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RFC: describe 'static requirement

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Niko Matsakis 2020-07-05 10:41:46 +00:00
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book/src/rfcs/RFC0006-Dynamic-Databases.md
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@ -634,6 +634,25 @@ we have to make a few changes:
* The `DatabaseStorageTypes` supertrait is removed (that trait is renamed and altered, see next section).
* The `salsa_event` method changes, as described in the User's guide.
### Salsa database trait requires `'static`, at least for now
One downside of this proposal is that the `salsa::Database` trait now has a
`'static` bound. This is a result of the lack of GATs -- in particular, the
queries expect a `Q::DynDb` as argument. In the query definition, we have
something like `type DynDb = dyn QueryGroupDatabase`, which in turn defaults to
`dyn::QueryGroupDatabase + 'static`.
At the moment, this limitation is harmless, since salsa databases don't support
generic parameters. But it would be good to lift in the future, especially as we
would like to support arena allocation and other such patterns. The limitation
could be overcome in the future by:
* converting to a GAT like `DynDb<'a>`, if those were available;
* or by simulating GATs by introducing a trait to carry the `DynDb` definition,
like `QueryDb<'a>`, where `Query` has the supertrait `for<'a> Self:
QueryDb<'a>`. This would permit the `DynDb` type to be referenced by writing
`<Q as QueryDb<'a>>::DynDb`.
### Salsa query group traits are extended with `HasQueryGroup` supertrait
When `#[salsa::query_group]` is applied to a trait, we currently generate a copy