2396: Switch to variant-granularity field type inference r=flodiebold a=matklad
r? @flodiebold
Previously, we had a `ty` query for each field. This PR switcthes to a query per struct, which returns an `ArenaMap` with `Ty`s.
I don't know which approach is better. What is bugging me about the original approach is that, if we do all queries on the "leaf" defs, in practice we get a ton of queries which repeatedly reach into the parent definition to compute module, resolver, etc. This *seems* wasteful (but I don't think this is really what causes any perf problems for us).
At the same time, I've been looking at Kotlin, and they seem to use the general pattern of analyzing the *parent* definition, and storing info about children into a `BindingContext`.
I don't really which way is preferable. I think I want to try this approach, where query granularity generally mirrors the data granularity. The primary motivation for me here is probably just hope that we can avoid adding a ton of helpers to a `StructField`, and maybe in general avoid the need to switch to a global `StructField`, using `LocalStructFieldId` most of the time internally.
For external API (ie, for `ra_ide_api`), I think we should continue with fine-grained `StructField::ty` approach, which internally fetches the table for the whole struct and indexes into it.
In terms of actual memory savings, the results are as follows:
```
This PR:
142kb FieldTypesQuery (deps)
38kb FieldTypesQuery
Status Quo:
208kb TypeForFieldQuery (deps)
18kb TypeForFieldQuery
```
Note how the table itself occupies more than twice as much space! I don't have an explanation for this: a plausible hypothesis is that single-field structs are very common and for them the table is a pessimisation.
THere's noticiable wallclock time difference.
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>
The current system with AstIds has two primaraly drawbacks:
* It is possible to manufacture IDs out of thin air.
For example, it's possible to create IDs for items which are not
considered in CrateDefMap due to cfg. Or it is possible to mixup
structs and unions, because they share ID space.
* Getting the ID of a parent requires a secondary index.
Instead, the plan is to pursue the more traditional approach, where
each items stores the id of the parent declaration. This makes
`FromSource` more awkward, but also more correct: now, to get from an
AST to HIR, we first do this recursively for the parent item, and the
just search the children of the parent for the matching def
