352: Add options to tracked funcitons for lru capacity r=nikomatsakis a=XFFXFF
fixes#344
Now we can write something like the following to set the lru capacity of tracked functions
```rust
#[salsa::tracked(lru=32)]
fn my_tracked_fn(db: &dyn crate::Db, ...) { }
```
some details:
* lru should not be combined with specify. We will report an error if people do #[salsa::tracked(lru = 32, specify)]
* set 0 as default capacity to disable LRU (Because I think doing this would make the code simpler when implementing `create_ingredients` of tracked functions).
* old salsa support to change lru capacity at runtime, [as noted here](https://salsa-rs.github.io/salsa/rfcs/RFC0004-LRU.html?highlight=change#reference-guide), but we do not support this now
Co-authored-by: XFFXFF <1247714429@qq.com>
Accumulators don't currently work across revisions
due to a few bugs. This commit adds 2 tests to show
the problems and reworks the implementation strategy.
We keep track of when the values in an accumulator were pushed
and reset the vector to empty when the push occurs in a new
revision.
We also ignore stale values from old revisions
(but update the revision when it is marked as validated).
Finally, we treat an accumulator as an untracked read,
which is quite conservative but correct. To get better
reuse, we would need to (a) somehow determine when different
values were pushed, e.g. by hashing or tracked the old values;
and (b) have some `DatabaseKeyIndex` we can use to identify
"the values pushed by this query".
Both of these would add overhead to accumulators and I didn'τ
feel like doing it, particularly since the main use case for
them is communicating errors and things which are not typically
used from within queries.
It turns out that we have some outputs (accumulators) for which
it only makes sense to have a `DependencyIndex` (they don't have
individual keys to identify).
We also track whether reset is required at the ingredient level.
For tracked struct fields, we were not using `push_mut`,
and I think that was an oversight.
The plan is to do a "dependent ingredient" interlinking pass
once the database is constructed.
I realized I can do this better:
* require that outputs are DatabaseKeyIndex, fewer unwraps,
more clearly justified
* when we validate result of tracked fn, also validate its outputs
(this is incompletely implemented, would ideally be separated
into its own commit, but I'm short for time)
The last step will allow us to keep the memoized results for
assigned values and means we don't have to eagerly clear them.
If we see an "assigned value" that is not verified in the current
revision, it can simply be considered dirty.
We can still delete them when entities are deleted, but they're
less special.
We don't do anything with this info right now besides log it,
but the logs show we are reporting it at the right times
in the `specify_tracked_fn_in_rev_1_but_not_2` test
(also fix an oversight in the test where it was creating a new input
each time).
Rename QueryInputs to QueryEdges.
Modify its fields to track both inputs and outputs.
The size of the struct doesn't actually change,
as the separator comes out of padding.
We will record each thing that gets *output* by the query.
We use a btree-set so that we can get a sorted list.
That will allow us to easily compare what is output between revisions.
We will use that to clear stale values.
We don't do anything with this info right now besides log it,
but you can see that we are reporting it at the right times
in the `specify_tracked_fn_in_rev_1_but_not_2` test
(also fix an oversight in the test where it was creating a new input
each time).
Rename QueryInputs to QueryEdges and modify its fields
to track both inputs and outputs. The size of the struct
doesn't actually change, the separator comes out of padding.
We will record each thing that gets *output* by the query.
Use a btree-set so that we can get a sorted list.
That will allow us to easily compare what is output between revisions.
We will use that to clear stale values.
