296: Slot no more: overhauled internal algorithm r=nikomatsakis a=nikomatsakis
This is the overhauled implementation that avoids slots, is more parallel friendly, and paves the way to fixed point and more expressive cycle handling.
We just spent 90 minutes going over it. [Some rough notes are available here,](https://hackmd.io/6x9f6mavTRS2imfG96tP5A) and a video will be posted soon.
You may find the [flowgraph useful](https://raw.githubusercontent.com/nikomatsakis/salsa/slot-no-more/book/src/derived-query-read.drawio.svg).
Co-authored-by: Niko Matsakis <niko@alum.mit.edu>
In cycle.md, change `[salsa::Cycle]`
to `salsa::Cycle` so mdbook doesn't get confused
and think there is a broken Markdown link.
The warning was as follows:
```
warning: Potential incomplete link
┌─ cycles.md:3:131
│
3 │ By default, when Salsa detects a cycle in the computation graph, Salsa will panic with a [`salsa::Cycle`] as the panic value. The [`salsa::Cycle`] structure that describes the cycle, which can be useful for diagnosing what went wrong.
│ ^^^^^^^^^^^^^^^^ Did you forget to define a URL for ``salsa::Cycle``?
│
= hint: declare the link's URL. For example: `[`salsa::Cycle`]: http://example.com/`
```
After fixing this problem, there are no more mdbook warnings.
It's simpler to just store a DatabaseKeyIndex. It may be somewhat
slower, we'll have to measure. But we can add back in this other
design later if we want.
This had two unexpected consequences, one unfortunate, one "medium":
* All `salsa::Database` must be `'static`. This falls out from
`Q::DynDb` not having access to any lifetimes, but also the defaulting
rules for `dyn QueryGroup` that make it `dyn QueryGroup + 'static`. We
don't really support generic databases anyway yet so this isn't a big
deal, and we can add workarounds later (ideally via GATs).
* It is now statically impossible to invoke `snapshot` from a query,
and so we don't need to test that it panics. This is because the
signature of `snapshot` returns a `Snapshot<Self>` and that is not
accessible to a `dyn QueryGroup` type. Similarly, invoking
`Runtime::snapshot` directly is not possible becaues it is
crate-private. So I removed the test. This seems ok, but eventually I
would like to expose ways for queries to do parallel
execution (matklad and I had talked about a "speculation" primitive
for enabling that).
* This commit is 99% boilerplate I did with search-and-replace. I also
rolled in a few other changes I might have preferred to factor out,
most notably removing the `GetQueryTable` plumbing trait in favor of
free-methods, but it was awkward to factor them out and get all the
generics right (so much simpler in this version).
- extend some of the empty sections, add a new common pattern
- also, show how to use anchors and include so we can test the sources
for common patterns
