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start chapter about interners

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Aleksey Kladov 2019-01-19 20:20:45 +03:00
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@ -351,11 +351,37 @@ declarations.
[`submodules_query`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/module_tree.rs#L41)
We store the resulting modules in a `Vec`-based indexed arena. The indices in
the arena becomes module identifiers.
the arena becomes module ids. And this brings us to the next topic:
assigning ids in the general case.
## Location Interner pattern
One way to assign ids is how we've dealt with modules: collect all items into a
single array in some specific order and use index in the array as an id. The
main drawback of this approach is that ids are not stable: adding a new item can
shift ids of all other items. This works for modules, because adding a module is
a comparatively rare operation, but would be less convenient for, for example
functions.
Another solution here is positional ids: we can identify a function as "the
function with name `foo` in a ModuleId(92) module". Such locations are stable:
adding a new function to the module (unless it is also named `foo`) does not
change the location. However, such "id" ceases to be a `Copy` integer and in
general can become pretty large if we account for nesting (third parameter of
the foo function of the bar impl in the baz module).
[`LocationInterner`] allows us to combine benefits of positional and numeric
ids. It is a bidirectional append only map between locations and consecutive
integers which can "intern" a location and return an integer id back. Salsa
database we use includes a couple of [interners]. How to "garbage collect"
unused locations is an open question.
[`LocationInterner`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_db/src/loc2id.rs#L65-L71
[interners]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/db.rs#L22-L23
For example, we use `LocationInterner` to assign ids to defs: functions,
structs, enums, etc.
## Macros and recursive locations
## Name resolution