items using link-section and then place all the data via alloc + ctor copies where needed (the scattered iterator won't need this, and the map needs at least one iteration anyways, so we might be able to have a custom impl there).
• All other platforms: generally will work via slices as expected. Similar single-threaded init with a late ctor to ctor's static will allow for guaranteed init before main without ordering issues.

[mmastrac]

Closing - PoC completed.